Air Elimination from Suction Piping

[dultmeier]

We have an existing 6x4 end suction centrifugal pump unloading rail cars of fertilizer. We are having problems with air getting into the suction pipe as rail cars getting emptied (unloading multiple rail cars at once with many 4" hoses feeding into an 8" dia. suction pipe). NPSHA is not our problem but the air is causing problems. We have installed a 3/4" air "bleed" valve on the discharge side of the pump (near the pump). Is 3/4" just too small for this air bleed valve? Also, would a float or other style air eliminator located at the highest point in the suction piping help? I want to be sure we use the correct style air eliminator so we don't bring more air in on the suction side.

Tom Hansen

 

[pennpiper]

You do not give us enough information to offer answers.
Please describe the configuration of the suction piping.
Are the cars top unloading or bottom unloading, etc.?
You say you have an 8" suction pipe but you do not give the size of the pump suction nozzle.

 

[BigInch]

If you want to give more info, go ahead and do it, but on what you've said already....

You've got a suction high point and air in the suction line. Suction lines are usually larger, hence slower velocity and easier to trap bubbles, so I'm wondering why the air is not being removed in the suction line right now? After the pump, its likely to be moving fast and possibly well entrained. Discharge doesn't seem like the best place.

Treat this as preliminary, if you plan on giving more info, but in the meantime you might consider doing this...

If you know the usual air to liquid ratio, size the valve for that air flowrate and install it at the high point of the suction line.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

Check the folowing link which addressed a similar problem sometime back.

http://www.eng-tips.com/viewthread.cfm?qid=181062

 

[dultmeier]

Sorry for the delay in getting more information. The pump unit is 6" suction. The suction pipe is 8" dia. We run approx. 15 each 4" hoses from the suction pipe to the bottom fitting of rail cars at one time. As some rail cars get emptied before others (operators don't get valves closed right away when a rail car is emptied) we get air into the suction pipe and onto the pump. We are using a small 3/4" ball valve to "bleed" air out on the discharge side of the pump. Not using any type of air eliminator on the suction side of the pump currently. We are trying to pump approx. 600 GPM but having trouble because of the air getting into the pump. The 3/4" manual "bleed" valve may be too small so I am thinking of increasing its size to 1.5" or 2" and then seeing if we still need an air eliminator on the suction pipe. Any additional thoughts would be much appreciated. This is a great site!

Tom Hansen

 

[4Pipes]

I assume the fluid is non-hazardous from your description. If it were a relatively low viscosity you might be able to use a suction drum of some form. Maybe a large diameter section of pipe. This would allow the air to separate. The dropping liquid level would be detected for operator warning. If the pressure in the "suction drum" was positive your could have a float type air eliminator. If it wasn't you would need some form of vacuum system or eductor to reduce the pressure in the "suction drum" to get the air out.

(An eductor driven by compressed air, water or steam driven or a mech vac pump if you have one. Maybe a small self priming centrifgul could be run in parallel as a jockey to your main pump but connected to the "air space" of the "suction drum". If its got enough suction lift, it'll pull the air out when there is air present and give you a few more gpm when there isn't. The power drawn by or the increased noise of the self priming jockey pump will warn the operator. If the self primer has a lower head than the main pumps, then you could connect the self primer to a second positive pressure "separator" with float type air vent and dump the clear flow back into the main suction line through a globe valve to give it a bit of back pressure).

A vacuum system could be driven by same level switch used to warn the operator.

Let us know how you got on.

PS. I have absolutely no operating experience of such a system so please excuse the whacky ideas.

 

[Artisi]

You will need some form of air elimination on the inlet side. For a trouble free system I would suggest a re-think and either replace the unit you have with a vacumm-primed pump set or convert your existing pump to vacumm primed if at all possible.

 

[25362]

It seems to me the amount of entrained air from emptied cars would be variable and limited by the sealing effect of the liquid downloaded from the various rail cars. Thus, IMHO, the preferred location of the vent valve (piped away to a safe point) would be at the top of the 8-in suction manifold closest to the pump.

 

[vpl]

dultmeier

I don't believe increasing the vent size on the pump outlet (discharge) will do you much good. As long as the inlet side has large pockets of air, the pump is going to have problems.

I'd recommend installing a loop seal (U-bend) on your suction pipe with your 3/4" vent at the top. Then, if the 3/4" isn't big enough, try a different valve. But only after you've gone to the "right" side of the pump.

Patricia Lougheed

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[dultmeier]

Thank you. I really want to stay with the current pump
so does anyone know of some good vacuum system
suppliers?

Tom Hansen

 

[Artisi]

Where on the planet are you? - this will make it easier to give you a suppliers name, otherwise see the link I posted earlier.

http://www.eng-tips.com/viewthread.cfm?qid=181062