In oil and gas facilities and plants, many pumps are operated in parallel. There are risks, potential issues and problems for parallel pump operation. Many pumps have been damaged because of poor combination, wrong selection or carelessness. However, general knowledge on pump parallel operation is low, and there has been limited publication on this important topic.
Unfortunately, many engineers seem to think pumps are like home appliances: someone can easily plug them in and put in parallel operation. This is not the case since pumps in parallel operation take suction from a common source and discharge into a common header. Another reason is that the pumps that interact with each other would be neglected. This is a major shortfall in pump engineering.
Different pumps present different behaviors when in parallel operation. As an example, “sensitive pumps,” which are more affected by operation far from best efficiency point (BEP), can be more affected. The effects of parallel operation on efficiency, reliability and performance should be carefully considered.
General Notes on Parallel Operation
There are many reasons to put pumps in parallel operation. Sometimes, pumps are put in parallel to cover a wide range of flow requirements (flow control). In other situations, they are installed for standby requirements or emergency backup. In other cases, for instance with an expansion or renovation program, a few new pumps are added to an existing set to increase the system capacity. In such a case, dissimilar pumps could be operated in parallel, which is a complicated matter.
In theory, when pumps run in parallel, they operate against the same discharge head, and the combination pump “head- capacity” performance curve is, theoretically, determined by adding the respective flow rates of each pump, at a series of specific head values. However, this is just theory. There are many practical concerns, risks, factors and parameters that should be considered.
If the pumps are not properly selected for parallel operation and all required provisions for such complicated operation are not foreseen, there might be issues. Inefficient operation, pump reliability issues, pump damage and operational problems are possible. For instance, when incorrectly selected pumps are operated in parallel, one of the pumps can be driven to operate outside of allowable operating range (far from BEP), even near shut-off point. This can result in over-heating and damage to the pump.
Parallel operation of identical pumps is a better-known topic than for dissimilar pumps. There are some well-established rules for identical pumps. For identical pumps, performance curves should be matched within specified tolerances, usually with tolerances less than 2 percent. In other words, although pumps are theoretically identical—because of different manufacturing and tolerances— in reality their curves are slightly different and should be limited to 2 percent. This is required to avoid problems.
Parallel Operation of Dissimilar Pumps
In many cases, dissimilar pumps need to operate in parallel. The head should be matched at the rated point of each pump—not at the same volumetric flow rate points. The parallel operation of dissimilar pumps is most often seen in renovation or expansion projects. For a revamp or renovation case, the head of the new pump (at rated point) should be matched with the head of the existing pump (rated point, each has its own volumetric flow rate) considering the elevation head difference and frictional head difference at downstream 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 within 3 to 4 percent. When dissimilar pumps are operated in parallel, care must be taken to avoid a situation that pushes any pump below its minimum allowable flow rate.
The pump manufacturer should be asked to explain all details and guarantee the trouble-free operation for all recommended combinations of parallel operation of pumps. Operating points for all scenarios should be plotted and compared between the minimum continuous flow and the end of the curve. The manufacturer should supply a study report for all possible parallel operation cases. The report should be carefully reviewed to ensure reliable parallel operation.
Parallel operation of dissimilar pumps is complicated, and some aspects of it—and all possible combinations—are often not understood even to pump experts and consultants.
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, over-heat and possibly fail. In other words, these pumps work against the same discharge head for a curve with more than one flow condition for a given head; so, a pump can be stuck near shut-off and could fail over time. 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.
A critical issue is the driver sizing for pumps in parallel operation. Electric motors of all pumps should be sized for the operation at the end of the 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, other pumps will try to ramp up and operate at the far right side of rated point or even close to the end of the curve for a short time. The electric motors should be sized accordingly, and the current set point should be set properly to avoid any trip.