Understanding the causes and effects of minimum flow damage in centrifugal pumps is critical for knowing how to prevent it. End users should also be familiar with the advantages and disadvantages of common minimum flow protection systems in order to select the best ones for their applications.
A common cause of premature wear in centrifugal pumps is oversizing or using too many pumps for a service. Operating a centrifugal pump below its allowable operating range as a result of oversizing or operating more pumps than necessary for an extended period of time is one of the most common causes of premature wear or failure of pump internals. These conditions can also damage seals and bearings. In addition to affecting the pump, oversizing or operating too many pumps can impact other system components and overall energy.
For example, a boiler feed system was designed with three pumps—two for parallel operation and one standby spare. The automatic recirculation valves used to protect the pumps were wearing prematurely compared with earlier plant history.
A review of the process flow rate showed that a single pump would support the current demand. With two pumps running, the process flow requirement was less than the minimum flow the pumps needed.
Both minimum flow valves were partially open to bypass continuously. Once the system changed to operating one pump rather than two, the system operated slightly above its best efficiency point (BEP), which stopped the rapid valve wear and decreased power consumption.
Another plant had two 100 percent pumps with continuous flow orifices to provide minimum safe flow. During startup and some low-load cases, however, the pumps experienced excessive vibration. Changing the orifices to increase bypass flow solved the vibration problem, but now the combined flow of the orifices and the process flow ran out too far on the curve and was below the needed pressure to sustain the process.
The solution was to run both pumps in parallel, which negated the standby pump concept and drastically increased the power cost of pump operation. To address this issue, personnel replaced the orifices with automatic recirculation valves that only opened when the process flow dropped to minimum flow. As a result, the plant was able to support the process by operating a single pump as originally intended.
Minimum Continuous Safe Flow
Minimum continuous safe flow (MCSF) is the flow at which a pump can operate continuously without excessive wear from hydraulic anomalies and temperature rise associated with low-flow conditions.
If the answer is yes to any of the following questions, a pump is probably operating below the safe minimum flow:
- Are the impeller vanes (first-stage impeller if a multistage pump) pitted or worn through?
- Are the wear rings or journal bearing bushings worn more on one side even though the pump shaft appears to be centered when in a static condition?
- Does pump noise and/or vibration increase more than expected at low process flow demands?
- Has the pump experienced shaft breakage that could not be explained?
- Has the pump casing and/or bearings shown signs of overheating?
If a system is experiencing any of these symptoms, operators can take several steps to operate the pump differently to protect it and improve performance.
The first step is to operate the fewest pumps required as load is reduced. This increases the flow per pump and is the most effective, easiest and least expensive corrective measure. Operating fewer pumps will normally
result in operating each pump closer to its BEP, consuming less power. All centrifugal pumps should have minimum flow protection.
If the existing pump is severely oversized, replacing it with a properly sized pump (or impeller in some cases) may be the only appropriate corrective action.
Select new pumps with the normal operating range between 80 and 110 percent of BEP, and protect them from minimum flow damage. The following consequences may result if MCSF of a pump does not occur.
Since the 1940s, it has been common practice to hold the temperature rise of the product being pumped to at most 15 F. On hydrocarbon applications, it is best to keep temperature rise under 10 F (and under 5 F when net positive suction head [NPSH] is critical).
The temperature rise is the result of hydraulic power losses within the pump. The difference between brake horsepower consumed and the water horsepower developed is converted into heat and transferred to the liquid being pumped. If the pump operates against a completely closed valve, the power loss becomes equal to the brake horsepower generated at shutoff. All of the horsepower is used to heat the volume of liquid within the pump casing, which results in a temperature rise.