Cavitation in Pipelines - Need Advice 2

[Ballintoy]

Assume there is a high point in a water system. By opening a series of hydrants we lower the hydraulic grade line to the point where it intersects the pipe at the high point - no problem yet. If we continue to open hydrants the negative pressure at the high point eventually reaches the point where cavitation is imminent. What's happening, physically happening, in the pipe if we allow the flow to continue or open additional hydrants. I use this as an example of a real situation I am looking at. I'm trying to understand what would haoppen if we have a line break. Modeling predicts a pressure of -20 psi or less at the high spot, but what does this mean? There is roughly a mile of line beyond the high spot. It would take a long time to empty.

 

[katmar]

This seemingly simple and common situation is actually rather complex and often overlooked. You are correct that if you increase the downstream demand by opening more hydrants the pressure will decrease. But of course the pressure at any point in the system can never go below -14.7 PSIG (0 PSIA) and in practice will not go below the vapor pressure of the water (0.36 PSIA at 70 deg F).

The reason your model shows -20 PSI is that it will assume that it is always liquid water flowing, and will calculate the pressure drop accordingly. What actually happens is that when the pressure gets below the vapor pressure of the water, the liquid turns to vapor (steam) and the flowrate drops off dramatically and the model is no longer a true representation of what is happening in the pipeline.

When you get to this stage and the water is boiling in the pipeline you can get very bad vibration and hammering problems.

My own background is in process plants rather than water reticulation, but in process plants we get a similar situation where cooling water is pumped up to 120 feet or so at the top of the plant, and then has to run down back to the cooling tower. The usual solution to this problem is to install a vent at the high point. This results in air being drawn in if the pressure goes too low. Having air in the system introduces a new problem because you now have two phase flow and the pipes have to be much bigger. And you lose any pressure you had in the pipe. This a whole new ball game which I will ignore for now.

I have seen vents on water pipelines, but I'm not sure if they work on the same basis as what we use in process plants.

Sorry I cannot give you a definitive solution to your problem, but I can confirm that you are correct to be concerned about what might happen. I suggest you call in someone with the right experience in addition to any good advice you might get here.

regards
Katmar

 

[BobPE]

Ballintoy:

Yor opening of hydrants does indeed lower the HGL and if this falls below the pipeline you will have vacuum at those points. Flow in the system is conserved and you will balance out and flow will be constant at the hydrants or additional hydrants at some point and the vaccum at the high point will serve as the primary limiting factor. Now depending on how fast you open these hydrants, you may induce column seperation, which is different that cavitation, but just as much of a concern.

If you had a line break, depending on where it occured, you would again induce column seperation at that high point where the high point serves as a hydraulic knee. At this point the water column would seperate and then snap back. Both the extreme vacuum of seperation and the high pressures of rejoining the column are what you need to be concerned about. This is typically why engineers design air and vacuum valves at these points. A column seperation condition needs air input at that point then the subsequent rejoining of the column needs to have air expelled at the point. Now the trick to surviving an event like this is the proper sizing of the air/vacuum valve by an engineer that knows what they are doing.

A lot of people confuse column seperation with cavitation, but they are two distinctly different things.

I hope I helped....

BobPE

 

[bimr]

You are probably aware that cavitation is something that occurs in pumps when operating outside of the recommended design range. When pumps operate at high speeds and at a capacity greater than the best efficiency point, pump cavitation is a danger. It occurs when the absolute pressure of the inlet drops below the vapor pressure of the pumped fluid, causing a hammering action when the bubbles collapse.

Vacuum valves are used at system high points to allow enough air to enter the force main to keep the pressure close to atmospheric. By keeping the pressure in the air cavity close to atmospheric, the velocity with which the water columns on either side of the cavity rejoin is reduced and some air-cushioning effect is obtained. This also limits the pressure increase in the system following collapse of the vapor cavity.

 

[BobPE]

birm:

cavitation can happen anyhwere in a system, pressure or gravity including pumps. It happens a lot in pipes and fittings, etc. where wall velocities induce head losses.

But I just wanted to clarify one thing in your post, to accomodate column seperation, you have to have air and vacuum relief. If you were only to have air enter (vacuum relief), the subsequent column collapse into the body of air would allow significant compression and which would produce the energy needed to seperate the column again. The goal is to rejoin the column the first time after it seperates and then keep it together.

BobPE

 

[Ballintoy]

Thanks to all for the advice. Just to clarify things, I've been retained to evaluate an existing water system. This is one of many actual and potential problems that I've identified.

 

[bimr]

Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure. The term cavitation implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse.

A one time event where you pull vacuum on a pipe is not considered to be cavitation.

In the water business where the customary water piping systems use 3 to 8 ft./sec pipe velocities, cavitation is generally found only in pumps.

 

[hydrae]

Ballintoy
One other item that may occur during these negative pressure events, the pipe may collapse, If it is a large diameter thin wall steel, again the need for properly designed and installed vacuum relief valves.
Hydrae

 

[JStephen]

One problem with assuming that the line breaks is that you don't really know what does happen. You might just assume that water runs out at that point, and pulls a vacuum and forms a big vapor pocket somewhere up the line. But you could have air running back up the line, and once that happens, you no longer know the pressures or the flow rates.