15 April 2007

Pump Action

We have two water pumps around the place: one providing irrigation water from the small Earth-dam next to the house, the other pumping our household water from the main storage tanks to a header-tank in the roof of the house.  Having installed our own plumbing, and water being a bit important, keeps us very tuned to the behaviour of the machinery we rely upon.  Let the water pump cycle even a fraction of a second too short, and I am instantly aware that there's a problem.

Over several weeks the house-pump has been cycling in ever shortening durations, and I'm well familiar with what that means...

Digression: The Inner Life of Pumps and Pressure Switches

The pump is controlled by a pressure-switch, which in turn relies on a rubber-bladder inside the pressure-dome. I've also heard it called a surge tank.  The rubber bladder gets filled with water, compressing the bubble of air between the bladder and pressure-dome, until there is sufficient back-pressure to overcome the spring in the pressure switch.  There are more modern electronic versions of pressure switches that do away with the need for pressure-domes, and I am told they're very reliable, but they were not around when we built and plumbed our house, and I see no real need for another expense to replace a perfectly serviceable setup.

Every so often, though, the bladder wears out and develops a pucture, and water seeps through to the wrong side of the bladder.  This means that there's less air to make everything work, and the pump trends towards switching in ever-shorter cycles, which, if neglected, will have the pump destroy itself.  So everything has to come apart, as pictured here, the bladder replaced, then put back together again.  Only this time, I was feeling a bit miffed at a bladder that has lasted a bare 18 months.  I should check inside the pressure-dome and check that there's not a rough spot sandpapering through the rubber.  The bladders are also getting quite expensive!

Invent Another Way

Out came an old bicycle puncture-repair kit.  Puncture located. Patch applied.  Whan, bang and we're back in action.  If it works.

After all, the whole thing is under about 4 bar of pressure.  And being flexed and bent all the time.  Neither of which is normal for a bicycle tube.

Only way to find out is to put it all back together; suck it and see.

Putting the bladder and pressure-dome back together, bolting the baseplate on securely, and threading the whole thing back onto the 4-way joint that ties everything together without causing any leaks is only part of the story, though.  The pressure-dome still needs to have air pumped into it through a tyre-valve on the top.  Normally about 1.5 to 2 bar of air pressure is plenty -- this is all about just having the air in place, rather than creating any tremendous pressure.  I have tried using a small 12-volt pressure pump such as you might carry in the pickup for emergency tyre pumping, but, to be honest, it takes so long to fill the pressure-dome that I'm better off using an ordinary handraulic tyre-pump.

There's Always Another Job Along the Way

Out comes the tyre-pump, only to find that the hose has perished and broken.  It's Saturday afternoon, and the hardware shops are all closed for the weekend.  Close examination reveals that most of the length of hose is still OK -- only one end was badly perished.  So, a little action with knife and a hose-clamp, and we're back in action.

Now we know why the shed is filled with a rich and varied assortment of plumbing bits, glues, wire, spare parts and piping of various descriptions.

Happily my patch-job seems to be holding well, though it remains to be seen how long it will last.  The saving of a couple of hundred bucks was a good win, but more importantly, the feeling that comes of having dealt with the problem, having refused to accept defeat in the face of niggling problems and difficulties -- that's the big win.

Post Script

I do plan to install a hand- or wind-driven backup pump for the house water.  That's partly the reason that the plumbing system was designed as a gravity-feed setup in the first place.  Right now we have water sufficient for about 3 or 4 days without power to pump water up to refill the header tank.  And it has happened a couple of times that we've been without power for that long as a result of storm damage.  In these situations we're still able to use inside water, bath, etc. where our much more modern and clever neighbours are forced to do without.

The catch is that I expressly do not want a manufactured pump, but something I can totally build myself from scratch, so that I am not at all reliant on any factory-engineered bits and pieces that would instantly become unobtainable should "everything go Pear-shaped."

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