Feedwater Pump Air Lock?

[daysaver1]

Feedwater to our 40' VFT boiler is via a 3/4" diameter piston with 2" stroke running at half engine rpm. With the 1/4" bypass needle valve cracked this pump has worked wonderfully for many hours of steaming. However after doing a little re-routing of the plumbing from pump to boiler, and cleaning the incoming filter and related appurtences, yesterday the pump, although seeming to be operating, didn't put any water in the boiler. Before heading out, I opened the 1/4" cock on the pump until water started running to ensure the pump and feed lines were wet. With 100% hand pump, we were worn out by the time we returned to the dock. Friends have talked about air lock. How does one know if this is happening, and if it is, what would one do about it?

[Mike Rometer]

It does sound likely. You need to have a bleed screw on a high point of the line to let the air out, while the pump fills the line. If you've a joint you can crack, near the top, that will prove it.

[DetroiTug]

That is why it is good to have a relief valve on the pump discharge set at 300psi or more. One can pull the relief valve to bleed air and see the pump working. Also it is good to have a pressure gauge at around 1000 psi for ram pumps to also indicate the pump is working. Lower pressure gauges will fail because of the wild fluctuations.

Sounds as though one of your check valves at the pump is possibly not seating.

-Ron

[gondolier88]

It's not just a matter of bleeding the highest point- if you have a flexible line anywhere on the feed line, or you have put a vertical dog-leg in the line then you could have a created an airlock.

Make sure the highest point on the line is the check valve on the boiler, make sure that the line has as little amount of right angled fittings on, bend pipe where possible. Make sure the pipe from the pump to the boiler has a rise all the way, it cannot rise and then fall as this will create an airlock.

Greg

[Maltelec]

We used to find the feed pump would air lock every time with a horizontal output (meaning the piston was vertical). Once we changed the output to be vertical, so the piston and valves would always be under water with any air moving away from the outlet valves, we never had a problem.

I always think an air lock in the pipework isn't a problem, but an air lock in the piston area and around the valve area would cause a problem.

[daysaver1]

Thank you all for your comments. Friday trials found our pump not pumping, so the forum query. A repair of the things I thought might be the fix saw us pumping by hand again Saturday without a solution. Saturday night I realized part of my re-plumbing included a "T" on the inlet side which will allow me to re-install our steam injector and the temporary plug was leaking air. Hence every input stroke of the piston brought more air than water which then merely compressed in the pump cylinder against the check when the piston came forward. Fixing the air leak last night followed by a four hour ride today (Sunday) was a real pleasure. All this so we can tow our boat 900 miles starting Wednesday to the NWSS meet in Anacortes, Wa this weekend. Once again the world is turning in greased grooves and life is good.

[Lopez Mike]

I have installed a squeeze bulb pump from an outboard motor gas tank in the suction line to my feed water pump. Of course that wouldn't help with an air leak like yours in the input line but it sure helps flush air out of the system when refilling the hot well after a layup.

Now I'm waiting to see if the ball checks in that gasoline bulb are stainless steel.

Mike