Water Hammer when filling pipe - Need Helpful Analogy

[ziptron]

Hi All,

We have a very long 8" diameter pipe that contains lots of twists and turns and the end of which is open to atmosphere. The pipe is normally empty, but it acts as a large discharge pipe from a tank. The tank discharge has a butterfly valve and when the tank is emptied, water flows through this pipe.

A discussion of water hammer came up today. I intuitively know that the butterfly valve needs to be open slowly, at least until stable flow through the pipe is established. This would be to prevent water hammer in the long discharge pipe. However, I'm having a hard time explaining this to a non-technical individual. What is a good way to explain water hammer in an empty pipe to someone?

They seem to understand how water hammer (or large thrust) is caused when a valve is abruptly shut, or flow is stopped/started, but cannot believe that water flowing through an empty pipe (that is open to atmosphere) can cause a water hammer.

My explanation was that in the initially empty pipe, at the start of flow, the water front will eventually hit an elbow and since it does not "know" where to go, it will reflect back and cause large trust forces on the elbow. These would be analogous to water hammer. However, once flow is established through the elbow, the water will "know" to turn that elbow (via hydro-static forces) and there will not be as much thrust on that elbow afterwards.

Am I out in left field with my explanation? Any analogies or references that you could think of would be very helpful.

Thanks in advance!

 

[hydtools]

Water hammer does not happen because the rising pressure due to the sudden change in direction has someplace to go. There will be reactions caused by the change in flow direction, but not hammer. Those reactions continue as long as flow continues.
Water hammer happens when flow is suddenly stopped and the resulting pressure rise has no place to go and is reflected back towards the flow source.

Ted

 

[ziptron]

That does not seem right, otherwise there would not need to be any reason to fill a large pipe slowly.

 

[hydtools]

Who makes the decision that the pipe needs to be filled slowly? And why?

Ted

 

[Compositepro]

Ziptron, your understanding of water hammer is flawed, but that is why you are asking. The forces you need to be concerned about are due to mass and velocity of the flowing water (mass times velocity is a quantity called momentum). Water hammer is usually due to stopping flow too quickly. This is because water is basically incompressible, so if you stop the flow in a pipe that contains 5,000 lbs. of water too quickly, that 5,000 lbs acts like a car hitting a wall. Your problem may not be with water hammer but you still have other things to be concerned about. The force that flow generates at an elbow is proportional to flow rate in lbs/sec times velocity in feet/sec. It does not depend on the rest of the water in the pipe, because only the water in the elbow is changing velocity. However, an empty pipe is much lighter than a full pipe so the empty pipe may be more affected by this force. This would be the only reason to open the valve slowly. This force is present as long as there is flow, not just at "the moment of impact".

A valve at the top of the pipeline can control how much water enters the pipe but will not control the velocity in the pipe if the pipe does not run full. It is like a roller coaster. Your valve controls how many cars per minute are launched but has no control of the speed of the cars after launch. For full control of the flow, the pipe must run full. In many cases this requires a valve at the bottom of the pipe. But if your slope is not too great that may not be necessary.

 

[bimr]

Ziptron, you are correct that a water hammer is possible. There have been books written about this complex subject.

Unless the pipeline was constructed with a constant downward slope, it may be possible to have a water hammer. It is also not likely that the pipeline was constructed with a constant downward slope with no pockets.

An air-water two-phase flow condition may occur in a pipeline. The two-phase flow may enable an air pocket to develop. The air pocket may be entrapped as a result of incomplete removal during the pipeline filling operation, incomplete water removal during the pipeline draining, off-gassing of air from solution, or a high spot in the pipeline. When the upstream valve is opened quickly, a sudden pressurization transient of the air pocket may induce an undesirable high-pressure surge, which may lead to deformation or rupture of the pipe, depending upon the amount and location of the air pocket as well as the pipeline configuration.

The intensity of the high-pressure surge may be affected by many variables including the water level in the tank, the speed that the valve is opened, the slope of the pipeline, the headloss in the pipeline, the length of the pipeline, siphoning, etc.

Water Hammer Generated During Pipeline Filling

Air Entrapment Video

If the water hammer is severe, consider installing an orifice in the pipeline to slow the fluid velocity.

 

[LittleInch]

ziptron, you haven't given us a lot of information here, but a few assumptions / questions

1) the pipe slopes down continuously as well as left and right ( otherwise it won't be "empty" and could have pockets of air and water as bimr suggests)
2) how long is "long"?
3) what is the actual effect ( a lot pof pipe movement / "banging"0 if you open too fast?

As others say above, what you are seeing is probably not water hammer in its many forms, but simply forces caused by change of momentum of a fast slug of water at the bends

Anyway - an analogy.

Your pipe is like a narrow winding road with solid walls. It has solid walls because the car / vehicle which drives down it ( the water) has no steering and needs to bounce off the walls at the bends (this is a magic indestructible car). If you set it off too fast by jamming down the accelerator (opening the valve full open) it hurtles into the bends at 70 mph causing lots of damage and force. As it is dragging behind it a long rope which is coiled up at the start, the car gradually slows down as the effort to pull the rope gradually gets higher and higher.

If on the other hand you set the cruise control at say 20 mph (opening the valve partially), the car still bounces off the walls a bit, but with much lower force. The car still pulls the rope Ok as the cruise control gradually increases the power, but stays at 20 mph.

In reality how you end up with a full pipe or whether the pipe is always running par tfull depend son many things including the head of water in the tank, the profile of your pipe, the final water velocity to sweep out air pockets (if there are any) etc etc

the key difference between just opening the valve full and filling slowly is the velocity of the initial water wave / plug and hence the energy in that water. When the pipe is empty there is little to no back pressure or friction so all the potential energy / pressure at the base of the tank goes into acceleration / velocity of the water. Only when it is full all the way to the end does the velocity reduce due to the frictional effect of flow in the pie.

Does that work?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ziptron]

Thanks everyone!

Apart from all of your very clear and logical answers, I was also doing some research and came across some interesting findings which helped clarify this in my head. At first, I pictured the water front more like a horizontal wall. However I found some references which show that water travels more like a wedge and less like a horizontal wall through the pipe. The wedge shape can cause air to become trapped in the flowing water. This is more likely to happen on a longer and larger diameter pipe. This trapped air then leads to pockets of compressed air that, when travelling round a corner, can accelerate the corner quickly lead to water acceleration and then to water hammer.

Long story short, fill a large empty pipe slowly

 

[bcd]

Why do you need to open the butterfly valve slowly to fill the pipe? It is not to avoid water hammer if the end of the pipe is open to atmosphere. It is to avoid dynamic torque on butterfly valves. When the pipe is unfilled and open to atmosphere (i.e., 0 psig, no back pressure), and you have a tank upstream supplying water at a relatively constant upstream pressure. You will have very high flow rate until the pipe fills and the pressure losses up to the exit discharge compile, create pressure and begin to limit the flow rate.

Flow around a butterfly valve disc generates torque. The amount of torque generated, and its direction (opening or closing the valve) depends upon the flow direction. With the full differential pressure across the disc, the flow can be sufficient to generate enough torque to twist and break the valve shaft. So fill the pipe slowly by slowly opening the valve.

 

[bimr]

Will agree that valves should be slowly operated to minimize the potential for water hammer. However, the link to Bray indicates that dynamic torque on a butterfly valve should not be a concern for this application. Dynamic torque is more of an issue when closing large butterfly valves against a fast moving fluid stream.

If a valve is closed too quickly (note that the posted application is opening, not closing), especially in long systems, large transient pressures are generated and travel as waves through the pipeline. These transient waves increase the pressure difference across the valve, which in turn increases the dynamic torque that is applied to the valve. The effects of the increased dynamic torque are generally more significant in larger butterfly valves (larger than 24-Inch) since dynamic torque is a function of the diameter raised to the third power.

Link to Bray

For this application, when the butterfly valve is opened quickly to full open, the dynamic torque will decrease to zero at full valve open before the fluid has begun to move. A dynamic torque problem would occur when one was attempting to close the valve against the maximum flow.

Agree that the valve should be opened slowly to fill the line and minimize the water hammer from trapped air, but would not expect a dynamic torque problem.