For submersible vertical turbine pumps in service, we recommend using local pressure instrumentation to aid in predictive maintenance and troubleshooting.

Pump Operating Point

Pumps are designed to achieve and operate at a specified design flow and differential pressure/head. Operating within 10% to 15% of the Best Efficiency Point (BEP) minimizes vibration associated with unbalanced internal forces. Note that the percentage deviation from BEP is measured in terms of BEP flow. The further the pump is operated from BEP, the less reliable it is.

The pump curve is the operation of the equipment when there is no problem, and the operating point of a well-performing pump can be predicted by the suction pressure and discharge pressure or flow. If the equipment fails, all three of the above parameters must be known to determine what the problem is with the pump. However, without measuring the above values, it is difficult to determine whether there is a problem with the submersible vertical turbine pump. Therefore, it is critical to install a flow meter and suction and discharge pressure gauges.

Once the flow rate and differential pressure/head are known, plot them on a graph. The plotted point will most likely be close to the pump curve. If so, you can immediately determine how far from BEP the equipment is operating. If this point is below the pump curve, it can be determined that the pump is not performing as designed and may have some form of internal damage.

If a pump is constantly running to the left of its BEP, it may be considered oversized and possible solutions include cutting the impeller.

If a submersible vertical turbine pump is habitually running to the right of its BEP, it may be considered undersized. Possible solutions include increasing the impeller diameter, increasing the pump speed, throttling the discharge valve or replacing the pump with one designed to produce a higher flow rate. Operating a pump close to its BEP is one of the best ways to ensure high reliability.

Net Positive Suction Head

Net Positive Suction Head (NPSH) is a measure of the tendency of a fluid to remain liquid. When NPSH is zero, the liquid is at its vapor pressure or boiling point. The Net Positive Suction Head Required (NPSHr) curve for a centrifugal pump defines the suction head required to prevent the liquid from vaporizing when passing through the low pressure point at the impeller suction hole.

The available net positive suction head (NPSHHa) must be greater than or equal to NPSHr to prevent cavitation - a phenomenon where bubbles form in the low pressure zone at the impeller suction bore and then collapse violently in the high pressure zone, causing material shedding and pump vibration, which can lead to bearing and mechanical seal failures in a small fraction of their typical life cycle. At high flow rates, the NPSHr values on the submersible vertical turbine pump curve increase exponentially.

A suction pressure gauge is the most practical and accurate way to measure NPSHa. There are many different reasons for a low NPSHa. However, the most common causes are a clogged suction line, a partially closed suction valve, and a clogged suction filter. Also, running the pump to the right of its BEP will increase the pump's NPSHr. A suction pressure gauge can be installed to help the user identify the problem.

Suction Filters

Many pumps use suction filters to prevent foreign matter from entering and damaging the impeller and volute. The problem is that they clog over time. When they clog, the pressure drop across the filter increases, which reduces the NPSHa. A second suction pressure gauge can be set upstream of the filter to compare to the pump's suction pressure gauge to determine if the filter is clogged. If the two gauges are not reading the same, it is clear that a filter plugging exists.

Seal Support Pressure Monitoring

While mechanical seals are not always the root cause, they are widely considered to be the most common point of failure for submersible vertical turbine pumps. API seal support piping programs are used to maintain proper lubrication, temperature, pressure and/or chemical compatibility. Maintaining the piping program is critical to maximizing reliability. Therefore, close attention must be paid to the instrumentation of the seal support system. External flushing, steam quench, seal pots, circulation systems and gas panels should all be equipped with pressure gauges.

Conclusion

Surveys show that less than 30% of centrifugal pumps are equipped with suction pressure gauges. However, no amount of instrumentation can prevent equipment failure if the data is not properly observed and used. Whether it is a new project or a retrofit project, the installation of appropriate in-situ instrumentation should be considered to ensure that users can perform proper troubleshooting and predictive maintenance on critical equipment.