6 replies to this topic

[frans6]

Hi,

I am reading up on cavitation and its damage done to pump impeller(pitting etc).

On the subject of entrained air(or nitrogen), there seems to be 2 schools of thought of whether it cause damage to pump impeller or not.

http://www.pump-zone...d-failures.html

http://www.cheresour...ugalpumpsb1.pdf

If we follow the mechanism of process bubble imploding and causing a microjet to hit on the impeller, resulting in pitting, then entrained air or nitrogen should not cause any pitting(or erosion) damage to the impeller?

This is because the dissolved air or nitrogen will not implode and should not cause any material damage to the impeller?

I would also like to ask if the pump sucks in foam that was formed as a result of the process liquid being agitated, will pitting damaged also result on the impeller? In our case, the process liquid contains nitrogen and the vessel where the process liquid is sent to the pump is also sealed with nitrogen.
Our pump impeller was badly pitted(sorry I left the pics in my office), and the vendor explained it was due to entrained nitrogen.

Pls comment.

[Art Montemayor]

fran:

Your statement: "This is because the dissolved air or nitrogen will not implode and should not cause any material damage to the impeller" is, in my opinion, precisely correct. There is a gross, semantical error in the Pump Zone article by Ross Mackay(which is addressed and critiqued in the reader response section) regarding the basic difference between having CONDENSABLE bubbles in the suction fluid and NON-CONDENSABLE bubbles in the suction fluid (such as air, nitrogen, etc.). The former can cause classical "cavitation" and resulting damage. The latter can cause air-binding (or loss of prime).

This whole issue reduces itself to simple basics: a pump is designed to pump LIQUIDS (or non-compressible fluids) and NOT VAPORS, GASES, OR NON-CONDENSABLES. Compressors are designed to transport the latter. That is why you should expect problems and troubles if you insist or try to "pump" a vapor or gas with a pump.

Foam is a practical compressible fluid. This is fairly obvious and should be well-understood and accepted. Therefore, it will be practically improbable to have a pump transport this type of fluid. You will loose pump prime if you try it. I seriously doubt you would suffer any pump impeller damage. You might damage or harm your seals and bearings, but trying to pump air or gas bubbles will not damage the metal surfaces of a pump. The pump will just spin harmlessly without developing a head. Your pump vendor is trying to pull your leg and see if you fall for the story he's fabricated. You are better off looking for the true culprit of the damage in other areas.

[katmar]

For good, hands-on, practical advice on pumps I always turn to the McNally Institute. See
http://www.mcnallyin...l#Air ingestion
This confirms that air entrainment can sound like cavitation, but is unlikely to result in physical damage.

The way I have viewed this phenomenon is that when a bubble generated by lowering the pressure below the vapor pressure of the liquid re-collapses it collapses down to nothing, but an air bubble that grows as it goes through the low pressure section of the pump will only collapse back to some smaller, but still finite, size as the pressure increases. This means that the liquid rushing into the collapsing bubble is cushioned like a shock absorber when a non-condensible gas like air is present.

[agorag]

You should've started with stating what is the NPSHR & NPSHA, details of the liquid (atleast say hydrocarbon or aqueous), the % of dissolved gases, the pumping temperature & the vapor pressure at this PT. All else can come later.

In a centrifugal (or even a positive displacement) pump, maximum pressure drop takes place as the fluid passes through the suction zone & enters the impeller eye. And if the pressure is lower than the VP at that PT, it WILL cavitate. Cavitation - in common terms - is boiling of liquids at a lower temperature, due to drop in pressure. Irrespective of what gas is dissolved, cavitation DOES leave cavities on the low pressure zones of the pump - impeller & casing.

Three factors affect the magnitude of the pits - extent of "boiling" of the liquid ie., extent of departure of actual pressure at impeller eye to the VP at PT, SG of the liquid - cavitation with aqeous liquids is higher than that with hydrocarbons and hardness of the impeller (or casing) material at the suction zone.

[frans6]

Hi Art and katmar,

Thanks for your confirmation on this matter as people in my company have different theories on what happen to the impeller which I think is wrong.
For example, entrained nitrogen bubbles being swinged by centrifugal force and hit the impeller, causing pitting.
The actual cause is still unknown.

I will "consult" the vendor on this matter as we are planning to buy a new impeller from them soon!

Hi agorag,

Process liquid is 50:50 mixture of NaNO2 and KNO3(used as coolant)
Temperature is ~300degC
Density is ~1.9ton/m3
I do not know the vapour pressure, but it does not seem volatile to me compared the hydrocarbons or even water.
NPSHA is 7m and NPSHr at design flow is ~5m. However, flowrate will drop steadily from design flow over a period of 3 years.

Hi All,

Another question is that after reading some links on the internet and books, I came across NPSH reduction chart which is sometimes used for hydrocarbons NPSHr reduction as specific volume of these liquid hydrocarbons are higher than that of cold water.

Thus, when lqiuid hydrocarbons flash to vapour, the margin between liquid and vapour specific volume is lower than that of cold water.
The % increase in volume when hydrocarbon flash is lower than that of cold water, resulting in lower requirement for NPSH(and consequently less damage).
I think of we define NPSHr as when the TDH drop by 3%, then we can see why hydrocarbons need correction from vendor pump curve.
Pls correct me if I am wrong, I read it all from the internet and based on my own understanding.

http://www.mcnallyin...tion_chart.html

The question to ask is there such a thing as NPSH addition?
If the margin between the specific volume of the process liquid and vapour is larger than that of cold water,do we need to
add it to NPSHr as specified on the pump curve?
Thus, infact, the required NPSH could be larger than that specified on the pump curve.

I am starting to think if this is a possible reason for pitting on the impeller.

Pls comment

[katmar]

The reason for the NPSHr reduction when working with hydrocarbons (as compared with water) is the generally higher molecular weight of the hydrocarbon. Because of the higher MW the vapour generated is more dense and consequently for each kg or lb flashed a smaller volume of vapour results. You have to offset against this the generally lower latent heat of vaporisation of hydrocarbons (again, compared with water) which would result in more vapour being generated.

Perhaps you would need to consider adding to the NPSHr if you were pumping liquid hydrogen or helium (i.e. low MW materials), but that would be such a specialised application that you would be using a pump designed for that specific duty.

Your pumping of this salt mixture at 300 C is also a rather specialised application and you should be wary of general rules of thumb in that case. Perhaps the collapsing of air bubbles with such a dense liquid does lead to cavitation. I'm afraid we are now outside my zone of experience and I suggest you get help from someone who has relevant experience. If your pump supplier has supplied into this application before your best option may be to go with his advice.

[agorag]

I presume it is for atomic reactors. And the SG is almost twice that of water. So, any implosions of bubbles due to cavitation will have a very strong effect on the pump's low pressure zones, as compared to water & HC.
As for NPSH addition, I dont know of any such thing. From fundamental principles, since pumps are tested with water at manufacturer's workshop and the specification sheet gives NPSHR in terms of head of water, so long the liquid pumped in the process is lighter than water and NPSHA is adequately higher, it is not a problem. By 'adequate', I mean 1M for small pumps.
Since NPSHA is 7m and NPSHr at rated flow is ~5m in this case, the margin appears to be sufficient for a 'standard' pump. However, if it happens to be a high energy pump (refer Alan Budris' Pump Users' handbook), the NPSH margin may not be sufficient and rather than margin, we'll have to discuss in terms of margin ratio.
Such cooling solutionsmay not be volatile at ambient conditions, but at 300C, I'm not sure. Vapor pressure must be considered at PT. Actually you must be able to find it from the pump specification sheet or your process manual.
To start with, why dont you hear the pump casing with a hearing stick when it is running for any cavitation or abnormal sounds? Vibration spectrum analysis will also help.

It is best to discuss everything with all data in place - Pump drawing, process specifications...
Such piecemeal discussions are usually counterproductive.