CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP 3

[vjr0512]

Dear all,
We have selected the pump API type VS4 for Pumping out the condensate collected by gravity flow in the Condensate Drum from process heat exchanger. Condensate drum is continuously vented to atmosphere through lock open valve. The Condensate temperature is 100 dec C . Pump has to start when the drum level is high and stop when level reaches the low low level. At low low level there will be around 500mm of water above the pump impeller centre line. The vendor is confirming that as the pump suction is flooded there is no chance of cavitation so long as the min submergence requirement of the 350mm above the impeller centre line is available. Incidentally the pump curve furnished by the vendor indicates a NPSHr of 2.2m at the pump operating point. Pl clarify whether this pump can work with out cavitation as it is having only liquid level sufficient to meet the minimum submergence requirement when operating near to the low level in the vessel.

 

[Midan]

I understand from the conditions you state that the condensate is stable at atmospheric pressure, if this is the case then the vapour pressure of it is lower than the atmospheric and hence the NPSHA calculation shall be:

NPSHA = (atm. Press. - vapour Press.)X 10 / S.G + Min. Submerge

say vapour pressure = 0.7 bara & Specific Gravity = 0.8

then;
NPSHA = (1-0.7) X 10 / 0.8 + 0.5 = 4.25 m

anyway; you can consult the vendor in this calculation and ask him to prove that NPSHA is higher than NPSHR

cheers

 

[BigInch]

Be sure to use your vapor pressure for your condensate at 100C. NPSHr is given assuming cold water and 100C condensate is not the same. Is the vendor giving NPSHr for water or hot condensate? If it's for cold water, you may need to take a more conservative approach.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[Pumpsonly]

The actual condensate temp.is only just slightly below the boiling point at the barometric pressure of the atm.
With the drum vented to atm. the NPSHA is
only the liquid height above the impeller center line.

 

[DubMac]

Minimum submergence requirements should not be confused with NPSH required.

The 350mm figure is the submergence required to prevent air vortexing into the pump suction. It is not measured from the impeller centerline as NPSH values are, but from the bottom of the suction bell or suction inlet.

Listen to your pump performance curve when it says you need 2.2m of NPSH....

 

[1gibson]

VS4 pumps can be tricky machines. Make sure you (immediately) review the maintenance requirements. Are the bearings fresh water lubricated, forced process fluid, grease, oil?

 

[Midan]

To Pumpsonly;

I just need to know how the condensate is at slightly below the boiling point and hence its vapour pressure is almost atmosphericthe while vessel is vented .
In this case I assume that the condensate shall evaporate and then the vessel should not be vented to atmosphere.

 

[rmw]

There have been some strange things said in this thread. DubMac got it right when he said that minimum submergence and NPSHr are two different things. I'll add entirely different. Based on your conditions, the only time you will have to worry about minimum submergence is should you ever be pumping atmospheric temperature water. It won't be your worry at conditions.

When your water is at (or nearly at) 100C, the NPSHr characteristic of the pump governs in your case and if the vendor stated value is accurate, at anything less than 2.2M of water level over the pump suction bell, your pump will be cavitating. I would not expect it to reach 350mm if the real NPSHr is 2.2M. It would vapor lock before you got there IMO.

The NPSH calculation takes temperature into account when it compares your fluid's vapor pressure to the pressure over the surface of the fluid. When you do your NPSHa calculation (to determine if it meets your NPSHr), you will see that your vapor pressure and atmospheric pressure are nearly equal (would be equal at 100C at sea level) and the only thing you have left is head above the suction (since you have no flow losses associated with suction piping as is true in the case of other types of pumps.

If your flow is variable, be sure to pay attention to the NPSHr when your pump runs out on its curve. Some pumps don't operate at just one point on their curve, and NPSHr sensitive pumps get in trouble out at high flows.

Also... Vertical pumps are sensitive to pit configuration, so check with your vendor to see what is required with respect to the flow channel for the fluid as it approaches the pump. Many pumps with improper provision will vortex at liquid levels higher than minimum submergence due to the swirl pattern set up in the pit by the pump impeller.

Back to your original question, either (1) your pump vendor is unaware that your actual conditons are at or near 100C or (2) he/she is woefully ignorant about pumps. (If you are talking to an agent and not a factory rep, watch that). Just for your own satisfaction, do a NPSHa calculation with water at atmospheric temperature and pressure. THEN, and only then, I suspect that minimum submergence will govern and that you will have adequate NPSHa. I'm guessing, but if you want to know, you can do the calculations.

rmw

rmw

 

[GPRnD]

It is incorrect to say that if you have NPSHa of 2.2m, that your pump will not be cavitating.

NPSHr is defined by API and HI as the point at which a 3% head drop has occurred. At that point you have a large and extensive cavitation field within the pump suction.

I am assuming you don't want to avoid cavitation entirely as this will require a NPSHa/NPSHr ratio > 3 ?

 

[SeanB]

Using a submerged pump for pumping hot condensate is not a good fit for this pump. With an NPSHr or 2.2 m your low level cut out will have to be above this to prevent the pump from cavitating. Typically, for this sort of application the condenate drum would be elevated with your pump sitting at grade.