Lube Oil Rundown Tank - System Design And Operational Aspects 725d29

PUMP SIZING – ADDING MIN. FLOW TO RATED FLOW – “WHEN” AND “WHY” 12 APR 2015 By:

Muhammad Imran – Mechanical Rotating Engineer – LUKOIL Overseas Services B.V Dubai

C

entrifugal Pumps are the most widely used Rotating Equipment in the process plants. The ultimate goal of using a Centrifugal Pumps is to maintain a certain flow rate (m3/h) in the system at a certain time. Flow rate demands can be intermittent or continuous, fixed or variable within a range from zero (no flow) to a certain maximum. Centrifugal pumps are flexible to meet the process demand variations; however it is inherent design feature of the centrifugal pumps that they are limited for a certain minimum flow rate called “minimum continuous flow - MCF” below which it is not recommended to operate the pump in a continuous manner. Unlike positive displacement pumps, centrifugal pumps behave differently when the system resistance (line backpressure) changes. An increase in the system resistance kills the pump differential head and the pump is able to maintain a lower flow rate in the system. Similarly, a decrease in the system resistance enables the pump to maintain a higher flow rate. This characteristic of centrifugal pumps is widely used in a controlled manner to achieve flow regulation in order to meet the process demand variations. There are possibilities when the system resistance may change in an uncontrolled fashion, for example, chocking of heat exchanger tubes in the downstream or a sudden closure of the discharge valve etc. This may cause that pump operating point to move further left beyond the MCF point and may even push the pump into shut-off condition resulting in a catastrophic pump failure. Pump selection becomes challenging when the process flow demand falls below the MCF capability of the available centrifugal pumps. In such cases, a larger centrifugal pump is selected with a particular system design such that the process demand is fulfilled without having to operate the pump below the MCF point. Various methods are used to prevent the pump operation below the MCF point and to avoid the operation at shut-off condition. Commonly used methods include “Minimum continuous flow control valve”, “Automatic recirculation valve” or using a “Minimum continuous flow by- line”, the last one being the simplest method used for the smaller size pumps. This paper describes how centrifugal pump protection is achieved by using MCF by- line and how the pump sizing is done for this particular case. MCF BY- LINE – HOW IT WORKS:

Figure 1-a shows schematic for a simple pumping system. In case of any increase in the system resistance, the pump operating point (red dot) moves upwards along the Q-H curve. If the system resistance keeps on increasing, the pump operating point keeps riding up the curve until it reaches the MCF point (black dot). If the system resistance keeps on increasing further, the operating point moves beyond the MCF point and may finally reach the shut off point if the pump discharge is fully blocked. 1|Page

Figure 1-a: Typical Pumping System without MCF Protection Figure 1-b shows a typical pumping system with MCF by- line. The by- line consists of a suitably sized orifice plate with an appropriate line size leading back to the suction system. The MCF by-pas line provides an additional flow path in the pump discharge system which remains continuously open thus ensures that certain minimum flow is always ed through the pump. In this case, even if the pump discharge is fully blocked, the system resistance does not increase beyond the MCF point of the pump and hence the operating point does not fall below the MCF region.

2|Page

Figure 1-a: Typical Pumping system with MCF By- Line PUMP SIZING FOR MCF BY- SYSTEM:

There are various applications where MCF by- line is the most suitable method for the centrifugal pump protection. There are cases when the rated flow rate is too small to be covered by the available centrifugal pumps, so a bigger capacity pump is selected and operated at a larger flow with continuous spill back for the excess flow. In another case, the pump controls are simplified by adopting the minimum continuous flow line. A good example is application of Jockey Pumps in the Fire Water Pump system when pump is continuously operated without using a pressure sensing line. In all these cases, the pump sizing is done based on “Rated Flow + Minimum Continuous Flow” rather than the “Rated Flow”. Following details are provided to explain how and why this pump sizing practice works to achieve the desired pump performance. 3|Page

Example: Cold Condensate Pumps – Steam Generation and Distribution System

Following figure shows section of data sheet with operating conditions for a centrifugal pump in the steam generation and distribution system of a Gas Treatment Plant.

Fig-2: Section of Pump Data Sheet – Cold Condensate Pumps The pumps in the above example were part of the system where the system resistance was variable. There was an all-time possibility of increase in the system resistance leading the pump operation below the MCF point or even pushing the pump into dead head (shut-off) condition when occasionally the system demand was zero. Therefore, pump MCF protection was required. However, considering that the pump was very small (flow / head: 11.8m3/h / 1113m) and a too small quantity of MCF (1.7m3/h), a minimum continuous flow by- line was selected as the best option for pump MCF protection. The pump selection was done based on flow rate “rated flow + minimum flow” instead of just “rated flow”. The following description explains why and how this pump selection basis works. Let us assume we select the pump based on “Rated flow” instead of “Rated flow + minimum flow”. When the minimum continuous flow by-pas line is added to this pump, the system behaves as if the discharge line size has increased. In other words, the system has an additional flow path which causes a decrease in the system resistance. In this case, the pump is able to maintain an overall higher flow rate in the system consuming additional chunk of the differential head produced by the pump causing the pump operating point to move down the curve. This causes loss of flow/head to the main process as shown Figure 2.

4|Page

Fig-2: Pump Selected for RF without Considering MCF & Operated with MCF By-

5|Page

The illustration in Figure 2 shows how the overall flow maintained by the pump is increased but the head/flow to the main process is decreased. Therefore, the pump behaves as an undersized pump for the required process demand. The correct way of sizing the pump in the above example is to add MCF to the RF, thus the pump should be sized for (11.8 + 1.7 = 13.5 m3/h). A pump sizing based on this approach would not cause any head/flow loss to the main process as illustrated in Figure 3 below.

Fig-3: Pump Selected for (RF + MCF) & Operated with MCF By- 6|Page

CONCLUDING REMARKS:

Pump MCF protection is a critical requirement for the centrifugal pumps and must be carefully addressed during the FEED and EPC detail engineering stage. Failing to provide MCF protection may result into catastrophic failure of the pump during operation. The process engineer must assess the pumping system and define the MCF protection requirement in the process data sheet for the pump and the same should be indicated in the P&ID’s. When a MCF by- line is required, the process and mechanical data sheet must say “Rated flow: X + MCF” where X = Rated Flow & MCF = Minimum Continuous Flow. Alternatively, a note may be added in the “Rated Flow” column to say “Pump is provided with MCF by- line, the pump MCF shall be added to the Rated Flow”. The Rotating Equipment Engineer must be qualified enough to interpret this requirement in the process data sheet and P&ID’s and must ensure this requirement is incorporated in the pump sizing during the TBE (technical bid evaluation) stage. The orifice plate required for the by- line must be included in the pump vendor scope. The pump vendor must do the orifice plate sizing and should provide the actual materials including the orifice plate with mating flanges, nuts & bolts and gaskets. The vendor documents must include the orifice plate sizing calculation and reference drawing. The process engineer should do the by- line sizing based on the pump data sheets & performance curves provided by the vendor.

About the author

Muhammad Imran is Mechanical Rotating Equipment Engineer presently working with LUKOIL International Services B.V based in Dubai. He has 15 years of professional experience in Oil & Gas and Petrochemical projects including EPC detail engineering, PRE-FEED, FEED and hands-on experience in the Pre-commissioning, Commissioning, Startup and initial operations. Before ing LUKOIL, he worked for WorleyParsons Qatar as Lead Mechanical Design Engineer for Rotating Equipment and Packages. He can be reached at “[email protected]”.

7|Page