In process operations, fluids and their movement and transfer from place to place, plays a large part in the process. Fluids are contained in various types of vessels and are transported in pipelines. In order to Start or Stop fluid flow, to Control flow and to Prevent Back-flow, various types of valves are installed in the piping. To prevent over-pressure in piping and vessels, we use Safety Relief valves and, to isolate equipment for maintenance etc, Block valves are installed.
Where fluids contain suspended solids - scale, dirt etc…, which must be removed, Filters and Strainers of various types are installed.
In steam systems, steam condensate (water), is a particular problem which can cause 'Water-Hammer' in piping and equipment. (This will be explained later).
To minimize this problem, the water must be drained from the steam lines. This is sometimes accomplished by regularly 'Blowing Down' - (draining the water manually), - from the system. Generally, steam systems are fitted with automatic drainage mechanisms called 'Steam-Traps'.
As previously stated, valves are used for the following purposes :
- To start and stop flow of fluids
- To regulate, throttle and control fluid flow or pressure
- To prevent back-flow
- To relieve excess pressure / vacuum
TYPES OF VALVES
1. THE GATE VALVE (See Figure : 13)
This is the most common type of valve in use in industry and is used to start or stop the flow of fluids. It gives a positive shut-off when closed and is often used as a 'Block Valve' for isolating systems.
The gate valve MUST be either FULLY CLOSED or FULLY OPEN and NEVER USED TO CONTROL FLOW, as the fluid velocity will erode the valve internals - gate and body seats .. etc.
A gate valve may be of the 'RISING STEM' type as in Figure: 13 or a 'NON-RISING STEM', in which the stem threads are 'Left-hand' threads set into the gate itself as in Figure: 14.
The valve consists of a body which houses the valve mechanism - the stem (or spindle), the disc(s) (or gate), and the disc and body seats. Gate valves can be operated by a hand-wheel, an electric motor (MOV), or by an air powered diaphragm, or motor, or a piston actuator.
THE RISING STEM GATE VALVE
Figure : 13
Figure : 14
2. THE GLOBE VALVE (See Figure : 15)
The Globe valve is used where control of fluid flow or pressure is required and it can be operated in any position between open and closed.
A Globe valve may be constructed with a single or double port and plug arrangement. The double port type is generally used in a CONTROL VALVE where accurate control of fluid is required. Due to the double valve plug arrangement, the internal pressure acts on each plug in opposition to each other, giving an internal pressure balance across the plugs. (See Fig: 20 on page 42 – Control Valve)
This gives a much smoother operation of the valve and better control of the process. Some control valves are 'Reverse Acting'. Where a valve normally opens when the plug rises, in the reverse acting valve, the valve closes on rising. The operation of the valve depends on process requirements. Also depending on requirements, a control valve may be set to open or close, on air failure to the diaphragm.
Figure : 15
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 :
- The 'LIFT' type. (Spring loaded 'BALL' & 'PISTON' Types).
- 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
8. SAFETY RELIEF VALVES (See Figure : 21)
Safety valves are installed on equipment to prevent over-pressure (or excess vacuum), which could cause damage, rupture or collapse of the equipment.
Where the type installed is to prevent excess vacuum, they are normally referred to as 'Vacuum Breakers'.
A safety valve will have a lifting set pressure higher (about 1.5 times) than the operating pressure of the system but well below the design maximum pressure allowed.
In the conventional type, the pressure of the valve discharge line will affect the set pressure of the valve by giving a back-pressure on the valve plug.
In the bellows type, the bellows isolates the plug from the back-pressure and therefore there is no effect on the valve setting.
The following diagram is of a conventional safety relief valve. All safety valves have the same purpose of protecting equipment from over-pressure. Different makers have their own designs.
SAFETY RELIEF VALVE
Figure : 21
1. STUFFING BOX and PACKING (See Figure : 22)
Packing is a means of preventing leakage around the valve stem to atmosphere. The packing is placed in the 'Stuffing Box' around the stem. It is then held in place and compressed by the 'Gland' and 'Gland Follower' to give a leak-tight seal.
However, having the packing too tight can cause the operation of the valve to be stiff. When tightening down on the packing does not stop leakage or causes valve operation to be stiff, the packing should be replaced.
In some larger valves, a 'Lantern Ring' is inserted between the turns of packing and is placed opposite to a lubrication point where grease can be injected in order to ease the valve operation.
Figure : 22
2. VALVE OPERATORS (See Figure: 23)
The most common valve operator is the 'Hand-wheel'. Other methods of valve operation can be as follows :
A. A Diaphragm
Here, the valve is operated by an air powered diaphragm. These are called 'Pneumatic Control Valves'. The air powered actuator can also take the form of a 'Piston' rather than a diaphragm. The operation of the valve, depending on requirements, may be 'Air to Open' (AO) or 'Air to Close' (AC).
B. An Electric Motor
This is called a 'Motor Operated Valve' (MOV) and may be installed to allow valve operation from a remote location and may then be referred to as a 'Remote Operated Valve' (ROV).
C. A Solenoid
This method of valve operation is used for a small valve placed in an air supply line to a shut-down device. The solenoid will be activated on an electrical signal from an automatic trip system and will open or close depending upon its function.
Figure: 23 -Diaphragm Operated Valve