Seeking help regarding water hammer phenomenon
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I see that there was already a post asking this question; since it was a long time ago, I’m afraid no one might have seen it, so I’m posting a new one to ask – it doesn’t count as repetition, lol. Below is the written explanation from the original post: “The water hammer effect is a metaphorical term. It refers to the severe water shock that occurs when water flow impacts the pipes during the start-up and shutdown of a water pump.” Inside the water pipe, the inner wall is smooth, allowing the water to flow freely. When an open valve suddenly closes or the feed water pump stops, the flow of water creates pressure on the valve and the pipe walls, primarily on the valve or the pump. Due to the smooth wall of the pipe, under the influence of inertia, the water flow rapidly reaches its maximum velocity, resulting in destructive effects. This is what is known in hydraulics as the \"water hammer effect\", or positive water hammer. On the contrary, a closed valve can also generate water hammer when it is suddenly opened or after the feed pump starts; this is known as negative water hammer, but its intensity is less than that of the former. ” I can understand water hammer that occurs when a valve closes suddenly or due to the sudden stoppage of a pump. But I don’t understand what he meant by \"negative water hammer\" – that water hammer is also generated when a closed valve suddenly opens or when a feed pump starts up; how does that work? I would be extremely grateful if experts could share their insights! :handshake
Where have all the experts gone?:'
Water hammer is a phenomenon of hydraulic transient. Rapid changes in the momentum of the fluid flowing in the pipeline occur due to the opening and closing of pumps, valves, etc. The pressure wave resulting from this propagates at the speed of sound upstream and downstream in the pipeline. The peak value of its pressure wave usually exceeds the pressure of a steady-state system. If water hammer occurs in a pipeline system transporting incompressible fluids, it can cause significant damage to such systems. The peak pressure can be calculated using the formula: Delta P = 0. 5 × Density × Sound speed × Flow velocity changes. For complex piping systems, it is necessary to use appropriate software for calculations. It should be noted that hydraulic changes can also lead to local pressure drops; when the pressure falls below the saturated vapor pressure of the fluid, bubbles form within the flow channels, causing the liquid flow to be segmented. This phenomenon is common in piping systems with significant height variations, and the water hammer that occurs when the segmented flows recombine can be very destructive. When designing systems for cooling water, steam, etc., the water hammer problem must be given careful consideration. Due to space constraints, I can only talk about these. That’s incorrect; please correct me.
I heard about it for the first time; it’s quite informative.
Support the opinion from the 4th floor; I really learned something new here: lol
Positive water hammer occurs when the pipeline is oriented upward or when there is positive pressure at the end of the pipeline. When the pump stops operating suddenly, the liquid in the pipeline flows back. Due to the action of the check valve, this flow is slowed down, which creates a force on the pipeline; in severe cases, this can damage the pipeline components. In engineering practices, swing-check valves can be used to prevent this phenomenon, thereby extending the time over which the flow is slowed down and reducing the impact force. Negative water hammer is also easy to understand; its mechanism of action is the exact opposite of that of positive water hammer. This occurs when the pipeline runs downward or when there is negative pressure at the end of the pipeline. Such negative water hammer typically arises in situations where the pipeline starts upward, reaches its highest point in the middle, and then goes downward at the end. When the pump stops suddenly, the liquid at the starting point flows back toward the pump, while the liquid at the end continues to move forward due to inertia, gravity, and negative pressure. As a result, extremely high negative pressure is generated at the highest point of the pipeline. If the pipeline is made of engineering plastic and is not resistant to negative pressure, it will be damaged in such cases. In engineering, this is prevented by installing an automatic exhaust and intake valve at the highest point.
The situations you mentioned are indeed easy to understand, but I still don’t get why the sudden opening of a valve or a centrifugal pump can cause negative water hammer As for positive water hammer, it is not entirely due to the pipe being oriented upward or to there being positive pressure at the end of the pipe. In fact, as long as there is flowing liquid, when the valve at the end is suddenly closed, the liquid in front of the valve continues to move forward due to inertia; although there is no macroscopic displacement, this causes the liquid in front of the valve to acquire high static pressure energy, which then causes it to rebound. When it reaches the other end, the same phenomenon occurs (assuming no energy loss). As a result, the liquid within this closed pipe undergoes periodic back-and-forth movements, exerting pressure on the pipes on both sides – and this is what constitutes water hammer.
The two situations I mentioned are those that pose the greatest threat to pipelines; other types of water hammer phenomena, as well as those that occur when steam mixes with water, can also cause serious damage if the design is not proper. The remaining cases are basically harmless. I’ve heard of the negative water hammer that occurs when a valve is opened for the first time, and it’s easy to understand: when a valve is opened, static pressure energy is converted into kinetic energy, causing the flow velocity of the liquid to be higher than that during normal pump operation. In other words, the fluid at the front has a higher flow velocity than the fluid behind it, which results in negative pressure at some point – that’s what is meant by negative water hammer. That’s why standard operating procedures require valves to be opened slowly.
What you say makes sense; thanks for the explanation! :victory:
The water hammer in cooling water pipes generally does not cause too much damage to the fittings; it is the steam pipes that, when first put into use or shut down, tend to cause significant damage to the equipment.
I feel like they have confused water hammer with water surge upstairs; the mechanism behind water surge is different from that of water hammer!
Thank you, I’ve learned something new again. I’ve learned a lot.
Negative water hammer is actually a phenomenon where, as the liquid inside the pump starts to flow, atmospheric pressure acts on the inlet pipeline due to the negative pressure in that pipeline