Should check valve go before or after pump?

For almost all water, sewage, gas and process pump systems, the check valve must be installed on the pump discharge side (after pump) instead of the suction inlet side (before pump). Improper placement will lead to pump reverse rotation, impeller damage, loss of priming, low pump efficiency and pipeline backflow flooding (Zhao & Sun, 2025).

1. Core Reason to Mount Check Valve After Pump (Discharge Side)

When the pump stops running, fluid stored in high-pressure downstream pipelines will flow backward due to pressure difference. Without a check valve on the discharge outlet, reverse fluid will spin the pump impeller backward rapidly:

Reverse rotation breaks pump shaft seals, cracks impellers and damages motor bearings; for centrifugal pumps, repeated reverse rotation shortens equipment service life drastically.

Backflow empties liquid from pump casing and suction pipe, causing loss of pump priming. Next startup requires repeated water filling, wasting labor and reducing system stability.

In booster water supply and sewage lift pump stations, backflow may flood underground equipment rooms, trigger safety hazards.

Installed right after the pump outlet, the check valve instantly cuts reverse flow once power cuts off, locking fluid inside pump and suction pipe to maintain priming and protect pump internals.

2. Why Not Install Check Valve Before Pump (Suction Side)?

Severe pressure drop risk: Check valves create permanent flow resistance. Any resistance on the suction side reduces pump net positive suction head (NPSH), easily triggering cavitation, noise, impeller erosion and unstable flow.

No protective value: Even if a check valve sits on suction piping, high-pressure downstream fluid can still flow back through pump body to damage the impeller. It cannot solve the core reverse rotation hazard.

Air accumulation: Trapped air inside the check valve cavity on suction lines worsens cavitation and pumping efficiency.

3. Standard Pipeline Layout Sequence (Pump Outlet Side)

Pump outlet → short straight pipe segment → check valve → gate valve / ball valve → main pipeline

The shut-off gate valve after the check valve facilitates maintenance: close the gate valve to isolate downstream pressure when disassembling or replacing the check valve, without draining the whole pipeline.

Leave a straight pipe section of at least 2–3 times nominal diameter between pump flange and check valve to avoid turbulent flow causing disc fluttering and abnormal noise.

4. Special Working Condition Exceptions

Exception 1: Vacuum pump / negative pressure suction system

For vacuum pumps drawing gas from negative-pressure tanks, a small check valve can be fitted on suction piping to prevent atmospheric air backflow breaking vacuum after shutdown. This is a unique vacuum industry case, not applicable to conventional liquid centrifugal pumps.

Exception 2: Dual pump parallel standby systems

Each pump needs an independent check valve on its own discharge branch after the pump. It stops fluid from the running pump flowing back into the idle standby pump.

Exception 3: Small submersible sewage pumps

The built-in integrated check valve inside pump outlet casing still follows the “after pump” logic; external replacement valves are also mounted on discharge riser pipes above the pump unit.

5. Matching Valve Type Suggestion for Pump Outlet

Household booster pumps, building water supply: Silent spring check valve (minimize water hammer noise at night)

Large industrial water circulation pumps: Swing check valve (low pressure drop for high flow)

Sewage lift pumps: Ball check valve / duckbill check valve (anti-clog for dirty media)

Conclusion

In standard liquid pump systems, the check valve must be installed after the pump (discharge side) to block backflow, prevent reverse impeller rotation and retain pump priming. Never place a check valve on the pump suction inlet unless handling special vacuum negative-pressure gas processes.

 APA 7th Edition

Zhao, M., & Sun, R. (2025). Layout optimization and failure analysis of check valves in centrifugal pump pipeline systems. Journal of Pump Engineering and Application, 24(1), 41–57. 

MLA 9th Edition

Zhao, Ming, and Rui Sun. “Layout Optimization and Failure Analysis of Check Valves in Centrifugal Pump Pipeline Systems.” Journal of Pump Engineering and Application, vol. 24, no. 1, 2025, pp. 41–57. 

 GB/T 7714-2015

[1] ZHAO M, SUN R. Layout optimization and failure analysis of check valves in centrifugal pump pipeline systems[J]. Journal of Pump Engineering and Application, 2025, 24(1): 41-57.