Why does the absence of a check valve at the pump outlet easily lead to water hammer?

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First, let’s look at the consequences of not installing a check valve in a water pump: Sudden changes such as opening/closing valves or shutting down the pump cause abrupt variations in the flow velocity within the pump’s discharge pipe. This, in turn, leads to changes in momentum per unit time, which inevitably generates corresponding inertial forces. As a result, there are alternating spikes and drops in pressure within the pipe. This phenomenon in which water flow velocity and pressure change over time and location is known as water hammer (or hydraulic shock). Water hammer is divided into start-up water hammer, valve-shutting water hammer, and pump-stop water hammer (caused by sudden power outages or similar reasons). The first two types of water hammer do not cause problems that threaten the safety of the unit under normal operating procedures. The water hammer pressure generated by the latter is often very high, leading to accidents. Therefore, the purpose of studying water hammer and calculating it is: ① to develop protective measures against damage to pipelines and units caused by the maximum water hammer pressure ； ②Propose protective measures to prevent the minimum water hammer pressure from causing unacceptable negative pressures in the pipeline and leading to pipe damage ； ③Prevent the destructive effects caused by the reverse rotation of the unit. In a water pump system, the characteristics of the pump serve as the boundary conditions at the starting end of the pipeline. Now let’s analyze the water hammer phenomenon (also known as the hydraulic transient process) that occurs when there is no check valve installed at the pump outlet; in the event of a sudden shutdown of the pump, the pump loses its driving force and the outlet valve cannot close in time, resulting in water flowing back through the pipeline. As shown in the diagram: 1. When the pump loses its power, its rotational speed drops sharply. The water inside the pump and the outlet pipeline continues to move in its original direction due to inertia, but its flow velocity decreases rapidly and the pressure drops, until the water stops flowing in the forward direction and the flow rate becomes zero. At this moment, the water pump is still rotating in the forward direction, and the water flow is in the positive direction (the flow of water from the pump toward the outlet tank is referred to as positive flow); this is what is known as the pump’s operating condition. 2. Under braking conditions, when the flow rate in the outlet pipe is zero and the water is at a static state, the hydrostatic head of the outlet pipe system causes a reverse flow to occur, with water flowing from the outlet pipe system back toward the pump. This reverse flow exerts a braking effect on the pump impeller, which is still rotating in the forward direction, forcing the speed of the rotor to decrease rapidly until it reaches zero. At this moment, due to the resistance of the rotating impeller against the backflow, the pressure at the pump outlet gradually increases. The transient operating condition of the water pump at this moment is referred to as the braking condition. 3. As the flow rate of the reverse water flow increases, the rotation speed of the pump starts to decrease from zero and rises rapidly. At the same time, the centrifugal force exerted by the rotating impeller on the water flow also increases, acting as a resistance to the reverse water flow. This resistance increases as the reverse rotation of the impeller accelerates, causing the pressure behind the pump to rise rapidly and reach a maximum at a certain point; correspondingly, the reverse rotational speed also reaches its maximum at that time. As the resistance increases, it also prevents any further increase in the reverse water flow rate; after reaching a certain maximum value, it gradually decreases. The energy acting on the impeller also decreases accordingly, causing the reverse rotation speed to drop gradually, until under the action of a stable head pressure, the torque exerted by the water flow on the impeller balances the resistance torque of the rotating parts of the unit. The unit then operates at a constant reverse flow rate and speed without any load, just like a hydro turbine, and the water hammer pressure disappears as a result. This transient operating condition is referred to as the turbine condition. Meanwhile, the pressure changes after the pump will propagate as water hammer waves along the discharge pipe toward the discharge piping system, and then be reflected back by the system, resulting in complex water hammer phenomena within the discharge pipe. Therefore, to prevent water hammer, a check valve must be installed in the pump outlet pipeline; a water hammer-resistant check valve can be used.