March pump losing prime when running pump wide open
 

So I have a march 809HS with chugger stainless head with the bigger march impeller. I had a problem on my last brew day when I was pumping wort from my BK through my plate chiller. I kept losing prime and could only have valve about 3/4 open.

So tonight i did experiment with just the pump. I filled my keggle with about 6 gal of room temperature water. I hooked up my 3/4" hose (1/2" male npt camlocks) from my BK to pump and 3/4" hose (1/2" male npt camlock) to the output of the pump. I could not open the valve on the output side of the pump to 100% open. I could only get it about 85 % - 90% open. Flow rate at this setting is about 5 gpm. Now if put at 1/2" hose (1/2" male npt camlock) on the output side of the pump (keeping the 3/4" hose on the input side)I can run the pump with the valve on the output 100% open.

Anyone have some incite into why I lose prime when using 3/4" hose on the output side but have no issue when using 1/2" hose on the output side?

Thanks
Tim

Maybe there is a problem with your camlocks, or the connections between tubes and vessels somewhere so that air is making it into your tubing? If you get a small air leak it can ruin your pumps' priming. Try eliminating extra components ( ie just use tubing, worm clamps, and the pump) and see what happens. Post up what happens.

Also, make sure there is no restriction on the input side of the pump. For example, the valve on the input kettle should have a sufficient diameter to provide all that wort/water without restriction. If you have multiple kettles that are different you could try using a different kettle as the input vessel.

Also, the pump needs to be below the input kettle by at least a few inches.

The 1/2" hose on the output will have more resistance to flow than the 3/4" and this effect is the same as if you partially closed the flow control valve on the pump output. It sounds like your pump is cavitating which is causing it to lose prime. I suspect your dip tube inside the kettle is excessively restrictive and this is causing the cavitation. What is the ID of the dip tube? I think that may reveal something.

FWIW, IMO for optimum pump performance it's best to use large ID hoses etc and keep them as short as conveniently possible. This is most important on the suction side of the pump.

Catt22: I have 1/2" copper diptub on the inside of the BK. Its shown in picture below except I don't use the bazooka screen anymore. Other than the 1/2" diptube is smaller than my 3/4" hose I don't see how the diptube is "excessively restricted".


http://www.homebrewtalk.com/gallery/...m/image003.jpg

You might have better luck by matching the ID of the input side tubing to the ID of your diptube. You could see if the diptube was the problem by just sticking the end of the tubing you connect to it into a bucket of water, and start pumping with that.

The 1/2" dip tube is plenty large enough. I was thinking you might have had a 3/8" OD (1/4" ID) tube installed. That eliminates one prime suspect. Now then, check to be sure that nothing is obstructing the flow on the suction side. Be sure the dip tube, valve, hoses and pump head are free of any hop debris etc. Particulates from whatever can sometimes cause a blockage in the pump head, so I would remove the pump head take a look inside.

One last thought. Pumping near boiling wort can sometimes cause cavitation. Vapor pockets can form when the pump suction is applied. IOW, it starts boiling again under the reduced pressure even though the burner has been shut down. The keg skirt and burner will remain very hot long after flame out and continue to heat the wort for awhile. I spray the skirt down with water to help ease the cooling and pumping process. I have a floor drain in my brewing area, so that's no problem to do.

rocky I am with cat22. For whatever reason, could be hose length, could be your diptube, etc., your output flow rate exceeds your input flow rate to the pump.

Why are you using 3/4" hoses if all your connections are 1/2" ID?

the pump is designed to be used with 1/2" line...by going larger you are allowing the pump to try and exceed its rated flow rates because there is less resistance on the outlet side. That puts a larger stress load on the mag drive and you could de-couple it.
Or....you could have a slightly less then normal magnetized impeller......when they run the magnetizer here they check every 10th or so piece. Most times they are way stronger then what we require...but every so often we do get a weak one come out. If you want to send it to me I can test it for torque. If not then just use 1/2" line and you probably will never have any problems.

-Walter

Thanks for the input Walter, it is much appreciated. I will try my next batch with the 1/2" line and if I still have issue (I don't think I will) I will send you the impeller.

Tim