What is vapor lock? 3

[dobit]

My co-workers mentioned this vapor lock twice and I've never heard of it. The first time was when condensate water from reactor jacket could not flow through plate & frame heat exchanger. Second time was we were pumping liquid propane into a reactor (temp = 125F) using positive dispalcement pump. The flow of propane stopped after few minutes.

Can someone explain what VAPOR LOCK is? Thanks,

Dabit

 

[StoneCold]

Vapor locking is a common place term that probably means something different for every instance it is used. Basically it means that vapor is somehow blocking the flow of fluid. Sometimes it is in a pump as your example where the check valves are not seating good enough for the pump to pump vapor, it will just surge back and forth and not actually pump anything until you can force some liquid through the pump to solve the problem.

In the heat exchanger example I would guess that you had air (vapor) trying to flow back up the inlet to the exhanger while liquid was trying to flow down, and you got "vapor locked"

Clear as mud

Regards
Stonecold

 

[25362]

In a gasoline engine: interruption of the flow of fuel caused by the formation of vapor or gas bubbles in the fuel-feeding system.

 

[Artisi]

Vapour lock is just a term to describe the phenomenon of air or gas taking the place of the liquid in a particular part of a system which causes flow to stop. The 2 previous posting give good examples.

Naresuan University
Phitsanulok
Thailand

 

[dobit]

Thank you guys,

 

[sshep]

I gained insight into a form of vapor lock and other interesting phenomenon by syphoning liquid from various items (aquarium, water beds, gas tanks, etc) using a hose of clear vinyl tubing.

When the hose has seperated liquid and vapor as a result of high and low pockets, the hydraulics are significantly different from a liquid full hose. If you have two liquid pockets, the amount of effort to suction lift (or blow) is doubled- when the first liquid pocket lifts 6"wc , the second pocket also lifts 6"wc. A few such pockets and it becomes impossible to start a syphon (or blow out a hose) by mouth- the hose is vapor locked. Routing of the hose to avoid vapor pockets is key to success.

In a plant you never get to see these things. Imagine how easy our job would be if steel were transparent.

best wishes, sshep

 

[pt1000]

In the process industrie "Vapor Lock" and "Cavitation" are well known. I've seen the replies and think the most common cause is not mentioned.
When pumping a fluid, both can occur.

Vapor Lock can happen when the flow is to low through the pump and the fluid reaches its boilingpoint. This is because the energy created by the pump itsself will heat up the product which starts to boil. A hot pumphousing, no flow and an indication of pressure which is mostly zero are the result. To prevent this, create more flow (via a by-pass or a pipe back to the tank) or less rpm's using a frequency controler. Lowering the temperature will help a bit but not for long.

Cavitation happens when there is to little NPSH
(sunctionpressure) inside the pump and the pressure drops to the boilingpoint. Small vaporbubbles will be created on the impellor, imploding where the pressure is higher again. It will give a rattling noise and a vibrating indication of pressure. Thing to do is to create more NPSH by opening sunctionvalves or running with a higher level or pressure on the sunctiontank. Lowering the producttemperature will allso work. (Closing the dischargevalve a bit will help because the pressure inside the pump rises but that's not the right way to do it)

Both Vapor Lock and cavitation will damage the pump in the end so precautions should be taken to prevent it.
Forgive my misspells, English is not my 1st language.

 

[zdas04]

I think the first response from StoneCold was right on the money--vapor lock is something different in each application.

I've seen it very clearly in water pipelines in hilly country. From monometer theory, we can tell that a line full of water would have the same pressure at a common elevation on either side of a hill, if the service pressure is 30 psi and the line is full then the pump discharge pressure will be 30 psi plus friction drop (the height of the hill is irrelevent). The "pressure rebound" on the downside of the hill relies on the line being full of liquid. If there is a gas bubble at the top of the hill then you have to overcome the hydrostatic head on the upside (a 1000 ft hill would need 431 psi pump discharge to top the hill, even if the line is flowing into an atmospheric tank at the bottom of the other side).

David