The humble pressure tank is probably the most misunderstood piece of equipment the we sell at Southern’s. There are many different ideas out there on what they do and how they work so I would like to explain the basics of how a pressure tank operates and what its function is, I will also give you some basic tips for maintenance.
What Is A Pressure Tank?
A pressure tank (or pressure vessel or bladder) is a metal or fibreglass container that can vary in size from 2 litres up to 500L. Inside the tank there is a rubber diaphragm that divides the tank into 2 halves, the wet half and dry half.
The dry section has a tyre valve attached to it and compressed air is pumped into it until the pressure is at a certain level, which we will talk about later. The wet half has a metal (or sometimes plastic) threaded outlet attached to it and this is connected to the pipework for a pressure pump.
Pressure tanks are generally made out of steel or fibre glass. The modern steel models are cheap and low maintenance and the diaphragm is set into them. The range in price from $90 for a 8L up to $720 for a 200L model. The quality brands (we only stock Grundfos and Davey) generally last but they can be prone to poor water quality and being out in the weather, especially near the coast. The heavier duty option is a fibre glass tank. These are generally more expensive and range from $550 for a 60L up to $1230 for a 250L model. The best part of these is that the bladder is generally replaceable if it bursts and the tank itself lasts for years as it can’t rust.
How Does A Pressure Pump Work?
A pressure pump is a pump with some sort of electrical switch on it which turns the pump off when the system pressure reaches a pre-set point, sometimes this is fixed and sometimes it is adjustable. When your pressure pump is running and a tap on the system is open, the water flows out of the tap.
When the tap shuts, then the water stops flowing and the pressure in the system builds. When the pressure reaches the “off” setting of the switch or controller, then this turns the pump off and the pipes remain pressurised. As water is not very compressible and pipes only store a small amount of water, when you open the tap again the pressure drops very quickly and only a small amount of water comes out. When the pressure drops to the “on” setting of the switch, then the pump turns on again and starts pumping.
How Do I Adjust My Pressure Tank?
Most pressure tanks come already pre set, this is usually to between 15-25 psi (100kpa – 175kpa)
- When you have installed the tank, you can then adjust the air pressure using a tyre gauge and a compressor or bike pump.
- Work out what pressure your pump is turning off at ( high pressure) and on at (low pressure) from its pressure switch.
- Make sure there is no water pressure in the system as this will give you a false reading on the air pressure. Turn the pump off at the switch and shut any taps on your supply.
- You want the air in the tank to be approx. 70% of the off pressure. For example your pump builds up pressure and shuts off at 500kpa, then the pressure drops and it turns on again at 200kpa. You want to be setting your air pressure at 350 kpa (500 x 0.7).
- Turn your pump back on and check if turns off and on as you want it to. If it is cycling, then you may need to adjust the air up or down.
I Have Been Told To “Set It At 20psi and Forget About It”….
With normal house pressure pumps, the operating pressure of these is usually between 200kpa and 350kpa and they shut off at a maximum of 400kpa. As these pumps are smaller in flow and work at lower pressures to an irrigation pump or a commercial pump, then there is less chance of damage occurring and they generally don’t cycle as much.
As well as that, most house pumps these days also are controlled by electronic pressure controllers ( or press controls) and these also measure water flow too so they don’t cycle as rapidly. Because of all this, a good rule of thumb for your house pump tank is 20-30 psi (140-200 kpa.) and should stop it cycling and remove most of the pressure fluctuations.
What Is “Cycling”?
When a pressure pump is operating, it produces a fixed amount of flow and pressure (an example being 40L/ min at 250kpa). If the tap that you have open only flows 15L/min then the pump is producing more volume than the system can release and the pressure builds.
Once it reaches the off pressure of the switch the pump turns off and the pressure starts to drop. The pressure then will rapidly drop in the pipes to the on pressure and the pump starts again. As the tap is flowing only a fixed amount, then the pump will keep doing this over and over, this is known as cycling. The rapid turning on and off is not good for electric motors, uses a lot of electricity and causes large fluctuations in the pressure which can be annoying. Cycling is even more important when the pump is a submersible bore pump as these are recommended to only start 100 times in a 24 hour period, if it is not controlled, permanent damage can be done to the motor.
How Do You Fix Cycling?
There Are 3 Ways To Fix Cycling:
- Select a pump that has a flow and pressure that is better suited to the job you are trying to do. If your sprinklers have a combined flow of 50L/min, then don’t use a pump that operates at 150L/min because it will cycle.
- Install a pump with a variable speed motor. As discussed in my previous blog, a variable speed pump measures the demand for flow and when less is required it slows down to meet the demand. This can be expensive and need a fairly complex piece of electrical equipment to work.
- Install a pressure tank on the delivery side of the pump, as shown in the diagram. When this happens it creates a “balloon” of water that can build up in the tank. Once this volume has built up, then the pressure switch turns off and the water then pushes out into the line providing a more constant flow and pressure. Once the tank has pushed its contents out into the pipe, then the pressure drops further and the pump switches on taking over. The pressure tank also provides a “shock absorber” effect in that the rubber diaphragm flexes so that pressure spikes are not as noticeable. It also stops the effect of water hammer. By having this flexible volume of water in the system, then the cycling amount is reduced or stops completely depending on how things are set.
Where Do I Install A Pressure Tank?
You install a pressure tank anywhere on the delivery or pressurised side of a system. All pressure pump systems need a check valve (or 1 way valve) installed in the pipework. This can be installed on the suction side (as a foot valve or on the pump inlet) or just after the pump on the outlet. The check valve stops water flowing backwards and the pressure dropping when the pumps turns off. This allows a pressure pump to build up pressure and let the automatic switch turn it off.
A pressure tank can be installed anywhere after this check valve. This means that when all the taps in the system are shut, then the pressure in the system builds. When the water pressure in the system builds to beyond the air pressure sored in the tank, then the air starts to compress and the water fills up the wet side of the tank. Once the wet side of the tank has compressed the air as much as it can, then the pressure builds a little bit more and then switches off.
You simply cut in a T piece or a tapping saddle into the outlet pipework and have a pipe or a piece of pressure hose across to the inlet of the tank. Some tanks (up to about 15L) are OK to mount on the pipe or on top of the pump. Once they get beyond this size, they usually have to be mounted on a base and most of the time they actually come with a base .
Pressure tanks are a cheap, simple and effective piece of gear that will help protect your pump and provide you with a good constant pressure and flow of water for whatever application you need.