A methodical approach to diagnosing pumping problems

By Darren McPherson

 

Commercial pump issues are often system issues 80 to 90 per cent of the time in my experience. That’s not to say that the pump is never the cause of the problem, it’s just easy to point to the pump before verifying the system meets the design for which the pump was selected.

Distinguishing between a pump problem and a system problem often requires a good bit of information gathering. Use a multi-step process when identifying issues involving commercial pumps.

Be methodical and collect as much information as possible. Panicking or jumping to conclusions is not helpful. In fact, jumping to a conclusion is counter-productive because it may cause us to overlook the real issue. The first step is to identify the problem. For example, they have a chilled water system and the water flow is low. While we can be fairly certain there’s a flow problem, we’re unsure of its cause.

 

Interview Process

At this point we need more information. Ask specific questions such as: is this a water-cooled system; if so, is the issue on the chilled water or condenser water side; how many pumps are in the system; and what model is the pump?

Once we have the pump model, we need to know the design operating conditions. These are the conditions for which the pump was specified. We also need to know the actual, real-time pump performance. We’ll compare design conditions and actual performance information later. This means that someone onsite will be required to measure the flow and pressure drop across the pump. If there are pumps in parallel, it’s best to measure flow across one pump at a time. This establishes the performance of the pumps individually. The preferred method to measure flow is a calibrated flow meter, either a permanently-installed unit or a strap on, ultrasonic meter. Measuring the Delta-P across the pump is an indicator, but it’s not conclusive.

 

Data Collection

The importance of photographs can’t be overstated. Photos can help determine if the system was piped according to the original design and identify easily-overlooked issues such as placement of meters. Collect as many images as possible and develop a file. Navigate the images and ask further questions that may arise. We also need access to the piping diagrams. If diagrams aren’t available, ask for a hand sketch.

The next step is to collect electrical data. Volt and amp readings should be taken at the motor input by a licensed electrician. If the pump is equipped with a VFD, the readings should be taken at the input of the drive, not the motor. This is because the VFD modifies the voltage going to the motor. There are other considerations when a VFD is present on the pump. Ideally, the electrical data at the drive should be taken at full speed. This means that the flow and pressure differential must also be measured at full speed.

Make note of the pump RPM. This allows us to verify that the pump is in fact operating at the correct speed and in the correct direction. The next thing we need is manufacturer data. It’s quite common for people to think they have pump model X, only to realize it’s a different pump when the pump tag is checked. This is another reason for photos. After the pump model is known, the single most important piece of manufacturer data is the pump curve. The installation and operation manual is also critical, as is the field data. Blank data sheets or reports may be available for download from the manufacturer.

 

Analyze the Data

Figure 1

Let’s assume the pump shown in Figure 1 is an end-suction pump. Referencing the pump curve, we find that in a single-pump configuration, this unit provides 1,000 GPM at 80 feet of head. The red line represents the pump curve and the blue line represents the system curve. In this example, the customer didn’t actually measure the flow. Instead, they measured the pressure differential across the pump and reported that it was 92 feet of head.

Keep in mind that there’s a correction needed here. An end-suction pump, typically has a larger inlet than outlet so we need to correct for velocity. They did this and 92 feet was correct, meaning that they intersected the pump curve line at 750 GPM. In this scenario, we find that the pump is operating on the pump curve but at a different pressure drop, which hints at a system issue instead of a pump issue.

Looking at the pump curve, this unit should be operating at 25 horsepower while providing 1,000 GPM at 80 feet of head. Given the field-collected flow and pressure, the pump curve shows the pump should be operating at roughly 21 horsepower. Determining the actual horsepower of the pump in the field requires a number of electrical calculations. Use the field data recorded by the electrician (at 1,760 RPM) to calculate the actual pump horsepower. The efficiency and power factor of the pump is listed on the pump’s nameplate. These numbers are a critical component in the calculation* (see next page).

When the horsepower was calculated for this example, we found that the pump was operating at 21 horsepower, as expected. This confirmed that the reason we’re not getting the proper flow is that there’s more system resistance than was originally anticipated.

Ask the people in the mechanical room for the shutoff head. Keep in mind that this number can be greater or less than the published value by as much as eight feet. To determine the shut-off head, throttle back the discharge valve and isolate it for a few seconds. The operator should be able to measure the pressure differential across the pump. In this case, the shut-off pressure should be around 97 feet, which will confirm that the impellor diameter is as specified (10.15 inches) and indicated on the pump curve.

 

Recommend a solution

Everything we’ve done so far leads us to believe there’s a pressure drop in the system beyond what was anticipated. Another look at the piping diagram and photos reveals there was only one difference between the piping diagram and what existed in the mechanical room. The photos revealed a basket strainer installed between the cooling tower and the suction side of the pump. We learned it was added after the initial installation and was not part of the design. Despite the fact that the published pressure data for the strainer was two feet, it was causing a 12 ft. pressure drop. The strainer could be removed or maybe it can be oversized, but suffice to say this was not a pump issue.

 

Additional considerations

There are a few other key elements to troubleshooting a pump issue.

  • The use of a glycol mixture instead of pure water will significantly raise pumping resistance within a hydronic system. The dilution of the glycol, along with the temperature of the system fluid, will have an impact. The more glycol used, and the lower the fluid’s temperature, the greater the resistance.
  • When measuring the pressure differential across the pump, the difference in elevation between the inlet gauge and the outlet gauge can have an impact on collected data. If the gauges are at different elevations, a correction needs to be made before the readings can be used in any calculations.
  • Having calibrated gauges and meters is the only way to ensure the readings taken are accurate. Most suppliers of gauges and meters will check the calibration of the instruments before shipping them. There are three types of pressure gauges readily available; conventional, compound and digital. If conventional gauges are used, keep in mind that the unit will read zero any time the pressure is at or below zero. It’s very unlikely that pressure will ever be zero.

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