【Common Issues in Haichuan Pipeline Design】-- What are the hazards of not installing a check valve at the pump outlet?

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During the design, installation, and inspection of pipelines, various minor issues that have been overlooked or not taken into account are often discovered. These issues are collected and organized into a series; this list is being updated continuously. Everyone is welcome to actively participate in discussions. It’s a compilation post on common problems in Haichuan pipeline design – it’s still being updated, and discussions are welcome: https://bbs.hcbbs.com/thread-5705828-1-1.html ------------------------------------------------------------------ Water hammer is a highly destructive fluid impact phenomenon in pipeline systems; it can cause pipeline vibration and increased noise at minimum, while at worst it can lead to pipeline rupture and damage to pumps. The absence of a check valve at the pump outlet is one of the most common causes of water hammer. In many operating conditions, operators tend to overlook the function of check valves, thinking that they are only used to prevent backflow of the medium, without realizing that one of their functions is to suppress water hammer. The following explains the reasons, starting from the principle of water hammer formation and the potential hazards in the absence of check valves, and taking into account the actual operating conditions, in order to provide a reference for the safe operation of pipeline systems. To understand this issue, it is first necessary to clarify the basic principle behind water hammer: the fluid in the pipeline possesses inertia and compressibility (although the compressibility of liquids is low, it cannot be ignored during transient processes). When the flow condition of a fluid changes suddenly (such as when a pump stops operating abruptly or a valve closes quickly), the kinetic energy of the fluid is instantly converted into pressure energy, resulting in an instantaneous pressure wave. This pressure wave propagates and reflects within the pipe, thereby causing the water hammer phenomenon. The check valve at the pump outlet is essentially a valve that automatically prevents the backflow of the medium. Its primary function is to quickly shut off the path for backflow in situations such as when the pump stops, thereby preventing the flow disturbance caused by backflow of the medium – which is a key factor in the formation of water hammer. When the pump is operating normally, the fluid flows steadily downstream through the pipes driven by the pump, with the system pressure and flow rate being in a state of relative balance. When the pump stops suddenly (such as due to power loss, failure, or intentional shutdown), the driving force disappears instantly. However, the fluid downstream continues to flow forward due to inertia, resulting in a rapid drop in pressure near the pump outlet and the formation of a low-pressure area (even liquid column separation). At this point, the downstream medium flows back toward the pump outlet under the effect of the pressure difference, resulting in backflow. The backflowing medium collides with the forward flow remaining in the pump impeller and pipes; simultaneously, the kinetic energy of the backflowing medium is rapidly converted into pressure energy, resulting in an instantaneous high-pressure shock wave. Lacking the obstruction provided by a check valve, this shock wave propagates rapidly along the pipeline, reflecting and building up repeatedly at weak points where the pipe geometry changes, such as bends, tees, valves, and pump bodies, thereby intensifying the water hammer effect. Especially in long-distance pipeline systems, high-head or high-flow pump systems, the backflow velocity is faster and the backflow volume is larger, resulting in higher peak pressures due to water hammer and more significant damage. Furthermore, in the absence of a check valve, the backflowing medium can cause the pump impeller to rotate in the opposite direction. This not only leads to abnormal wear of the mechanical seal and bearings, but may also result in severe collisions between the impeller and the pump casing, even leading to breakage, thereby exacerbating the destructive effects of water hammer. After installing an appropriate check valve, when the pump stops operating and the flow rate drops to the shutdown setting, the check valve closes rapidly due to its own gravity, spring force, or the force of the backflowing fluid, thereby blocking the path for backflow and preventing sudden changes in flow kinetic energy; this helps to suppress the occurrence and escalation of water hammer at its source. It should be specifically noted that the selection of check valves must be matched to the operating parameters of the pump system (such as flow rate, head, properties of the fluid, and pressure rating). The closing characteristics (such as rapid closing or two-stage closing) should also be chosen appropriately, so that the closing speed is in harmony with the rate at which backflow occurs; this is necessary to ensure effective hammer prevention. If the check valve is improperly selected or installed, it may instead cause more severe water hammer during valve closure. Therefore, installing a check valve at the pump outlet is not redundant; rather, it is a crucial measure in system design to suppress water hammer and protect the pipes and the pump itself. Ignoring this step can easily lead to safety accidents and cause significant losses.