Water Velocity
Water Velocity
(OP)
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.
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.





RE: Water Velocity
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Water Velocity
The phenomena is "cavitation" - where the liquid vapourizes at the vena contracta then then collapses again.
See page 136:
ht
RE: Water Velocity
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
www.muleshoe-eng.com
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RE: Water Velocity
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Water Velocity
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.
RE: Water Velocity
http://w
RE: Water Velocity
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.
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"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/
RE: Water Velocity
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.
RE: Water Velocity
RE: Water Velocity
BTW, none of which have anything to do with water's vapor pressure.
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"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/
RE: Water Velocity
Are you looking at starting or stopping a pump????
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"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/
RE: Water Velocity
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.
RE: Water Velocity
**********************
"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/
RE: Water Velocity
I don't expect water hammer from the pump on/off.
RE: Water Velocity
**********************
"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/