October 08, 2007

Parallel Pumping - Its a Team Effort

When multiple pumps operate continuously as part of a parallel pumping system, there can be opportunities for significant energy savings. For example, lead and spare (or lag) pumps are frequently operated together when a single pump could meet process flow rate requirements. This can result from a common misconception—that operating two identical pumps in parallel doubles the flow rate.
Although parallel operation does increase the flow rate, it also causes greater fluid friction losses, results in a higher discharge pressure, reduces the flow rate provided by each pump, and alters the efficiency of each pump. In addition, more energy is required to transfer a given fluid volume.



Example
A split-case centrifugal pump operates close to its BEP while providing a flow rate of 2,000 gallons per minute at a total head of 138 feet. The static head is 100 ft. The pump operates at an efficiency of 90% while pumping fluid with a specific gravity of 1. With a drive motor efficiency of 94%, the pumping plant requires 61.4 kW of input power.

When an identical parallel pump is switched on, the operating point of the composite system shifts to 2,500 gpm at 159 ft of head.

Each pump now operates at 80% efficiency while providing a capacity of 1,250 gpm. Although the fluid flow rate increases by only 25%, the electric power required by the pumping system increases by 62.2%:

P2 pumps = 0.746 kW/hp x (2,500 gpm x 159 ft) / (3,960 x 0.8 x 0.94) = 99.6 kW

For fluid transfer applications, it is helpful to examine the energy required per million gallons of fluid pumped.

When a single pump is operating, the energy intensity (EI) is as follows:

EI1 = 61.4 kW / (2,000 gpm x 60 minutes/hour x million gallons/106 gallons) = 512 kWh/million gallons

When both pumps are operating, the EI increases as follows:

EI2 = 99.6 kW / (2,500 gpm x 60 minutes/hour x million gallons/106 gallons) = 665 kWh/million gallons

When both pumps are operating in parallel, approximately 30% more energy is required to pump the same volume of fluid. The electrical demand charge (kW draw) increases by more than 62%. If the current practice or baseline energy consumption is the result of operating both pumps in parallel, pumping energy use will decrease by 23% if process requirements allow the plant to use a single pump.

REASONS

The reason for this can be as below
  1. Due to incorrect piping layout for combined system. Generally the interconnection of the individual discharge headers to the common header is made perpendicular (TEE Connection). In such a case when the common header is not large enough to accomodate flow pulses / higher volumes then restriction in the discharge flow is very common for parallel pumps. This causes ~5 - 15% loss in system efficiency. Convert them to tangential entry to reduce entry losses & better ejecting effect to other pumps in the flow direction.



  2. Deviation of pump operation from BEP due to change in system curve after some time OR may be due to alteration made in the system which are not taken care of for changes & identification of BEP in the entire system vs pump curve. It may be due to addition of some other pump in the same system but at farther location which is often neglected as a parallel pump due to poor visibility.

  3. After few months/years of operation, generally in cooling water pumps, internal corrosion / pitting leads to variation in pump curve shifting it towards lower side of the original curve. This reduces the capacity & head of that particular pump. However, when this weared pump runs in parallel with a better pump, the better pump affects it in 2 ways - First its head is higher so it presses hard the other (damaged or weared) pump reducing its capacity further. - Second its capacity is already lower due to wear. This have double impact on overall performance. In some cases it may happen that weared pump contributes hardly 10-20%in flow consuming equivalent power.

  4. So the perception that we have two identical pumps which were working fine initially, will now also be working at best, is not good due to differential changes even in similar pumps lead to variations & they dont remain identical after some time of operation.

  5. If motor used is of different speed (which is not very common) OR even with same speed motors slip may be slightly different altering their speed in the range of 1%. This will also shift the curve of identical pumps leading to drop in efficiency. So we should check motors speed also.

  6. Use of VFD changes Pump curve by altering its speed in exponential manner. However, the system curve for many process applications, Circulating pumps, Filling pumps & specially for Boiler feed water pumps is different than exponentiality. In such cases VFD use may result in drop in overall system performance. So be careful in sugesting VFD in parallel pumping system.
All of them or anyone of them can be as severe as resulting in major loss in EI. The term Energy intensity as defined above is introduced for better understanding in terms of entire system. Our target should be to improve EI rather than improving only efficiency of the pump OR flow or Head.

More info next time........

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