Performance Evaluation Parallel Pumping & reducing cost of operation

Abstract 

Over the lifetime of a typical pump, the amount of energy used can exceed the initial cost by as much as 15-fold. The performance optimization of pumps offers tremendous potential for energy conservation. By understanding the relation between energy and functionality, we can make informed decisions about procurement, installation, maintenance and operation of pump systems. Optimizing the energy performance of pumps, either stand alone or parallel or series operation, essentially requires that a “System Approach” is taken.

There is a tremendous energy saving potential in parallel pumping systems in industrial pumps, water supply pumps etc., The energy saving potential varies from 15% to as high as 30% of the actual energy consumption in pumping systems.

 This study is focuses on parallel pumping system and provide areas of improvement.

What is parallel Pumping?  

Parallel pumping refers to two or more pumps that are installed with the same suction and discharge headers. The pumps must be selected to provide system design flow and head at the discharge header.

The simplest parallel-pump installations use two small equally sized pumps each selected to provide full system head from Point B to Point A (as shown in figure-1), but only half of full system flow. When both pumps are operating, the system experiences the combined effect of both pumps.

Reason behind Parallel Pumping

Normally single pump is preferable for operation to control the system with equal capacity standby. However, pumping capacities are two large and single pump is not available the two parallel pump operation is specified. Sometime site electrical infrastructure is also limitation for large single pump operation.

 A combination of factors including desired process reliability, service factor, operating condition, maintenance philosophy and cost.

In certain applications redundancy is essential considering the criticality of process requirement. Any breakdown in a critical application may lead to plant shutdown and huge loss in terms of production, energy and manpower. With a multiple pump arrangement, one pump can be repaired without disturbing the system.

Benefits of Parallel Pumping

The use of several pumps in parallel broadens the range of flow that can be delivered to the system. Additionally, by energizing and de-energizing pumps, the operating point of each pump can be kept more closely near its Best Efficiency Point (BEP). Caution should be used in operating parallel pumps to ensure that the minimum flow requirement is not violated for any pump. Sometimes single pump size is the restriction and we have to go for parallel pumping.

Sometimes use two pumps in parallel even though one pump could handle 100-percent head and flow. If two 50-percent pumps can meet design conditions by operating together, and one of the pumps potentially could provide more than 50 percent of the flow. 

However, if either of the pumps fails, full flow cannot be provided. In general, two 50-percent pumps cost only 79 percent as much as two 100- percent pumps.

 If three 50-percent pumps in parallel are provided which are contributing a significant fraction of design flow, any two of these pumps can meet 100 percent of the flow, which allows one pump to be standby. Initial costs for improved redundancy can be surprisingly low. In general, three 50-percent pumps cost only 18 percent more than two 100-percent pumps.

However, this reduction in initial cost is not free of cost. This may increase energy consumption per unit quantity of flow because of reduced operating efficiency level in parallel pumping.

Maintenance in Parallel Pumping 

It is ideal to operate pump close to its BEP which lowers bearing wear and permits the pumps to run more smoothly. If perfect selection of pump is made, multiple pump configurations allow each pump to be operated close to its BEP, a potential advantage from multiple pumps is a higher overall efficiency level. By energizing or de-energizing pumps as necessary to meet changes in system demand, each pump can operate over a smaller region of its performance curve, ideally around the BEP.

Alternatively, a single pump would have to operate over a larger range forcing it to operate further away from the BEP, this is in case of large process variation or part load operation. Which results into increase in operation and maintenance cost. However, this efficiency advantage depends on the pump curves, the system curve, and the demand change that is being met.

In case of variable flow requirement, parallel pumping reduces reliance on energy dissipating flow control options such as bypass lines and throttle valves. The use of a single, large pump during low flow demand conditions forces the excess flow to be throttled or bypassed. Throttling the flow wears the throttle valves and creates energy losses. Similarly, bypassing the flow is highly inefficient; since all the energy used to push the excess flow through the bypass lines is wasted.

 In systems, with high friction components, alternatives such as adjustable speed motors tend to provide a more efficient solution to variable demand requirements.

Basic behind Parallel Pumping 

The total system flow rate is equal to total flow rates or contributions from each pump at the discharge pressure. Parallel pumps provide balanced and equal flow rates when same models are used with same impeller diameter and rotational speed. A recommended design practice is to have parallel pumps moved from beyond Best Efficiency Point (BEP) when single pump is running and to the left of BEP at the highest flow when more than one pump are running in parallel. In an ideal scenario, the pumps will have highest average operating efficiency for overall flow rate v/s time profile.

There is a myth about parallel pumping 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 which in turn results in a higher discharge pressure and reduces the flow rate provided by each pump, and finally alters the efficiency of each pump. Finally we are in a situation where we consume more energy to transfer a given fluid volume

a) Similar pumps in parallel operation

 This is advisable and most of parallel pumping operating under this philosophy. This is very much clear from the figure-2 that the flow is not doubling in parallel pump operation and it is slight less than double flow.

 The amount of flow is dictated by the system curve as it is moving upward, the steepness is goes off and the pump flow increase in minimum. As it is clearly visible in figure 1, the three-pump operation only have incremental change in flow.

 b) Dissimilar pumps in parallel operation

Pumps having with different shutoff heads, the larger pump provides the high backpressure to other pump and try to close the check valve and reduces the output form second pump. Hence, manipulation of delivery valve is important in non-identical pump in parallel operation.

 Hence, the non-identical pump with different flow can be sued in parallel operation as long as these have similar shut off heads.

 c) Constant and variable pumps in parallel operation

The objective in using parallel constant and variable speed drive is to minimize system operating cost.

 For example, there are two pumps in parallel and

each has been selected for full system head and half of full system flow. The system also requires a controller and sensor to provide automatic operation, but initial costs are reduced because only one variable-speed drive is needed, and the constant-speed pump requires only a standard motor starter.

 In a variable-volume hydronic system, the variable-speed pump handles part-load flows up to its full speed capability and the constant-speed pump is turned off. If flow increases, the constant-speed pump stages on, and the variable-speed pump slows to meet the actual flow requirement. As flow continues to increase, the variable-speed pump again speeds up to the full revolutions-per-minute capacity so that at design flow both pumps are operating at full speed, splitting the flow between them.

It is important to understand that parallel pumps operating with variable speed drives and discharging into a common header behave similarly when operating a pump against a static head. The first pump discharging to the header pressurizes the header. The second and subsequent pumps that come online must then pump into a pressurized header. The more pumps that are running, the higher the pressure in the header and it limit the chances of saving energy by using a variable speed drive.

 Optimization of the variable speed system is essential and specialist advice should be sought however as a common rule all the pumps should run to a similar characteristic, which usually means running identical pumps at an identical speed. Similarly, it is not recommended to run a fixed speed pump in parallel with a variable speed pump of the same size. The potential problem that can rise and must be avoided is that one of the pumps operates at no flow.

Selection Aspects 

While selecting and operating pumps in parallel operation following three aspects need to be considered.

a) Design aspect

b) Maintenance aspect

c) Energy Efficiency aspect

Energy Effectiveness 

Pump selection for parallel operation is key to minimize the energy costs. However, many parallel systems are not operated in the most energy efficiency ways or combinations. A more optimum pump selection and operating plan can, therefore, provide substantial cost saving opportunities. The way to achieve optimum parallel pump selection is to calculate the energy effectiveness (gpm/kW) of each pump and then single pump or pump combination that yields higher energy effectiveness. The energy effectiveness of a pump over its flow range can be determined from the following equation.

kWh/GPM = 5310XnpXnd/H

Where np = Pump efficiency nd = drive efficiency H= Head developed in feet

Energy effectiveness selection for parallel pumping installation, the following guideline should be followed.

1.      Calculate gpm/kW, flow rate curve for each pump and combination

2.      Operate minimum number of pumps for any system required flow

3.      Select highest energy effectiveness (normally lowest head/capacity) pumps for any flow requirment.

Recent in cooling tower pumps study, it was observed 3 pumps operating in parallel. The capacity of each pump is 650 m3/hr and operating head is 35 meter.

The analysis of pumping system is done and shown below in the table.

Conclusion 

1.      Avoid parallel operation of dissimilar pumps wherever possible.

2.      Avoid parallel operation of pumps with drooping performance curve wherever possible.

3.      Always calculate energy effectiveness while making decision of parallel pumping operation and compare the same with single pump operation.

4.      Pumps should operate on right side of BEP when operating at low flow rate (Fewer pumps are running in parallel) and should operate on left side of BEP

5.      when operating at higher flow rates. This will allow pumps to operate with higher average operating efficiency.

6.      Parallel pumping is best suitable for static head dominated systems.

7.      The motor provided for parallel pumps must be sized for single pump operating mode.

8.      Pumps with flat performance characteristic curves should not be run in parallel.

9.      When dissimilar pumps are operated in parallel, avoid running pumps below minimum allowable flow rate.