Pumps are used in many upstream facilities and units, and many of them operate in parallel. There are risks and potential problems associated with parallel operation of pumps. Pumps can be damaged because of poor design, wrong selection or operational carelessness in parallel operation. To mitigate these risks and dangers in operation, end users should consider many factors and parameters. General knowledge of pump parallel operation is limited, and there have been relatively few publications on this important topic.
Unfortunately, some engineers think pumps are like home appliances that you can easily plug in and operate. This is not the case. Usually the fact that pumps in parallel operation take suction from a common source and that they discharge into a common header is disregarded. The fact that the pumps interact with each other is similarly neglected. This is a major shortfall in pump engineering all over the world.
Parallel operation and its effects on efficiency, reliability and performance should be carefully considered. Different pumps present different behaviors when in parallel operation. As an example, so-called “sensitive pumps,” which are more affected by operation far from the best efficiency point (BEP), can also be more affected by parallel operation.
General Notes on Parallel Operation
There are many reasons to put pumps in parallel operation. Sometimes, pumps need to cover a wide range of flow requirements. In other situations, they are installed in parallel to meet standby requirements or for emergency backup. In an expansion or renovation, new pumps can be added to an existing unit to increase the capacity of the pumping system. In such cases, dissimilar pumps can be operated in parallel, creating a complicated pumping environment.
In theory, when pumps run in parallel, they operate against the same discharge head, and the combination pump head-capacity performance curve is determined by adding the respective flow rates of each pump at a series of specific head values. Again, this is theory. There are many practical factors and parameters that should be considered.
If the pumps are not properly selected for parallel operation and all required provisions are not accounted for, there can be problems. Some examples include inefficient operation, pump reliability issues, pump damage and operational problems. For instance, when poorly selected pumps are operated in parallel, one of the pumps might be driven to operate outside the allowable operating range (far from the BEP), even near the shut-off point, which can result in overheating and pump damage.
For identical pumps, performance curves should be matched within specified tolerances, as an indication with tolerances less than 2 percent. Although pumps are theoretically identical, because of different manufacturing and tolerances, their curves are slightly different. The differences between curves should be limited to 2 percent.
Parallel Operation of Dissimilar Pumps
In many cases, dissimilar pumps operate in parallel. The head should be matched at the rated point of each pump (not at the same volumetric flow rate points). This kind of parallel operation is most often seen in a revamp, renovation or expansion project. For a revamp/renovation case, the head of the new pump rated point should be matched to the head of the existing pump rated point—each has its own volumetric flow rate—considering the elevation head difference and frictional head difference for each pump. The surrounding area of rated points should offer a suitable operating condition for all pumps.
As an indication, curves of different pumps in the expected area of operation should be matched to within 3 to 4 percent. When dissimilar pumps operate in parallel, end users must be careful to avoid an operating situation that pushes any pump below its minimum allowable flow rate. The pump manufacturer should explain all details and guarantee trouble-free operation of all combinations of parallel operations of pumps. In addition, operating points for all scenarios should be plotted and compared between the minimum continuous flow and the end of curve.
A study for all possible parallel operation cases should be undertaken to ensure trouble-free operation.
Performance Curve for Parallel Operation
The shape of the head-capacity performance curve should be carefully considered when selecting pumps for parallel operation. If the pump curve droops (the head drops) as the flow is reduced toward the shut-off point, a second pump may not be able to move from the shut-off point. This pump could run near shut-off, overheat and possibly fail.
Pumps in parallel work against the same discharge head. For a curve with more than one flow condition for a given head, a pump can get stuck near shut-off, and it could fail. As a general rule, the performance curve should continuously rise to the shut-off point. Pumps with head-capacity curves that droop toward the shut-off point or have more than one flow condition for a given head should not be operated in parallel.
Driver Sizing for Parallel Operation
Electric motors of all pumps should be sized for operation at the end of curve to allow all pumps to continue operation at the far right of the rated point for a short time, without tripping on motor current limit. There is usually an electric motor current limit with trip to protect electric motors. If the electric motor is undersized, this trip can be activated by transient operation at the right side of the curve. This is particularly important for the transient cases in parallel operation.
If one pump is tripped, the other pumps will try to ramp up and operate at the far right side of the rated point (or even close to the end of curve) for a short time. Electric motors should be sized accordingly, and the current set point should be set to avoid trip.
This case study examines an upstream facility using four 200 kilowatt (kW) pumps and three 250 kW pumps in parallel operation. The service is a critical pumping application in an upstream unit. The case is parallel operation of dissimilar pumps. The first four pumps (200 kW) were provided in the initial upstream development. A wide range of capacity coverage was required, which was only possible using four pumps. Another three pumps (250 kW) were added by another pump manufacturer five years later in a major expansion project for the upstream facility. The seven pumps must operate in parallel for peak production.
A series of simulations were conducted to assess the parallel operation and evaluate different operational scenarios. The first simulation scenario was defined when all seven pumps were in operation and suddenly a 250 kW pump was tripped. When only one pump tripped, the other operating pumps ramped up the lost capacity in accordance with their performance curves. The power consumption of the running pumps stayed within the limit set by their motor rating: 200 kW for small pumps and 250 kW for large pumps. Based on simulations, within 10 minutes, the operation was stabilized without any loss in the critical flow.
The second simulation showed what would occur if two large (250 kW) pumps were tripped together (or tripped back to back). In this scenario, when the second large pump (250 kW) is tripped, the power consumption for the large pump is moved to 219 kW based on simulation results (operation at the right side of the BEP), which is well within the motor rating limit (250 kW). However, the power consumption of the small pumps is 191 kW, which is dangerously close to the motor rating (200 kW). There is only a 4.5 percent margin to the motor rating. This 4.5 percent margin is for a relatively stabilized operation.
In transient cases, one small pump likely will pass the electric motor current limit because of unstable and dynamic effects. This will probably trip another pump and consequently all the small motors. This will lead to the trip of all pumps and consequently the whole upstream facility. If transient and dynamic effects can be managed, the operation can be continued on the assumption that the electric motors will not trip. However, this is not the case, and in such an emergency the entire facility should be tripped.
If the small pumps were sized properly (more than 200 kW), when the second pump tripped, the production could be continued. The loss of two large pumps cannot sustain production at peak capacity. However, with some new set points and some minor modifications, the control system could smoothly bring the entire facility to its new capacity, which is slightly less than the peak capacity.
If dynamic situations can be managed and transient cases can be controlled (without trip), the control system can effectively handle the trip of two large pumps. Most likely, because the motors of the small pumps will trip, the control system will not get the opportunity to stabilize the operation. Unfortunately, the electric motors of the small pumps were not sized for the end of the curve. This major shortfall resulted in an issue in the parallel operation. This is a good example of the effects of undersized electric motors on upstream operation.
In the initial development of the upstream facility, small pumps (200kW) were undersized, and now there is no possibility to replace them because it is too expensive from an operational point of view. Lesson learned: For all pumps, drivers should be sized for the end of the curve. This will result in a more reliable and stable operation, especially for pumps in parallel operation.