Function of the Check Valve at Pump Discharge

Water hammer is a highly destructive fluid shock phenomenon in piping systems. In mild cases, it causes pipe vibration and increased noise; in severe cases, it results in pipe rupture and pump damage. Failure to install a check valve at the pump discharge is one of the most common triggers of water hammer. Under many operating conditions, operators tend to underestimate the function of check valves, believing they only prevent medium backflow. Few realize that one of their critical functions is to suppress water hammer. This article explains the mechanism based on the water hammer formation principle and potential hazards without check valves, combined with practical working conditions, to provide references for the safe operation of piping systems.

To understand this issue, we first clarify the basic principle of water hammer formation: fluid inside pipelines has inertia and compressibility. Although liquid has low compressibility, this property cannot be ignored during transient processes. When the fluid flow state changes abruptly (e.g., sudden pump shutdown or rapid valve closure), the fluid’s kinetic energy instantly converts into pressure energy and generates transient pressure waves. The propagation and reflection of these pressure waves within the pipeline constitute water hammer. Essentially, the check valve installed at the pump discharge is an automatic valve for preventing medium backflow. Its core function is to quickly cut off the backflow passage under conditions such as pump shutdown, blocking the key link of water hammer formation — flow abrupt change induced by medium backflow.

During normal pump operation, the medium flows steadily downstream through the pipeline driven by the pump, maintaining a relative balance of system pressure and flow velocity. When the pump stops suddenly (due to power loss, malfunction or routine shutdown), the driving force disappears immediately. However, the downstream medium keeps moving forward by inertia, causing a sharp pressure drop near the pump outlet and forming a low-pressure zone (even liquid column separation). Driven by pressure difference, the downstream medium then flows backward toward the pump outlet, creating backflow.

The backflow medium impacts the pump impeller and the residual forward-flowing fluid in the pipeline. Meanwhile, the kinetic energy of backflow rapidly transforms into pressure energy, forming instantaneous high-pressure shock waves. Without the isolation of a check valve, these shock waves travel rapidly along the pipeline, repeatedly reflecting and superposing at weak points with structural discontinuities such as elbows, tees, valves and pump casings, which intensifies the water hammer effect. Especially in long-distance pipelines, high-head or large-flow pump systems, backflow occurs at higher speed with larger volume, producing higher peak water hammer pressure and more devastating damage.

Additionally, without a check valve, backflow will force the pump impeller to rotate in reverse. This not only accelerates abnormal wear of mechanical seals and bearings, but may also trigger violent collision or even fracture between the impeller and pump casing, further aggravating the damage caused by water hammer. After a properly selected check valve is fitted, when the pump stops and flow rate drops to the preset closing value, the check valve closes rapidly under its own weight, spring force or the thrust of backflow. It blocks the backflow path and prevents abrupt variation of flow kinetic energy, so as to restrain the occurrence and escalation of water hammer at the source.

It should be emphasized that check valve selection must match the operating parameters of the pump system (e.g. flow rate, head, medium properties, pressure class). Its closing characteristics (quick closing, two-stage closing, etc.) shall be properly selected to match the valve closing speed with the backflow development speed, so as to achieve effective water hammer protection. Improper selection or installation of check valves may instead induce more violent valve-closure water hammer. Therefore, installing a check valve at pump discharge is not redundant. It is an essential measure in system design to mitigate water hammer and protect pipelines and pumps. Neglecting this component may easily lead to safety incidents and heavy economic losses.