3. THE NEEDLE VALVE (See Figure : 16)

This is simply a variation of the Globe valve and, as its name implies consists of a narrow, tapered plug and port arrangement. The needle valve is, in itself, small in size and is used for very fine and normally manual, control of fluid flow.

4. THE ANGLE VALVE (See Figure : 17)

Another variation of the Globe valve which is often used for compressor recycle systems. It is also often used for dirty fluids. In this valve, the fluid flow enters from below the plug and passes through an angle of 90° to the outlet. An angle valve may be manually or automatically controlled.

5. THE PLUG or BALL VALVE (See Figure : 18)

These valves give a more streamline flow in their operation. The valve port is about the same size as the internal pipe diameter. They are operated simply by turning a handle through 90°. They can give a fairly tight shut-off and some degree of control.

6. THE BUTTERFLY VALVE (See Figure : 19)

The Butterfly valve is designed to give full flow with some control capability. Normally, this type of valve does not give positive shut-off but, present-day technology, using new Polymers and higher temperature ratings, has enabled the construction of butterfly valves having a 'Bubble-tight' shut-off.

7. CHECK or NON-RETURN VALVES (NRV's) (See Figure : 20)

These valves will allow fluid flow in ONE DIRECTION ONLY. Their purpose is to prevent backflow - for example -in pump discharge lines and where back-flow is undesirable. When flow stops, the weight of the piston, ball or flapper closes the valve. If the pressure downstream of the valve, is higher than the upstream side, the valve will close more firmly and back-flow cannot occur.

There are two main types of check valve :

1. The 'LIFT' type. (Spring loaded 'BALL' & 'PISTON' Types).
2. The 'SWING' (or Flapper Type).

THE NEEDLE VALVE

Figure : 16

THE ANGLE VALVE

Figure : 17

THE PLUG VALVE & BALL VALVE

Figure : 18

THE BUTTERFLY VALVE (SIMPLIFIED)

Figure : 19

CHECK or NON-RETURN VALVES (NRV's)



Figure : 20