Air Eliminator Causes Cavitation? 1

[DavidMBrown]

Pumping Jet-A fuel from one storgae tank to another,
we have the following setup:

Tank ->
Floating Suction ->
Check valve ->
Strainer ->
Positive Displacement Pump ->
Air Eliminator (NEW) ->
Filter/Separator ->
Relaxation Chamber ->
Meter ->
Tank

Everything has been working fine, but the owner insisted we add a bulk air eliminator, which we installed immediately after the pump. Now, when the discharging tank fuel level drops below a certain value we get cavitation in the pump.

When the cavitation happens, the vacuum at the pump inlet will have been holding steady at 3"-5" Hg, and then will very quickly jump up to over 10" Hg. This does not indicate normal conditions because the vacuum should increase linearly with the decreasing fuel level in the discharging tank. Do you agree?

Does anyone have any experience with any situation where changes on the pressure side of a positive displacement pump caused problems on the suction side?

 

[hacksaw]

you may be getting a lot more air in the system than you thought.

how is the air separator piped and why is it upstream of the relaxation chamber?

 

[DavidMBrown]

The air eliminator is a 30-gallon pressure vessel with two floats. One at mid-height opens the discharge valve when it gets submerged by fuel. Then second, near the top, closes the air vent at the top when it gets submerged.

We will have run up to 8000 gallons through the system when the problem occurs, so I don't think it's related to air. For example, we might empty several compartments of a transport, each time getting a slug of air, and not get the problem. Or, we might run a similar volume of fuel from one tank to another, with no air entrainment at all. By this time the eliminator should be in steady state.

The eliminator is upstream of the filter/separator and relax chamber to minimize the chance of fires in the filter. Sometimes, though usually for much larger systems, compressing air and vapors in an empty filter can cause this problem.

 

[danthemeterman]

What is the history of the pump, and does it have a built in internal relief. also does the system include an external bypass?

 

[DavidMBrown]

The pump has an internal bypass. It also had an external bypass, but the owner's engineer insisted we remove it. The pressure settings on the two relief valves were not close so the second bypass was not causing a problem. The water slug valve was doubling as a raft of flow control valve, causing the pressure at the pump to be approximately 60 psi. The internal bypass was probably cracked open most of the time.

 

[hacksaw]

internal recycles are notoriously unreliable. you might check it next time you are down

 

[danthemeterman]

Hacksaw is right. The problem with no ex. bypass is when your control valve resticts or stages you need an ex. bypass to remove cavitation. There really is no such thing as internal bypass, that is called internal relief, it is there only to protect the integrity of the pump. In most F. O. the internal relief will open at anywhere from +30 - + 75% of the differential pressure, while the automatic ex. bypass will open @ +25 % of the operating pressure. I hope this helps somewhat in the future, it will help you maintain a lower cost of maintanance to your pumps. as far as your suction have you figure your NPSH for your model of pump.

 

[DavidMBrown]

Thanks for your help, guys. What we did was install a variable speed drive so the pump turns at the correct speed and we don't need the rate of flow control valve any more. This also lowers the back pressure on the pump significantly, down to about 30 psi. The cavitation problem has gone away. If it comes back again we will consider putting the external bypass back in.