Frequently used types of valves are Gate Valve, Globe Valve, Ball Valve, Check Valve, Butterfly Valve, Plug Valve, Needle Valve etc.
American Standards
- ASME B16.10 : Face to face and end to end dimensions of valves.
- ASME B16.34 : Valves, Flanged, Threaded and Welding Ends.
Specification
Valve specification includes Valve Type, Valve Body Type, Valve End Type, Valve Stem Type, Valve Operator Type, Valve Trim, Valve Sealing System, Valve Material of Construction, Valve Dimensional Standard.
End Types
Valve End Types incude Flanged, Buttwelding, Socket Welding, Screwed, Hose Connection, Victaulic Coupling etc.
Trim
Valve Trim includes Seat Rings, Disk or Facing, Stem, Stem Guide Sleeves. Trim material should have wear resistance, galling resistance, corrosion resistance, toughness, coefficient of expansion almost same as valve body material.
Stem
Valve Stem types are Rising Stem and Non-Rising Stem. Rising stems are preferred as their position is visible. Rising stems are either inside screw or outside screw. Both gate and globe valves are rising stem valves.
For small bore valves, handwheel also raises with rising stem. Only gate valves are non-rising stem. In non-rising stem gate valve, seat rises on the stem inside the valve as handwheel is rotated. In non-rising stem gate valve, screw is inside bonnet and in contact with fluid, hence threads may corrode, accumulate dirt and operation can become difficult.
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Non-rising stem valves are used where there is a space constraint. In non-rising stem gate valve, valve position is not visible form outside, hence it is important to put a sign where such valve is used.
Valve stem extensions are used where standard valve operator is out of reach. Rods fitted with universal joints can also be used to bring a valve hand wheel within an operator’s reach. Chain operator is used if handwheel height is out of reach. Chain is brought within 3 feet of working floor height.
Operator
Handwheel is the most common type of valve operator. Hammerblow handwheel can be used for additional torque. Gear operator valves used where operating torque is very high.
- Gear operator : 125#,150#,300# : 14″ and higher
- Gear operator : 400#, 600# : 8″ and higher
- Gear operator : 900#, 1500# : 6″ and higher
- Gear operator : 2500# : 4″ and higher
Electrical operators along with gears are used for remotely operated valves. Pneumatic operator are used where flammable vapor may be present. Quick acting operator used for gate and globe valves as steam and air whistles.
Sealing System
Standard valve sealing system consist of packing which is compacted around the stem with the use of packing gland. As the packing wears and valve leaks around the stem, the gland can be tightened progressively to increase the pressure on the packing, deforming it against the sealing surface. Valve stem lubrication is very important for long term operation of a valve. Lantern ring, placed between stuffing box rings, can be used as a lubricant injection point or drain collection point. Flexible diaphragm sealing system uses a pack-less metal diaphragm held between body and bonnet engagement, which prevent escape of hazardous liquid or gas media escaping to atmosphere.
In bellows sealing system, a bellow is welded to both the valve bonnet and valve stem. Bellow seal used in axial stem movement valves like gate valve and globe valve.
Pressure seal construction is adopted for Valves for high pressure service, typically in excess of above 170 bar. In the pressure seal Bonnet type, the body-Bonnet joints seal improves as the internal pressure in the Valve increases, compared to other constructions where the increase in internal pressure tends to create leaks in the body-Bonnet joint.
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Bonnets
Types of bonnets are bolted bonnet, screwed bonnet, breechlock bonnet and Union bonnet. Bolted bonnet is most used valve bonnet type in hydrocarbon industry. U-bolt or clamp type bonnet is used for small gate valve at moderate pressures, to facilitate frequent cleaning and inspection. Breechlock bonnet involves seal welding of the bonnet with the body, and are heavier, infrequently used, more expensive construction for high pressure use.
Screwed bonnets may occasionally stick and turn when a valve is opened and best reserved non-hazardous services. Union Bonnets are more suitable for small valves requiring frequent dismantling than the simple screwed type.