Sizing Centrifugal Pumps for Variable Flow

As a plant or process manager, sizing pumps for variable demand is one of those seemingly small decisions that can make a surprising impact on your future maintenance budget and energy bills.

While it’s simpler to predict and control flow rate on positive displacement type pumps, centrifugal pumps remain the go-to for high-flow or high-head applications.  Most centrifugal pumps have the advantage of being highly configurable, and the vast range of stock options includes colossal pumps for raw water, to specialised variants optimised for abrasive, solids-laden slurries.

Whether you are handling fluctuating quarry dewatering demands, variable wastewater treatment flows, or controlling process throughput; selecting the wrong-sized pump can lead to premature seal failures, worn impellers, and wasted power. But, get it right, and your improvement reaps rewards in ongoing cost savings and production gains.

Selecting a pump for variable demand—and deciding how to control its flow—is certainly worth allocating some time for.

Get in touch with a pump specialist for quick advice or to arrange a pump audit.

 

Step-by-Step Pump Selection

Map the demand profile

• Note minimum and maximum peak flows
• Identify the most frequent flow rate point
• Any foreseeable changes in future conditions

Develop the system curve

• Calculate friction losses at your identified flow rates

 

The Golden Rule: Prioritise Normal Flow, then Check Minimum & Maximums

It’s tempting to size a pump for absolute maximum-case flow and leaving it to run flat-out at that. However, you end up with a machine that spends most of its working life choked back, operating far away from its Best Efficiency Point (BEP), or frantically cycling between on and off.

Instead, begin by plotting measured or forecasted flow requirements against time. It could be a fancy histogram of multiple readings taken at regular intervals, or your best use of existing information available.

For example, a simple analysis might show:

Operating condition	Flow	Approx. operating time
Minimum	50 m³/h	10%
Normal	100 m³/h	70%
Peak	150 m³/h	20%

The normal operating condition represents the majority of uptime, but the pump must still be capable of meeting the peak requirement and minimal flow without issue.

In addition to the minimum stable flow rate (MSFR), each pump configuration has a specific minimum supply pressure (NPSHr).

Finally, if you’re pumping a slurry, you might need to check that the solids don’t settle out at low flow.

Of course, flow-rate is only one side of the performance coin. The other side is pressure; we want enough to power the fluid up to its final discharge, plus residual pressure to power any process equipment it is feeding. Too much terminal pressure will likely cause issues for any connected equipment, and in today’s energy-scarce world, its’ a massive cost burden if just thundering into a tank.

The relationship between flow and pressure (head) is known as the system curve.

 

Finding the System Curve

Your system curve is made up of static head pressure and friction loss.

Static head is simply the gain in height from pump inlet to system discharge. The rule-of-thumb is 1 bar pressure for every 10 metres of vertical height.

The pressure required to overcome friction losses depends on pipe material, diameter, bends, valves etc – it can be estimated using online calculators, or you can send us the details to run through our modelling tools.

Critically, for our subject of variable flows, friction loss increases with velocity. Fast flow through a narrow pipeline requires more power.

Total Dynamic Head (TDH) is static head + friction loss + discharge/terminal pressure required.

To calculate the Total Dynamic Head (TDH) required at the different flow rates you want, you need to know the:

• Static head (vertical distance)
• Pipe friction (surface material, diameter, length)
• Friction loss from bends, valves & fittings
• Required discharge/terminal pressure

Plot the TDH for each flow-rate; this will create points that can be joined to plot the curve.

Once you have your system curve, you can send it (along with details of the fluid) to your pump supplier and let them recommend a matching pump size. However, if you want to see how this is done, read on.

 

The Pump Curve

Every centrifugal pump configuration has its own characteristic performance curve, showing the relationship between flow and head at a specified speed or power frequency. Manufacturers provide these based on clear water at ambient temperature – conversion factors can be applied to match your fluid’s characteristics.

Pump curves also provide information about efficiency, Net Positive Suction Head requirements (min. supply pressure), and other operating limits.

The objective is not simply to find a pump that can deliver the maximum required flow. Rather, it is to select a pump and impeller combination that covers the complete system duty range within the pump’s Preferred Operating Region (POR). Within this POR will be the pump’s Best Efficiency Point (BEP) – the sweet spot of efficiency.

The duty point (or operating point) is the specific point where the system curve and pump curve intersect.  On a variable-demand system, the best we can do when sizing a pump is to match the most common duty-point as near as possible to the pump’s BEP.  High and low flow variance has to be dealt with by making changes to either the pump curve or the system curve.

• Pump performance: increasing or decreasing power (e.g. VFD)
• System performance: increasing or decreasing friction or head (Valves, bypass loops, etc)

For more on controlling the performance of fixed size pumps, read “Centrifugal Pump Controls for Variable Demand”

 

BEP (Best Efficiency Point) and POR (Preferred Operating Range) in Demanding Applications

High-viscosity sludge, abrasive slurries, hot fluids and volatile chemicals change the pump’s BEP and narrow its operating range.  This makes correct sizing even more critical.

For demanding fluids, the POR is often reduced from the standard rule-of-thumb for operational flow rate (70% – 120% of BEP) down to 80% – 110%, or even 85% – 105%.

In high static-head applications, running multiple smaller fixed-speed pumps in cascade or choosing a correctly trimmed fixed-speed pump may yield better reliability and lower capital expenditure than a single large VFD installation.

See our article “Centrifugal Pump Control for Variable Demands”

Top Tips: Never buy a larger pump “just in case” without evaluating flow-rate and variable speed control. Operating an oversized pump costs significantly more over its lifespan than investing in correct initial sizing. If you need a ‘buffer’ in the system, consider holding tanks that absorb the peaks and troughs in flow demand. A series of smaller pumps operating across buffer tanks can bring cost, resilience, and flexibility benefits.

 

Get in touch

Need support evaluating your system curves or selecting energy-efficient drive options? Contact the engineering team at Atlantic Pumps to discuss your application and recommendations for pump sizing.