Design against gaseous cavitation
Design against gaseous cavitation
(OP)
Hello All,
At my company we recently had an interesting issue with cavitating pumps.
Two 100% pumps were installed in a chilled water plant and were operating quite happily until the late addition of a thermal energy storage tank (TES, large tank, open to the atmosphere).
Pump Flow rate = 6000 gpm
Pump media = water
media temperature = 55-75 F
NPSHr at Flowrate = 28
NPSHa at flowrate = 38
Once the TES tank was brought online the pumps began cavitating, even though the NPSHa was above required. This was confirmed with a pressure measurement in the field.
The pump manufacturer installed air taps to allow small amounts of air entrainment into the pump suction, however the client rejected this as a long term solution due to long term corrosion concerns from the injected air (oxygen). The final solution is to install VFD's and operate both pumps at 50% where the NPSHr is only 15 ft.
All sign point towards the pumps suffering from gaseous cavitations due to dissolved air in the water coming in and out of solution in the pumps areas of low pressure.
My question is how does one design against the gaseous cavitations failure mode in chilled water cooling tower and TES tank applications? All the information I have found to date states gaseous cavitations will occur in these applications, but there is very little direction on how to prevent its occurrence during design. Allowing its occurrence is not an option due to customer perception issues even if the risk of damage is low. Any guidance would be greatly appreciated.
At my company we recently had an interesting issue with cavitating pumps.
Two 100% pumps were installed in a chilled water plant and were operating quite happily until the late addition of a thermal energy storage tank (TES, large tank, open to the atmosphere).
Pump Flow rate = 6000 gpm
Pump media = water
media temperature = 55-75 F
NPSHr at Flowrate = 28
NPSHa at flowrate = 38
Once the TES tank was brought online the pumps began cavitating, even though the NPSHa was above required. This was confirmed with a pressure measurement in the field.
The pump manufacturer installed air taps to allow small amounts of air entrainment into the pump suction, however the client rejected this as a long term solution due to long term corrosion concerns from the injected air (oxygen). The final solution is to install VFD's and operate both pumps at 50% where the NPSHr is only 15 ft.
All sign point towards the pumps suffering from gaseous cavitations due to dissolved air in the water coming in and out of solution in the pumps areas of low pressure.
My question is how does one design against the gaseous cavitations failure mode in chilled water cooling tower and TES tank applications? All the information I have found to date states gaseous cavitations will occur in these applications, but there is very little direction on how to prevent its occurrence during design. Allowing its occurrence is not an option due to customer perception issues even if the risk of damage is low. Any guidance would be greatly appreciated.





RE: Design against gaseous cavitation
http://www.expertctr.com/ch_5.php
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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: Design against gaseous cavitation
RE: Design against gaseous cavitation
The diffusers are designed to handle 17000 gpm (withou air entrainment)in this tank by the tank vendor.
RE: Design against gaseous cavitation
The air can come out of solution in low pressure areas, but then what does it do? It goes back into solution as the pressure rises within the pump. Or it stays a gas and is pushed around by the liquid. I can see it possibly causing lubrication problems in places that were designed to be wet, but it doesn't sound like that is what you are seeing. Can you upload a picture?
David
RE: Design against gaseous cavitation
RE: Design against gaseous cavitation
Why do you think it is cavitating - noise, loss of performance ???? this could give a lead to the problem
What has changed other than fitting the tank - has the inlet pipe work changed in any way - any changes on the inlet side ie, valves, bends etc?
Have you / did you measured flow, head and power - before and after the change?
RE: Design against gaseous cavitation
RE: Design against gaseous cavitation
RE: Design against gaseous cavitation
RE: Design against gaseous cavitation
David
RE: Design against gaseous cavitation
**********************
"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: Design against gaseous cavitation
Google " Henry's Law " and do some calculations and depending on your system and how much air is initially dissolved and/or entrained, and you might be surprised at the CFM of air passing through the pump. I just did this for a pump handling 6K GPM and I was (surprised that is.)
rmw
RE: Design against gaseous cavitation
If this was the case (air accumulating in the impeller eye) pump performance wouldn't droop and as the percentage of entrained air increases, which it does until flow stops altogether if sufficient air enters the inlet.
It would be great if flow remained at duty even though air was being entrained -- wouldn't that overcome a lot of problems and entrained air would no longer be a problem.
RE: Design against gaseous cavitation
**********************
"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: Design against gaseous cavitation
Another possibility (?). If the installation of the TES caused a sharp projection in the path of the flowing water into the pumps it could be a cause. When a liquid flows along a solid boundary, it separates at all points at which this boundary has a discontinuity, such as a cusp, or sharp edge.
I quote from Sam Yedidiah's Centrifugal Pumps User's Guidebook, Chapman & Hall:
"When a liquid that has been in direct contact with a solid wall flows past a sharp edge, it separates from the wall, creating an empty space (vacuum) between the flowing liquid and the solid wall. This causes some of the liquid to evaporate into that empty space and to be carried away in the form of vapor-filled bubbles. When these bubbles enter the zone of higher pressure they collapse vigorously, causing typical cavitation damage."
RE: Design against gaseous cavitation
**********************
"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: Design against gaseous cavitation
BigInch, you're absolutely right. As always, I'll appreciate reading your instructive comments. As you say: dissolved gases coming out of solution adversely affect pump efficiency, but do not cause cavitation. Therefore, please consider ColonelSanders83 opening paragraph:
I imagined (and I may be wrong) that if, because of the TES addition, the suction piping contains a "T" near the pump inlets, it might create precavitation. Kindly comment.
RE: Design against gaseous cavitation
I wouldn't think that a "T" would create precavitation and if it did I wouldn't expect it to carry over into the pump inlet and cause any problems, but a "T" would certainly create extremely disturbed flow into the pump which could result in all sorts of hydraulic problems.
RE: Design against gaseous cavitation
Artisi, thanks. In the same chapter Sam Yedidiah tells us that he came across a cavitation case involving an unconventional inducer, tested in a water loop, that created vapor-filled bubbles at the sharp edges of the blades. The bubbles were carried away with the flowing liquid.
This effect continued to occur even after removing all dissolved air from the loop and after the suction pressure was increased to 20 m above atmospheric pressure.
He also suggests (to those who can) reading the details in the ASME Cavitation and Multiphase Forum, FED Vol. 194, pp.101-103, Lake Tahoe, Nov. 1994.
RE: Design against gaseous cavitation
**********************
"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: Design against gaseous cavitation
I apologize for the tardy reply, I had some nasty production fire that needed quelling.
I have attached a quick layout of the system piping. I would like everyone to temper their exercise regimes for the day and read the following before jumping to conclusions.
This system was laid out and set in stone by the sales end of the business, complete with major equipment purchases before engineering ever got a chance to evaluate it. I am aware that many don'ts are shown in the layout, engineering was told to deal with it. As I said in my initial post, I was tasked to design against this failure in the future.
So far there have been three useful responses in this thread. One by compisitepro with the statement that "dissolved gas will raise the vapor pressure of the liquid" and JMW's reference to Henry's law. These two posts led me to this site
http:/
This had some useful information. but no detailed design calculations.
BigInche's most recent point regarding areas of low pressure cavitations before the pump is interesting, but this does not explain why it is happening now after the TES tank installation.
The pump vendor has stated that we do not have cavitations in the pump. They have stated that the noise, which both sounds and acts exactly like cavitations is just "turbulence noise". This statement doesn't help with future prevention of the problem and they really had no insite as to why it was happening.
Another point of interest is that the Nss of the pump is at 8215, which may be negatively affecting things, still working on that.
I appreciate any thoughts and comments that allow calculations to be performed in determining the problem (or even ones that prove the pump is ok, at least then I can move on knowing that this isn't the problem). Again I need to design against this failure mode in the future.
Always remember, free advice is worth exactly what you pay for it!
RE: Design against gaseous cavitation
RE: Design against gaseous cavitation
The temperatures are correct.
Always remember, free advice is worth exactly what you pay for it!
RE: Design against gaseous cavitation
Cavitation cannot happen if:
Atmospheric Pressure + Static Height – Inlet Friction – NPSHr is greater than the Vapor Pressure of the water at the pumping temperature.
The only value you usually don't calculate is the NPSHr, as you get this from your pump supplier.
What if the NPSHr value your supplier passed on to you is wrong (I mean wrong for the specific pump you got, because of a construction fault)?
RE: Design against gaseous cavitation
Has the addition of the tank adversely affected the previous NPSHA in any manner? Has the partial pressure of air in the water increased over what it was previously? Is theremore contact surface area and/or time being allowed for air to cross a fluid interface than was available previously?
What did the previous configuration look like?
**********************
"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: Design against gaseous cavitation
The previous configuration was a closed loop, with no exposure of the water to the atmosphere.
The system was calculated to have more NPSHA (approx. 31') then NPSHr (approx. 25') when the TES tank is installed.
If we isolate the TES tank and go back to a closed loop the problem disappears (suction pressure at the pump suction also increases to 10 psi giving an NPSHA of 54').
During noisy operation with the TES tank we suffer a loss of 100-200 gpm of flow.
If we lower the suction pressure to below NPSHr we get much noisier response from the pump combined with large vibrations (classical vaporous cavitations).
All the signs point toward the pump operating in the gaseous cavitations range when the TES tank is operational.
So far I found this paper to be exceedingly useful and I am making up a spreadsheet to run the numbers
"Cope with dissolved gases in pump calculations" Chen, Chyuan-Chung, Chemical Engineering; Oct 1993; 100, 10;
Always remember, free advice is worth exactly what you pay for it!
RE: Design against gaseous cavitation
**********************
"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: Design against gaseous cavitation
Have you run the pumps against an increased discharge head, if yes- what was the outcome, more or less noise?
Seems you have 3 likely problems or even a combination of more than 1.
Insufficient NPSHa
Air entrainment
Disturbed flow into the pump inlet
RE: Design against gaseous cavitation
The pipe work in the vicinity of the pumps has not been changed. The TES tank ties into the suction line some 500 ft away.
Running the pumps up against a higher discharge head results in the pumps running slightly quieter and corresponds to a slight increase in suction pressure.
Insufficient NPSHa - I agree, the research I have done so far leads me to believe this is caused by the air dissolved in the water raising the effective vapor pressure.
Air entrainment - as I explained above this is not really a problem with our suction diffuser in the tank being so grossly oversized.
Disturbed flow in the pump inlet - this does not explain why the noise goes away upon TES tank isolation, even though the suction piping remains identical.
Always remember, free advice is worth exactly what you pay for it!
RE: Design against gaseous cavitation
As you consider air entrainment not possible because of the oversized tank allowing any air to escape then I don't really see that you would have enough dissolved air to cause any major problem.
Seems it only leaves NPSHa as the likely cause as there is no problem when you revert to a closed system with increased NPSHa and increasing the discharge head also quietens the pump.
What condition are the pumps in with regard to impeller leading edges, wear ring clearances etc. it is possible that NPSHr is actually higher than you think / have been advised, have the pumps been NPSHr tested?
RE: Design against gaseous cavitation
Has a tank discharge coefficient been used to reduce NPSH when including the tank? Do you have a suction pressure gage?
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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: Design against gaseous cavitation
The atmospheric TES changes the scenario in a way a gas source is now available to dissolve in water (in a closed loop this was not possible).
In the impeller of the pump a separation of the gas phase from the liquid phase takes place, and this could produce a choke noisy effect which also affects the range of capacities over which the pump can operate(ColonelSanders83 in one of his previous post stated "we suffer a loss of 100-200 gpm of flow").
RE: Design against gaseous cavitation
**********************
"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: Design against gaseous cavitation
**********************
"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: Design against gaseous cavitation
Point 18. in
ht
is quite explicit.
RE: Design against gaseous cavitation
From the NPSHr definition made by HI (Hydraulic Institute) it is possible to see the discharge head reduction even below the 3% threshold as due to a cavitation phenomenon.
I think the link below could clarify the situation.
http:
RE: Design against gaseous cavitation
Yes I know about that and agree completely. That was what I have already said at least once above. It causes a reduction in capacity, because air replaces the volume of liquid that would be otherwise pumped. Totally agree.
ione,
Yes, I know aobut that and have already mentioned the fact that the above NPSHA equation would not suffice in all cases without a substantial safety factor. I know that cavitation can begin at 30% higher than NPSHR in some situations. Totally agree. But I don't think, perhaps wrongly, that it is the introductioin of air that sometimes causes that to be 30% higher at times.
I clarify. My Question Now is only,
that I don't understand why or how excess AIR from any source LOWERS the NPSHA in any other manner than by reducing the efficiency of the pumping action and hence head, ie. by replacing liquid with air. Specifically how it increases the "effective" vapor pressure or affects some other mechinism (other than by fluid replacement) in actually reducing the NPSHR. I think that's what somebody stated or implied above. If I've got that wrong then, Thanks for getting this far with me.
**********************
"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: Design against gaseous cavitation
RE: Design against gaseous cavitation
I tried to give you a star now 3 times, but unfortunately I keep getting an error with the star page popup.
"Internet Explorer cannot display the webpage'.
Bad luck.
I'll report the error.
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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: Design against gaseous cavitation
Cannot really understand whether you've found the paper of some interest or whether you're simply pulling my legs. Anyway it doesn't matter, I just wanted to give my small and probably trivial contribution to this thread which is becoming somewhat intriguing.
RE: Design against gaseous cavitation
As I have stated previously we have a suction pressure gauge.
When the TES tank is operating and the pump is "cavitating" the suction pressure gage reads 0-2 psig (it bounces around)
When we Isolate the TES tank the "cavitations" go away and the suction pressure gauge reads 10-11 psi.
Ione: the 70 degrees quoted is the warmest the water ever gets, due to volume comparisons in the system and the fact that water at this temperature is a sponge for air, the system quickly reaches equilibrium. There is no place in the system where the air can ever come back out of solution, except at the low pressure points of the pump.
According to "Cope with dissolved gases in pump calculations" by C.C. Chen the work previous to his approximated an equivalent vapor pressure based on experiment to be approximately the average of the vapor pressure of the liquid and the pressure of the gas it is exposed to. C.C. Chen's work went through and detailed the equations and step necessary to accurately calculate the equivalent vapor pressure (working on this, however it will take time).
For our system the NPSHa was calculated to be about 31', however this was based on the vapor pressure of pure water from the steam tables, (I previously posted a link saying why that was incorrect). For our case pure water has a vapor pressure of about 0.5 psia whereas the air rich water has an equivalent vapor pressure of 6.73 psia. If the equivalent pressure is used in the calculation of NPSHa we find that we only have 15.5 ft of head available and this fails the NPSHr of the pump (25ft), and leads to the noise and flow issue we are having. This shows we will suffer gaseous cavitations in the pump when operating the TES tank.
When the TES tank is isolated our suction pressure goes up by about 10 psi, using the same method above this yields 38.1 ft of NPSHa, well above the required 25 ft of the pump.
It is important not to confuse vaporous and gaseous effects in the pump. they must be accounted for separately using the appropriate vapor pressure, although it is apparent to me that for chilled water systems, the gaseous requirements will govern.
Always remember, free advice is worth exactly what you pay for it!
RE: Design against gaseous cavitation
Looks like the NPSHA is starting to add up, or not, as may be the case. Very interesting.
**********************
"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: Design against gaseous cavitation
RE: Design against gaseous cavitation
The difference is one of an open loop and a closed loop with the subsequent increase in suction pressure associated with the closed loop.
As I stated above, with the open system suction pressure is 0-2 psig and when the system operates from a closed loop the suction pressure is 10-11 psig.
Always remember, free advice is worth exactly what you pay for it!
RE: Design against gaseous cavitation
Well then do you think we're finally getting back to something relating to the piping configuration to and from the tank? You must be losing some head there.
What's is the tank outlet valve's head loss and the tank's outlet nozzle flow coefficient? Please don't tell me there is 1000 feet of pipe there too.
**********************
"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: Design against gaseous cavitation
I'm staying with an NPSHa problem at this stage.
Where on the curve is the pump running in relation to BEP?
RE: Design against gaseous cavitation
Please see my above post detailing that the TES tank is 500 ft away. This is what accounts for the differences in suction pressures in the two configurations (closed loop and TES tank operation).
The real issue is that the pump sounded like it was cavitating when the NPSHa to NPSHr comparison said it should have been fine.
The use of the equivalent vapor pressure for the air saturated water shows that the pump will indeed operate noisily from gases coming out of solution when the TES tank is operating. It also shows it will be adequate when it operates from a closed loop.
The papers I have referenced, esp. the one from C.C. Chen details out how to correctly calculate the equivalent liquid vapor pressure. I can then use the equivalent vapor pressure to calculate the correct NPSHa for this system, which will be substantially lower the NPSHa calculated using the values for a pure liquid (see above).
This allows me to design against the failure mode of "gaseous cavitations" or "noisy pump operation", which was my original question. (I admit gaseous cavitations is probably a misnomer but I didn't make the name).
Always remember, free advice is worth exactly what you pay for it!
RE: Design against gaseous cavitation
**********************
"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: Design against gaseous cavitation
If there are any further thoughts on the matter feel free to add.
Always remember, free advice is worth exactly what you pay for it!
RE: Design against gaseous cavitation
**********************
"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: Design against gaseous cavitation
Pls take a look at page 119 of the attached paper (it's quite recent).