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.
Low Discharge Pressure
In this example, we investigate the possible causes of low discharge pressure in the same pumping system, and through the process of elimination we will narrow in on the probable cause. The system represented in Figure 1 has two identical pumps in parallel. One pump is operating and the other is on standby. The pumps are alternated each month to balance the number of operating hours between them. After shifting from the PU-101A to the PU-101B pump, the plant operator checked the system. Table 1 compares the observed results with the validated results. The discrepancy was turned over to the plant engineer to troubleshoot.
Because the levels and pressures in the supply and destination tanks are identical to the validated tank levels and pressures, we can rule out a change in the static head for a change in the pump discharge pressure. Also, because the set point of the flow control valve FCV-101 is set to 1,000 gpm and there is no change in the pump suction pressure reading, we can rule out problems in the pump suction line.
Another possible cause for a reduction in pump discharge pressure is cavitation. When cavitation occurs in a centrifugal pump, the flow rate decreases because some liquid becomes vapor. Because the flow control loop is set to maintain the flow rate, the control valve opens further and allows more flow. Because the pump is already cavitating, the increase in flow rate causes even more cavitation and results in reduced discharge pressure.
Discussions with the operator revealed that the pump did not sound like it was cavitating. The only sure way to identify cavitation is to calculate the net positive suction head available (NPSHA) at the pump suction and compare it to the manufacturer's supplied curve for net positive suction head required (NPSHR). Quick calculations showed that the NPSHA at the pump suction was 46 feet of fluid, and the NPSHR from the pump curve is only 8.3 feet of fluid. As a result, pump cavitation can be ruled out.
Change in the Process Fluid
Changes in process fluid can cause a reduction in discharge pressure. A change in the process fluid's viscosity can also cause major changes in pump performance.
Pumping a process fluid with a higher viscosity than that of water will produce less head at a given flow rate while reducing the pump's efficiency. The method for calculating the viscosity correction for pumps is outlined in the American National Standards Institute/Hydraulic Institute (ANSI/HI) 9.6.7 Effects of Liquid Viscosity on Rotodynamic Pump Performance standard. The method is much more complex than correcting the pump's differential pressure for changes in density, but most manufacturers' supplied selection software automatically perform viscosity corrections based on the ANSI/HI standard
The plant process engineer was asked if there were any changes to the system's process fluid. The process engineer stated that no change that could affect the fluid viscosity had occurred.
One possible cause for a reduction in pump head is a worn impeller. This can be caused by a pump cavitating for an extended period of time. Also, process fluid containing solids or abrasives can cause erosion to the impeller. The pump was not experiencing cavitation and the process fluid is a clear liquid with no grit, ruling out erosion to the pump impeller.
Internal leakage within a pump can cause a reduction in the pump's discharge pressure. The impeller's rotation imparts kinetic energy to the fluid within the pump. The impeller's shape and volute convert that added kinetic energy to pressure energy. Because the suction is at low pressure and discharge is at high pressure, there is internal leakage from the pump discharge to the pump suction around the impeller. That internal leakage is minimized in the pump design by having a close running clearance in the leakage joint.
In an open and semi-open impeller, the leakage joint is between the rotating impeller and the stationary pump casing. For an enclosed impeller, the leakage joint occurs across the pump's wear rings. If there is excessive clearance in the leakage joint, the pump will not operate per the manufacturer's pump curve. Excessive internal leakage causes a reduction in pump head, flow rate and pump efficiency. The shape of the pump curve for the increased internal leakage is not a concern, but what we are able to gain from the model is that the pump is not operating as supplied and needs to be repaired.
We can find out if the problem is internal leakage if we take the pump out of service and check for the leakage joint clearance. In this example, the clearance in the pump's leakage joint was greater than the manufacturer's recommendation. Once the clearance was adjusted, the pump was placed back in service and the operating data matched the curve.
Looking at the Motor
Another way to find out if a pump is operating properly is to measure the pump drive's power consumption. The motor's installed power meter can be used to determine the power consumed. The current motor power can be compared to the motor value obtained during the validation process. A motor drawing more or less power than determined during the system validation can indicate a problem with the pump element.
As we demonstrated in the previous two articles on troubleshooting fluid piping systems, we were able to determine that a mathematical model of a piping system accurately represents the operation of fluid piping systems under a range of expected system operation. As a result, once the model is validated to accurately reflect the physical system, the model can be used for troubleshooting.
In this article, we looked at two observed conditions dealing with a pump in a system. During the troubleshooting process, we considered a variety of possible conditions that can affect the system. When troubleshooting, end users must first consider all possible causes for the expected problem and then eliminate them one at a time based on knowledge of the individual system components and their interaction within a system.
Next month, we will look at more system-related conditions dealing with the process elements consisting of tanks and vessels, pipelines, and process equipment. Once finished with the process equipment, we will evaluate the control elements to round out our system troubleshooting.