Hitachi 300 Gx Air Charger Function

[T_Dog]

We have a 300GX pump that had a defective air line between the suction side (right above the one way check valve) and the diaphragm on the pressure side. Turns out the replacement hose is leaking now. The symptom is that the pump loses prime and there is high pressure air on the suction side when you take the primer cap off. Empty the water out of the air line and tighten it back down, prime it back up, and it runs for a month or so.

Can any of you pump gurus out there explain the operation of this air charger system, especially the diaphragm area, to me? It is baffling to me and rather problematic. If it wasn't there, I think the pump would be much better off.

[T_Dog]

Well, not even the pump shops know how that air charger system works. A local shop recommended replacing the Hitachi with a Lucky Star the next time there is a problem. I took the air line off and used silicone sealer to make sure it won't leak when I re-installed it and so far no problems.

[Pib]

If you are talking a small diameter (about pencil diameter) black/grey flexible PVC hose that runs between the manifould suction/water inlet and to the top of the air control valve, it is not an air line...it's a water line. When your pump turns on an sucks water in, water suction is applied down that pencil size line to the top of the air control valve....all the water/air is sucked out of the top of the valve and it moves upward causing an air suction on the under side of the rubber diaphragm which it turns makes a ball valve open and sucks in a little bit of air form ouside which is then fed into the little pressure tank the pump is setting on...the air in the pressure tank is compressed which provides that energy storage/cushion/spring to continue to push water out of the tank when the pump is not running. This little bit of air fed into the tank by the air control valve replaces the little bit of air that is naturally absorbed within the tank...because within the tank the air is compressed on top of the water....there is nothing separating the air and water to prevent slight/slow air into water absorption....in some more expensive pressure tanks they have a rubber bladder or diaphragm separating the water and air in the pressure tank which negates the need for an air control valve.

Without the air control valve working properly (and it needs that little hose to do that) your pump will become water logged after a few weeks to a month...when it's water logged that means it will be running the great, great majority of the time even when turning on a tap to just get a few ounces of water. Normally, such a pump will have a run-off cycle of approx. 50-50 when running a tap(s)...say run 5 seconds then off 5 seconds....and it also won't kick-on if drawing just a little bit of water...usually up to around a liter of water...then it kicks on (this assumes that liter had almost been drawn down when getting some water a little earlier).

Those little hoses are usually a bead between them and the screw down cap to ensure they don't leak...the bead type connector goes around the hose and when the hose nut is tighten down the bead cause a tight fit between the cap and hose for not leaks. Somchai the repairman replaced my neighbor's hose without using beads and it was leaking again within a week....he had just used a bunch of Teflon tape. He got called back to fix it again and it was pointed out where he errored ...he went and got some proper connectors, installed them, no leaks.

It also sounds like the rubber diaphragm in the air control valve may be ruptured which would allow leakage between your suction side and your high pressure side...loss of some suction...if an underground tank or well is the source of your water I guess you could lose all suction and have to do the priming thing like you have been doing....but if use a above ground tank and your pump is lower than the storage tank you shouldn't lose suction....but a bad air control valve would still cause your pressure tank to become water logged after a few weeks.

[T_Dog]

Pib,

I'll have to noodle on that a while and try to see where the air might go in the ball valve at the bottom. I understand everything else you are saying and that is why it is called an air charger system I guess. Our pump works great until I see little bead of water on the fittings. The hose ends are true compression fittings and the bibs (sliding compression clamps) are indeed there on both ends in addition to the threaded ends. The cycle on and off is always fine so we have a good air charge. When we do lose prime, there is a huge amount of air pressure at the top of the pump on the suction side where you prime it. I appreciate your explanation and it helps.

[Pib]

If were are talking the same thing there would be high pressure where your prime even with a perfectly working pump since that priming area feeds the pump vanes which is all part of the high pressure side. That pressure would only bleed off if the check valve on the suction input was dirty/damaged/missing which would also end up causing you to lose suction unless you had another check valve install in a line external to the pump.

In that same general area were the air control valve hose connects into the suction inlet there should also be a "check valve" assembly within which is a plastic & rubber valve with a spring on top of it which forms the check valve. If the check valve is not working/seat properly I could see where it would allow high pressure backflow which would mess up your prime, cause loss of high pressure making the pump turn on for no reason, and I guess allow high pressure feed back into you air control valve assembly which should only be dealing with the lower pressure on the suction side. I guess the exact symptoms would depend on how large and frequent (or intermittent) the check valve leak is. The spring in the check valve assembly can be removed under certain circumstances like when pumping from a water source over 3M below the pump...but the spring should be installed when the water source is less than 3M below the pump or when the water source is above the tank like in a water storage tank.

P.S. Yea, compression fitting for the air control valve hose was the word on the tip of my tongue but I just couldn't get it out; so I used the term bead type connector.

Edited May 1, 2013 by Pib

[T_Dog]

If were are talking the same thing there would be high pressure where your prime even with a perfectly working pump since that priming area feeds the pump vanes which is all part of the high pressure side. That pressure would only bleed off if the check valve on the suction input was dirty/damaged/missing which would also end up causing you to lose suction unless you had another check valve install in a line external to the pump.

In that same general area were the air control valve hose connects into the suction inlet there should also be a "check valve" assembly within which is a plastic & rubber valve with a spring on top of it which forms the check valve. If the check valve is not working/seat properly I could see where it would allow high pressure backflow which would mess up your prime, cause loss of high pressure making the pump turn on for no reason, and I guess allow high pressure feed back into you air control valve assembly which should only be dealing with the lower pressure on the suction side. I guess the exact symptoms would depend on how large and frequent (or intermittent) the check valve leak is. The spring in the check valve assembly can be removed under certain circumstances like when pumping from a water source over 3M below the pump...but the spring should be installed when the water source is less than 3M below the pump or when the water source is above the tank like in a water storage tank.

P.S. Yea, compression fitting for the air control valve hose was the word on the tip of my tongue but I just couldn't get it out; so I used the term bead type connector.

Okay, that pressure up there might be normal there after all. I am not using the spring on the check valve as we are pulling up a two inch bore so lots of back pressure to keep it closed well. All the fittings are good and leak free. The only time we have problems is when (it seems) the small air charger hose leaks, but I think I have that tamed now.

Taking another look at the air charger system it all makes sense now after your comments. If there is equal pressure on both sides of the diaphragm, the charge valve inside remains closed so no water leaks out. Once the pump spins and suction appears at the top, the diaphragm moves opening the charge valve which lets a tiny amount of air into the suction side. The air goes up the tube to the top and then through the pump into the top of the tank where it maintains air pressure.

I am thinking that our loss of prime problem is coincidental to the small tube leaking. Should make no difference at all.

Thanks again for the inputs.

[Pib]

Pretty close on the air charger valve but no air goes up through the tube...the tube just has water in it during the suction activity/all the time. When suction is applied through that tube (i.e., pump turns on) to the air charger valve assembly that suction pulls the diaphragm up which was being held down by a diaphragm spring....Ok, when the diaphragm is pulled up it creates a vacuum/suction underneath it when also causes a little ball valve (which also has a little spring) to briefly open up to the atmosphere and suck in a little bit of air which also causes the ball spring to close the ball valve....when the pump turns off the suction goes away on top of the air control valve diaphragm and the its spring then pushs it back down which forces that little bit of air into the top of the pressure tank (or the part of the intake manifold leading to the top of tank.

Now the way I described above is how my Mitsubishi pump works which is the same basic design although the air control valve for your Hitachi is probably looks a little different and mounted in a slightly different place...my air control valve screws into the top of the pressure tank and the small tube on the top of the valve goes to the pump water/suction inlet.....on some other designs the air control valve is mounted on the manifold assembler.

But when the dust settles air control valves basically add a little bit of air to the pressure tank to offset the little bit of air absorption that naturally occurs in the tank between the water and air...and all of this occurs with the air control valve when suction is created with the pump's on and off cycle....keeps the pressure tank from getting water logged.

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