Water Velocity 4

[PEDARRIN2]

I have searched the forum and haven't found anything.

I am reviewing some sprinkler hydraulic calcs where at a pressure reducing valve - the flow velocity is in the range of 30-40 fps.

We have a specified maximum of 30 fps in sprinkler designs. I was told the rationale was that at about 32 fps that water vaporizes - but I cannot find any technical corraboration for this.

Other than the obvious noise, erosion, water hammer effects of water flowing that fast - is the water vaporization a valid concern.

If so, could somebody point me to a technical reference that backs that up - either through direct statement or a calculation approach that I could use that would verify this upper limit.

 

[katmar]

I cannot see any relationship between the flow velocity and the vaporization of water. Looks like someone trying the old "Bull$#*7 baffles Brains" routine.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[AllHandlesTaken]

No baffling.

The phenomena is "cavitation" - where the liquid vapourizes at the vena contracta then then collapses again.

See page 136:

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

 

[zdas04]

Cavitation is real. It is defined as "the formation and subsequent collapse of condensable vapor bubbles in the flow". This kind of phase change causes damage when liquid rushes in to fill the voids at very high velocity. It regularly rips material off of control surfaces.

The effect in the reference that AllHandlesTaken provided is very specific to a reduction in the flow path followed by the flow opening back up to the full-flow area. If you use full-opening valves (e.g., ball or gate), and the only vena contracta is the spray nozzles then your velocity shouldn't be a problem. This is a problem that can be approximated with Bernoulli's equation--look for a minimum pressure below the saturation pressure for your fluid temperature.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Remember, being ignorant isn't your fault; staying ignorant is.

 

[katmar]

Sure you can get cavitation in valves, but how do you tie that to 32 fps? It must depend on the water pressure and temperature, the valve type, the valve opening %, the pressure drop across the valve etc etc. I would not accept an arbitrary limit like this without a decent explanation. As the OP has already noted, this is a high velocity for water and may not be a good idea anyway, but let's have some sound engineering behind the decision.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[PEDARRIN2]

To give a little more specifics.

The building is a multistorey building with a fire pump. I have to have 100 psi at the top of the building (~170 feet above the level of my fire pump) for fire hoses. I have sprinklers on the lower floors that are only rated to 175 psi. So the pressure on these floors has to be reduced when it comes off the standpipe.

The fluid is water at ambient temperature.

We are coming off a 6" standpipe, through a 2.5" pressure reducing valve, and then to/through a 4" black steel pipe to sprinklers. The valve is the size it is because that is the only size this particular valve for this particular application comes in. This fact is what I think is driving the problem.

Per the hydraulic calcs sheet I am reviewing,

Velocity is 48.1 fps
Flow is 718.087 gpm
P(in) is 115.7 psi
P(out) is 83.9 psi

The valve is a 2.5" Potter Roemer 4036 (field adjustible pressure regulating valve) - from cut sheet states at full flow, the valve acts as a straight pattern globe valve.

Just looking at the flow pattern of the valve (definitely not a straight through full port type valve) and the pressure drop - I think there is a problem.

I am just looking for the technical approach or definitive resource I can use to arrive at the response I think I have to make.

I don't want somebody to calc the solution - just point me in the right direction or point me to a resource that states I will have a problem with that velocity - although I don't think there will be such a resource for a general problem like katmar suggested.

 

[AllHandlesTaken]

Yes there are references, for example:

http://www.osti.gov/energycitations/servlets/purl/10155405-SCfePH/10155405.pdf

 

[BigInch]

In this case it doesn't have to be cavitation. Stopping flow at 32 fps by fast valve closure can (and most certainly will) reduce pressure to values below the vapor pressure of water, hence it is vaporizing by column separation, ie. without having a pressure drop caused by "cavitation" across a valve. Its vaporization by extreme water hammer effect.

The "water hammer" equation (Joukowsky Equation) predicts pressure increase ahead of a moving column of water coming to rest and the pressure decrease behind the water column accelerating away from the valve on the downstream side. The pressure increase and decrease is taken from the average steady state operating pressure at a point in the pipeline where the transient passes. Hence the wave trough may easily equal 0 psiA at lower steady state operating pressures, thereby vaporizing the water. As the column runs away and slowly decreases velocity to zero, the pressure behind eventually increases to above vapor pressure, which collapses the column and reverses the water in the opposite direction. That collapse can be rather violent and cause very high pressures as the column stops again. Water hammer can begin to cause problems at velocities as low as 7 fps, or lower in some circumstances. 32 fps velocities should only be used in liquid pipes and pipelines with very very high maximum allowable operating pressure to normal max operating pressures. A ratio probably in the vicinity of at least 1.5 or even higher.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[PEDARRIN2]

In this case, we are not looking at any valves closing - so water hammer and any subsequent vaporization will not occur.

When looking at it closer, the thought that the water would vaporize in a pipe - no constrictions, no valves - at 32 ft/s was seen to have no technical basis.

Thanks for the assistance.

 

[Compositepro]

There is nothing special about 32 ft/sec. except someones erroneous association with the gravitational constant and the resultant fact that a column of water cannot rise more that 32 ft. by suction.

 

[BigInch]

Whoooo. Good point up to where you mention the "resultant fact" of water rising to 32 ft. That happens only to be because its demineralized water, temperature is 33 deg F and atmospheric pressure is 14.69 psia. Nothing to do with gravity, other than gravity's effect on mass to give the water its specific weight. And lets not forget gravitational acceleration is 32.174 feet per second per second ie. sec^2 ; very different from the was 32 ft/sec velocity that was initially mentioned above.

BTW, none of which have anything to do with water's vapor pressure.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

"we are not looking at any valves closing- so water hammer and any subsequent vaporization will not occur. "

Are you looking at starting or stopping a pump????

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[PEDARRIN2]

The fire pump will start when the sprinkler(s) open due to heat from a fire.

I don't think water hammer will occur from starting the pump - but I could be wrong.

No valves will be closing so there would not be any water hammer due to sudden stopping of the flow

Anyway the valves are going to be changed - since the manufacturer would only recommend them up to 500 gpm.

 

[BigInch]

With that design velocity, I'd bet you are wrong... but I might be wrong too, I just don't think so. The electric pump will probably start fast enough to do it, the diesel.. maybe not... depends. What I would do is to check it out. Do a hammer analysis and sleep at night.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[PEDARRIN2]

Fire pump controller allows for slower speed up and no slam check valves will help when the pump stops.

I don't expect water hammer from the pump on/off.

 

[BigInch]

Hope you're right.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/