Assessing Pump System Performance


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

Determining which pumps and their associated systems to assess is the first step in the process.

First of Two Parts

What Is a Pump Assessment?

A pump assessment is a mini performance test of the pump within the system in which it normally functions. For a pump with a small range of operational flowrates, this test might be done by taking data at one flowrate to represent the pump's normal operation.

In more complex systems, where the pump may operate over a wide range of flows and heads, the assessment may require several individual tests encompassing the range of the pump's performance requirements, or it may require continuous monitoring over a period of time to capture the operational extremes. The assessment generally requires measuring the pump's flowrate, head and power input to determine its actual operating point within its system.

Why perform a pump system assessment?

The two most common reasons to perform a pump system assessment are to enhance the reliability and/or efficiency of the pump and system.

Reliability. A pump's reliability is related to how close its actual operating point or range is to its best efficiency point (BEP). Reliability improves when the pump operates near its BEP. The BEP, as the name implies, is the flowrate and head level at which the pump achieves its highest efficiency.

Operating significantly away from this point leads to axial and radial loads from internal hydraulic forces. These forces cause shaft deflection and bearing loads which in turn can lead to seal failure, bearing failure, internal rubbing and vibration problems. Even pumps that have been properly installed and maintained experience greater reliability problems if operated away from their BEP. In addition, problems related to cavitation and suction recirculation can be exacerbated by off-design operation.

Efficiency. In today's greener world, performing a pump assessment to determine if the pump is operating as efficiently as expected has become common. Most facility managers assume that their pumps are running pretty close to their BEP. In reality, the average pump in the field operates at 44 percent efficiency . This is well below the average BEP of those pumps. In a quick study of the average best efficiency weighted by sales volume of each size of ANSI pump sold in one year from three of the largest suppliers of these pumps in the U.S. I found that the average best efficiency of this small segment of the pump population is 65 percent. These pumps are often smaller in size and designed primarily with reliability, not efficiency, in mind. So, this efficiency sample was intentionally not taken from a high efficiency portion of the population but still shows a 21 percent efficiency deficit between the average best efficiency and the average operating efficiency. This 21 percent deficit seems compelling enough to want to examine the pumps.

However, this is only the beginning of the good news. As the view is broadened and the whole system is examined, partially closed valves, open bypass lines, excessive flowrates and other issues may exist. System efficiency, rather than pump efficiency, is where the real efficiency deficit lies. It is not uncommon to find systems in which pump energy costs can be cut by 50 percent when the proper pump is installed and the system is operated properly.

A common example of a system that continuously wastes energy is one in which a throttling valve is used to control flowrate. Most operators are shocked when they learn the actual operating cost of a partially closed valve. Consider a system with a valve that is partially closed (10 psi pressure drop) and 500 gallons per minute (gpm) passing through the valve. If the pump and motor have efficiencies of 65 percent and 95 percent respectively, the system runs continuously, and the cost of power is $0.08 per kilowatt hour. The cost of energy loss across that valve is $2,790 yearly.

How many partially closed valves are operated in facilities around the world? Operating a pump and controlling its flowrate with a throttling valve is akin to driving home with a brick on your car's accelerator and using the brake to control your speed. The valve dissipates energy in the flow, wasting it as friction. Eliminating as much friction as practical is paramount to optimizing a pump system's efficiency.

Why Do Pipe Systems Calculations and Reality Differ?

Pipe systems loss calculations are based on average friction loss data on the pipe and fittings. The generic pipe and fitting data cover a range of manufacturers, and casting and manufacturing variations even from a single manufacturer cause data variations.

A rarely noticed note found at the bottom of some pipe friction loss tables warns that the values in the table should have a safety margin of 15 percent to 20 percent added to cover variation. To improve the accuracy of your calculation, system loss calculations should be run with pipe and fitting loss data from the actual manufacturers being used on the project.

A second reason that design and reality fail to match is the difference between design and as-built conditions. It is often instructive to take a piping and instrumentation drawing and walk down the system. Often, components and branches that are not shown on the drawing exist in the system.

Another reason that the pump and system can be mismatched is a change in the function of the system over time. Often, systems are tasked to pump different fluids or provide flow to more or fewer systems than planned in the original design.

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See also:

Upstream Pumping Solutions

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