There are a large variety of valves and valve configurations to suit all services and conditions; different uses (on/off, control), different fluids (liquid, gas etc; combustible, toxic, corrosive etc) different materials and different pressure and temperature conditions. Valves are for starting or stopping flow, regulating or throttling flow, preventing back flow or relieving and regulating pressure in fluid or gaseous handling applications. Common valve types include: Ball,Butterfly,Check, Diaphragm, Gate, Globe, Knife Gate, Parallel Slide, Pinch, Piston, Plug, Sluice, etc.
Valves can be grouped into two main types:
Shut off valves: They are ON/OFF valves either allow full flow or shut off completely. Shut off valves can be operated by hand or automatically. They are not used for controlling the flow.
eg: Gate valves
Control valves: These valves act as the correcting element of a control loop. Control valves are normally operated automatically, there are also hand operated valves.
There are many control valves types
Safety & Relief valve
A pressure Relief Valve is a safety device designed to protect a pressurized vessel or system during an over pressure event and there are three types of valves e.g
- Self acting relief valve
- Self acting safety
- Power relief valve
Types of control valve Remarks :-
Different types of control valve different application in used as per process requirements like how much opening timing , pressure drop across the control valve ,Quick opening, Noise factor , cost etc
Globe/Stop Valves: – The flow path through globe valves follows a changing course, thereby causing increased resistance to flow and considerable pressure drop. Because of the seating arrangements, globe valves are the most suitable for throttling flow, however avoid extremely close throttling when the repeatable pressure drop exceeds 20%. Close throttling creates excess noise, vibration and possible damage to valves and piping.
The valve is named after is globular body. Compared to gate valves, globe valves are designed to open and close more quickly. Their flow characteristics can be changed by re-configuring their discs. Advantages – best shut off (not drip tight above 50NB) & regulating.
Disadvantages – high pressure drop (head loss), unidirectional.
The butterfly valve derives its name from the wing-like action of the disc which operates at right angles to the fl ow. It’s main advantage is a seating surface which is not critical. It is designed for fl ow isolation. The disc impinges against a resilient liner to provide bubble tightness with low operating torque. Compact and with a simple construction, butterfly valves facilitate easy pipe arrangement. Due to disc, they have slightly reduced fl ow characteristics.
Advantages – quick acting, good regulating characteristics, compact & light, low pressure isolation. Butterfly valves are simple, reliable and range in size from 40mm to 1000mm and beyond. They can be controlled by a notched lever, handwheel as well as by pneumatic or electric actuator. A shaft turns a disc 90º within a pipe. The disc angle within the pipe can provide a restriction varying from drip-tight through to almost full fl ow (except very small sizes).
Because of their excellent operating characteristics, ball valves are used for the broadest spectrum of isolation applications and are available in a wide range of sizes and materials and are available in full fl ow and full through conduit.
Advantages – quick acting, straight through fl ow in either direction, low pressure drop, bubble tight shut off & operating torque, easily actuated.
Disadvantages – temperature limitiations on seating material, long “relative” face to face dimension.
Plug valves are valves with cylindrical or conically-tapered “plugs” which can be rotated inside the valve body to control flow through the valve. The plugs in plug valves have one or more hollow passageways going sideways through the plug, so that fluid can flow through the plug when the valve is open. Plug valves are simple and often economical.
Pinch valves employ an elastic tubing (sleeve/hose) and a device that directly contacts the tubing (body). Forcing the tubing together will create a seal that is equivalent to the tubing’s permeability.
Air-operated pinch valves consist of an elasticised reinforced rubber hose, a type of housing, and two socket end covers (or flanges). In air-operated pinch valves, the rubber hoses are usually press-fitted and centered into the housing ends by the socket covers. There is no additional actuator, the valve closes as soon as there is a pressurized air supply into the body. When the air supply becomes interrupted and the volume of air exhausts, the elastic rubber hose starts to open due to the force of the process flow.
An instrument needle valve uses a tapered pin to gradually open a space for fine control of flow. The flow can be controlled and regulated with the use of a spindle. A needle valve has a relatively small orifice with a long, tapered seat, and a needle-shaped plunger on the end of a screw, which exactly fits the seat.
As the screw is turned and the plunger retracted, flow between the seat and the plunger is possible; however, until the plunger is completely retracted, the fluid flow is significantly impeded. Since it takes many turns of the fine-threaded screw to retract the plunger, precise regulation of the flow rate is easily possible.
The virtue of the needle valve is from the vernier effect of the ratio between the needle’s length and its diameter, or the difference in diameter between needle and seat. A long travel axially (the control input) makes for a very small and precise change radially (affecting the resultant flow).
Needle valves may also be used in vacuum systems, when a precise control of gas flow is required, at low pressure, such as when filling gas-filled vacuum tubes, gas lasers and similar devices.
Sometimes referred to as a non-return valve, the check valve prevents back-flow in the piping by constantly keeping fluid flowing in one direction. Check valves operate automatically. Some piston/disc check valves are spring loaded for fast operation, (minimum cracking pressure should be specified). Vertical downwards flow requires a spring loaded check valve.
Wedge Gate Valves: – Commonly used in industrial piping for stop or isolating – to turn on and shut off the flow as opposed to regulating flow. Gate valves are named from the gate-like disc which operates at a right angle to the path of the flow. Gate valves are general service valves that can be made in a broad spectrum of sizes using a variety of different materials. Wedge gate valves are metal seated but are also available with resilient seat insert if drip tight shut off is required. They can meet the demands of a wide range of pressure and temperature conditions and is available in full port. Advantages – low pressure drop, straight through flow either direction. Disadvantages – slow acting, bulky. Not drip tight shut off (over 150NB). Do not partially open as this will cause damage to seat/disc.
Knife Gate Valves: – Useful for many applications in larger sized pipework (50mm up). Unlike traditional gate valves, they are able to throttle (at lower pressures) depending on line media and degree of opening. Metal seated knife gate valves are not leak tight shut off. Some knife gate valves have a resilient seat in order to ensure they close drip-tight. Available in v-port, o-port and lined they are ideally suited for the control of effluent, slurries, waste products, semi solids, pulp, bulk powders. Most knife gate valves are designed for single flow direction.
Parallel Slide Gate Valves: – Popular in steam applications as the energised disc design handles thermal expansion without sticking like wedge gate valves. Another advantage is lower torque then wedge gate valves especially in venturi (Ferranti) reduced bore configuration. Parallel slide valves consist of two parallel gates that are energised against the seat at all times by springs or a wedging spreader bar between the two gates. No mechanical stress is exerted between the discs, and the valve is not subjected to dangerous strains in opening and closing. This design of valve maintains fluid tightness without the aid of wedging action. These valves are used for saturated and super heated steam.
Pipeline Slab Gate Valves: – Available in parallel solid slab and expanding 2 piece wedging slab. Both styles protect the seat area from the flow in all operating positions. These valves have a full through conduit configuration with a hole in the slab. Slab style gate valves have seats that are spring energised. The expanding slab features two opposed sliding v-shape segments that maintain pressure against the seats. These valves are for API6D pipeline applications but are also used for API6A wellhead valves. All these valves are made in metal to metal and soft seat configuration.