How to Properly Size a Split Case Pump for Your System

Posted by angroup on December 8, 2023
split case centrifugal pump

When selecting a split case pump for a system, properly sizing the pump is crucial to ensure optimal performance and efficiency. An undersized pump will fail to provide adequate flow and pressure, while an oversized pump wastes energy and money. In this article, we will walk through the key factors to consider when sizing a split case pump, including flow rate requirements, head pressure, net positive suction head (NPSH), and pump efficiency. We will also provide a step-by-step guide to selecting the right pump size using manufacturer pump curves.

Determining Flow Rate Requirements

The first step in pump sizing is accurately determining the system's flow rate requirements in gallons per minute (GPM). Consider the following when assessing flow rate needs:

  • Purpose of the pumping system. For example, a HVAC system has different requirements than a water treatment plant.
  • Size of pipes, valves, strainers and other components. These impose limits on optimal flow rates.
  • Future expansion plans. Size for higher capacity if system upgrades are expected.
  • Safety factor. Adding ~10% to anticipated peak flow rates gives a buffer.

Be sure to account for the entire system's demand, including all branches, accessories, and losses due to friction, valves, etc. It is better to overestimate required flow rather than undersize it at this stage.

Understanding Total Head Pressure

In addition to flow rate, determining the total dynamic head (TDH) or total head pressure is essential for proper pump sizing. TDH refers to the total equivalent height that a pump must develop to overcome friction losses and elevate water to the discharge point. It is measured in feet or meters of fluid.

When calculating TDH, consider all components that contribute resistance such as:

  • Static lift: Elevation difference between suction reservoir surface and discharge outlet.
  • Pipe and fitting friction losses. Increased loss accompanies smaller diameters and longer runs.
  • Component losses: Losses from elbows, valves, strainers, filters, etc. Consult manufacturer specs.
  • System backpressure: Any pressure on discharge side opposing flow (boiler, filters, nozzles).

Add all these factors to get the system TDH. A detailed pipe schematic is extremely useful. Again, build in safety margin on initial estimate.

Determining Required Pump Suction Conditions

In addition to flow rate and TDH, properly sizing a split case pump relies on analyzing required net positive suction head available (NPSHA). NPSHA refers to the absolute pressure at the pump inlet needed to prevent cavitation. It accounts for components like:

  • Atmospheric pressure
  • Static suction lift
  • Piping friction losses
  • Safety allowances

The pump's NPSH required (NPSHR) value must be lower than the system's NPSHA. Overlooking NPSH when selecting pumps often causes cavitation and poor performance.

Reviewing Pump Efficiency

Finally, examine the specified efficiency or optimal efficiency operating range of the pumps under consideration. Pump efficiency indicates the percentage of shaft horsepower converted to hydraulic power. Peak efficiencies range from 60-80% for split case models.

Choose a pump that reaches maximum efficiency close to the intended operating point. Efficiency drops quickly on either side of best efficiency point (BEP). Consider installing multiple pumps for systems with fluctuating demands.

Proper pump sizing requires matching all these parameters appropriately. Now we can explain the step-by-step process for using them to select the right pump.

centrifugal split casing pump

How to Size a Split Case Pump

Follow this straightforward methodology when reviewing split case pump options:

Step 1: Determine System Requirements

  • Flow rate = size for peak demands + safety factor
  • TDH = all components contributing to head losses
  • NPSHA > NPSHR to avoid cavitation

Step 2: Gather Pump Specifications

  • Obtain pump performance curves from manufacturers showing various sizes
  • Note flow rate, head, efficiency, power, NPSHR at curve points

Step 3: Overlay System Curve on Pump Curves

  • Plot the system TDH and flow rate on same axes as the pump curves
  • Ensure it intersects within a pump's operable range

Step 4: Read Intersection Point Capacity

  • Identify where system curve crosses selected pump curve
  • Check if flow, TDH, efficiency are acceptable

Step 5: Calculate Required Pump Power

  • Note brake horsepower (BHP) or kilowatts (kW) at intersection
  • BHP = Flow (gpm) x TDH (ft) / 3,960 x Pump Efficiency

Step 6: Adjust Pump Size As Needed

  • If unsuitable match, try different pump or impeller size
  • Confirm match with multiple pumps from different manufacturers

By following these key sizing steps, you can zero in on a correctly sized split case pump that meets the system requirements right within the pump's best efficiency zone. Adjust size or add multiple units as necessary until finding the optimal arrangement.

Conclusion

We have covered the essential factors involved in properly sizing a split case pump, including flow rate, TDH, NPSH, and efficiency. While it may seem complicated, breaking down the system needs and methodically reviewing pump specification curves simplifies the pump selection process considerably. Paying close attention to detail in assessing the system and avoiding guesses or rules of thumb will lead to superior pump performance and energy savings over the long run. Matching pumps precisely using the head, capacity, and efficiency data available from manufacturers is well worth the effort during initial selection.

An Pump Machinery is a professional split case pump manufacturer. If you require assistance with split case pump, please contact us at anpump5@angroupcn.com or +86 15032857866; our technical sales engineers have considerable product knowledge and will be able to assist you.

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