Water Hammer Calculations - Part 2

[PEDARRIN2]

thread378-399464

In my previous thread, i was looking for guidance on how to calculate what was happening on the upstream side of a quick closing valve. Thanks for all who provided insight.

But there is another aspect of water hammer I have not seen much information/guidance I can use to design to minimize the effects. That is the resultant column separation that can occur on the downstream side of the valve.

I understand the flow momentum of the water downstream of the valve causes the pressure to fall below the vapor pressure of water and forms a vacuum (cavitation) which eventually reverses direction as the vapor bubbles implode and another positive pressure water hammer occurs on that side of the valve.

But I have not found any calculation methods which can provide numerical values for the pressures involved, something similar to Joukowski.

Are there sources I could look for, hopefully with examples?

Thanks in advance

 

[bimr]

Column separation is a phenomenon associated with sloping pipes, not generally associated with valve closing. Valve closing is a different transient phenomenon that may be minimized through modification of the valve closing period.

Pipelines may slope and down surges result when power fails or when valves at the upstream end (the usual configuration) close quickly. Under some conditions, the downsurges can cause column separation, a condition to be avoided at any cost by a proper control strategy.

Column separation is more likely to occur if there are knees in the pipeline and is almost certain to occur at the high point. Column separation may also occur a other places along the pipe. The location and extent depends on the relative relationship between the dynamic head, the total head, and the profile of the pipeline.

There is no simple, easy way to perform reliable transient analyses. Computer modeling is the most effective means available, but there are practical constraints on time and cost.

Do not to analyze systems where pumping systems with flow less than 100 gal/min. Discharge piping is usually such that the velocity is low and transient pressures are low. Even if transient pressures are high, small diameter 4-in piping has a high pressure rating and can usually withstand the pressure.

Watters, G. Z., Analysis and Control of Unsteady Flow in Pipelines, 2nd ed., Butterworth-Heinemann, Stoneham MA (1984).
Chaudhry, M. H., Applied Hydraulic Transients, Van Nostrand Reinhold, New York (1979).
Wylie, E. B., and V. L. Streeter, Fluid Transients, Feb Press, Ann Arbor, MI.

 

[LittleInch]

I agree it's not common, but when you get fairly fast flowing lies with a relatively low pressure ( e.g. loading lines of a few km long) you can get this phenomenon and the pressure spike can be huge. The same thing can occur when a pump stops fairly quickly.

I think it really needs a transient program to give you the right result.

In general to prevent this the same things apply, i.e. close the valve(s) slower, close them in a different sequence or timed delay, add a vacuum breaker (not normally a great idea), add a differential relief valve to allow a limited amount of liquid past the closing valve, use a pressurised bladder to fill the void, etc etc

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

 

[BigInch]

Downstream column separation is difficult to model exactly, even with speciality computer program. It still requires tuning the governing parameters to match a known response to assure correct results. Pressures can reach extreme levels depending on how fast the vapour column collapses.

Conservatively you should consider that column separation happens whenever a valve is closed on a fast moving liquid. Rather than try to analyse column separation, I think it is much easier to try to prevent it from happening by controlling flow in the downstream section, such that upstream segment inlet pressures never drop below vapour pressures, i.e. shut down a pipeline by slowly positioning downstream valves first, keeping all upstream pressures always above vapour pressure. Work upstream as transient pressures in downstream segments subside.

If you must close upstream valves first and quickly, the general effect upstream will be a pressure increase to a level above the normal operating pressure level at that point. That effect is mirrored with a negative wave downstream until vapour pressure is reached. So, if you can keep the increase in pressure upstream to less than 1/2 * (normal operating pressure - vapour pressure), you can usually presume that the pressure downstream of the valve will remain above that same value as the valve is closed.

 

[PEDARRIN2]

I agree it is not something common, and I have read articles about how/when it happens in utility systems. It seems one method of minimizing is to add vacuum breaking valves that introduce air into the system. That is not something I want to do inside the building.

I did find an article describing a couple of cases where it occurred in the plumbing, or rather the irrigation of a building.

http://www.pmengineer.com/articles/89218-water-hammer-from-a-different-angle

Also found, the device that was used to mitigate it.

http://www.siouxchief.com/docs/defa...um-breaker-arrester---vacurester.pdf?sfvrsn=6

So they would put an water hammer arrester upstream of the valve and the vacuum arrestor downstream of the valve.

I was just wondering if there was a way to determine the size of the arrester that was not related just to pipe size and did not involve complicated computer software which probably costs more than the valve and may only be used once a year.

 

[MortenA]

Its common enough in fire water ring mains e.g. if you have a water filled system with jockey pumps and deluge valves. If the elevation difference it big enough you may get column separation at high elevation during the time it takes from the deluge valves opens and the firewater pumps starts because the capacity of a deluge valves is higher than the jockey pump. <then once the pressure comes back when the main firewater pumps kick in you can have a fairly big surge due to column separation. However, vacuum breaks and/or check valves can mitigate this in most cases.

 

[gcastan]

there was a steam/water study done at MIT... I don't know if this would help:

http://dspace.mit.edu/handle/1721.1/60599

Regards

Gabriel Castaneda, P.E.

http://www.gabcheminc.com

 

[sairanji]

Pedarrin,

I am checking for the same for the dowstream of the valve. Did you happen to get a formulae or method to calculate the negative pressure downstream?


Regards
Ranjit