Design against gaseous cavitation 5

[ColonelSanders83]

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.

 

[BigInch]

Any change of direction with sufficient velocity would tend to induce a low pressure area on the inside curvature of the streamlines. If the low pressure region is below the vapor pressure of the product, in addition to impacting the pump with the usual unbalanced pressures across the intake flow area created by a closeby change of direction, the additional possibility of increased detrimental effects from precavitation should be investigated.

**********************
"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/

 

[ColonelSanders83]

Gentlemen,

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://pump-zone.com/pumps/centrifu...n-npsh-and-example-calculations-of-npsha.html

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!

 

[chicopee]

I am confused about your diagram. Is the TES tank on the discharge or suction side of the pump? Also I am questioning the media temperature 55/75 dF that you show. If cavitation is the problem, I would expect the watertemperature range to be higher.

 

[ColonelSanders83]

The TES tank is on the suction side of the piping.

The temperatures are correct.

Always remember, free advice is worth exactly what you pay for it!

 

[ione]

Just a possibility.

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)?

 

[BigInch]

I wouldn't say cavitation can't happen if you meet that equation, unless you add a safety factor to that of about 2X, and it still might be a problem under certain conditions. And its still conditional on the fact that you have evaluated all the detrimental effects from fittings and changes of direction properly.

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/

 

[ColonelSanders83]

BigInch,

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!

 

[BigInch]

Will you post the spreadsheet???

**********************
"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/

 

[Artisi]

Has the approach pipe work configuration close to the pump been changed?

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

 

[ColonelSanders83]

Artisi,

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!

 

[Artisi]

Fair comment on air entrainment and disturbed flow.

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?

 

[BigInch]

I would't have thought that, but it does appear that it is having some effect. What mechinism is responsible for dissolved air increasing the vapor pressure of the water? I thought that should remain the same, if temperature remains constant.


Has a tank discharge coefficient been used to reduce NPSH when including the tank? Do you have a suction pressure gage?

**********************
"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/

 

[ione]

Water is somehow cooled in the atmospheric TES, and temperature reduction leads to higher gas solubility.
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”).

 

[BigInch]

Yes, but he also mentioned 6000 gpm, so that 100-200 is exactly the percent flowrate range that I would expect to lose (2 to 5%) from the usual range of additional air in relatively highly aerated cool water being released in the suction piping as it goes around fittings and enters the pump (not related to vapor pressue cavitation). I still don't understand exactly why the likelyhood of cavitation is increased by the addition of that air as claimed. I still think its a separate problem, but I have not read the articles mentioned above yet. I don't understand the purposed interrelationship free air to "cavitation". I was hoping for a summary of the how and why of those claims.

**********************
"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]

I'm not an expert with water, as with petroleum products we usually have a higher margin of safety in the onset of cavitation than when dealing with water and other fluids, so just trying to get a handle on the reasoning behind those thoughts above.

**********************
"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/

 

[25362]

Point 18. in

http://www.lightmypump.com/centrifugal-pump-tips.htm

is quite explicit.

 

[ione]

BigInch,

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://www.waterworld.com/index/dis...tion-air-on-centrifugal-pump-performance.html

 

[BigInch]

25362,

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/

 

[ione]

Let's try with this

 

[BigInch]

THAT'S IT! THE quasi-steady bubble theory is exactly what I needed to hear. Good work!

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.

**********************
"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/