CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
(OP)
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.
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.





RE: CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
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
RE: CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
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.
RE: CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
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....
RE: CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
RE: CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
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.
RE: CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
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
RE: CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP
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 ?
RE: CAVITATION: NPSHR AND MINIMUM SUBMERGENCE IN VS4 PUMP