Sunday, May 13, 2012

Fixing a Pressure Pump

Today I did something a bit different and did some mechanical work on a pump that we use to supply our washing machine and toilet with rainwater from tanks.  Recently it has been making some strange sounds, and sometimes it doesn't cut in at all.

Pump without the water outlet

The pump is part of a standard set-up, water comes from the rainwater tank and is pressurised and sent to where it needs to go.  A pressure switch on the outlet port of the pump turns the pump on when it senses a drop in line pressure created by a tap being opened, it then cuts out when a certain pressure is reached.  To stabilise the system, and stop the pump constantly cycling, a pressure tank is connected to the output.  This consists of a pressurised bladder of air in a tank, as the water is pressurised it compresses the bladder.  If you want to think of it in electrical terms, the pressure tank acts like a capacitor and the pressure switch adds hysteresis, creating an oscillator.  The larger the capacitance, or pressure tank in this case, the lower the oscillation frequency.

After a few tests I could tell that the pressure tank was fine and only needed to be topped up with some air.  The strange sounds that I heard sounded like something was rubbing when the motor turned.  This could have been a dry bearing, something wrapped around the impeller shaft, or a object obstructing the cooling fan.  The only way to find out was to remove the pump and pull it apart.

To start with I removed the outlet of the pump to make sure there was nothing wrapped around the impeller causing the sound I was hearing.  There was a bit of slime, but in general it was clean as you can see in the image below.


Pump Impeller
The next most likely culprit was the cooling fan that blows air over the motor casing.  After removing the cowling I was able access to the fan and found a build up of dirt.  What's basically happening is air with dust in it gets draw into the fan, the dirt being heavier hits the blades and gets flung into the cowling where it builds up until it rubs against the blades and causes a rubbing sound.  Pieces then break off and rattle about the fan housing.  If your really unlucky the pieces jam the fan blade and stall the motor.

Dirt on the fan and cowling

Dirt on the fan blades

After a clean up with an old toothbrush it was as good as new.  The cowling was replaced and the pump was reconnected.  It worked perfectly.  The pump was pumping and the strange sound had disappeared.  Then it stopped working again.  I tapped the cowling off a little bit and all was good again for a while.  From what I can see, I think the cowling is a tight fit and if it is slightly misaligned it will touch the fan blades.  The next step is to remove it completely, let it go for a week and see if it works.  If it does, which I think it will, I'll have to make my own fan cover or file a tiny bit off each of the blades.

Update - 20 May 2012
With further investigation and testing I have discovered the fault with the motor.  The problem and how to fix it can be found here.

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