Centrifugal Compressor Basics 3v1b3

CENTRIFUGAL COMPRESSOR BASICS

TRAINING Why and How? Why: How: 1. Practical aspects 2. Basics

SBW compressor in Iran Performance in Lavan Performance in Abadan

Compressor Definition: Increases the pressure of a gas.

Delivers the gas. Increases pressure and delivers the gas.

Compressor application: Dentist, Oil, gas, petrochemical, aviation

Compression How compression or delivery is achieved? Positive-displacement compressors confine successive volumes of fluid within a closed space in which the pressure of the fluid is increased as the volume of the closed space is decreased. Dynamictype compressors use rotating vanes or impellers to impart velocity and pressure to the fluid.

Compressors Classification Positive displacement Dynamics

Compressor Classification

Reciprocating Compressor

Screw Compressors

Axial Compressors

Integrally geared centrifugal compressor

Compressor Comparision

Compressors Range

Compressor Pr. Ratio

Centrifugal Compressor Centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diff (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants.Their application can be from 100 horsepower (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa). . They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines. Sometimes the capacity of the compressors is written in NM3/hr. Here 'N' stands for normal temperature pressure (20°C and 1 atm ) for example 5500 NM3/hr.

Howe C. Compressor works

Howe C. Compressor works

Howe C. Compressor works

Howe C. Compressor works

Centrifugal compressor component Casing Impeller Rotor Bearings Sealing system Balance system Control system Protection system

Compressor casing Horizontally (axially) split case A horizontally split case is split parallel to the axis of the rotor. The upper half of the case is bolted and doweled to the lower half. Access to the internals of the compressor for inspection and maintenance is facilitated with this case design (especially when the process piping connections are located on the bottom half of the case). The horizontally split design is inherently pressure-limited to prevent gas leakage at the case split t.

Vertically (radially) split case This case is split perpendicular to the axis of the rotor. Heads (end covers) are installed at both ends for pressure containment. The vertically split case configuration is capable of handling higher pressures than the horizontally split type. The rotor and stationary internals are assembled as a cylindrical inner bundle that is inserted axially through one end of the case. Inspection and maintenance of a radially split centrifugal compressor require that the inner bundle be removed for disassembly. Removal of the inner bundle requires that sufficient space be provided in the layout of the compressor installation.

Horizontal split compressor

Vertical split compressor

Impellers

Impeller type

Impeller type

Rotor

Rotor –Impeller-Balance disk

C. Compressor Major Components

Rotor Axial Balance

Rotor axial balance

Back to Back Balance

Back to Back Compressors

Compressor Sealing Internal sealing

Shaft sealing

Internal Sealing

Internal Sealing

Shaft Sealing Wet sealing system Dry sealing system

Wet seal

DRY GAS SEAL It will be explained in BCL compressor description.

Dry Gas Seal Beginning in the late 1980s, the compressor industry began to embrace the application of dry gas seal technology to the critical function of shaft sealing. The seal consists of a rotating disc running very close to a stationary ring. The rotating disc face contains special grooves that generate an axial (“lift”) force during rotation. The stationary ring is backed by a quantity of coil springs that force it tightly against the rotating disc when the compressor is at rest. The lift force compresses the coil springs slightly, resulting in the very small running clearance between the two faces. This small clearance effectively limits gas leakage from the compressor seals. The small amount of gas leakage exits the compressor through auxiliary seal piping, where it is then either sent to a flare system or to some other recovery system. Usually the two compressor seals (inlet and discharge ends of the compressor) are subjected to the gas suction pressure. A thrust balance line subjects the discharge end dry gas seal to inlet pressure, thereby avoiding the need to seal the higher discharge pressure. Dry gas seals require clean and dry gas for reliable operation. Seal gas is normally taken from the compressor discharge and then cooled and filtered as part of an external seal gas processing system. A seal reference pressure is measured just inboard of the dry gas seal, and a pressure regulating valve supplies the seal gas to the sealing faces at a pressure slightly above the reference pressure. This system ensures that the seals are not exposed to untreated process gas containing liquids or particulate matter that could damage the seals. Although dry gas seals are relatively expensive, their auxiliary system is less complex, physically smaller, and less expensive than the auxiliary system required by the predecessor liquid film design.

Dry Gas Seal: Single seal

Dry Gas Seal Dual seal

DGS: Tandem arrangement without intermediate labyrinth

Tandem arrangement with intermediate labyrinth

DGS: Tandem arrangement with intermediate labyrinth

Tandem arrangement with intermediate labyrinth The seal consists of two single seals arranged in tandem with an intermediate labyrinth. During normal operation the pressure is reduced via the sliding faces of the main seal to the pressure of the vent system, which exceeds the atmosphere pressure only slightly. The seal gas pressure ("Secondary seal gas") at the second seal exceeds the pressure at connection » B « ("Primary vent") only slightly. The product side seal is supplied with clean gas ("Primary seal gas") via connection » A «. The seal gas ("Secondary seal gas") is fed in via connection » C «. The seal gas pressure ("Secondary seal gas") slightly exceeds the pressure of the primary vent » B «, i.e. a flow direction through the intermediate labyrinth towards the the vent is ensured.

Thus the leakage of the product-side seal is carried off via connection » B « and a product leakage to the atmosphere is avoided if the system is operated as directed. If no gas is fed in at connection » C « some leakage of the seal gas will get to connection » S « via the secondary seal during normal operation. The function will be like that of a tandem arrangement without an intermediate labyrinth; the existing labyrinth can be used as additional throttle or monitoring device for the secondary seal.

DGS: Tandem arrangement with intermediate labyrinth

DGS

DGS

Centrifugal Compressor Theory Thermodynamics Process

Polytropic head

PV^n=C P1V1^n=P2V2^n=C n= 0 p1 =p2 Isobaric process n= 1 T1=T2 Isothermal n= K Adiabatic process = no heat transform

Head Generated

Compressor Performance Curves

Compressor Standards Important API 617 definitions: Compressor rated point Maximum allowable temp., pressure Maximum continuous speed, Normal speed,100% speed, Trip speed, Critical speed

API617

Notes to API

Surge and surge line The surge limit defines the flow at which, for a given speed, the operation of the compressor becomes unstable. At flow rates below the surge limit the characteristic curve actually droops toward zero flow after having reached its maximum point at the surge limit. Because operation below the surge limit is unstable, this portion of the curve is not shown in Fig. 8. When the flow is reduced below the surge limit, the pressure at the discharge of the compressor exceeds the pressure-making capability of the compressor, causing a momentary reversal of flow. When this flow reversal occurs, the pressure of the discharge system is reduced, allowing the compressor to resume delivering flow until the discharge pressure again increases, and the surge cycle repeats. Surging usually creates a clearly audible noise. Prolonged operation in this unstable mode can cause serious mechanical damage to the compressor. When operating in a surge condition, the compressor discharge temperature increases significantly and the compressor experiences erratic and severe vibration levels that can cause mechanical damage—particularly to the internal seals.

Surge Control

Stone wall(Chock) The stonewall limit of the performance curve defines the flow at which the gas velocity at one of the impellers approaches the velocity of sound for the gas at the conditions within the compressor where this sonic condition is first encountered. At the stonewall (or choke) flow the pressure vs. volume curve becomes essentially vertical, and it is not possible to develop head or pressure at any greater flow. When the required operating flow exceeds the stonewall limit, the only remedy is to reconfigure the compressor with impellers (and matched stationary hardware) designed for larger flow rates.

Safety and monitoring devices Centrifugal compressors are equipped with instrumentation to monitor mechanical health. Vibration monitoring is accomplished by eddy current probes installed at each of the compressor bearings. Vibration amplitude is measured at each radial bearing, and the axial position of the rotor is measured at the thrust disc or shaft end. The trend of radial vibration amplitude provides insight into the condition of the compressor regarding rotor balance and alignment. When a problem arises, the vibration frequency spectrum can also be analyzed to provide useful diagnostic information. The axial position probe monitors the state of thrust bearing wear. Each of the bearings is also fitted with temperature-sensing devices. By trending the thrust bearing pad temperatures, it is possible to discern the condition of the internal seals because changes in seal condition affect thrust loads and, therefore, bearing temperature. Alarm and shutdown settings for high bearing vibration and temperature are established in the compressor control system. External to the compressor are numerous other alarm and shutdown safeguards. As a minimum, low lube oil pressure, low seal gas pressure differential, overspeed, high discharge gas temperature, high and low suction and discharge pressures, and high liquid level in the suction scrubber are monitored and will initiate a shutdown when necessary.

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