npsh(r)??

[bloch2006]

hi friends:

i have got manufacturing recomendation that npsh (r)=1.5m.

so when i install the pump from where i must measure at suction line to provide this no. ????

please clarify me this .if there is article clarify this point i will be appressiate.

best regrds

 

[katmar]

For everything you could ever want to know (and then some) on centrifugal pumps go to

http://www.mcnallyinstitute.com/home-html/Technical_paper_index.html

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Ashereng]

1.5 m NPSHr probably means you need 1.5 m of water column of pressure at your suction to the impeller. For practical purposes, I use at the suction flange, unless I am really close with my NPSHa.

Most pump manufacturers test with water, so I assume the 1.5m of NPSHr is water based.

You may want to confirm.

Have you done a search of Eng-Tips fora? This topic has been discussed in many threads already.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[25362]

In any case NPSH available should exceed NPSH required by an adequate margin to avoid erosive cavitation. A margin that varies as function of rotating speed, cavitation-resistant metallurgy, fluid corrosivity, operating conditions, dissolved gases, and thermo-physical properties of the fluid.
Sulzer Brothers Ltd say:

Cavitation erosion hardly ever occurs when pumping hydrocarbon media.

 

[Ashereng]

25362,

Sulzer is correct. The effect of cavitation between water and HC is different. For whatever reason (my chemistry is not so good), I have been told that HC cavitation effect is less troublesome than that of water.

I think it has either something to do with HC bubbles forming or collapsing that is the difference.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Montemayor]

Ashereng:

You state: "1.5 m NPSHr probably means you need 1.5 m of water column of pressure at your suction to the impeller." I believe the unit of head height is that of the flowing fluid being handled - not that of water (unless, of course, the flowing fluid is water). I don't think the fluid has been identified as water - yet.

 

[bingopin]

Cavitation with a hydrocarbon can be as damaging as cavitation with water but usually isn't. In many cases it is because of pumping a mixture if hydrocarbons that flash at different pressures. With water, all of the material in the low pressure area flashes at once, hence, usually see the more significaant damage with water. As my mentor always said, "hot water can be one of the most difficult liquids to pump".

 

[Ashereng]

Montemayor,

Most pump manufacturers test with water, so I assume the 1.5m of NPSHr is water based.

Yes, you are correct. I am assuming.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[zdas04]

I've seen pump curves generated for specific fluids, but you have to ask for them. Typically, when their literature says 1.5m of NPSH-r, they mean 1.5m of water at 15C (or some other "standard" temperature).

bingopin,
I've heard the same thing and it makes sense. Any crude is a complex mix of stuff with a wide range of boiling points and when the light ends flash, the latent heat of vaporization lowers the temperature slighly and it can be a while (miliseconds sometimes) before the next constiuent flashes. The impact tends to be less.

David

 

[25362]

Cavitation damage is the result of high-velocity liquid jets impinging on solid surfaces as a result of bubbles' implosion.

Since impact forces are proportional to velocity, density and flow rate, I assume hydrocarbons (pure or mixed) would probably be less devastating than water also because of the following:

a. a smaller bubble volume for equal pressure reductions (at saturation) resulting in decreased jet flow rates
b. lower densities
c. probably attenuated velocities due to higher viscosities

Any comment ?

 

[katmar]

a. a smaller bubble volume for equal pressure reductions (at saturation) resulting in decreased jet flow rates

I think this is probably a large part of the reason. For the non-chemical engineers amongst us - the reason for the small bubbles is that hydrocarbons have higher molecular weights than water (in general) and this results in higher vapor densities. On the other hand, hydrocarbons have lower latent heats of vaporisation which would result in more vapor being generated i.e. resulting in bigger bubbles.

I believe David has highlighted the major reason for hydrocarbons being less prone to cavitation i.e. because the liquid contains more than one component the collapse of the bubble does not occur all at once at a single temperature or pressure - it will contract relatively slowly over a range of tremperature or pressure.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[25362]

For what it's worth, the speed of sound waves in liquid hydrocarbons drops with increasing temperatures. In water it increases. Wonder whether this also applies to shock waves.

 

[WebWizz]

If you want to calculate NPSH-available for an actual application, there is a free calculator routine at:

http://www.engineeringpage.com

choose pumps > pump condition

Wim

 

[stanier]

Cavitation and the Centrifugal Pump by Grist covers the subject. Invest in this book.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au