Knowledge of Pump Problems Often Lacking

Maintenance personnel in industrial plants and facilities know a great deal about pumps. They can replace seals and bearings in a jiffy, remove and install pumps and generally fix anything that goes wrong mechanically. They know how, because many of them do it a lot—on the same pumps, month after month. Plant engineers know how to read a data sheet and (often with the help of a pump manufacturer) can spec pumps to meet flow, pressure and service requirements. Pumps can be properly sized initially for ideal conditions when a system is designed. The only problem is those conditions in the process are dynamic and rarely stay the same over the life of the pump, so it may not be the ideal pump size for the conditions five years from now. But it can be difficult for these maintenance technicians and plant engineers to tell when a pump is having problems, performing badly or is about to fail. So many pumps run to failure. A big part of the problem is the lack of sensors on many pumps, resulting in insufficient data to detect common problems, such as:

• vibration that can damage pumps, pipes and foundations
• cavitation that can destroy impellers and volutes
• “dead head” operation (zero flow) that can overheat the liquid, maybe causing the liquid to flash to vapor
• seal pot leaks of toxic, hazardous
• or corrosive fluids
• excessive pump case pressure or pressure spikes that can damage pump seals
• excessive temperature within a motor that can cause damage

Without flow, pressure or temperature sensors—and the knowledge of what to do with the data provided by these sensors—many pumps fail unexpectedly and upset processes, use too much energy or require excessive maintenance. And when a pump fails, it can shut down a process or an entire plant, cause spills, fires or explosions and harm personnel. The solution is twofold: add instrumentation to pumps, and train maintenance personnel and engineers on how to diagnose pump problems using data from the instruments.

Adding Instrumentation

Most pumps already have (or should have) a flow meter to monitor pump discharge flow rates. Pumps also need a differential pressure (DP) instrument and a temperature instrument. A DP instrument can monitor the suction and discharge pressure—i.e., the differential pressure—of a pump. Too high or too low suction and discharge pressures can cause or indicate various pump issues such as cavitation, loss of flow, mechanical failure, vibration issues, excessive noise, or bearing and sealing wear. And some newer electronic DP devices have built-in temperature measurement capabilities (see Image 1). Temperature instruments can measure pump, fluid and motor temperatures. To avoid cavitation, the net positive suction head (NPSH) available must be greater than or equal to the NPSH required with a safety margin. Monitoring the suction head identifies conditions that can damage the pump.Several factors can change the NPSH required, including increases in flow rate and changes to the pressure or liquid level in a supply tank in front of the pump. “Smart” flow meters are available that can diagnose problems such as entrained air, vibration (which could be caused by pump cavitation), coating, corrosion and inhomogeneous or unsuitable media (see Image 2). Data from a Coriolis flow meter can detect an empty pipe, density shift, temperature shift and other conditions. Data from flow, temperature and DP instruments is sent to the control system and routed to pump analysis software. This software—which is available in many asset management, condition monitoring or maintenance platforms—typically incorporates pump curves from all pumps being monitored. This lets the software compare the pump’s actual performance to what it should be. Pump software is also available from the Department of Energy (DOE). The Pumping System Assessment Tool (PSAT) is a free online software tool to help industrial users assess the efficiency of pumping system operations. PSAT uses achievable pump performance data from Hydraulic Institute standards and motor performance data from the MotorMaster+ database to calculate potential energy and associated cost savings. It can be downloaded at www.energy.gov/eere/amo/articles/pumping-system-assessment-tool. The savings from reduced maintenance, longer pump life, fewer process upsets and improved efficiency far outweigh any modest investment in pump instrumentation.

Training the Pump Experts

Even if pumps are instrumented properly and the data from these instruments is analyzed with a pump monitoring system, the engineers and maintenance personnel may not know what to do with the data without proper training. In some cases, the instruments indicate a problem, but expertise is needed to determine the best fix. Pump training is available from instrument vendors, the DOE, pump specialists and educational institutions. For example, the DOE offers a free qualification program in the use of PSAT software. Those who successfully complete the PSAT qualification workshop and exam receive recognition from DOE as Qualified Pump System Specialists. For more information, go to energy.gov/eere/amo/articles/training-pumping-systems. Many companies also offer pump training courses and process training units (Image 3). One subject covered in many courses is how to analyze pump and system curves (Figure 1). While this was covered in the August issue of Pumps & Systems, training courses can go into more detail. For example, engineers and technicians who attend the one training seminar complete an evaluation at the end of the training. Many participants comment they learned their process pumps are constantly dragged all over the performance curve. Most come into the class believing the pump operates at a particular set of head and flow coordinates. They believe this because most pump engineers show a static system curve. Some pump engineers eliminate the system curve and draw a simple triangle on the pump curve indicating the primary head and flow coordinates, as though the pump has only one duty. It is true that some pumps perform one primary duty like recirculating water in a cooling tower reservoir or moving a liquid (orange juice or laundry soap) from a holding tank to a bottling carousel. Some pipe systems are simple and static. However, refinery and chemical process systems are dynamic. Levels constantly rise and drop in tanks. Pressures and temperatures constantly vary in reactors, boilers and pressure vessels. Valves open and shut all day. Filters and strainer baskets are clean in the morning and clogged with debris at midday. Then, new equipment (check valve, heat exchanger, in-line mixer, flow meter) is installed into an existing pipe system, altering the resistance and the pump’s performance. A process system curve is more like the wagging tail of a happy puppy, and the pump does what the system makes it do. A pump seminar shows engineers how to design a pump into a truly dynamic system. Operators learn to coax a misbehaving pump away from vibrations, cavitation and premature failure. The seminar shows engineers and technicians alike how to contribute to pump reliability.

Summary

A modest investment in instruments, software and training can improve the reliability and performance of expensive pumps and pumping systems. Spending a few thousand dollars in these areas can protect a $50,000 pump from serious damage, prevent process upsets, help protect personnel from injuries and stop spillage of dangerous materials. Pumps are vital, expensive components consuming a great deal of energy, and they are expensive to rebuild or replace. Adding a few instruments to a system—and learning how to interpret the data from these instruments—can help pumps run more efficiently and identify problems before they cause incidents.

References

Monitoring Pumps with Pressure Transmitters, Pumps & Systems, August 2017 https://www.pumpsandsystems.com/pumps/august-2017-pump-monitoring-electronic-differential-pressure-transmitters Pump sensors can improve system operation and detect dangerous faults, Pumps & Systems, June 2015. http://www.pumpsandsystems.com/instrumentationcontrols/june-2015-protect-pumps-one-essential-tool?page=2 How Smart Instruments Enable Improved Pumping, automation.com. https://www.automation.com/portals/process-automation/fluid-power-valves-pumps/how-smart-instruments-enable-improved-pumping