Equilibrium ball valve

An equilibrium ball valve is a type of ball valve which utilises the pressure of the incoming water to help close the valve.

Contents – equilibrium ball valve


In a standard ball valve, the incoming water enters the valve on one side of the washer only. As the water level in the cistern rises, the float raises the float arm, pushing a washer onto the valve seating, and closing the valve. Thus, the buoyancy of the float has to be sufficient enough to overcome the pressure of the incoming water.

This is where equilibrium ball valves do things differently. An equilibrium ball valve allows the incoming water to move to the other side of the washer. This keeps the water pressure equal on either side – in other words, in a state of equilibrium. By equalising the water pressure on either side of the washer, the upward force of the float only needs to be strong enough to raise the float arm. It does not need to overcome the pressure of the incoming water.

Equilibrium ball valves are useful in areas where the water pressure is very high, or for cisterns with a large diameter inlet pipe. They have also been used in order to overcome water hammer.


There are several different types of equilibrium ball valve in existence. Brass 1/2″ valves are available and are similar in appearance to Portsmouth pattern ball valves. Larger bores are also available, e.g. 1.5 inch, 2 inch, and the cost of these can run into hundreds of pounds each. However, these are obviously only used for very large cisterns, such as those in communal accommodation, e.g. a block of flats.

One of the main advantages of equilibrium ball valves is that because the float only has to lift the arm of the valve and not force it against the incoming water, the arm can be much lighter and more compact in comparison to traditional ball valves. This design concept is evident in the Torbeck line of float valves. These are are intended for use in toilet cisterns.