When should pumps be operated in parallel versus series?

Parallel operation is preferred when higher total flow capacity is required or for variable process demand turn-down. Series operation is used when higher total dynamic head is needed to overcome high elevation static head or long pipe friction resistance.

Why does running two identical pumps in parallel not double system flow?

System pipe friction losses increase quadratically with flow rate. As combined flow increases, the operating point shifts up the steepening system curve, resulting in higher system head and a lower flow rate per pump than when operating individually.

What hazard occurs when operating unequal pumps in parallel?

If two pumps with significantly different shutoff heads run in parallel and system demand drops, the stronger pump can build system pressure higher than the shutoff head of the weaker pump. This causes zero flow or reverse flow through the weaker pump, causing overheating and rapid mechanical seal failure.

Pump parallel and series operation

Design and operational aspects

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Section summary
1. Definition : what is parallel and series pumping ?
2. Operation, flow and head calculation
3. Interactive Parallel/Series Calculator & Mathematical Equations
4. Precautions & Reliability Issues

1. What is parallel and series pumping ?

It is sometimes necessary to use pumps in parallel or in series.

Parallel operation is the fact to put in operation 2 pumps besides each other on 2 parallel pipes that are merging after the pumps. It allows to add the pump capacities.

Series operation is the fact to put in operation 2 pumps one after the other on a single circuit. It allows to add the pump heads.

Some precautions are however necessary when operating pumps in parallel or series (see corresponding section).

2. Pump parallel and series operation guide

How to operate pumps in parallel or series ?

All calculations are performed thanks to the pump characteristics established by the supplier. This document is necessary for design parallel and series pumping systems. A typical pump characteristic curve is represented below, for a centrifugal pump.

Pump Characteristic

Figure 1 : Pump NPSH, required and available

Reading and understanding the pump characteristic curve is a very important 1st step for pipe circuit calculation.

Pump Parallel operation - calculation, advantage and disadvantage

In pump parallel operation, both pumps will deliver the flow they have been designed for, at the given head of the combined flow. The characteristic of the parallel pumping system can be calculated by adding the capacity of each pump at a given head.

Pump in parallel characteristic

Figure 2 : Building resulting characteristic of 2 pumps in parallel

However, it must be noted the following : the combined flow is requiring a higher head for a given system curve, thus the combined capacity \(Q_3\) of the system is not equal to \(Q_1 + Q_2\) when the pumps are running one by one. This is illustrated on the graph below by the system curve.

Flow in a system with pump in parallel

Figure 3 : Flow in a system with pump in parallel

Pump Series operation - calculation, advantage and disadvantage

In pump series operation, at a given capacity, the 1st pump will deliver its head and send the fluid to the next pump, then the 2nd pump will add its own head. The characteristic of the series pumping system can be calculated by adding the head of each pump at a given capacity.

Pump in series characteristic

Figure 4 : Building resulting characteristic of 2 pumps in series

To understand which head and flow the system will deliver, the system curve must be added to the pump characteristic.

Flow in a system with pump in series

Figure 5 : Flow in a system with pump in series

Mathematical Equations & Curves Analysis

Centrifugal pump characteristic curves are modeled quadratic relationships between total dynamic head \(H\) and volumetric flow rate \(Q\):

\[ H_{pump}(Q) = H_0 - k \cdot Q^2 \]

Where \(H_0\) is the pump shutoff head (head at zero flow), and \(k\) is the pump hydraulic resistance factor derived from the rated operating point \((Q_r, H_r)\):

\[ k = \frac{H_0 - H_r}{Q_r^2} \]

1. Parallel Pump Combination:

At a constant system head \(H\), total flow rate is the sum of individual pump flow rates:

\[ Q_{parallel}(H) = Q_1(H) + Q_2(H) = \sqrt{\frac{H_{01} - H}{k_1}} + \sqrt{\frac{H_{02} - H}{k_2}} \quad (\text{for } H \le H_{01}, H_{02}) \]

2. Series Pump Combination:

At a constant system flow rate \(Q\), total dynamic head is the sum of individual pump heads:

\[ H_{series}(Q) = H_1(Q) + H_2(Q) = (H_{01} + H_{02}) - (k_1 + k_2) \cdot Q^2 \]

3. System Curve & Operating Point Intersection:

The system dynamic loss curve is represented by static elevation head \(H_{stat}\) plus friction losses:

\[ H_{sys}(Q) = H_{stat} + k_{sys} \cdot Q^2 \]

⚡ Interactive Pump Parallel & Series Calculator

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

Unit System: SI Metric (m, m³/h, kW) US Customary (ft, gpm, hp)

System Arrangement & Fluid Properties

Configuration Mode:

Fluid Density (kg/m³):

Pump 1 Characteristics

Shutoff Head H₀₁ (m):

Rated Flow Q_r1 (m³/h):

Rated Head H_r1 (m):

Efficiency η₁ (%):

Pump 2 Characteristics

Shutoff Head H₀₂ (m):

Rated Flow Q_r2 (m³/h):

Rated Head H_r2 (m):

Efficiency η₂ (%):

Piping & System Curve Requirements

Static Elevation Head H_stat (m):

Design Target Flow Q_req (m³/h):

Total System Head at Q_req (m):

Operating Point Summary

Operating Total System Flow Rate (Q_op): -

Operating Total Dynamic Head (H_op): -

Pump 1 Individual Flow / Head: -

Pump 2 Individual Flow / Head: -

Combined Brake Power Draw: -

Capacity Gain vs Standalone Pump 1: -

💡 Industrial Best Practices & Engineering Rules of Thumb

Matching Shutoff Heads in Parallel: Always select parallel pumps with matching or near-identical shutoff heads (\(\Delta H_0 < 5\%\)). If shutoff heads differ significantly, low system demand forces the system pressure above the weaker pump's shutoff head, leading to zero flow, recirculation, dead-heading, and thermal destruction.
Non-Return Check Valves: Every parallel pump branch MUST install a dedicated check valve on its discharge nozzle to prevent high-pressure backflow through idle pumps.
Series Pump Casing Pressure Ratings: In series installations, the second-stage pump body and shaft seals experience total system pressure (\(P_{suction,2} \approx P_{discharge,1}\)). Ensure the 2nd pump casing is rated for full cumulative discharge pressure.
Law of Diminishing Returns in Parallel: Doubling identical pumps in parallel NEVER doubles process flow unless the system curve is completely flat (\(H_{stat}\) dominates, friction \(\approx 0\)). In friction-dominated systems, expect a 20% to 35% capacity increase per added pump.
Motor Power Limits in Single Operation: Parallel pumps selected to run together operate at lower flow per pump. If a single pump is operated alone, its operating point shifts right along its curve toward higher flow and lower head, which may overload the drive motor if non-overloading impellers are not used.

3. Pumps in series and parallel problems - Reliability and efficiency issues

What can happen when putting pumps in series or parallel ?

Precautions must be taken especially for parallel operations. Indeed, if the head of both pumps is very different, when the system is put in operation, the following can happen : the head delivered by one of the pumps exceeds the head delivered by the other one. In such a case, a recirculation will happen where the flow in the pump with the smaller head will go backwards. To avoid such issue, it is important to draw the characteristics and system curve as explained above, and make sure that both pumps don't have performances too far one from the other.