This series focuses on evaluating piping systems for which calculated results do not match displayed plant instrumentation. Part 1 ("Troubleshooting Pumps in a System," Pumps & Systems, December 2015) examined the components of the pump and drive, and the conclusion of this series will examine possible causes for an increased discharge pressure.
Reduction in Flow Rate
One possible cause of increased discharge pressure could be a lower flow rate through the pump. The pump curve in Figure 2 shows that decreasing the flow rate causes the pump to operate further back on its curve. This would increase the pump head and discharge pressure.
This cause, however, can be ruled out for a variety of reasons. First, with a flow rate of less than 1,000 gallons per minute (gpm), less head loss would occur in the suction pipeline, resulting in a higher pressure at the pump suction pressure gauge that was not observed. Second, the pump curve for the 14.125-inch impeller shows that the pump's shutoff head is 212 feet, a 16-foot head increase over the normal pump head of 192 feet at 1,000 gpm. A fluid density of 62 pounds per cubic foot (lb/ft3) would result in a pressure increase of 8.6 pounds per square inch (psi), which is less than the observed 16 psi pressure increase. Finally, because the control loop is set to 1,000 gpm and the flow indicator FT-101 shows 1,000 gpm, we have independent validation that the flow rate is accurate.
Change of Process Fluid
Another possible cause for a higher pressure is a change in the process fluid. An increase in the density of the process fluid will not affect the pump head, but it will affect the pump's differential pressure.
A few quick calculations determined that a density increase of the process fluid from the normal 62 lb/ft3 to 73 lb/ft3 would result in an increase of 16 psi on the pump discharge. The change in fluid density would also cause an increase in the pump suction pressure because of the liquid level in the supply tank. This is a rather small change in suction pressure, and it may be masked by normal system operation. As a result, a change in fluid density is a viable cause for the discharge pressure increase.
This change in density could occur with a change in the process fluid, the fluid's concentration or the fluid temperature in tank TK-101 as seen in Figure 1. A discussion with the plant process engineer revealed that no change in process fluid that could have caused an increase in the pump discharge pressure had occurred in the system in the past six months.
Increased Impeller Diameter
The operating data shows that an increase of 16 psi for a fluid with a density of 62 lb/ft3 results in an increase of 37 feet of fluid. The supplied pump curve reveals that an impeller diameter of 15 inches results in an increase of approximately 37 feet of pump head. Because changing an impeller's diameter requires disassembling the pump, the plant engineer checked with the maintenance department to see if any work had been performed on the pump. Maintenance records showed that the impeller was replaced four months earlier. The damaged impeller was replaced with an impeller diameter of 15 inches instead of the previously used 14.125-inch diameter.
The original purchase order for the pump specified a 15-inch impeller, but a last-minute design change resulted in a 14.125-inch impeller. The maintenance department checked the original purchase order for the impeller diameter and did not see the revision during installation.
The maintenance record was updated to reflect the 14.125-inch impeller so the same problem would not occur. Maintenance was interested in keeping the impeller at 15 inches until it needed to be repaired again. A quick calculation determined that keeping the impeller at 15 inches instead of the specified 14.125 inches would result in a $10,000 increase in annual pumping cost. Because the pump was in service for five years before needing an impeller replacement, plant management decided to take the corrective action of trimming the impeller to 14.125 inches during an upcoming planned maintenance outage.