NPSHa Vertical Turbine Pumps 1

[SweetDewMe]

Before a week ago I had no idea what NPSH was. We didn't cover it in Fluid Mechanics in school (Im a recent ME grad) and I had never worked with pumps before. So I am just now learning about pumps and NPSH. I am comfertable calculating NPSHa for centrifugal pumps but I am confused about calculating it for vertical turbine pumps.

Correct me if Im wrong, but NPSH is basically calculating how close the liquid is to boiling due to low pressure in the intake. With vertical turbine pumps the impellers are submerged so you would never experience low pressures in the intake. Why then is NPSH even calculated for vertical turbine systems? It seems like the major question is if the pump is powerful enough to push the liquid up the piping to the level at where it is discharged and be able to overcome any friction losses in the piping.

OK, so I guess after all that my main question is: is NPSH calculated for vertical turbine pumps and if so, how?

Thanks

 

[rmw]

The NPSHa calculation gives you the resulting head of liquid that must be standing above the inlet of a pump in order to maintain that liquid above its flash point in the low pressure area of the inlet to the impeller. The requiremenet is usually given by the pump manufacturer based on empirical data or actual testing.

This is true whether or not the pump is horizontal or vertical.

Do an advanced search on this forum for NPSH. There have been many many threads devoted to the topic.

Don't feel intimidated by being new. If you do the search you can see that plenty of folks with lots more experience struggle with NPSH also. Ask more questins if you don't find the answers after the search.

rmw

 

[SweetDewMe]

Thanks for the quick response. I did search on here for about 15 minutes and didn't find the answer to my question. If anyone could point me to a thread that discusses this I would appreciate it.

 

[dcasto]

RMW said it all. NPSH is the same for any pump, vertcle, horizontal, split flow. The thing about verticle pumps is that the suction is at the lower end and you cah hve the impellars placed on a shaft far enough down such that the suction flange can require 0 NPSH. The pump is then lower and even underground from the source.

 

[Artisi]

For vertical turbine pumps which usually has the impeller/s below the free standing water level you have to treat this a little differently.

Assume you are at sea level - then the NPSHa is - atmospheric pressure at the water level plus the vertical distance from the water level to the impeller eye.

So roughly speaking if the impeller is positioned 10ft below the water level the NPSHa is approx 44ft - less any vapour pressure, inlet losses etc.

Or more precisely:

NPSHa = Ha - Hvpa (plus or minus Hst)- Hfs

 

[bingopin]

"The NPSHa calculation gives you the resulting head of liquid that must be standing above the inlet of a pump in order to maintain that liquid above its flash point in the low pressure area of the inlet to the impeller. The requiremenet is usually given by the pump manufacturer based on empirical data or actual testing"

Didn't you mean to say NPSHr?

 

[SweetDewMe]

ARTISI,

Thanks for your help! Your explaination is perfect. I also found some more information that supports what you said and it makes sense to me now. If you have an impeller that sticks 10 feet under the surface of the water, the higher pressure down there actually improves your cavitation situation, or makes it less likely to occur than if the pressure wasn't there. This is reflected by the higher NPSHa value.

Thanks guys. This site is great!

 

[BigInch]

Artisi, good work *.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[EdStainless]

The only practical difference with verticals is that is usually harder to not have enough pressure at the intake. But many of us have seen it done. Pump down so that there isn't enough submergence and they will cavatate like any other pump.

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[Artisi]

To be on the safe side, for vetically installed pumps your calculation should be based on the minimum water level to ensure you always have sufficient NPSHa, also the pump inlet must be positioned at a point below the low water level water to ensure that vortices are not created resulting in air being drawn from the surface, this is not to be confused with NPSH as it is completely different consideration.

 

[djack77494]

I think this situation is being overanalyzed. For a free standing vertical turbine pump set into an open-to-the-atmosphere sump, I have never even attempted an NPSH calculation. The liquid is typically subcooled (or at worst is at its bubble point at the surface). There is no suction piping. The pump vendor typically lets you know what SUBMERGENCE is needed, and this takes the place of an NPSH calculation. Just ensure the needed submergence, and follow prudent standards for sump design. While I guess that's somewhat similar to what's been said above, I just hate to see the word "NPSH" used; I'd guess that 99.9% of sump mounted vertical turbine pump applications involve aqueous solutions, and much of the rigorous analysis can be greatly simplified.

 

[Artisi]

Sorry to disagree but, submergence will never take the place of an NPSH calculation which is very simple in an open pit / vertical pump arrangement, it is important to remember that submergence may only be a few feet in some installations.

On very large high flow mixed and axial flow pumps NPSHr can be quite high right of BEP where pumps can run at times of upset in a plant, therefore NPSHa is a critical component of correct pump / sump design.
However, will agree the point with you that having sufficient submergence is usually ok to ensure cavitation free operation.

 

[JJPellin]

In my experience with large vertical pumps in open water sumps, I have seen all of the following:

1. Pumps that cavitated because they had inadequate NPSH available but did meet the OEM requirement for submergence.

2. Pumps that cavitated because they had inadequate submergence but did meet the OEM requirement for NPSH available.

3. Pumps that cavitated despite the fact that they had adequate NPSH available and adequate submergence but had suction recirculation cavitation resulting from pre-rotation of the fluid because of a poorly designed sump or a poorly designed impeller.

All of these are important. Submergence might be the most common cause of problems, but it is not the only cause.

Johnny Pellin

 

[Whyknot]

Johnny is correct. Be sure your sump is designed properly. How the water comes into the sump can be very important. A water fall effect can cause air entrainment in your pumpage causing cavatation.

 

[Artisi]

You shouldn't confuse air entrainment with cavitation as they are 2 different animals.
As a matter of fact, introducing air to the inlet of a cavitating pump is one way to reduce the cavitation effect.

 

[Whyknot]

Air entrainment defines a variety of conditions where the vapor bubbles are already in the liquid before it reaches the pump. When they arrive in the eye of the impeller, exactly the same thing happens as if they were created at that point. In other words, they are subjected to the increasing pressure at the start of the vanes and are then imploded, causing the identical damage as cavitation, and at the same location. This condition can often be a result of pumping fermenting liquids or foaming agents found in a wide variety of industries.It can also be a result of pumping a liquid,such as condensate,that is close to it's boiling point.However, air entrainment is most frequently caused by turbulence in the suction line,or even at the suction source..

 

[Artisi]

Cannot agree that air-entrainment causes the same damage (if any) as cavitation.

From NcNally Institute with some addition (xxx) by myself.

Air ingestion.

---------------- Both vaporization (NPSHa/r problem) and air ingestion (entrainment) have an adverse affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity.

Although air ingestion and vaporization can both occur, they have separate solutions. Air ingestion is not as severe as vaporization and seldom causes damage, but it does lower the capacity of the pump.

 

[djack77494]

If you can accept discussing an aqueous system in an open sump design, then, I would contend, when you satisfy the pump vendor's submergence requirement you will have also met the pump's NPSHr requirement. The only losses are the acceleration of the water into the pump's impeller, and friction losses at the bell. Both are characteristics of the pump itself (at a given point on the curve). That's why I stated that trying to analyze for NPSH is "overkill".

The only adjustment I might make is to alter the manufacturer's recommended minimum submergence to consider any vapor pressure effects if the system was at elevated temperature. Thus, the vendor's recommended submergence based on 60F water should be increased if the water is actually at 160F. The water's vapor pressure in that case would have gone from 0.26 to 4.74 psia, and roughly (4.74-0.26)*2.31 or 10.35 ft.

OK - that's a lot. I hadn't anticipated much change at the start, so I'll retreat a bit and concede that for significant changes in conditions one should look at the NPSH effect.

 

[JJPellin]

This could be an unfortunate byproduct of an attempt to use good standards and hold pump manufactures to the requirements of the standards. We have standards that set limits on suction specific speed and require flow within a certain range from BEP depending on Nss. In order to meet these standards, a pump manufacturer could be encouraged to provide a pump with a higher NPSH(required) in order to get a lower Nss. This could result in a situation where NPSH becomes the constraint rather than submergence. And, as you noted, an increase in water temperature could shift the constraint from one to the other.

To make the situation even worse, this type of pump is often tested in an open sump in relatively cold water with no NPSH test possible. They might test for submergence, but NPSH requirements have to be taken on their word, untested in your specific pump.


Johnny Pellin