Pump Ghost Stories

The April 2004 issue of CSE featured the article “Busting Ghosts,” an indepth description of pump system troubleshooting-shaking the ghosts out of pumps. Here, the author offers some actual case histories of successful solutions to pump problems.

Case History – Horizontal phosphate rock slurry pump
Service: Transfer
Fluid: Phosphate rock slurry
Density: 25% solids by weight
S.G.: 1.18
D50 particle size: 16 mesh
Design Flow: 1,200 gpm – selected by the plant engineer
Design Head: 69 ft. TDH – selected by the plant engineer
Pump selected: 6 x 8 – 17
Type of pump: Horizontal end-suction slurry pump
Pump Impeller dia.: 17
Pump Speed: 850 rpm
Driver: Electric motor with a V-belt drive

The pump specified was a reasonable selection. The design point of operation would be to the left of the pump’s best efficiency point (BEP). Pump rotational speed was well within good design parameters for a slurry pump.

So, what could go wrong? The symptoms of poor pump performance as reported by plant personnel were: very little to no flow through the system, pump making a very loud screaming noise and motor would intermittently kick-out on overload.

The plant engineer, who had just installed this brand new pump in a brand new system that did not work correctly, called the pump consultant to the plant. The pump consultant and plant engineer went to the job site and confirmed system symptoms as reported by the operators and maintenance personnel.

The following pump and system checks were performed, or at least attempted, during initial visit:

• Motor speed was confirmed to be near design.
• Pump speed was confirmed to be near design.
• No discharge pressure readings were taken, because gauges had not been installed.
• Fluid density could not be confirmed, because little or no flow was getting to the plant.
• There was no method to accurately measure flow.
• Amp readings were taken and seemed a little high for design but not appreciably high given the limited accuracy of all “field” measurements anyway.
• Valves were confirmed to be open. There were no discharge valves. Slurries can tend to eat away most things in their path. Therefore, many slurry systems do not have discharge valves in them unless absolutely necessary.
• It was confirmed that there were no obstructions in the lines.

The plant engineer and consultant went back to the office to look at calculations. As-built drawings were reviewed and system calculations were verified. A full system curve was drawn over the pump curve. Based upon the confirmed system calculations the pump appeared to be the correct selection for this application and nothing seemed to indicate cause of symptoms.

So, what went wrong? A trip back out to the job site to look for additional clues resulted in several hours of standing around waiting for some kind of divine inspiration. No lightning bolts struck, so it was suggested to go to lunch. Returning from lunch, the pump consultant and engineer stopped back at the job site and proceeded towards the installation. During their walk back toward the pump, the pump guy paused for a moment to enjoy the day and happened to look up at the plant structure. He asked the engineer how high the plant was. The engineer informed the consultant that his as-built drawings indicated that the plant is approximately 50 ft. high. The consultant, looking at the plant, remarked to the engineer that it sure “looks taller than that.” The engineer took another look at the plant and replied, “Now that you mention it, the plant does look higher than 50 feet.”

The consultant suggested that it would be worthwhile to accurately measure the plant height. The pump was pumping to the top of the plant with calculations based upon 50-ft. static head. If the plant were in fact taller, it would affect the system curve, which in turn would affect pump performance.

Plant personnel measured the plant and found it to be approximately 72 ft. from ground level to the point of pump discharge. This equated to the actual static system head being approximately 70 ft. after liquid levels were taken into account. A new system curve was produced based on the new static head.

From the intersection of the revised system curve over the pump curve, one could see that the pump was operating back towards its shut-off point. Slurry flow demands a minimum fluid velocity to maintain slurry suspension to prevent slurry settling. Because there was so little flow, the slurry was settling out in the pipe and the pump. This produced extreme drag on the pumps impeller, which resulted in the screaming belts trying to drive the impeller through the settled slurry in the pump. The motor tended towards overload due to the extreme drag on the impeller from the settled slurry. This explained the symptoms experienced. Little to no flow is obvious.

Solution to problem: Change belts and sheaves to speed the pump up and increase the motor horsepower. Also, check with the pump supplier to make sure pump can tolerate additional loads. The moral of this story is to be cautious of “as-built” drawings when performing pump system calculations.

Case History – Horizontal phosphate ball-mill rock slurry pump
Service: Ball mill feeding attack tank
Fluid: Phosphate rock slurry
Density: 68% solids by weight
S.G.: 1.7
TPH: 115 Tons per hr.
D50 particle size: 28 mesh
Design Flow: 400 gpm – selected by the plant engineer
Design Head: 65 ft. TDH – selected by the plant engineer
Pump selected: 4 x 3 – 10
Type of pump: Horizontal end-suction slurry pump
Materials of Pump: Rubber lined
Pump Impeller dia.: 9.6 in.
Pump Speed: 1,450 rpm
Driver: Electric motor with a V-Belt drive

In this case, too, the pump specified seemed to be a reasonable selection. The design point of operation would be to the left of the pump’s BEP. Pump rotational speed is well within good design parameters for a slurry pump of this size. However, the system performed poorly, with plant personnel reporting the following symptoms:

• Excessive wear on pump liners. Expected life was six to 12 months. Reported life was in the two- to three-month range.
• Motor kicking out on overload protection intermittently.

The plant engineer, who had installed this system, called a pump consultant to the plant. They went to the job site to confirm system symptoms as reported by the operators and maintenance personnel. The following pump and system checks were performed, or attempted, during initial visit:

• Motor speed was confirmed to be near design.
• Pump speed was confirmed to be near design.
• Suction/discharge pressure gauges confirmed expected pressures.
• Fluid density was confirmed.
• Flow rate was confirmed near design by calculating gpm from tons being recorded going into plant compared to density recorded.
• Amp readings were taken and seemed in line for service.
• Valves to pumps were confirmed to be open. There were no discharge valves.
• There were no obstructions in lines.
• Belt tension appeared to be acceptable.

The plant engineer and consultant went back to office to look at calculations. A full system curve was drawn over the pump curve. One could see from this that the system curve did not overlap the selected pump curve at a reasonable position on the pump curve. More detective work was required.

The worn-out pump components were analyzed in the field by the consultant for possible clues of premature failure. The rubber felt to be in good condition. There did not seem to be any chemical attack or high temperature failure. The location of the worn-out material did not provide any indication for the premature failure. The pump components appeared to be just worn-out, as if they had already experienced about six to 12 months in service instead of the two to three months actual service.

The intermittent motor overload was a possible clue. There was nothing to indicate a cause for motor overload. The motor selected was conservatively almost three times more horsepower than required for the design service. Further inquiries into the motor overload situation were made. Checking with the operations personnel into exactly when the motor overloaded revealed some additional clues. The motor overload only occurred periodically when the operators would speed up the pump using the variable frequency drive installed in the system. It seemed that the operators would watch the tons going into the reactor and when, for any reason, the tons would drop, they would immediately crank up the pump speed as high as it would go to get the system back on design. Investigations into the VFD revealed that the operators could in fact speed the pump up to almost 1,950 rpm. They would leave the pump operating at that speed until they were convinced the system was getting sufficient supply. Sometimes the operators might forget to slow the pump down and operate the pump at that speed for quite a while. This proved to be a daily method of operation.

With this bit of information the pump curve and the system curve were used as the tool for diagnosing the high wear problem.

This case is different from the first. The pump curve, rather than the system curve, is different from what we thought it was. We must now look at the operating point for the pump given this new pump speed.

The new pump curve is at 1,950 rpm and where the pump curve intersects the system curve. This is where that pump will operate unless some artificial changes are made to the system to alter the system curve. From the actual operating point we can see that the pump is actual pumping 800 gpm instead of the 400 gpm as was designed. The pump is now handling approximately 230 tons per hour as opposed to the 115 tons per hour as planned. Plus the pump operating speed of 1,950 rpm is at the upper limit for this pump design. Combining the increased rotating speed of over 30% with the increased tons of slurry pumped, we see the reason for the rapid wear on the slurry pump components.

The solution to this problem was to place restrictions on the operator’s ability to increase the pump speed. Educate the operators on the effects of prolonged operation of the slurry pump at elevated speeds. Even when you think you have the correct information, you may not.

Case History – Two pumps in parallel. 600 equivalent length of 16″ schedule 40 pipe.

Service: Water supply pumps
Fluid: Water
S.G.: 1.0
Design Flow/pump: 12,000 gpm – selected by the plant engineer
Design Head/pump: 96 ft. TDH – selected by the plant engineer
Pump selected: 16 x 18 – 18
Type of pump: Horizontal split-case pump
Materials of Pump: Cast iron with bronze internals
Pump Impeller dia.: 17 in.
Pump Speed: 1,180 rpm
Driver: Electric motor directed connected to pump. Constant fixed speed

In this case, the plant complained to pump vendor that the system functions fine with one pump operating. But with a second pump on line, they didn’t get double the flow.

Many plants have systems with two pumps operating in parallel. Typically, one pump will operate most of the time with the second pump coming online only when there is an increased system demand for the fluid or product being pumped. What happens to the pumping system when two pumps are operating in parallel? Why doesn’t one always get twice the flow with two pumps pumping in parallel?

Design point is at BEP, which for handling water is a good point to be on the pump curve. Taking a look at the system curve with the pump curve we see that the pump selection is confirmed to be acceptable. We will have a very good operating pumping system when we operate one pump. Now let’s investigate how the pumping system will function when we turn on the second pump in parallel. Pumps in parallel add their representative flows at the same pump head.

Here we can see the combined pump curve as if the two pumps were functioning as one single pump. Inserting the second pump into the mix, we see that the two pumps will intersect the system at approximately 16,000 gpm. As we have determined before, the system will drive the pumps to function at precisely where the pump curve intersects the system curve. Therefore, this is where our parallel pumping system will operate. We will only pump 4,000 gpm more with two pumps operating that we did with only one pump operating. Go tell your plant manager that is the most you are going to get no matter how much you beat up on the poor pump guy. To get twice the flow with this system is not possible. The system will have to be modified and or the pump driver system will have to be changed. The piping could be changed to > 18 in. and or a VFD could be added to increase the speed of the pumps. Caution here to check with your pump manufacturer prior to increasing pumps speeds.

The moral is that the whole is not always the sum of the parts. You will rarely achieve double the flow of a single pump online with two pumps online in a parallel pumping system unless you have some kind of speed control to increase and decrease your pump speeds as needed to achieve your end results.

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