Pump System Energy Assessment


Written by:
Greg Case, Pump Design, Development & Diagnostics, LLC

Improved reliability, performance and energy efficiency can result from pump system assessment.

Second of Two Parts

In Part One of this series (Pumps & Systems, November 2010), a method was developed to determine which pump systems in a facility offer the best opportunity for significant cost reduction through energy conservation and reliability improvement.

After selecting a candidate system, data must be taken to determine its actual operating point or operating range. For a system with a small operating range, only a simple, one-point test may be needed. Systems with a larger range of operation may require a performance test at each of several operating conditions to understand the scope of operation. More complex systems or systems of which little is known about the operating range may need continuous monitoring over a period of time to capture the operational extremes.

Data Needed to Assess a Pump System

A pump assessment is a mini performance test of the pump within the system in which it normally functions. The assessment generally requires measuring the pump's flow rate, head and power input to determine its actual operating point within its system. Specifically, collected data should include:

1.      Pressures

a. Suction

b. Discharge

2.      Flow rate

3.      Pump input power

4.      Shaft speed

5.      Fluid properties at the operating condition(s)

6.      Pump performance curve (from the manufacturer)

7.      Motor performance—Possibly the VFD performance also (from the manufacturer)

WarningBefore any data collection is begun, all data must be collected by trained and qualified personnel. Data collectors will be connecting test equipment to live electrical wires and to rotating equipment. All company, local, state and federal safety protocols must be followed including but not limited to NFPA 70E and OSHA 29CFR 1910.335. If any reservations in the collectors' ability to gather these measurements safely arise, consult a professional for assistance. 

Pressure Gauges

Pressure gauges are often permanently installed on pumps but these may not be the gauges that should be used during an assessment. Because of pressure spikes, water hammer, fouling, etc., these gauges are often inaccurate. If safety permits, mount accurate pressure gauges onto the system. The gauges should be of good quality and accurate enough to read pump suction and discharge pressures within +/- 3 percent.

Be aware that the suction pressure may be below atmospheric, requiring a compound gauge (able to read both positive and vacuum pressures). The gauges should be mounted as close to the pump as possible to eliminate potential errors caused by losses in pipe fittings.

The head level in and out of the pump must be determined. The difference between the inlet head (input energy) and the discharge head (exiting energy) is the amount of energy added to the flow measured in foot-pounds per pound. If only the inlet or outlet head or pressure is known, determining the amount of energy transferred to the fluid is impossible. The formula to convert pressure to head is (see nomenclature listing below for definition of terms):

hg = (P x 2.31)/SG

If a suction gauge is not available, as on a vertical immersion sump pump, use the level of the fluid above the inlet to the impeller to determine the suction head level. The equation to determine suction head from a fluid level measurement is:

Hg = [(GP x 2.31)/SG] + hvs ± Zs - hfs

Flow Rate

The next required measurement is the flow rate through the pump system. This can be determined by an inline flow meter that is permanently installed in the system or by one installed temporarily for the test. Whichever method is used, the flow meter needs to be installed correctly with the proper amount of straight pipe before and after the meter.

Temporary flow meters must not change the system loss characteristics during the measurement. A popular type of temporary flow meter is an ultrasonic flow meter. It uses changes in a sound signal passed through the pipe and the flow to determine the velocity of the flow in the pipe. This is non-invasive and does not require the pipe to be penetrated. However, they can be temperamental and error prone when misapplied.

Another type, common in some industries, is a pitot tube that measures velocity by determining the difference between the total and static pressures in a flow. There are several other insertion type meters, including paddlewheel and turbine-type meters. All these meters require that the internal diameter of the pipe be known so that the velocity and the pipe's internal cross sectional area can be used to find the volumetric flow rate.

 

Flow rate = velocity x cross sectional area

Cross-sectional area = (Π x D2)/4  (for round pipe)

 

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