In that elusive perfect process world, a pump always operates at its best efficiency point (BEP) ," the flow rate at the point of highest efficiency. In the real world, that's almost never the case. In fact, the average pump efficiency in industrial applications is estimated to be below 40 percent.1 Process variables, pump over-sizing and other factors can cause a pump to perform at a less-than-optimum level. A key factor in improving mean time between failure (MRBF) is determining where your pump actually is operating.
Many pump problems actually are pump and system interaction problems. This is especially true for pumps with chronic failures. When pumps operate away from their BEP ," and most industrial pump installations do ," several problems can result. (See the table.) Maintenance, repair and operation costs can go through the roof. Pump operation away from the BEP also can waste energy ," a major concern, considering that pump systems currently account for 20 percent to 50 percent of the energy used in industrial facilities.2
Another common problem you can diagnose easily by determining the actual operating condition of the pump is low net positive suction head available (NPSHA). Low NPSHA can be a major problem for centrifugal pumps. Too often the operating point and NPSHA are calculated prior to facility construction. Pumps are purchased based on the design calculations and usually are not checked again. The factory might test the pumps upon purchase to verify performance and net positive suction head required (NPSHR), but almost never checks the system.
Anyone who has been involved in facility design and fabrication knows paper design and physical reality often differ greatly. Consider the stacks of engineering change orders generated by most projects. An accurate determination of your pump's operating point can be essential to improving the MTBF, reducing maintenance costs and increasing equipment availability.
What to measure ," and where
You can determine a pump's operating point with reasonable accuracy by measuring suction pressure, discharge pressure, fluid temperature, flow rate, amperage, voltage and shaft speed.
Pressure is one of the most commonly measured process variables. Most plants have hundreds of pressure gauges. Unfortunately, many of these have broken lenses and wiggling needles. These gauges often do not give an accurate representation of system pressure. Therefore, every maintenance department should purchase a set of quality gauges for use only in the troubleshooting and performance verification process.
Fig. 1 illustrates the system that can be used to mount the test gauges. These gauges should be kept in calibration, clearly marked as test gauges only, and stored in a clean restricted-access area such as a locked maintenance cabinet.
Figure 1. Test Gauge Setup
Every facility should have high-quality gauges dedicated solely to trouble-shooting and performance verification.
The pressure snubber protects the gauge from process pressure spikes. If you buy a high-quality gauge, then spend an extra $15 to protect it from spikes. A gauge isolator also is recommended. It can prevent gauge or process contamination. It also allows the use of less noble metallurgy for the gauge, which might represent a significant cost savings if your process requires exotic metallurgy. The valve is essential for the isolation of the test gauges and must meet all process fire and safety requirements.
You may take pressure measurements at any location close to the pump. Many process pumps have tapped pressure connections at the suction and discharge flanges. For performance verification purposes, these will suffice.
If you have to install taps for the pressure gauges, the Hydraulic Institute standards require the suction and discharge pressure tap locations to be two diameters of straight pipe from the pump suction and discharge. Locate the taps as close as possible to locations specified in the Hydraulic Institute Standards to get the most accurate results.
Temperature is probably the most commonly measured process variable. A temperature reading of 5 Degrees F generally will be accurate enough. The exception is when the process fluid nears its boiling point ," greater accuracy is needed in this case. You might require a calibrated thermocouple or resistance temperature detector (RTD) to obtain the necessary accuracy. If the temperature probe is located in a thermal well, be sure to allow sufficient time for the process temperature to stabilize before taking a measurement.
Take the temperature measurements as close as possible to the suction flange of the pump. This location allows an accurate determination of the fluid's vapor pressure, enabling accurate NPSHA prediction.
Flow often can be the most difficult variable to measure. The high cost of transducers discourages flowmeter use in most installations. If no flowmeter is provided in the system, a portable ultrasonic flowmeter is a good option. You generally can rent these units from a local representative for $150 to $200 per day.
Most insertion-type flowmeters used as transducers for process measurement are not terribly accurate. However, they can provide valuable data. A magnetic, turbine or vortex flowmeter in the system provides a very accurate method for determining the actual flow rate.
Take the flow measurements in a location that represents the total flow from the pump. Flowmeter requirements should dictate its placement. Make sure the required number of straight pipe diameters are provided before and after the flowmeter, and follow all of the other manufacturer's recommendations to properly locate and mount your flowmeter.
You can measure amperage with any common clamp-on ammeter ," most maintenance departments have one, and most electric motor data can be correlated using amperage. Take the measurements as close as possible to the motor junction box. Hazardous environments and other safety considerations can restrict motor current checks at the motor junction box. It is often a good idea to check all three motor leads to make sure load is balanced across them.