Building Construction, Renovation & Maintenance

Knocking in Water Pipes


water pump with pressure tank

Pressure Tanks can stop water hammer in pipes

Water hammer or knocking in water pipes are a common problem that can be solved by installing a pressure tank which is screwed onto the top of a water pump or into the water pipes to absorb the shocks that cause water hammer.

Pressure tanks can also stop water pumps switching on and off quickly, known as 'cycling.' Here we look at pressure tanks, what they do and how they work.


See the full Fixed Abode article "Knock Knock" here


Why do water pipes sometimes knock?

This is a phenomenon known as water hammer or, more technically, 'hydraulic shock' and is the result of a sudden build up or release of pressure in your water pipes.

It all goes back to an inescapable quality of water - it is pretty solid really (yes I know - a solid liquid) and not compressible (well - only a very small amount), you can't squeeze it. It might flow very easily but get it into a confined space and its volume remains virtually the same. The fact that you cannot compress water is useful, it means we can pump it and we can use it to push things but no matter how much pressure we put into it its volume may change very, very slightly but to all intents and purposes it remains the same.

Let us consider what this means in practice. If we pump water into a container such as a tank or a pipe, or for that matter, your water system at home it will take a certain amount of water to fill the space available. Once full we keep pumping and the pressure in the water increases. Very, very little additional water is needed to increase the pressure. This also means that it only takes a very small release of water to release the pressure. Any very small leak will immediately release the water pressure in the tank, or for that matter, your whole water system.

While this can be very useful from other points of view it can also create difficulties.

Balancing the output from the water pump

Why? Well its like this, if you have a water pump to pressurise your water system this will give you a good energetic shower.

For most houses we need a pump that is large enough to supply all the demands we might have at any one time. Someone might be showering while in the laundry the washing machine is running, someone has just been to the toilet so the cistern is filling, he goes off to top up the swimming pool or wash the car when someone else decides to get up, walks into the shower and, turning on the tap, there is no water issuing forth from the trendy but very water demanding “rain effect” shower head.

To avoid this we need a water pump that is large enough to supply all these needs at once.

Great - but now we have a further problem. Usually there is only one person showering or one tap on at any one time and our large pump is pumping far more water than can physically squeeze itself out of a single shower head.

To pump only a low flow of water our large pump must switch itself on and off. To do this water pumps have two pressure switches on them. They work like this: if you open a tap the water pressure in the system drops instantly, one pressure switch senses low pressure and switches the pump on. The pressure in the system increases and when it is high enough the second pressure switch switches the pump off.

We now have another problem. It will only take the removal of a very small amount of water to remove all the pressure from the water system. So we turn on our shower and within a split second the small amount of water is gone, the pressure has dropped and our larget pump bursts into action attempting to pump a large amount of water down that little pipe to your shower. In a split second the small amount of water has been replaced, the pressure is back up and the pump switches off again with a "Knock” that reverberates through the house

This is what makes the water hammer. Water hammer can also occur when you turn a tap off suddenly, this is particularly the case with modern quarter turn taps which tend to cut off the water flow very quickly.

But, just a minute, we haven't finished yet. The shower is still running and once again a small amount of water has gone and the pump switches back on. This process continues ad nauseum, the pump switches on and off, it is ”cycling”.

Water Hammer Can Cause Damage to your plumbing

The shock loading to the water pipes is not only very annoying, it can cause damage to your plumbing particularly to joints and/or fittings

So how do we avoid this? We install a pressure tank.

A pressure tank provides a cushion

We add some air into the system. Air (and all gasses for that matter), unlike water, are very, very compressible. Have you ever seen how much air you can get into a car tyre? Rather a lot I can tell you.

Not only that, because air is compressible it makes things soft and squishy, it acts like a cushion. In former times air was used for the suspension in cars most notably by the French who widely used air suspenion in Citroen cars.

As I have said, in the case of water there is a very small difference in volume between high and low pressure. With air, there is a huge difference in volume between high and low pressure.

So how do we use this?

If you have a look on the top of your water pump you may see a round tank about the size of a football. Small pumps may have a small tank and large systems may have very large tanks although the football size is most common.

This is a pressure tank, it is a fairly simple device that takes a lot of the load off your water supply system by introducing a rubber bag of air which acts as a cushion in our water system.

How does a pressure tank work?

A Pressure tank has a rubber bladder inside it which is filled with air to a pressure of around 20 pounds per square inch.

As our water pump pumps up the pressure in our water system it also pumps water into the pressure tank which progressively compresses the bladder of air. This will take time so our pump can run for a longer period before reaching the high pressure and switching off.

Now when we turn on the shower the water pressure is released and the bladder full of air gradually expands pushing the water out of the tank to our shower until it the water reaches the lower pressure when the pump switches on again.

The pressure tank does 2 things: it slows down the time between the water pressure pump switching on and off and it acts as a cushion stopping sudden pressure changes in the water pipes that cause that knock

These pressure tanks are also known as accumulators they collectand save pressure.

Identfying pressure problems in your water system

How do you know if there is a problem?

If your water pressure pump is rapidly clicking on and off then there is something wrong in your water system. A bit of investigation can help to isolate the problem.

The first thing to do is to check your pump and make sure it is the water pressure pump and not the well pump that is the problem (the well pump is the one that pumps water from your well or bore to your storage tank).

Ok so you are sure it is the pressure pump.

Next make sure anything that may be using water, taps, toilets, showers, etc. are all turned off. Check the pump - it should be off. If the pump is running constantly or clicking on and off then you most likely have a leak in the system somewhere. The slower the clicking the smaller the leak.

Alright so the pressure pump is off. Now turn on a shower or a single tap. The pump should start. If it is a small pump such as a Shimizu it will probably be running constantly. If it is a larger pump it will probably be cycling, switching on and off at a slow pace as it alternately pumps up the pressure tank then allows it to empty.

If it is cycling quickly then the problem will probably be the setting of the 'on' and 'off' pressure switches on the pump or the operation of the pressure tank. These are not really things you can do yourself and you should call a plumber.



Copyright © Phil Wilson March 2011
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