Mitsubishi Water Pump - Air Tank Full Of Water

[PETERTHEEATER]

It seems that I will never be free of pump problems! One of my Mitsubishi house pumps is playing up again.

The pump is a 450 model about 5 years old. Having sussed and fixed previous problems (with help from Thai Visa members) I now have a problem with the air tank filling with water over a couple of days. With reduced air cushion the pump motor overheats and the thermal cut-out operates. After around 20 minutes cooling time the pump will run again.

The pump sucks water from a sub-level tank maximum lift about 3 metres.

I have never seen a diagram of the internal arrangement of this type of pump but I understand that the air tank contains a 'bladder' or diaphragm separating water from air and that the air pressurizes the tank in order to provide small demands without the pump running.

I now find that when I drain the tank by removing the drain plug a large volume of water is released under pressure. I now do this every day to avoid 'no water' problems in the house. I suspect, but don't know that the bladder may have a hole in it or even a tear. So, a couple of questions:

What else could be causing the tank to fill?

Can the bladder be replaced or does the whole tank have to be replaced?

Any other tips before I commit to buying a new pump?

I'm irritated because I have a similar (but smaller) pump in another property which has been running daily without maintenance - other than the odd re-priming - for nearly 20 years. My other pump, same model, used to supply the garden ring main has been trouble free.

[chantorn]

If it is 5 years old, I suggest just buy a new one. Buy one with exactly the same model, so replacement make easy. Also keep the old one as spare part. It is not that expensive.

[simoncnx]

If it is 5 years old, I suggest just buy a new one. Buy one with exactly the same model, so replacement make easy. Also keep the old one as spare part. It is not that expensive.

make sure there is a non-return valve at the bottom of the pipe in your supply tank and that it works

[PETERTHEEATER]

If it is 5 years old, I suggest just buy a new one. Buy one with exactly the same model, so replacement make easy. Also keep the old one as spare part. It is not that expensive.

make sure there is a non-return valve at the bottom of the pipe in your supply tank and that it works

Yes, there is a foot valve and I assume that it is serviceable. The supply tank was drained and cleaned 5 months ago.

You do not explain how a faulty (or lack of) foot valve causes the air tank to fill with water.

[PETERTHEEATER]

If it is 5 years old, I suggest just buy a new one. Buy one with exactly the same model, so replacement make easy. Also keep the old one as spare part. It is not that expensive.

Not that expensive? A replacement, same model is around 12k baht! Since this pump is only 5 years old and I have others in various properties which have been running for over 20 years I think the actual problem is something simple. As a retired aeronautical engineer, I have plenty of knowledge and experience of hydraulic systems but no-one seems to be able to explain the the specific workings of this type of pump to enable an accurate diagnosis.

The usual train of Thai 'experts' have been and gone having adjusted the pressure switch, replaced the pressure switch, resealed the pump impeller etc etc but no-one really knows what they are doing.

[wimpy]

Take it to a pump specialist. If you are in Chiang Mai, I could tell you where to go. Sounds like it needs a new rubber bladder.

[Artisi]

I am assuming this doesn't have an internal bladder, just an airpocket over water, however, not being an expert in the inner workings of these pump unit I can only suggest that there must be an internal non return valve on the high pressure side of the unit leaking back into the tank displacing the air.

[Lantern]

Sure sounds like the diaphram has a hole in it.

[CDNinKS]

I'm pretty sure these small pumps with integral pressure tanks all have bladders.

Here is a link that has some good info.

Edited April 27, 2010 by CDNinKS

[lopburi3]

Unless they have changed those tanks did not have bladders - but they did have an air pressure control on the top of tank that can become dirty or fail which automatically supplies the proper amount of air cushion for the tank. It is a common failing and any shop should have the replacement available - or you can take apart and clean it as I did many years ago. Believe it was a small black or gray disc shaped thing that screws into a tank access hole.

[PETERTHEEATER]

I'm pretty sure these small pumps with integral pressure tanks all have bladders.

Here is a link that has some good info.

Thanks CDNinKS, that website explanation helps a lot.

Thanks also to Lopburi, I shall take a closer look at that part which is under a cap on the suction side of the pump.

[PETERTHEEATER]

Having read the extensive information available on the website which was pointed out, I am fairly sure that the bladder is leaking so I am committed to buying a new pump and getting the old one repaired although I think that a new tank will be required due to it being of welded construction with no access to fit a new bladder.

I will let you know the outcome once I install the new pump and run it for a few days!

Thanks to all constructive posters.

[PETERTHEEATER]

Hah! I bought a new pump, same model, for direct connection but decided to leave the old pump alone until it 'failed' before replacing it. That was over a week ago and the old pump is performing normally! It's probably scared that it will be out of a job and thought that I was bluffing!