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]

You'll find a few suggestions here,

http://www.expertctr.com/ch_5.php

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

I would suggest it is an air entrainment problem and not an NPSHa/r consideration. Can stilling baffles or similar be installed in the TES to allow the air a chance to escape before the inlet to the pump/s.

 

[ColonelSanders83]

We don't belive its an air entrainment problem, the TES tank is filled to a static liquid level with the suction diffusers in the tank situated far enough below the liquid level to prent vortices.

The diffusers are designed to handle 17000 gpm (withou air entrainment)in this tank by the tank vendor.

 

[zdas04]

"Gaseous Cavititation"? I don't think so. Cavitation is defined as "the formation and subsequent collapse of condensible vapor bubbles in a liquid". Disolved air would not condense at any temperature where your water is liquid. Dissolved gases do not participate in 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

 

[Artisi]

I was refering to introduced air which becomes entrained within the pumped water, not air entrained via vortices at the inlets.

 

[Artisi]

Thinking further on your problem:

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?

 

[Compositepro]

Disolved gasses are a major factor in cavitation and npsha. The bubbles formed are still mostly water vapor , not air or noncondensible gas.

 

[Artisi]

Sorry, have to disagree, entrained air and cavitation are two completely seperate issues affecting pump performance especially considering that entraining into the pump inlet is a way of softening the effects "real" cavitation.

 

[Compositepro]

Dissolved gas is not the same as entrained air bubbles. The effect of dissolved gasses is the same as increasing the vapor pressure of the liquid.

 

[zdas04]

Compositepro, I have to disagree as well. The key phrase in the definition of "cavitation" is "subsequent collapse". That collapse can only happen with condensation. Yes, dissolved gases can change the boiling point of a liquid, but that is at best a secondary effect that may make cavitation worse, but is unlikely to be the reason that cavitation happens.

David

 

[BigInch]

Disolved gases coming out of solution adversely affect pump efficiency, but do not cause 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/

 

[rmw]

What dissolved gasses coming out of solution in a pump suction is to occupy space in the suction piping and impeller eye that should be occupied by liquid and cause the velocity of the fluid to have to increase to meet the volume demands of the pump. That increase in velocity can then produce the conditions that lead to cavitation. So it isn't cavitation but it might take a pump that was right on the verge of cavitating and push it over the threshold.

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

 

[Artisi]

I don't think that flow increases to maintain performance due to entrained air expanding in the impeller eye, although you could have a situation where an air pocket in the approach pipe system could cause a local area to be subjected to cavitation resulting from a velocity increase.

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.

 

[BigInch]

Velocity remains appx. the same at the expense of liquid flow.

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

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."

 

[BigInch]

That's quite a bit different than entrained air. The liquid has already been cavitated by the sharp edge. Precavitation, if you will.

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

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:

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.

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.

 

[Artisi]

Same question I posed way-back re any changes to the inlet pipe work - seems ColonelSanders might be too busy frying chicken to answer.
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.

 

[25362]

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.