Centrifugal pump. Source: GN Solids Control Co., Ltd.

For many engineers, pump sizing can seem confusing and difficult to understand. And yes, size does matter when specifying a pump. Problems can arise if the pump is either oversized or undersized.

Oversizing a pump beyond a safety factor can lead a motor to be loaded less than 50 percent of full load during operation. This will cause the motor to produce a lower power factor, greatly increasing operating expenses. According to process industry estimates, this costs companies billions of dollars each year. Additional problems can also occur due to oversizing, including premature bearing failure, excessive vibration, cavitation issues and other problems, increasing maintenance costs and loss of production due to equipment downtime.

Certain pumps, such as progressive cavity pumps, can have issues due to undersizing. Since the flow of a progressive cavity pump is a function of cavity size, pumps that are undersized and have too small a cavity for the application need to pump faster. This can lead to a reduction in suction, increased wear and, ultimately, pump failure. Correctly sizing the pump is critical to the efficiency of the system.

Specifying pump size is critical to ensure the proper flow rate and the differential head required for the application, the two most important specs when designing a pump system.

Flow rate, or volumetric flow rate (Q), is the volume of fluid that passes through a cross-sectional area per specified unit of time. Pump volumetric flow rate is typically measured in cubic feet per second (ft3/s) or gallons per minute (GPM) in U.S. customary and imperial units, and cubic meters per second (m3/s) in SI units. Flow rate requirements are usually a function of how much product the system needs to produce.

• Flow rate (Q) = V/t = Volume/time

Head, or static head, is the height that a pump could raise fluid if it were pumping straight up in the air. Head is the same for all fluid types, regardless of their specific gravity. Head is a function of suction, the lower the suction the lower the head, and the higher the suction the higher the head.

Discharge head as a function of suction; Source: www.pumpfundamentals.com

Total dynamic head (TDH) is the height the pump can raise fluid, taking friction losses into account.

To calculate total dynamic head, first, calculate the frictional head loss. Since frictional losses are dependent on flow, you need to know what the desired flow is. Online calculators, such as this one on FreeCalc.com, can help perform these calculations easily. Variables that factor into this calculation include nominal fluid viscosity and specific gravity, pipe size, pipe schedule, pipe material, total pipe length, number and type of pipe fittings, and number and type of valves. The result will be the head loss due to friction, measured in feet (or meters).

Once the frictional head loss is calculated, add it to the static head to determine the TDH.

Once the head is determined, you may want to convert it to pressure. This is easy using one of the following formulas, depending on the preferred units.

• Pressure (psi) = 0.433 x head (ft) x Specific Gravity of fluid
• Pressure (bar) = 0.0981 x head (m) x Specific Gravity of fluid
• Pressure (kg/cm2) = 0.1 x head (m) x Specific Gravity of fluid

The following formulas convert pressure to head:

• Head (ft) = 2.31 x pressure (psi) / Specific Gravity of fluid
• Head (m) = 10.197 x pressure (bar) / Specific Gravity of fluid
• Head (m) = 10 x pressure (kg.cm2) / Specific Gravity of fluid

It is important to enter the correct specifications to properly size the pump for the given application and media. When a pump is an incorrect size, it is most commonly oversized. Signs of an oversized pump include excessive noise or vibration, frequent bearing or seal failure, throttled flow control valves and intermittent operation.

If a pump is determined to be oversized, there are a few remedies. According to Flow Control Magazine, the following five troubleshooting methods can improve the efficiency of an oversized pump system.

1. Install a new pump that is properly sized
2. Add a flow recirculation line to help improve the pump efficiency
3. Trim the impeller to reduce energy consumption
4. Remove control valves by installing a variable frequency drive
5. Lower the pump speed

The inclination for those new to specifying pumps may be “the bigger the better,” or the tendency to build in too much of a safety factor. This is clearly not a good strategy and will end up compromising the pump system and incurring more costs down the road. When it comes to proper pump sizing, yes, size does matter!

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