N2 Fitting and Machining Theory|Hands-On! 87


When, however, there is at least one non-return valve in the system. Te valve closes automatically due to the weight of the liquid trying to drop down. Tis should be one at the base or pick up point, called a foot valve and at least one just beyond the pump to save the energy lost when the liquid was allowed to return down the pipe.


In the non-return valve, shown in Figure 6.4, the liquid would only be able to flow from leſt to right. Were this valve in a vertical pipe line it would be fitted on the side that is now on the leſt, at the bottom and the side on the right would be at the top.


Butterfly valves


A butterfly valve consists of a housing that is mounted between the supply or inlet and the delivery or outlet.


Butterfly valves are commonly found in air or other gas flow systems. At the centre of the housing is a disc that rotates about it’s own centre. When closed it forms a complete gas seal. In some cases the disc is rubber coated or may just be coated on the outer edge.


Te control arm that opens and closes the valve to the exact degree required, is oſten a spring loaded pulley that is activated by a cable as in the case of a carburetor in a petrol engine or the acceleration valve in a fuel injection system.


Ball valves


Ball valves work on the principle of a ball in the centre of the valve housing. Te ball has a large hole in it through which the liquid can flow without obstruction when the hole is lined up with the direction of flow. When you want to reduce the rate of flow, you merely start turning the handle on top which rotates the direction of the ball and causes a reduced open area for the liquid to flow through. Less and less liquid will flow the more the ball is rotated until the passage is completely blocked and no flow takes place.


Foot valves


A foot valve is usually fitted to the end of a pick up pipe that is found below the surface of the liquid a pump is trying to pick up.


A sieve is normally fitted over the intake side of the foot valve to prevent any solid muck entering the pipe line at this point.


Figure 6.5 A ball valve in the open position


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