Pump Monitoring with Electronic Differential

Pump Monitoring with Electronic Differential

Pressure Transmitters

An electronic differential pressure transmitter can monitor a pumping system, detecting problems, provide information, improve efficiency and providesavings.

By: Bruce Morgan, EMC Industrial Grp

In water and waste we incur thousandsof dollars inmaintenance costs to keep pumping systems in good health.We are quite good at monitoring the discharge pressure of pumps but we could learn more about our pump system with the use of an electronic differential pressure transmitter (EdP). An EdP device not only monitors instantaneous pump performance, but can also be used to identifyissues related to pump health and efficiency.

Monitoring Differential Pressure

An EdPtransmitter is primarily used to measure level and differentialpressure in tanks, but itcan also be used in pumping systems (Figure 2).An EdP transmitter can monitor the suction and dischargepressure—i.e., the differential pressure of a pump. Too high ortoo low suction and discharge pressures can cause various pumpissues such as cavitation, loss of flow, mechanical failure,vibration issues, excessive noise, and bearing sealing wear.

All pumps perform best when they are at, or very close to, the Best Efficiency Point (BEP) on their pump curve (Figure 1). The BEP is given by the manufacturer on its pump curve. It is typically 70-85% of the shut-off head or maximum differential pressure between the suction and discharge. Helping the pump operate at the BEP is the single most important aspect for prolonging pump life.

The EdP transmitteroutputs values for suction pressure, discharge pressure andthe calculated differential pressure. High and lowalarms can be set for any of the monitored values. The EdP has a4-20mA HART output allowing for connection to an automation system via a single mA loop.

Figure 1: The Best Efficiency Point (BEP) is 70-85% of the maximumdifferential pressure between the suction and discharge pressure.
Figure 2:An EdP transmitter monitors both a pump’s suction and discharge pressure.

The DP or gauge value from the EdP canbe used to set alarms which in turn helpmonitor problems such as cavitation. Cavitationoccurs if thepressure falls below a liquid’s vapour pressure in the impeller. It may also occur if air becomesentrained in the medium. Monitoring thedischarge pressure and comparing it to limit values set at theautomation system will reveal whencavitation is occurring. Monitoring pressure in the suction linecan indicate problems such as leaks or a stuck orclogged valve. If suctionpressure drops below preset limits, theEdP transmitter cantrigger an alarm.

Setting the Right Flow Rate

An incorrect flow rate setting is one of thetypical causes of inefficient pumpoperation. The following is an example ofhow to use the EdP to set the correct flowrate for a given pump. From the pump curve, first note:

1. Impeller size

2. Rated power

3. Impeller speed

4. Expected flow at BEP

The pump curve is typically provided bythe manufacturer (Figure 4). Run thepump with media flowing through thesystem at an arbitrary flow. Let the flowstabilize. Note the reported differentialpressure. This is the total head ordifferential pressure provided by the pump. Assuming 120 feet of water is the totalhead generated by this 7 hp pump,point A is the intersection with the 70%performance curve. The correspondingvalue on the X axis (30 gpm) is the flowrate for the pump at an efficiency of 70%, corresponding to a Net Positive Suction Head (NPSHr) of 10 feet of water.

Figure 3: Endress+Hauser FMD71 EdP differential pressure transmitter mounted across a pump.
Figure 4: Pump curve for determining flow rate of a pump with a 10-inch impeller.

To increase efficiency and run thispump at the same total head, the flowrate needs to be increased to 55 gpm(rightmost red dotted line in Figure 4),so that the intersection with the 85%efficiency curve will be at point B. Theusers should set high and low alarms onthe EdP output to keep the DP tightlycontrolled while increasing the flowrate.If the pump doesn’t generate 120 feettotalhead at the 55 gpm flow rate, thiscould be your first indication of anoperational or mechanical issue. Checkthe suction pressure and ensure it isgreater than NPSHr. Check thedischarge pressure and calculate the HPgenerated and verify it matches thelisted HP on the pump curve for theoperating flow rate.

Monitoring Total Head

Assume 55 gpm is the operating flowrate for a 6 hp pump with an 8-inchimpeller. The expected intersection onthe performance curve for this pump(Figure 5) would be point A. Thecorresponding expected total head onthe Y axis would be 65 feet of water.The EdP transmitter sends its outputs toan automation system that has controllogic and calculations to convert thetransmitter data to total head. Theautomation system compares the EdPtransmitter outputs to the expectedtotal head—65 feet of water in thiscase—and adjusts pump speedaccordingly. This alters the pump speedto maintain the pump at BEP, which is70% according to the manufacturer’spump curve.If higher pumpefficiency is desired, thepump must be run at a higher speed, ora larger impeller diameter needs to beused.With a known flow rate, differentimpeller sizes can be tried to arrive at adesired total head and correspondingefficiency. Referring to Figure 4, if 55gpm is the known flow rate, then an8-inch impeller can be used to achieve70% efficiency. If a higher efficiency isdesired, then a 10-inch impeller can beused to achieve an efficiency of 75%,and a 12-inch impeller would achieve85%. The corresponding total headwould then be 100 feet of water.

Figure 5: Pump curve for monitoring total head.

Return on Investment

An EdP transmitter can pay for itself ina matter of months. Let’s use anexample of a pump in a Waste water application. The council spends approximately $1,000per year for seal failure andreplacement because it operates itspumps below the BEP.Point A represents the BEP of a 7 hppump with a 10-inch impeller operatingat 75% efficiency. If the pump is incontinuous operation, the energyconsumption per year would be $1,310at 10 cents per kWh. Point Brepresents the deviation from optimum,operating at only 70% efficiency.The total cost of operating the pump at70% efficiency is $2,151, where lostpower output per year is $1,151 andrepairs due to seal failure, vibrationissues, other cavitation damages andassociated downtime are $1,000. Totalcost, considering a system of fivepumps, is more than $12,000 per year.The average cost of an EdPtransmitter is $5,000. If the efficiency of the pumpsis increased to 75%, using thetechniques described in this article,annual savings amount to about $8,000per yearproviding areturn on investment in about eightmonths.