2 replies to this topic

[Santoshp9]

I have cooling water pump pumping 1400m³/hr@3.5kg/cm2.,many time we observed vapor locks in discharge line.
Can someone tell me why vapor locks takes place in cooling water discharge line .

[Art Montemayor]

Santos:

Vapor lock is defined and understood by me as follows: “a vapor formation within a liquid fluid that is flowing and, because of its compressibility, either causes the flow to stop, to decrease, or to fluctuate.”

Are you observing the cessation, decrease, or fluctuation of cooling water in the pump’s discharge line? What is the temperature of your cooling water in the discharge line? If your cooling water is coming from a cooling water tower (which would be the normal installation), then it virtually impossible for the cooling water to “vapor lock” since the water would have to be at least at a temperature of 147.5 ^oC before it could vaporize at a pressure of 3.5 kg/cm²(g).

How is it that you have “observed” the formation of vapor lock in the discharge line? No one, to my knowledge has ever seen a vapor lock – except perhaps for those operating with glass piping. Vapor lock is not observed – it is usually deduced.

You might have air or non-condensables trapped in your discharge line – especially if you have high points in your system that might be trapping air and forming a barrier for liquid flow. You should be venting all high points in your CW system. This is standard startup practice in order to ensure that you have a 100% liquid full piping system. That is a basic requirement for all liquid piping networks.

It is very possible for vapor lock to occur in a pump’s suction line. That is the reason we have to have a NPSH_A that is larger than the pump’s NPSH_R. But it should not be possible to form a vapor lock in the pump’s discharge line.

[breizh]

As explained by Art , you don't have vapor locked but probably air carried over . You should check the way the pump is started up ( at site ) , if from control room that may happen . Make sure the suction is flooded , air released prior to start .I had similar experience with cooling water pumps.

Hope this helps

Breizh