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Page 1 of 10 Hydrocarbon & Related Projects Larsen & Toubro Limited Baroda , India
PROCESS DESIGN PACKAGE
95-PDS-P
PRESSURE VESSEL & PIPING DESIGN ALL RIGHTS RESERVED THIS DOCUMENT AND ANY DATA AND INFORMATION CONTAINED THEREIN ARE CONFIDENTIAL AND THE PROPERTY OF L&T-HARP AND THE COPYRIGHT THEREIN IS VESTED IN L&T. NO PART OF THIS DOCUMENT, DATA, OR INFORMATION SHALL BE DISCLOSED TO OTHERS OR REPRODUCED IN ANY MANNER OR USED FOR ANY PURPOSE WHATSOEVER, EXCEPT WITH THE PRIOR WRITTEN PERMISSION OF L&T.
1.0 Introduction This chapter deals on code requirements for Pressure vessels & Piping design used in Refining & Petrochemical Industry.
2.0 Pressure vessel design 2.1 Code Requirements ASME Codes. 1. SECTION VIII, DIVISION 1. All process vessels should be designed according to the ASME Section VIII, Unfired Pressure Vessel Code (Latest Edition) and/or the codes which govern in the particular country in which the unit is to be installed. This does not apply to small vessels made of pipe and atmospheric vessels handling water and injection chamicals (see Section I.A.2 and 3) 2. SECTION I. All vessels used for the purpose of generating steam with a process fluid or within a process heater will be designed according to the ASME Section I, Boiler Code (Laters Edition). In some instances, where steam is generated by a process stream, without the use of direct fire, the vessels may be designed according to the Section VIII Code if requested in writing by the customer. 3. SECTION VIII,DIVIDION 2. In cases of vessels operating at pressures above 1000 PSIG and less than 900 oF., consideration should be given to deg to the ASME section VIII, DIvision 2 Code. This code is similar to the ASME Section III, Nuclear Code and permits using a safety factor of 3 rather than the usual 4 in the design of the vessel. There are special requirements on the fabrication designer and material testing to use Division 2 which can use up much of the savings due to the higher allowable stress. 4. NON-CODE VESSELS. Many atmospheric vessels handling water and injection chemicals need not be code vessels. There is a nominal savings due to not having to be fabricated in a Code shop and not requiring a Code stamp. 5. OTHER CODES. API 510 Pressure vessel inspection code and IS 2825 Pressure vessel design code.
2.2. Design Conditions 2.2.1 PRESSURE a. NORMAL DESIGN. The design pressure should , normally be a minimum of 25 PSI or 10% above the maximum operating pressure, whichever is greater. Minimum design pressure is normally 50 PSIG. b. VERY LARGE VESSELS. In cases of very large vessels, operating at low pressures. The 25 PSI increment is sometimes used for designed above operating pressure, rather than the minimum of 50 PSIG. c. HIGH PRESSURE. At operating pressures above 1000 PSIG the design conditions are taken as minimum of 100 PSI or 5% above operating pressure, whichever is to obtain bubble tightness at 95% of set pressure.
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Page 2 of 10 d. VACCUM. Vessels operating under any condition of vaccum should be designed for full vaccum. e. POSSIBLE VACCUM. Vessels not operating under a vaccum, but subject to a vacuum condition due to failure of a control or heat source, should be designed for full vaccum or provided with a vacuum breaking device. Deg for vacuum usually only requires the addition of stiffening rings to the shall for most vessels designed for a reasonable pressure and shop fabricated sizes. f. LIQUID FILLED VESSELS . For vessels operating liquid full where a sudden drainage or other circumstance, such as flow stoppage and subsequent cooling should produce a vaccum condition, should be designed for vaccum. Design pressure is usually given for top and bottom separately. g. HYDROSTATIC TEST. In some instance, the design pressure may be dictated by the static head of liquid required for hydrostatic testing of the vessel. In cases of extreamely tall vessels in non-critical service, such as blowdown sumps, a pneumatic test is sometimes used to avoid this requirement.
2.2.2. Temperature a. NORMAL DESIGN. The design temperature should be about 50oF above the normal operating temperature. Consideration should be given to failure of coolers ahead of vessels which could require a greater increment than 50oF. b. HIGH TEMPERATUE. In cases of reactors operating at high temperatures and/or high pressure, where the allowable stress drops rapidly with each temperature increment, they are usually designed for the maximum expected. operating temperature at end of run. This practice is usually conservative if the reactions are endothermic. Consideration should be given to the accuracy of the temperature sensing element so as not to exceed the design temperature without sensing it. c. FLANGE RATING CONSIDERATION. In setting the design temperature of vessels, the effect on flange rating should always be taken into so as not to increase costs unduly. d. COLD WALL DESIGN. When temperatures are well above levels which can be reasonably designed for and in some instances to reduce alloy and thickness requirments in high pressure vessels, cold wall vessels are used ( internally insulated) where the outside shell is designed for 300 F.
2.2. 3 Corrosion Allowance a. NORMAL CORROSION ALLOWANCE. The normal corrosion allowance on carbon steel vessels in non-corrosive service is 1/8 inch. Vessels are normally designed for 10 years service. b. CLADDING. Vessels in severely corrosive service are usually alloys clad or "weld - overlayed" for protection. Where cladding or weld-overlay is used to protect against corrosion no corrosion allowance is added to the base metal. c. CEMENT LINING Where corrosion aqueous phases are handled at low temperatures they are sometimes protected with a layer of "acid proof cement."
2.3 Sizing of Vessels 1. Reactors, Fixed Bed. a. DIAMETER SELECTION. The diameter is picked as small a possible to keep wall thickness to a minimum. Pressure drop is the prime consideration, during normal operation, and when catalyst is coked or fouled at end of run. Consideration should also be given to shipping of vessel. Railways can ship up to about 13 feet in diameter.If brought to site by ship, reators up to about 16 feet O.D. have been handled. b. L/D RATIO. The diameter is normally chosen to give bed length to diameter ratio of about 3. Some processes may be as low as 1 and units such as Isomax can have ratios as high as 5. c. OUTAGE ALLOWANCE. The space above the catalyst bed should be great enough for a man to work in while installing balls, baskets, distributors, etc. that catalyst and balls must be spread level across file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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Page 3 of 10 the bed. d. CATALYST . The space below the catalyst is usually determined by the catalyst media and by-ing considerations. e. SLUMPING. Catalyst slumping should be taken into on deep beds. It is primarily a function of growth of the shell when heated and is about 1/4 inch per foot of catalyst. f. COLD WALL. Coldwall reactors usually have 5 inches of refractory lining, and an inner alloy liner about 1 inch inside of the refractory and sealed to the shell at one end to prevent by-ing the catalyst bed and thereby heating the shell. The inside diameter will, therefore, be about one foot less than the shell I.D.
2.3.2. Fractionators a. DIAMETER SELECTION. The diameter is designed for a maximum of 75% of flood. Care should be used with columns operating above 200 PSIG to select the most conservative method of design for high vapor density. Consideration should also be given to shipping of the vessel. Railways can handle up to about a 13 foot diameter. If it cannot be shipped, it must be field rather than shop fabricated which increases cost appreciably. b. TRAY SPACING. Tray spacing should be 24 inches for easy maintenance. For small columns with trays on rods and spacers this can be reduced to 18 inches, For columns in clean service and more than about 40 trays, spacing can be 18 inches also. Tray spacing at the feed point should be at least 24 inches and can be more if required for the distributor and manway. Also refer Tower Chapter for process guidelines. c. TRAY NUMBERING. Trays should be numbered starting from the top. Some designers number it from bottom also. d. TOP SPACE. The top space in fractionators is normally 3 feet from the top tray to the tangent line. More may be required when the vapor line is located on the shell, to prevent entrainment. e. BOTTOM SPACE. The bottom space is usually a minimum of 7 feet so that a man can stand in the bottom of the column. Reboiler return nozzle should be below bottom tray seal pan and above the maximum operating level of bottom level controller. f. SIDE-CUTS. Side draws from columns can be taken from a well on a tray immediately below a downcomer, when surge time is not required, and from a centerwell where surge is required. A centerwell should always be used when a pump takes suction direct from the column. The centerwell should have a vapor pipe with at least 10% of the column cross sectional area and provide enough space above the hat to give good vapor distribution to the tray. Vapor side-cuts usually require an accumulator pipe and about 5 to 7 foot tray spacing. g. SURGE CAPACITY. Surge capacty in the bottom of the column should be a minimum of 1 minute from the normal level. If the bottoms is feeding a reactor or a fractionator making a critical split, the time should be 5 minutes. Most columns can operate well with 2 minutes. Type of control should be taken of, as wall as destination and heat sensitivity of bottoms. Occasionally, a column will have so small a bottoms stream that the residence time is excessive and cause fouling of the reboiler. Adding a diluent to the feed can sometimes help. Time is based on volume from normal level and is usually based on net bottoms. See also Drum chapter for detail design practices. h. SWAGED COLUMN. Fractionating columns are often owaged above the feed point to reduce the cost of the column where the loadings are light. They are sometimes swaged out at the bottom toaccomodate stab-in reboilers or to provide additional surge time in the botom. Swaging is not usually done unless the column diameter can always section. On columns below 5 feet the reduction should always be 1 foot or more. i. L/D AND BRACING. Columns with a length to diameter ratio of less than 20 will usually not need bracing to precent away. Above this ratio, some columns may require bracing to other columns and in extreme cases guy wires. 2.3.3. Overhead Receivers. a. DIAMETER. The diameter of the receiver is usually set by vapor velocity, but may be determined sometimes by liquid residence time if vapor rate is low or nothing. The vapor velocity is kept below 0.157 (dl/dv)1/2 FPS. An old rule, not usually followed today, was that the receiver diameter was the same as the column diameter so that the heads could all be rolled together and thereby save money. This can still be used as a rough check. b. LENGTH. The length of an overhead receiver is usually about 3 times the diameter; however, if the diameter has been set by the vapor velocity the length will minutes on reflux and 5 minutes on net overhead, one-half full. It is preferable to have 5 minutes on total overhead, half full, so as to allow time to start the spare pump during a pump failure before the receiver overflows. If water settling required, residence time of hydrocarbon is usually 10 to 15 minutes. Residence time of the water should be 5 to 10 minutes, in a boot. Consider density of hydrocarbon file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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and any emulsion stabilizers present when setting time.
3.0 Features of Vessels 3.1 Types of Heads a. ELLIPTICAL. Normally 2:1 elliptical heads are used on most process vessels. The thickness of this type head will be almost the same as the shell. b. DISHED. Dished heads can be used in low pressure service when more economical. c. HEMISPHERICAL. Hemispherical heads are used when pressures are above 1000 PSIG.The thickness will be about one-half of the shell thickness, if designed to the same allowable stress. 3.2. Types of shell Construction a. ROLLED. Most vessels are fabricated by rolling of plate to the prescribed diameter. When plate thickness is above 2 to 3 inches the number of fabricators able to roll is rather small. b. FORGED. Some thick walled vessels are made by hollow forging. c. MULTILAYER. Many of the heavy walled reactors are now being fabricated by multilayer construction. One method (CB&I) buildsup layers of about 1/4 inch plate by shrinking one rolled ring over another until the desired thickness is reached. This method has the longitudinal weld t in each layer staggered 90 o. Another method, used in japan, coils 3 mm plate over a rolled core untill the desired thickness is reached and then is topped with a binding ring about one inch thick. This method has practically no longitudinal ts which have twice the stress of the transverse ts. 3.3. Types of Vessels s A. SKIRT. Most verical vessels are ed with a skirt type . The bottom nozzles should project through the skirt, if possible, to avoid flanges inside the skirt and the hazard of leaks. b. PIPE LEGS. Pipe legs are sometimes used on short drums at low pressures to save cost and avoid flang inside skirts where vessels are linked with alloy. c. TABLE TOP. Table top is used where access to the bottom of a vessel is required. It is the most expensive type of . The vessel can be ed on the table with lugs or a short skirt. d. SADDLES. Most horizontal vessels are ed on saddles which set on concrete blocks. 3.4. Nozzles a VENTS All vessels (except reactors with hydrogen recycle ) should have a vent, either on the vessel or on piping near the vessel at the highest point for venting to the flare or atmosphere. Frationator receiver vants should be large enough to depressure cloumns, with no net gas lines, in about 5 minuts during an emergency. b. DRAINS All vessels (except reactors with hydrogen recycle) should have a drain, either on the vessel or on the piping near the vessel at the lowest point for draining to the sewer or pumpout system. A 1-1/2 drain is the minimum shown for the bottom of a fractionator, but should be much largher for very large colmuns. c. LEVEL. Level measurement connections are provided at the bottom of all fractionator and other vessel in which a vapor-liquid or liquid-liquid interface exists. Gage glass connection are 1 inch nozzles and are usually taken off the same nozzles used for the level controller if both are provided. Level control nozzles are a minimum of 11/2 inches. Some require that the gage glass and controller connections be separate. The lower connection should be taken off the side or end of horizontal vessels to prevent pocketing of solids or water d. STANDPIPES. Standpipes are used on the inside of some bottom connections, if a water drain is provided, to prevent water from being withdrawn with a hydrocarbon. e. STEAMOUT. Normally steamout connections on vessels need not be provided unless requested by the customer. We normally assume steamout can be accomplished thru the level glass nozzles on the vessels, or through drain connections. f. CATALYST WITHDRAWAL. Catalyst withdrawal nozzles are used in radial flow reactors where the size of the file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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centrepipe will not permit pulling it up to unload catalyst. They are also used on most downflow reactors. g. TOO MANY NOZZLES. According to the ASME Code, all nozzles in vessel heads must be inside of a circle of diameter 0.8D. On very small columns, it is sometimes necessary to install the vapor outlet on the side rather than in the head to satisfy this. h. MAXIMUM SIZE. The maximum size nozzle which generally can be installed in a vessel is 1/2 in the diameter of the vessel. Larger nozzles can be installed but special calculations and inspections are required. I. MINIMUM SIZE. The smallest connections we use on any pressure vessel is one inch. They are also always falnged nozzles. Coupling connections for gage glasses, etc. are only used for non-code type vessels not overating under pressure. 3.5. MANWAYS a. FRATIONATOR. All fractionators should have an 18 inch I.D. manway at the top, an 18 or 16 inch I.D. at the feed tray and a 16 inch I.D.at the bottom. Care should be taken in specifying the maximum size tray section which can be installed in the Tray specification. Sometimes the manways will have to be larger to permit installation of a distributor. b. DRUMS All recievers, drums and separators should have a minimum of one 16 inch I.D. manway for inspection of the vessel. This may have to be larger sometimes to permit installations of internals. c. SMALL VESSELS. In cases where a vessels is too small to enable a man toenter,the Code permits providing 2 handholes to inspect the vessel. Locate so as to beable to use one as a light source for the other. They should be at least 6 inches I.D. d. REACTORS. Reactors usually combine the top manway with the inlet piping and use it for also loading catalyst. The size should be a minimum of 16 inches I.D. and is usually larger to permit installations of internals. In some reactors or caly towers a manway is provided on the side at the bottom of the reactor to unload unrecoverable catalyst or clay directly into a truck. 3. 6. Distributors a. FRACTIONATOR. All fractionators should have distributors on the feed and reflux streams in order to properly distribute the liquid onto the tray. The best type is a tee properly located which can handle all - liquid or mixed phase stream. Care should be taken not to run distributor pipes through downcomers which might interface with tray hydraulics. The distributo pipe should not be located too close to the tray, particularly if perpendicular to the liquid flow, where it will interefere with the flow of liquid and froth. If a vapor side-cut is taken from a column, an accumulator (slotted pipe) is usually used to avoid upsetting the tray hydraulics. b. DRUM. Vertical receivers or separators should use a tee distributor located close to wall. Horizontal receivers normally use a vertical pipe one size larger than the inlet nozzle. If the feed is all liquid, the pipe may be slotted with a closed end or it can be open-ended with no slots. The latter are used where subcooling is employed to overcome pressure, total condensing, fractionator overhead. Closed and slotted pipes are the most common for mixed phase and totally condensed feeds. The slotted area is made atleast twice the area of the inlet pipe. On a totally condensed fractionator overhead, the liquid is sometimes brought in the bottom of the receiver with only a standpipe to prevent water from running back into the incoming feed. Any feed stream entering in the vapor section of a receiver, particularly if oxygen can be present, should have a distributor to prevent free fall of liquid through the vapor section where static charge can build up and be an explosive hazard. c. SETTLERS. Liquid-Liquid settlers sometimes have a slotted pipe distributor located horizontally at the interface in the vessel so as to least disturb the settling action. d. REACTORS. Reactors use three or four types of distributors. The feed distributor is in most cases a velocity breaker to prevent impingment on the catalyst or liquid- vapor distributor at the top of of the catalyst bed. In some cases a liquid-vapor mixture is distributed with a mixing device at the inlet. Another device often used at the top and intermediate distribution points in reactors with mixed phase streams is a perforated plate, with raised vapor pipes, calculated to give at least 1 inch or liquid head at the lowest liquid rate. At the highest liquid rate the liquid overflows, from the top plate, through liquid overflows,onto a second perforated plate with 2 to 3 times the orifice area of the top plate and is distributed by the liquid head which may be up in the liquid overflow pipe. The pressure drop through the vapor pipes should be less than 0.1 inches of the flowing liquid so as not to interefere with liquid distribution. Quench distributors are used with os without redistributor trays, and are normally located in the catalyst bed. Suitable means must be used to give uniform flow and temperature of the quench stream. A double concentric pipe is sometimes employed which satisfies both of the requirements just mentioned as well as preventing catalyst attrition from high velocity exit from the distributor pipe. Liquid quench is normally injected into a vapor space file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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between catalyst beds with a spray nozzle. Distribution of liquid and vapor is one of the most difficult and important problems faced by the designer. If good distribution is not accomplished, the best catalyst may not perform and may even coke rapidly due to stagnant areas. In some processes dead pockets can cause demethylation reactions which have been known to damage reactor walls from the large exothermic heat of reaction. 3. 7. Catalyst s and Hold-Downs. a. GRATING AND SCREEN. This type of is used for multiple bed reactors where the grating is usually hinged for dropping the catalyst. b. CERAMIC BALLS. This is the most common for down-flow reactors and utilizes an elephant stool as an outlet containment. c. HOLD-DOWNS. Usually most catalyst beds have a layer of 1/4 inch and one of 3/4 inch ceramic balls to act as hold-down and also as a velocity breaker. On up-flow type reactors a grating and screen hold-down is used which can be fastened in place. 3.8. Vortex Breakers a. BOTTOM. Vortex breakers are used in any vessel outlet nozzle from which a pump is taking suction. They should also be used if gas entrainment would present a problem in downstream equipment to which an outlet is feeding. The standard VB is made of two plates forming a cross extending down into the nozzle and overlapping the nozzle above the outlet. b. SIDE. If a pump suction is taker off the side of a vessel, a single plate is sometimes installed to act as a VB when the outlet is close to a liquid-vapor interface. c. LARGE NOZZLES. On very large outlet nozzles a cover plate is sometimes installed over the standard VB to prevent vortexing into the quarter sections of the VB. 3.9 Mesh Blankets. a. DEMISTERS. Demisters are used to remove entrained liquid in vapor streams where such liquid could cause damage to downstream equipment such as compressors or result in loss of product to fuel, etc. They are usually stainless steel, except, where chlorides are present they should be monel. b. COALESCERS. Coalescers are used to coalesce fine droplets of water from liquid hydrocarbons in a shorter period of time than could be accomplished in an empty vessel. The usual material is stainless steel although monel and fiber-glass have been used. If an organic liquid is disparsed in water and coaleacing is needed the material blankets should be made of a material such as Teflon, or other organic material, which is perferentially wetted by the dispersed phase. 3.10. Vessel Boots. a. TYPES. Vessel bbots are used to permit controlled withdrawal of a water phasefrom a vessel. They are welded to the vessel when no corrosion problem is anticipated and are usually made of pipe. If corrosion is a problem, the boot is flanged to the vessel and may be disposable or alloy lined b. INCREASED TIME. When increased settling time is desired, over what is available in the boot alone, the controlled level may be up in the vessel about 6 inches. The hydro-carbon outlet stream should have a higher standpipe in this case to avoid entrainment. 3.11. Fireproofing and Insulation. a. FIREPROOFING. Fireproofing is applied to all steel vessel s, such as skirts and table tops, which could cause overturning of the vessel from failure during a fire. b. INSULATION. Insulation is called for whenever heat loss would affect the process Insulation for personnel protection is left to the contractor and is not shown in our specs or drawings. 3.12. Davits and Platforms. a. DAVITS . Davits are a type of crane installed at the top of all fractionators to facilitate raising of trays to the top. They are also installed on manways where required to manipulate the covers when vertical. file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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b. PLATFORMS. Platforms are installed at all manways and wherecer required to reach equipment, such as controls, more than about 8 feet above grade. 3.13. Stiffener and Insulation Rings. a. STIFFENER RINGS. Stiffener rings are made from angle irons, channels or teesections and are used to strenghen vessels for vacuum service. b. INSULATION RINGS. Insulation rings are angle irons used to block insulation on the walls of vessels.
4.0 FABRICATION PROCEDURES FOR PRESSURE VESSELS 4.1. Post Weld Heat Treatment (PWHT) and Radiographing
1. When to stress Relieve or PWHT a. CODE THICKNESS LIMITS. Residual stresses and charges in metallurgical structure produced by welding and forming particularly in thick walled vessels render the vessel prone to cracking failure. Therefore, the ASME Code requires stress relief above the following thicknesses for carbon steel and alloys: Carbon Steel above 11/2 inches Carbon Steel -1/2Mo above 5/8 inches Chrome, Moly Alloys Varies with alloys content Austenitic St. Steel. not required b. PROCESS REASONS. Carbon steel vessels handling acidic materials, such as fluorides, wet hydrogen sulpfide, hot caustic, etc. should be stress relieved to prevent stress corrosion, and/or cracking. 2. RADIOGRAPHY TESTING a. SPOT RADIOGRAHPING. This is an optional (also sometimes code requirement) inspection tool and credit for t efficiency is allowed. It is an aid to quality control and is usually a 6 inch spot check for every 50 feet or less of weld by each operator and weld procedure. b. FULL RADIOGRAPHY AND THICKNESS LIMITS. The ASME section VIII Code requires a full radiographing of butt-welded ts of all vessels whose thicknesses exceed the following : Carbon Steel above 11/4 inches Carbon Steel , 1/2%Mo above 3/4 inches 1 1/4% Cr-1/2% Mo above 5/8 inches 21/4% to 13% Cr all thicknesses Austenitic St. Steel. not required c. MANDATORY RADIOGRAPHING. All welded ed in vessels used to contain Lathal substances must be radiographed. Hydrocarbons are not considered to be lethal. ts in unfired steam boilers, the design pressure of which exceeds 50 psi, should be rediographed. d. NO RADIOGRAPHI. No rediographic examination of welded ts is required when the vessel is designed for external pressure only. 3. t Efficiency a. The following are mt efficiencies for double welded butt ts as allowed by the ASME Code : Full Radiograph 1.0 Spot Radiograph 0.85 No Radiographing 0.70 4.2. Other Inspection Methods file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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1. ULTRASONIC EXAMINATION. This methos is required by the Division 2 Code for all plate and forgings greater than 4 inches in thickness. It is also a valuable tool for checking welds in thick walled vessels where X-rays may not show defects and for periodic inspection of equipment to determine thickness and flaws. Our inspections normally use a proprietary tester called SONORAY. 2. MAGMAFLUXING. The method also referred to as "the magnetic particular test" is used for detecting cracks and discontinuitries in metals. An iron power is spread over the surface to be tested. The piece is then magnetized and any crack on the surface forms two poles, which attacks the iron particulars and exposes the crack. Any crack beyond the limit of magnetic penetration will not be revealed. 3. DIE OR LIQUID PENETRANT. These methods are used to detect discontinuities that are open to the metal surface. The dye method used a red dye which penetrates into the cracks and than stains a white coating or developer which is put on after cleaning with a special solution. The liquid penetrant method uses a fluorecent material which penetrates into the cracks and is then viewed under "black light " to expose the cracks. 4.3. Mill Scale and Rust Removal 1. Reason for Removal a. PREVENT PLUGGING. In processes using solvents which are goog descalin agents and which could cause plugging of small holes in extractor trays, valves and other suspectible places. b. PREVENT DAMAGE. Reciprocating compressor valves are particularly susceptible to damage from scale and other foreign materials carried into them with the flowing gas. c. PROTECT PROCESS. Certain processes, such as Butamer, Penes and ALkar are susceptible ato catalyst deterioration or corrosion due to water which is formed when the recycle hydrogen reacts with iron oxide to form water. 2. Methods of Removal a. ACIDIZING. This is the most common of removing scale and/or rust. Inhibited hydrochloric acid is the acid most used. The inhibitors, which are proprietary substances such as gelatin, are only effective in protecting carbon steel and all stainless steel valve trim, etc. should be removal from the acidizing environment. In systems containing substantial amounts of stainless steel, ammoniated critic acid is used for acidizing because it does not attack the stainless. b. SANDBLASTING AND SHOTBLASTING. These are effective methods of removing scale and rust from vessel walls, but cannot be used for pipe. They have been used for processes in place of acidizing to save cost and the problem of acid disposal. Large filters are placed in the lines permanently, to remove scale from the lines. c. REDUCTION. In some processes requiring removal of all traces of rust, the iron oxide is converted to the chloride salt by pretreating the reactor circuit, prior to installation of catalyst, with hot recycle hydrogen to which has been added chlorine or anhydrous HCL.
5.0.PIPING DESIGN 5.1. Code Requilrements and Standards 1. PIPING CODE ASA 31.3. All process piping should be designed according to the Petroleum Refinery Piping Code ASA 31.3 and the Standard Engg. Practices. 2.. FLANGE RATINGS ASA B16.5. All flanges shall conform to ASA B16.5 Standards and the ASME Section I and VIII Codes. Note that the ASME Section I Code derates the ASA flange ratings for boiler feedwater and blowdown services. 5.2. Design Conditions 1. PRESSURE. The design pressure of piping should be equal to the maximum design pressure of the equipment to which it connects. It should also be designed to the same margin above operating conditions as vessels are, if this is greater than the former. 2. TEMPERATURE. The design temperature of piping should be equal to the design temperature of the equipment to which it connects. It should also be designed to the same conditions above operating conditions as file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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vessels are, if this is greater than the former. The code allows the design temperature of flanges, for fluids above 32oF, to be taken as 90% of the fluid temperature if not insulated, The bolting, if uninsulated, can be designed for 80% of the fluid temperature. 5.3. Pressure Drop a. LIQUID. Liquid pressure drops are chosen to prevent vaporization and erosion and to give economical pumping costs. b. VAPOR. Vapor pressure drops are chosen to prevent erosion and high velocity sound and also to give economical compression costs. c. MIXED PHASE. Mixed phase pressure drops are usually set by the vapor velocities when a substantial amount of vapor is present. d. FITTING LOSS. On large size pipe, be careful to consider the pressure drop due to fittings, since a few elbows can amount to many equivalent feet. e. EXPANSION AND CONTRACTION LOSSES. On large size pipe be careful to consider the pressure drop or velocity heads due to expansion and contraction. Watch for manifold losses. For details refer Chapter 1 Fluid flow 5.4. Velocity a. EROSION. The most important consideration when pressure drop is not governing is erosion of the pipe. b. SOLIDS. Velocity high enough to prevent settling of solids is sometimes a consideration. c. WATER HAMMER. On long liquid handling lines where quick shut-off can occur, consideration should be given to preventing "water hammer" by keeping velocity low. d. CRITICAL VELOCITY. In vapor lines, velocities should be kept well below the critical velocity , the speed of sound. Whenever the upstream pressure is more than twice the pressure drop across an orifice, the speed of sound in the flowing gas is reached. Dropping the downstream pressure will not increase the flow of gas through the orifice. e. MIXED PHASE FLOW. If upflow of mixed phase flow in low pressure lines cannot be avoided, velocity should be kept high enough to prevent slug flow. 5.5. Types of Flanges 1. FLAT FACE. Flat face flanges are standard on cast iron fittings and are only used for water and air service. Where cast iron valves will be flanged to steel flanges, such as the water lines, in and out, of water coolers or condensers, the steel flanges are specified with flat face. This is to prevent breaking the cast iron flange when it is drawn up to a raised face steel flange. 2. RAISED FACE. Raised face flanges are standard on most process lines. 3. RING TYPE. Ring type fianges are used for high temperature, high pressure service and services where safety factors and toxic fluide arde being handled. UOP normally uses RTJ flanges in hydrogen service above 250 psig. For 300 pound flanges in hydrogen service, use 125 RMS raised face flanges with Flexitallic gasket in place of RTJ. 4. DISSIMILAR MATERIAL FLANGES. Where Austenitic and Ferritic flanges are ed in high temperature service, RTJ are not used above 4 inches. Instead, a modified type of Raised Face is used with Flexitallic gasketing. 5.6 Gaskets 1.. DOUBLE ARMORED. Double armored gaskets are standard for all Raised Face Flanges in process service. 2. ASBESTOS. Plain asbestos gaskets are only used for utility types of services. 3. RTJ. The Oval type of Ring Gasket made of a softer material than the flange is standard for RTJ. 4. FLEXITALLIC. Flexitallic gaskets designated for refinery service and with a retaining ring are used wherever a ring type t would be used normally, but for some reason is not desired. The raised face flanges used with flexitallic gaskets should be 125 RMS finished rather than & phono graphic finish used with armored or asbestos gaskets.
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5.7. Bolting 1. STUD BOLTS. Bolting of all process lines is with Stud type bolts rather than machine bolts. Machine bolts have the disadvantage of high stresses in the forged heads, do not seat as well as a nut, and are not as accessible as a stud bolt from which the nuts can be removed from either side. 2. BOLTING MATERIAL. Process line flange bolting is a minimum alloy of ASTM A-193-40T 2H (carbon steel ) nuts for process temperature up to about 1000oF.Higher alloys, including Austenitic steels, are used for higher temperatures. 3. BELLVILLE WASHERS. For some high temperature services, springs and special washers or spacers are used to minimize the effects of differential expansion of the bolts and flanges. 5.8. Flange Ratings 1. STANDARD. Standard flange ratings used in steel and alloy services are 150,300,600,900,1200,1500, and 2500 pounds. Flanges for higher ratings must be designed special, according to the ASMF Code. 2. CAST IRON. Cast iron flange ratings are 125, and 250 pounds and match the 150 and 300 pound steel flanges, respectively. 3. BOILER SERVICE. Flange ratings are derated in feedwater and blowdown services in the ASME Section I Code. 4. TEMPERATURE LIMIT. Good Engg. Practices does not use 150 pound flanges in services designed above 700oF, regardless of the design pressure. 5. RELIEF VALVE SERVICE. Normally less than 300 pound flanges are not used in relief valve service. Nozzles for relief valves are specified as I.D. rather than norminal pipe size. 6. DESIGN TEMPERATURE. The piping code allows the design temperature to be 90 percent of the fluid temperature for uninsulated flanged valves, flanged fittings and flanges.
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PROCESS DESIGN PACKAGE
95-PDS-P
PRESSURE VESSEL & PIPING DESIGN ALL RIGHTS RESERVED THIS DOCUMENT AND ANY DATA AND INFORMATION CONTAINED THEREIN ARE CONFIDENTIAL AND THE PROPERTY OF L&T-HARP AND THE COPYRIGHT THEREIN IS VESTED IN L&T. NO PART OF THIS DOCUMENT, DATA, OR INFORMATION SHALL BE DISCLOSED TO OTHERS OR REPRODUCED IN ANY MANNER OR USED FOR ANY PURPOSE WHATSOEVER, EXCEPT WITH THE PRIOR WRITTEN PERMISSION OF L&T.
1.0 Introduction This chapter deals on code requirements for Pressure vessels & Piping design used in Refining & Petrochemical Industry.
2.0 Pressure vessel design 2.1 Code Requirements ASME Codes. 1. SECTION VIII, DIVISION 1. All process vessels should be designed according to the ASME Section VIII, Unfired Pressure Vessel Code (Latest Edition) and/or the codes which govern in the particular country in which the unit is to be installed. This does not apply to small vessels made of pipe and atmospheric vessels handling water and injection chamicals (see Section I.A.2 and 3) 2. SECTION I. All vessels used for the purpose of generating steam with a process fluid or within a process heater will be designed according to the ASME Section I, Boiler Code (Laters Edition). In some instances, where steam is generated by a process stream, without the use of direct fire, the vessels may be designed according to the Section VIII Code if requested in writing by the customer. 3. SECTION VIII,DIVIDION 2. In cases of vessels operating at pressures above 1000 PSIG and less than 900 oF., consideration should be given to deg to the ASME section VIII, DIvision 2 Code. This code is similar to the ASME Section III, Nuclear Code and permits using a safety factor of 3 rather than the usual 4 in the design of the vessel. There are special requirements on the fabrication designer and material testing to use Division 2 which can use up much of the savings due to the higher allowable stress. 4. NON-CODE VESSELS. Many atmospheric vessels handling water and injection chemicals need not be code vessels. There is a nominal savings due to not having to be fabricated in a Code shop and not requiring a Code stamp. 5. OTHER CODES. API 510 Pressure vessel inspection code and IS 2825 Pressure vessel design code.
2.2. Design Conditions 2.2.1 PRESSURE a. NORMAL DESIGN. The design pressure should , normally be a minimum of 25 PSI or 10% above the maximum operating pressure, whichever is greater. Minimum design pressure is normally 50 PSIG. b. VERY LARGE VESSELS. In cases of very large vessels, operating at low pressures. The 25 PSI increment is sometimes used for designed above operating pressure, rather than the minimum of 50 PSIG. c. HIGH PRESSURE. At operating pressures above 1000 PSIG the design conditions are taken as minimum of 100 PSI or 5% above operating pressure, whichever is to obtain bubble tightness at 95% of set pressure.
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Page 2 of 10 d. VACCUM. Vessels operating under any condition of vaccum should be designed for full vaccum. e. POSSIBLE VACCUM. Vessels not operating under a vaccum, but subject to a vacuum condition due to failure of a control or heat source, should be designed for full vaccum or provided with a vacuum breaking device. Deg for vacuum usually only requires the addition of stiffening rings to the shall for most vessels designed for a reasonable pressure and shop fabricated sizes. f. LIQUID FILLED VESSELS . For vessels operating liquid full where a sudden drainage or other circumstance, such as flow stoppage and subsequent cooling should produce a vaccum condition, should be designed for vaccum. Design pressure is usually given for top and bottom separately. g. HYDROSTATIC TEST. In some instance, the design pressure may be dictated by the static head of liquid required for hydrostatic testing of the vessel. In cases of extreamely tall vessels in non-critical service, such as blowdown sumps, a pneumatic test is sometimes used to avoid this requirement.
2.2.2. Temperature a. NORMAL DESIGN. The design temperature should be about 50oF above the normal operating temperature. Consideration should be given to failure of coolers ahead of vessels which could require a greater increment than 50oF. b. HIGH TEMPERATUE. In cases of reactors operating at high temperatures and/or high pressure, where the allowable stress drops rapidly with each temperature increment, they are usually designed for the maximum expected. operating temperature at end of run. This practice is usually conservative if the reactions are endothermic. Consideration should be given to the accuracy of the temperature sensing element so as not to exceed the design temperature without sensing it. c. FLANGE RATING CONSIDERATION. In setting the design temperature of vessels, the effect on flange rating should always be taken into so as not to increase costs unduly. d. COLD WALL DESIGN. When temperatures are well above levels which can be reasonably designed for and in some instances to reduce alloy and thickness requirments in high pressure vessels, cold wall vessels are used ( internally insulated) where the outside shell is designed for 300 F.
2.2. 3 Corrosion Allowance a. NORMAL CORROSION ALLOWANCE. The normal corrosion allowance on carbon steel vessels in non-corrosive service is 1/8 inch. Vessels are normally designed for 10 years service. b. CLADDING. Vessels in severely corrosive service are usually alloys clad or "weld - overlayed" for protection. Where cladding or weld-overlay is used to protect against corrosion no corrosion allowance is added to the base metal. c. CEMENT LINING Where corrosion aqueous phases are handled at low temperatures they are sometimes protected with a layer of "acid proof cement."
2.3 Sizing of Vessels 1. Reactors, Fixed Bed. a. DIAMETER SELECTION. The diameter is picked as small a possible to keep wall thickness to a minimum. Pressure drop is the prime consideration, during normal operation, and when catalyst is coked or fouled at end of run. Consideration should also be given to shipping of vessel. Railways can ship up to about 13 feet in diameter.If brought to site by ship, reators up to about 16 feet O.D. have been handled. b. L/D RATIO. The diameter is normally chosen to give bed length to diameter ratio of about 3. Some processes may be as low as 1 and units such as Isomax can have ratios as high as 5. c. OUTAGE ALLOWANCE. The space above the catalyst bed should be great enough for a man to work in while installing balls, baskets, distributors, etc. that catalyst and balls must be spread level across file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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Page 3 of 10 the bed. d. CATALYST . The space below the catalyst is usually determined by the catalyst media and by-ing considerations. e. SLUMPING. Catalyst slumping should be taken into on deep beds. It is primarily a function of growth of the shell when heated and is about 1/4 inch per foot of catalyst. f. COLD WALL. Coldwall reactors usually have 5 inches of refractory lining, and an inner alloy liner about 1 inch inside of the refractory and sealed to the shell at one end to prevent by-ing the catalyst bed and thereby heating the shell. The inside diameter will, therefore, be about one foot less than the shell I.D.
2.3.2. Fractionators a. DIAMETER SELECTION. The diameter is designed for a maximum of 75% of flood. Care should be used with columns operating above 200 PSIG to select the most conservative method of design for high vapor density. Consideration should also be given to shipping of the vessel. Railways can handle up to about a 13 foot diameter. If it cannot be shipped, it must be field rather than shop fabricated which increases cost appreciably. b. TRAY SPACING. Tray spacing should be 24 inches for easy maintenance. For small columns with trays on rods and spacers this can be reduced to 18 inches, For columns in clean service and more than about 40 trays, spacing can be 18 inches also. Tray spacing at the feed point should be at least 24 inches and can be more if required for the distributor and manway. Also refer Tower Chapter for process guidelines. c. TRAY NUMBERING. Trays should be numbered starting from the top. Some designers number it from bottom also. d. TOP SPACE. The top space in fractionators is normally 3 feet from the top tray to the tangent line. More may be required when the vapor line is located on the shell, to prevent entrainment. e. BOTTOM SPACE. The bottom space is usually a minimum of 7 feet so that a man can stand in the bottom of the column. Reboiler return nozzle should be below bottom tray seal pan and above the maximum operating level of bottom level controller. f. SIDE-CUTS. Side draws from columns can be taken from a well on a tray immediately below a downcomer, when surge time is not required, and from a centerwell where surge is required. A centerwell should always be used when a pump takes suction direct from the column. The centerwell should have a vapor pipe with at least 10% of the column cross sectional area and provide enough space above the hat to give good vapor distribution to the tray. Vapor side-cuts usually require an accumulator pipe and about 5 to 7 foot tray spacing. g. SURGE CAPACITY. Surge capacty in the bottom of the column should be a minimum of 1 minute from the normal level. If the bottoms is feeding a reactor or a fractionator making a critical split, the time should be 5 minutes. Most columns can operate well with 2 minutes. Type of control should be taken of, as wall as destination and heat sensitivity of bottoms. Occasionally, a column will have so small a bottoms stream that the residence time is excessive and cause fouling of the reboiler. Adding a diluent to the feed can sometimes help. Time is based on volume from normal level and is usually based on net bottoms. See also Drum chapter for detail design practices. h. SWAGED COLUMN. Fractionating columns are often owaged above the feed point to reduce the cost of the column where the loadings are light. They are sometimes swaged out at the bottom toaccomodate stab-in reboilers or to provide additional surge time in the botom. Swaging is not usually done unless the column diameter can always section. On columns below 5 feet the reduction should always be 1 foot or more. i. L/D AND BRACING. Columns with a length to diameter ratio of less than 20 will usually not need bracing to precent away. Above this ratio, some columns may require bracing to other columns and in extreme cases guy wires. 2.3.3. Overhead Receivers. a. DIAMETER. The diameter of the receiver is usually set by vapor velocity, but may be determined sometimes by liquid residence time if vapor rate is low or nothing. The vapor velocity is kept below 0.157 (dl/dv)1/2 FPS. An old rule, not usually followed today, was that the receiver diameter was the same as the column diameter so that the heads could all be rolled together and thereby save money. This can still be used as a rough check. b. LENGTH. The length of an overhead receiver is usually about 3 times the diameter; however, if the diameter has been set by the vapor velocity the length will minutes on reflux and 5 minutes on net overhead, one-half full. It is preferable to have 5 minutes on total overhead, half full, so as to allow time to start the spare pump during a pump failure before the receiver overflows. If water settling required, residence time of hydrocarbon is usually 10 to 15 minutes. Residence time of the water should be 5 to 10 minutes, in a boot. Consider density of hydrocarbon file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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and any emulsion stabilizers present when setting time.
3.0 Features of Vessels 3.1 Types of Heads a. ELLIPTICAL. Normally 2:1 elliptical heads are used on most process vessels. The thickness of this type head will be almost the same as the shell. b. DISHED. Dished heads can be used in low pressure service when more economical. c. HEMISPHERICAL. Hemispherical heads are used when pressures are above 1000 PSIG.The thickness will be about one-half of the shell thickness, if designed to the same allowable stress. 3.2. Types of shell Construction a. ROLLED. Most vessels are fabricated by rolling of plate to the prescribed diameter. When plate thickness is above 2 to 3 inches the number of fabricators able to roll is rather small. b. FORGED. Some thick walled vessels are made by hollow forging. c. MULTILAYER. Many of the heavy walled reactors are now being fabricated by multilayer construction. One method (CB&I) buildsup layers of about 1/4 inch plate by shrinking one rolled ring over another until the desired thickness is reached. This method has the longitudinal weld t in each layer staggered 90 o. Another method, used in japan, coils 3 mm plate over a rolled core untill the desired thickness is reached and then is topped with a binding ring about one inch thick. This method has practically no longitudinal ts which have twice the stress of the transverse ts. 3.3. Types of Vessels s A. SKIRT. Most verical vessels are ed with a skirt type . The bottom nozzles should project through the skirt, if possible, to avoid flanges inside the skirt and the hazard of leaks. b. PIPE LEGS. Pipe legs are sometimes used on short drums at low pressures to save cost and avoid flang inside skirts where vessels are linked with alloy. c. TABLE TOP. Table top is used where access to the bottom of a vessel is required. It is the most expensive type of . The vessel can be ed on the table with lugs or a short skirt. d. SADDLES. Most horizontal vessels are ed on saddles which set on concrete blocks. 3.4. Nozzles a VENTS All vessels (except reactors with hydrogen recycle ) should have a vent, either on the vessel or on piping near the vessel at the highest point for venting to the flare or atmosphere. Frationator receiver vants should be large enough to depressure cloumns, with no net gas lines, in about 5 minuts during an emergency. b. DRAINS All vessels (except reactors with hydrogen recycle) should have a drain, either on the vessel or on the piping near the vessel at the lowest point for draining to the sewer or pumpout system. A 1-1/2 drain is the minimum shown for the bottom of a fractionator, but should be much largher for very large colmuns. c. LEVEL. Level measurement connections are provided at the bottom of all fractionator and other vessel in which a vapor-liquid or liquid-liquid interface exists. Gage glass connection are 1 inch nozzles and are usually taken off the same nozzles used for the level controller if both are provided. Level control nozzles are a minimum of 11/2 inches. Some require that the gage glass and controller connections be separate. The lower connection should be taken off the side or end of horizontal vessels to prevent pocketing of solids or water d. STANDPIPES. Standpipes are used on the inside of some bottom connections, if a water drain is provided, to prevent water from being withdrawn with a hydrocarbon. e. STEAMOUT. Normally steamout connections on vessels need not be provided unless requested by the customer. We normally assume steamout can be accomplished thru the level glass nozzles on the vessels, or through drain connections. f. CATALYST WITHDRAWAL. Catalyst withdrawal nozzles are used in radial flow reactors where the size of the file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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centrepipe will not permit pulling it up to unload catalyst. They are also used on most downflow reactors. g. TOO MANY NOZZLES. According to the ASME Code, all nozzles in vessel heads must be inside of a circle of diameter 0.8D. On very small columns, it is sometimes necessary to install the vapor outlet on the side rather than in the head to satisfy this. h. MAXIMUM SIZE. The maximum size nozzle which generally can be installed in a vessel is 1/2 in the diameter of the vessel. Larger nozzles can be installed but special calculations and inspections are required. I. MINIMUM SIZE. The smallest connections we use on any pressure vessel is one inch. They are also always falnged nozzles. Coupling connections for gage glasses, etc. are only used for non-code type vessels not overating under pressure. 3.5. MANWAYS a. FRATIONATOR. All fractionators should have an 18 inch I.D. manway at the top, an 18 or 16 inch I.D. at the feed tray and a 16 inch I.D.at the bottom. Care should be taken in specifying the maximum size tray section which can be installed in the Tray specification. Sometimes the manways will have to be larger to permit installation of a distributor. b. DRUMS All recievers, drums and separators should have a minimum of one 16 inch I.D. manway for inspection of the vessel. This may have to be larger sometimes to permit installations of internals. c. SMALL VESSELS. In cases where a vessels is too small to enable a man toenter,the Code permits providing 2 handholes to inspect the vessel. Locate so as to beable to use one as a light source for the other. They should be at least 6 inches I.D. d. REACTORS. Reactors usually combine the top manway with the inlet piping and use it for also loading catalyst. The size should be a minimum of 16 inches I.D. and is usually larger to permit installations of internals. In some reactors or caly towers a manway is provided on the side at the bottom of the reactor to unload unrecoverable catalyst or clay directly into a truck. 3. 6. Distributors a. FRACTIONATOR. All fractionators should have distributors on the feed and reflux streams in order to properly distribute the liquid onto the tray. The best type is a tee properly located which can handle all - liquid or mixed phase stream. Care should be taken not to run distributor pipes through downcomers which might interface with tray hydraulics. The distributo pipe should not be located too close to the tray, particularly if perpendicular to the liquid flow, where it will interefere with the flow of liquid and froth. If a vapor side-cut is taken from a column, an accumulator (slotted pipe) is usually used to avoid upsetting the tray hydraulics. b. DRUM. Vertical receivers or separators should use a tee distributor located close to wall. Horizontal receivers normally use a vertical pipe one size larger than the inlet nozzle. If the feed is all liquid, the pipe may be slotted with a closed end or it can be open-ended with no slots. The latter are used where subcooling is employed to overcome pressure, total condensing, fractionator overhead. Closed and slotted pipes are the most common for mixed phase and totally condensed feeds. The slotted area is made atleast twice the area of the inlet pipe. On a totally condensed fractionator overhead, the liquid is sometimes brought in the bottom of the receiver with only a standpipe to prevent water from running back into the incoming feed. Any feed stream entering in the vapor section of a receiver, particularly if oxygen can be present, should have a distributor to prevent free fall of liquid through the vapor section where static charge can build up and be an explosive hazard. c. SETTLERS. Liquid-Liquid settlers sometimes have a slotted pipe distributor located horizontally at the interface in the vessel so as to least disturb the settling action. d. REACTORS. Reactors use three or four types of distributors. The feed distributor is in most cases a velocity breaker to prevent impingment on the catalyst or liquid- vapor distributor at the top of of the catalyst bed. In some cases a liquid-vapor mixture is distributed with a mixing device at the inlet. Another device often used at the top and intermediate distribution points in reactors with mixed phase streams is a perforated plate, with raised vapor pipes, calculated to give at least 1 inch or liquid head at the lowest liquid rate. At the highest liquid rate the liquid overflows, from the top plate, through liquid overflows,onto a second perforated plate with 2 to 3 times the orifice area of the top plate and is distributed by the liquid head which may be up in the liquid overflow pipe. The pressure drop through the vapor pipes should be less than 0.1 inches of the flowing liquid so as not to interefere with liquid distribution. Quench distributors are used with os without redistributor trays, and are normally located in the catalyst bed. Suitable means must be used to give uniform flow and temperature of the quench stream. A double concentric pipe is sometimes employed which satisfies both of the requirements just mentioned as well as preventing catalyst attrition from high velocity exit from the distributor pipe. Liquid quench is normally injected into a vapor space file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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between catalyst beds with a spray nozzle. Distribution of liquid and vapor is one of the most difficult and important problems faced by the designer. If good distribution is not accomplished, the best catalyst may not perform and may even coke rapidly due to stagnant areas. In some processes dead pockets can cause demethylation reactions which have been known to damage reactor walls from the large exothermic heat of reaction. 3. 7. Catalyst s and Hold-Downs. a. GRATING AND SCREEN. This type of is used for multiple bed reactors where the grating is usually hinged for dropping the catalyst. b. CERAMIC BALLS. This is the most common for down-flow reactors and utilizes an elephant stool as an outlet containment. c. HOLD-DOWNS. Usually most catalyst beds have a layer of 1/4 inch and one of 3/4 inch ceramic balls to act as hold-down and also as a velocity breaker. On up-flow type reactors a grating and screen hold-down is used which can be fastened in place. 3.8. Vortex Breakers a. BOTTOM. Vortex breakers are used in any vessel outlet nozzle from which a pump is taking suction. They should also be used if gas entrainment would present a problem in downstream equipment to which an outlet is feeding. The standard VB is made of two plates forming a cross extending down into the nozzle and overlapping the nozzle above the outlet. b. SIDE. If a pump suction is taker off the side of a vessel, a single plate is sometimes installed to act as a VB when the outlet is close to a liquid-vapor interface. c. LARGE NOZZLES. On very large outlet nozzles a cover plate is sometimes installed over the standard VB to prevent vortexing into the quarter sections of the VB. 3.9 Mesh Blankets. a. DEMISTERS. Demisters are used to remove entrained liquid in vapor streams where such liquid could cause damage to downstream equipment such as compressors or result in loss of product to fuel, etc. They are usually stainless steel, except, where chlorides are present they should be monel. b. COALESCERS. Coalescers are used to coalesce fine droplets of water from liquid hydrocarbons in a shorter period of time than could be accomplished in an empty vessel. The usual material is stainless steel although monel and fiber-glass have been used. If an organic liquid is disparsed in water and coaleacing is needed the material blankets should be made of a material such as Teflon, or other organic material, which is perferentially wetted by the dispersed phase. 3.10. Vessel Boots. a. TYPES. Vessel bbots are used to permit controlled withdrawal of a water phasefrom a vessel. They are welded to the vessel when no corrosion problem is anticipated and are usually made of pipe. If corrosion is a problem, the boot is flanged to the vessel and may be disposable or alloy lined b. INCREASED TIME. When increased settling time is desired, over what is available in the boot alone, the controlled level may be up in the vessel about 6 inches. The hydro-carbon outlet stream should have a higher standpipe in this case to avoid entrainment. 3.11. Fireproofing and Insulation. a. FIREPROOFING. Fireproofing is applied to all steel vessel s, such as skirts and table tops, which could cause overturning of the vessel from failure during a fire. b. INSULATION. Insulation is called for whenever heat loss would affect the process Insulation for personnel protection is left to the contractor and is not shown in our specs or drawings. 3.12. Davits and Platforms. a. DAVITS . Davits are a type of crane installed at the top of all fractionators to facilitate raising of trays to the top. They are also installed on manways where required to manipulate the covers when vertical. file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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b. PLATFORMS. Platforms are installed at all manways and wherecer required to reach equipment, such as controls, more than about 8 feet above grade. 3.13. Stiffener and Insulation Rings. a. STIFFENER RINGS. Stiffener rings are made from angle irons, channels or teesections and are used to strenghen vessels for vacuum service. b. INSULATION RINGS. Insulation rings are angle irons used to block insulation on the walls of vessels.
4.0 FABRICATION PROCEDURES FOR PRESSURE VESSELS 4.1. Post Weld Heat Treatment (PWHT) and Radiographing
1. When to stress Relieve or PWHT a. CODE THICKNESS LIMITS. Residual stresses and charges in metallurgical structure produced by welding and forming particularly in thick walled vessels render the vessel prone to cracking failure. Therefore, the ASME Code requires stress relief above the following thicknesses for carbon steel and alloys: Carbon Steel above 11/2 inches Carbon Steel -1/2Mo above 5/8 inches Chrome, Moly Alloys Varies with alloys content Austenitic St. Steel. not required b. PROCESS REASONS. Carbon steel vessels handling acidic materials, such as fluorides, wet hydrogen sulpfide, hot caustic, etc. should be stress relieved to prevent stress corrosion, and/or cracking. 2. RADIOGRAPHY TESTING a. SPOT RADIOGRAHPING. This is an optional (also sometimes code requirement) inspection tool and credit for t efficiency is allowed. It is an aid to quality control and is usually a 6 inch spot check for every 50 feet or less of weld by each operator and weld procedure. b. FULL RADIOGRAPHY AND THICKNESS LIMITS. The ASME section VIII Code requires a full radiographing of butt-welded ts of all vessels whose thicknesses exceed the following : Carbon Steel above 11/4 inches Carbon Steel , 1/2%Mo above 3/4 inches 1 1/4% Cr-1/2% Mo above 5/8 inches 21/4% to 13% Cr all thicknesses Austenitic St. Steel. not required c. MANDATORY RADIOGRAPHING. All welded ed in vessels used to contain Lathal substances must be radiographed. Hydrocarbons are not considered to be lethal. ts in unfired steam boilers, the design pressure of which exceeds 50 psi, should be rediographed. d. NO RADIOGRAPHI. No rediographic examination of welded ts is required when the vessel is designed for external pressure only. 3. t Efficiency a. The following are mt efficiencies for double welded butt ts as allowed by the ASME Code : Full Radiograph 1.0 Spot Radiograph 0.85 No Radiographing 0.70 4.2. Other Inspection Methods file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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1. ULTRASONIC EXAMINATION. This methos is required by the Division 2 Code for all plate and forgings greater than 4 inches in thickness. It is also a valuable tool for checking welds in thick walled vessels where X-rays may not show defects and for periodic inspection of equipment to determine thickness and flaws. Our inspections normally use a proprietary tester called SONORAY. 2. MAGMAFLUXING. The method also referred to as "the magnetic particular test" is used for detecting cracks and discontinuitries in metals. An iron power is spread over the surface to be tested. The piece is then magnetized and any crack on the surface forms two poles, which attacks the iron particulars and exposes the crack. Any crack beyond the limit of magnetic penetration will not be revealed. 3. DIE OR LIQUID PENETRANT. These methods are used to detect discontinuities that are open to the metal surface. The dye method used a red dye which penetrates into the cracks and than stains a white coating or developer which is put on after cleaning with a special solution. The liquid penetrant method uses a fluorecent material which penetrates into the cracks and is then viewed under "black light " to expose the cracks. 4.3. Mill Scale and Rust Removal 1. Reason for Removal a. PREVENT PLUGGING. In processes using solvents which are goog descalin agents and which could cause plugging of small holes in extractor trays, valves and other suspectible places. b. PREVENT DAMAGE. Reciprocating compressor valves are particularly susceptible to damage from scale and other foreign materials carried into them with the flowing gas. c. PROTECT PROCESS. Certain processes, such as Butamer, Penes and ALkar are susceptible ato catalyst deterioration or corrosion due to water which is formed when the recycle hydrogen reacts with iron oxide to form water. 2. Methods of Removal a. ACIDIZING. This is the most common of removing scale and/or rust. Inhibited hydrochloric acid is the acid most used. The inhibitors, which are proprietary substances such as gelatin, are only effective in protecting carbon steel and all stainless steel valve trim, etc. should be removal from the acidizing environment. In systems containing substantial amounts of stainless steel, ammoniated critic acid is used for acidizing because it does not attack the stainless. b. SANDBLASTING AND SHOTBLASTING. These are effective methods of removing scale and rust from vessel walls, but cannot be used for pipe. They have been used for processes in place of acidizing to save cost and the problem of acid disposal. Large filters are placed in the lines permanently, to remove scale from the lines. c. REDUCTION. In some processes requiring removal of all traces of rust, the iron oxide is converted to the chloride salt by pretreating the reactor circuit, prior to installation of catalyst, with hot recycle hydrogen to which has been added chlorine or anhydrous HCL.
5.0.PIPING DESIGN 5.1. Code Requilrements and Standards 1. PIPING CODE ASA 31.3. All process piping should be designed according to the Petroleum Refinery Piping Code ASA 31.3 and the Standard Engg. Practices. 2.. FLANGE RATINGS ASA B16.5. All flanges shall conform to ASA B16.5 Standards and the ASME Section I and VIII Codes. Note that the ASME Section I Code derates the ASA flange ratings for boiler feedwater and blowdown services. 5.2. Design Conditions 1. PRESSURE. The design pressure of piping should be equal to the maximum design pressure of the equipment to which it connects. It should also be designed to the same margin above operating conditions as vessels are, if this is greater than the former. 2. TEMPERATURE. The design temperature of piping should be equal to the design temperature of the equipment to which it connects. It should also be designed to the same conditions above operating conditions as file://D:\WINNT\Profiles\jjsnl\Desktop\StandardsFS_files\annex
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vessels are, if this is greater than the former. The code allows the design temperature of flanges, for fluids above 32oF, to be taken as 90% of the fluid temperature if not insulated, The bolting, if uninsulated, can be designed for 80% of the fluid temperature. 5.3. Pressure Drop a. LIQUID. Liquid pressure drops are chosen to prevent vaporization and erosion and to give economical pumping costs. b. VAPOR. Vapor pressure drops are chosen to prevent erosion and high velocity sound and also to give economical compression costs. c. MIXED PHASE. Mixed phase pressure drops are usually set by the vapor velocities when a substantial amount of vapor is present. d. FITTING LOSS. On large size pipe, be careful to consider the pressure drop due to fittings, since a few elbows can amount to many equivalent feet. e. EXPANSION AND CONTRACTION LOSSES. On large size pipe be careful to consider the pressure drop or velocity heads due to expansion and contraction. Watch for manifold losses. For details refer Chapter 1 Fluid flow 5.4. Velocity a. EROSION. The most important consideration when pressure drop is not governing is erosion of the pipe. b. SOLIDS. Velocity high enough to prevent settling of solids is sometimes a consideration. c. WATER HAMMER. On long liquid handling lines where quick shut-off can occur, consideration should be given to preventing "water hammer" by keeping velocity low. d. CRITICAL VELOCITY. In vapor lines, velocities should be kept well below the critical velocity , the speed of sound. Whenever the upstream pressure is more than twice the pressure drop across an orifice, the speed of sound in the flowing gas is reached. Dropping the downstream pressure will not increase the flow of gas through the orifice. e. MIXED PHASE FLOW. If upflow of mixed phase flow in low pressure lines cannot be avoided, velocity should be kept high enough to prevent slug flow. 5.5. Types of Flanges 1. FLAT FACE. Flat face flanges are standard on cast iron fittings and are only used for water and air service. Where cast iron valves will be flanged to steel flanges, such as the water lines, in and out, of water coolers or condensers, the steel flanges are specified with flat face. This is to prevent breaking the cast iron flange when it is drawn up to a raised face steel flange. 2. RAISED FACE. Raised face flanges are standard on most process lines. 3. RING TYPE. Ring type fianges are used for high temperature, high pressure service and services where safety factors and toxic fluide arde being handled. UOP normally uses RTJ flanges in hydrogen service above 250 psig. For 300 pound flanges in hydrogen service, use 125 RMS raised face flanges with Flexitallic gasket in place of RTJ. 4. DISSIMILAR MATERIAL FLANGES. Where Austenitic and Ferritic flanges are ed in high temperature service, RTJ are not used above 4 inches. Instead, a modified type of Raised Face is used with Flexitallic gasketing. 5.6 Gaskets 1.. DOUBLE ARMORED. Double armored gaskets are standard for all Raised Face Flanges in process service. 2. ASBESTOS. Plain asbestos gaskets are only used for utility types of services. 3. RTJ. The Oval type of Ring Gasket made of a softer material than the flange is standard for RTJ. 4. FLEXITALLIC. Flexitallic gaskets designated for refinery service and with a retaining ring are used wherever a ring type t would be used normally, but for some reason is not desired. The raised face flanges used with flexitallic gaskets should be 125 RMS finished rather than & phono graphic finish used with armored or asbestos gaskets.
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5.7. Bolting 1. STUD BOLTS. Bolting of all process lines is with Stud type bolts rather than machine bolts. Machine bolts have the disadvantage of high stresses in the forged heads, do not seat as well as a nut, and are not as accessible as a stud bolt from which the nuts can be removed from either side. 2. BOLTING MATERIAL. Process line flange bolting is a minimum alloy of ASTM A-193-40T 2H (carbon steel ) nuts for process temperature up to about 1000oF.Higher alloys, including Austenitic steels, are used for higher temperatures. 3. BELLVILLE WASHERS. For some high temperature services, springs and special washers or spacers are used to minimize the effects of differential expansion of the bolts and flanges. 5.8. Flange Ratings 1. STANDARD. Standard flange ratings used in steel and alloy services are 150,300,600,900,1200,1500, and 2500 pounds. Flanges for higher ratings must be designed special, according to the ASMF Code. 2. CAST IRON. Cast iron flange ratings are 125, and 250 pounds and match the 150 and 300 pound steel flanges, respectively. 3. BOILER SERVICE. Flange ratings are derated in feedwater and blowdown services in the ASME Section I Code. 4. TEMPERATURE LIMIT. Good Engg. Practices does not use 150 pound flanges in services designed above 700oF, regardless of the design pressure. 5. RELIEF VALVE SERVICE. Normally less than 300 pound flanges are not used in relief valve service. Nozzles for relief valves are specified as I.D. rather than norminal pipe size. 6. DESIGN TEMPERATURE. The piping code allows the design temperature to be 90 percent of the fluid temperature for uninsulated flanged valves, flanged fittings and flanges.
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