Valve basic

Valve definition

Valve is any device that stops, starts or changes the direction or magnitude of flowing fluid (gases, liquids, fluidized solids, or slurries)

Valve Component

Regardless of its type or design, manual valve and control valves have standardized nomenclature for its components

Valve component

Note: General description below refer to nomenclature numbering as per figure above

1. Body

The valve’s body is the outer casing of valve’s internal parts or trim, which act as pressure boundaries of the medium.

Valve bodies are usually made from metallic or thermoplastic . Carbon steel and Stainless steel are quite common application. For specific application with relatively higher corrosion and or erosion rate, relatively high and or cryogenic application, as well as special chemical application end user normally choose different material. Monel valves are normally used in hydrofluoric acid (HF) Alkylation plants. Cast Iron and Aluminium bronze valve for seawater application. Due to corrosion resistant properties, duplex as well as super duplex valves are commonly used for brackish application. Hastelloy valves are often used in high temperature applications. Thermoplastic bodies are used for relatively low pressures and temperatures.

2. Bonnet

A bonnet acts as a cover on the valve body. It is normally bolted onto or welded towards the body. Some valves do not have bonnet,  for example single piece ball valve, and plug valves. Normally bonnet (to body connection) used as access to assemble valve internal for production and maintenance purposes.

3. Trim

Trim consists of stem/spindle, Seat seating surface, obturator (valve closure member e.g. ball, disc, gate, globe, plug), bushing or a deposited weld for the backseat and stem hole guide, and small internal parts that normally contact the service fluid. In some context ones also may refer this as wetted parts

As display on picture above, Valves’ trim consist of component as follow:

3.1. Obturator (valve closure member)

An obturator is a movable obstruction inside the stationary body that adjustably via the movement of the stem to restricts flow through the valve. Depending on the type of valve, a disc can move linearly inside a valve (further called as “linear motion” valve), or rotate on the stem (further called as “quarter turn” valve, or rotate on a hinge or trunnion (as in a check valve).

3.2 Seat

The seat is the interior surface area which contacts the obturator to form a seal. This sealing capability generated from seating load due to friction between seat and obturator. Such seat load designed in way within the allowable range of elastic deformation (for some extended period) for both seat and obturator material to perform its sealing function without mechanical defect.

Seats are classified by means as follow:

  • Integrated Seat, Either the internal surface of the body act as seat or originally seat made from different components however fuser or welded towards the body internal surface prior full assembly. Among many reason, the most common of this type of seat selection are due to economical values of complete valve and leakage of medium through the gap between seat and body (shall possible) is not desirable. (Thermoplastic  or rubber) lined valves and vulcanized seat are part of this type.
  • Replaceable Seat, which can be differed into two categories as per valve sealing component material
    • Metal seat(s). Seat component are made from metallic material. To prevent Galling due to metal to metal contact, usually seats are made or layered with material that has lower hardness level in comparison with its’ obturator hardness. Unless agreed otherwise, all metal seated valves have a tolerable leakage rate in conjunction agreed standards applied.
    • Soft seat(s). Usually fitted to the valve body or Seat holder. These are made from either polymeric such as PTFE, PFA or elasmotirc material such as NBR, EPDM, or FKM depending on the maximum designed temperature and type of services.

A closed soft seated valve is much less liable to leak when shut while hard seated valves are more durable. Gate, globe, and check valves are usually hard seated while butterfly, ball, plug, and diaphragm valves are usually soft seated