Vapour Pressure = Rated Suction pressure of the pump 2

[SENGUTTUVAN]

In one of the Process data sheet for a closed drain drum vertical pump - Amine service ( handling water ( case 1 ) and Hydrocarbon ( case 2), it has been indicated that the vapour pressure of the Hydrocarbon is 1.23 kg/cm2 (a) and the suction pressure ( rated) is 0.2 kg/cm2 (g). If both these pressures are same , how will be the pump performing? Is there any mistake in the spec? The Pump capacity is 10 m3/hr and the diff pressure is 81 Metres head. The temperature of the Hydrocarbon is 41deg C. Another question is, for this application , the spec indicates VS4 type pump whereas one of the pump vendor has selected Vs1 type pump. Which is better. Thanks to clarify.

 

[MortenA]

I don't know if your numbers are correct - but if the liquid vapour pressure is the same as the NPSHr - then you are in trouble because some pressure loss must be expected from the source to the pump. Unless you also have a elevation difference (including level in suction vessel) that you have not told us about.

The temperature as such is of no importance unless that referenced vapour pressure is measured at a different temperature.

Don't wast any more time here - set up your NPSHa calculation and compare it to your NPSHr value!

Best regards

Morten

 

[SENGUTTUVAN]

Thank U mortenA for your reply. The figures are as given in the Process data sheet. The NPSHA as given in the data sheet as Flooded. The NPSHR as asked by the pump manufacturer is 2.3 M. The NPSH Margin is more than 1 Metre. My query is , how can we select a pump for a rated Suction pressure of 0.2 kg/cm2g which is equal to the vapour pressure of the pumping fluid. Will it not lead to cavitation? What I am asking may be a fundamental question but missing somewhere. Thanks to clarify.

 

[quark]

The temperature as such is of no importance unless that referenced vapour pressure is measured at a different temperature.

Read that statement carefully. If the vapor pressure is 1.23 kg/cm² at 41⁰C then you have trouble with cavitation, as you expect.

 

[rzrbk]

Many vertical canned pumps have zero NPSH requirment measured from the inlet flange. The depth of the flooded can gives additional NPSH to the impeller eye.

If this is the case you may be OK, but I would still prefer some margin (liquid level above inlet flange) for reliability.

 

[MortenA]

As I said i think theres problems i was trying to tell him to look at the numbers in more details (do the calculation).

My comment about the temperature refers to the fact that we are only given tidbits of information - e.g. the HC vapour pressure is 1.23 kg/cm2 (a) and that the temperature is 41ºC - but not a clear indication that the referenced vapour pressure is measure at the same temperature as the HC actual operating temperature. Should the referenced vapour pressure have been measured (or calculated) at a different temperature then the vapour pressure may be different.

Last rzrbk spotted that the pump may actaully be a submerged pump and then there may be no problem at all if you can submerge it enough.

Best regards

Morten

 

[SENGUTTUVAN]

Thanks MortenA and rzrbk. The vapour pressure is calculated at the same temp of actual oper pressure.What I understand is that since it is vertical submersible, used for draining the drain ( as indicated in the first instance) vessel( as and when it fills up) , i think the suction pressure ( rated ) would have been given for max condition. But this cannot really work out as it will lead to cavitation. As rzrbk pointed out , there should be a min level cut off to the pump for auto stoppage so that sufficient suction head is maintained through out. The ramaining liquid in the vessel boot need to be siphoned out. Am I correct?