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COMMON MCF BYPASS LINE FOR MULTIPLE PUMPS

OPERATING IN PARALLEL 26 JUN 2015

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

Design optimization is a largely practiced tool applied for reducing the project costs. There are cases when an optimized design approach not only reduces the cost but also results into operational and maintenance simplicity. Design optimization practices often involve prudent elimination of certain system components or part of a system such that the original functionality of the overall system remains unaffected. In order to ensure integrity of original design, the designer should have a thorough understanding of all the direct and indirect impacts of the areas of optimization being considered. There are cases when a system is optimized aiming at a cost saving, however the basic functionality of the system is compromised due to lack of understanding of the design approach implemented. As a fundamental requirement, centrifugal pumps should not be continuously operated below the minimum continuous flow (MCF) point and should not be operated at all under a blocked discharge (shut-off) condition. To meet this requirement, centrifugal pumps are provided with MCF protection which is one of a pump safety design feature. Ideally each pump should have its own independent MCF protection. However, there are cases when a common MCF protection can be used for a number of pumps. This paper elaborates the design aspects which should be considered while using a common MCF protection for multiple pumps connected in parallel. The MCF protection for centrifugal pumps has various design alternatives such as continuous spill back line with orifice, bypass line with flow control valve or bypass line with Automatic Recirculation valve (ARV). Each design has its own pros and cons; however these details are not part of this discussion. This paper describes basic understanding for the pumps operating in parallel as a refresher and then moves on to the focal point of discussion restricting to the main topic. Also, an in depth discussion on design and operation of multiple pumps connected in parallel is not part of the subject. Note: The schematics and curves used in this paper are typical representations for the sake of illustration only and does not show the real pump performance curves or the system P&ID’s.

MULTIPLE PUMPS OPERATION BASICS – PARALLEL ARRANGEMENT:

When two or more pumps are connected in a system such that they take suction from a common header and discharge into a common header, the pumps are considered as connected in parallel. Centrifugal pumps may be connected in parallel to achieve one or more of the design objectives. Since pumps connected in parallel have common suction header (same suction pressure) and common discharge header (same discharge pressure), the total differential head across each pump remains the same, however the total flow rate in the discharge header is additive (sum of individual flow rate contributed by each pump).

Figure 1 shows a typical pumping system with two pumps connected in parallel arrangement.

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Figure 1: Typical Pumping System – Two Pumps Connected in Parallel Figure 2 shows typical Head-Flow curve for a centrifugal pump. For the sake of discussion, assume that the two pumps shown in Figure 1 are identical and have a performance curve shown in Figure 2 with point “M” as the minimum continuous flow (MCF) point.

Figure 2: Typical Head-Flow curve for Centrifugal Pump A or Pump B

The total performance for parallel operation of two pumps in Figure 1 can be represented by a composite or combined performance curve. The combined performance curve can be obtained by using

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the single pump performance curve shown in Figure 2 such that each point on single pump performance curve shifts to the combined performance curve based on the following facts:

1- Total differential head H [ m ] remains constant

2- Flow rate Q [ m3/h ] doubles (theoretically) at each point from single pump curve to combined curve

Figure 3 shows the combined or total performance of pumping system in Figure 1.

Figure 3: Total Performance for Parallel Operation of Pump A & Pump B

Each pump in parallel operation, for 2 pumps in this case, contributes theoretically one half of the total flow on the combined performance curve. If “R” shows the rated flow point on the combined performance curve at “x [m3/h]” and differential head “h [m]”, then flow contributed by each Pump A and B is “x / 2 [m3/h]” at the same differential head “h [m]”. Figure 4 illustrates the rated point on combined performance curve and the corresponding operating point of each pump on its individual performance curve.

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Figure 4: Rated Point for Pump A & Pump B, Individual Curve and Combined Curve

The combined performance of pumps operating in parallel can be regulated by throttling a valve in the common discharge header. For example, if we start closing the valve in the common discharge header, system resistance curve becomes steeper and steeper causing the operating point on combined performance curve to ride up the curve. Likewise, the operating point of individual pump (Pump A or Pump B) keeps riding on its own performance curve as illustrated in Figure 5 below.

Figure 5: Flow Regulation in Parallel Operation

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The concept of flow contribution by any number of pumps in parallel operation is generalized as below.

IF Number of Pumps Operating in Parallel = n

IF Total flow from “n” pumps operating in Parallel = Q [ m3/h ]

RULE Flow contributed by each Pump = Q / n [ m3/h ]. . . . . theoretically

COMMON MCF BYPASS LINE FOR MULTIPLE PUMPS IN PARALLEL OPERATION:

The MCF bypass line for a number of pumps operating in parallel may be combined into a common MCF bypass line. However, this design approach should be implemented with proper consideration. For example, if the MCF bypass line is provided with continuous spill back using an orifice, the orifice size and the bypass line size should be calculated based on the following rule:

IF Number of Pumps Operating in Parallel = n

IF MCF for each Individual Pump = q [ m3/h ]

RULE MCF for Orifice sizing and bypass line sizing = n x q [ m3/h ]

Let’s consider a case when pumps in Figure 1 are provided with a common MCF bypass line using continuous spill back arrangement shown in Figure 6 below.

Figure 6: Common MCF Bypass with Continuous Spillback

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If the MCF for each individual Pump is q [ m3/h ], then Orifice size and the bypass line size calculation should be done based on a MCF flow value of 2 x q [ m3/h ]. As discussed earlier, each pump contributes one half (1/2) of the total flow in the common discharge header. When flow is regulated, operating point rides up the curve on combined performance curve as well as on each individual pump performance curve. The flow should not be regulated below the MCF of each individual pump which is q [ m3/h ]. This implies that the lowest permitted flow on the combined performance curve is 2 x q [ m3/h ]. If the common bypass system has been sized to pass a minimum flow of 2 x q [ m3/h ] and the pumps face a blocked discharge condition, the operating point will ride up the curve from rated point “R” to the point “k” on combined performance curve. At this stage, each individual pump will be operating at point “M” without further reduction in the flow through the pumps. Hence the pumps are protected and operating point never falls below individual pump MCF limit.

Figure 7-a: Pump Performance (blocked discharge) when Orifice and bypass line is Sized for 2 x MCF What if Orifice and bypass line is sized for a Single Pump MCF? Consider a case when the Orifice and bypass line has been sized based on the single pump MCF (i.e. q m3/h). In this case, if the pumps see a blocked discharge condition, the operating point on the combined performance curve will ride up the curve from rated point “R” to point “k-1. The total flow through the common bypass line will be q [ m3/h ] with each individual pump contributing one half of the total flow q/2 [ m3/h ]. ”. Hence ach individual pump will be operating on its curve at point “M-1” which is half way inside the MCF flow region of each pump. So each pump will be operating inside its MCF region despite having the MCF protection, the MCF protection will be ineffective.

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Figure 7-b: Pump Performance (blocked discharge) when Orifice and MCF bypass is Sized for 1 x MCF What if the System has Flow Control Valve Instead of Continuous Spillback? If flow control valve is used instead of continuous spill back, the controller set point should be considered as below: Controller Set Point = n x q [m3/h]

Figure 8: Common MCF Bypass with Flow Control System

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CONCLUDING REMARKS:

Design optimizations should be applied with proper considerations such that the system functionality based on the original design is not affected. Multiple pumps operating in parallel may have a common MCF protection, however proper design consideration should be taken into account. About the author

Muhammad Imran is Mechanical Rotating Equipment Engineer presently working with LUKOIL Overseas Services B.V based in Dubai. He has more than 14 years of professional experience in Oil & Gas and Petrochemical projects including EPC detail engineering, Concept Optimization, FEED and hands-on experience in the Pre-commissioning, Commissioning, Startup and initial operations. Before joining LUKOIL, he worked for WorleyParsons Qatar as Lead Mechanical Design Engineer for Rotating Equipment and Packages. He can be reached at “muhundis@gmail.com”.