Hydraulic Valve parts

The main part of the valve consists of the valve body and bonnet. These two parts form the vessel or casing that holds the fluid going through the valve. Valve bodies are usually metallic. Brass, bronze, cast or ductileiron, steel, alloy steels and stainless steels are very common. Plastic bodies are used for relatively low pressures and temperatures. PVC, PP, PVDF and glass-reinforced nylon are common plastics used for valve bodies.

Ports

Integral to the valve body are the passages that allow flow into and out of the valve. These are called ports. These ports are obstructed or opened up by the valve member or disc to control the fluid flow. Valves with two or three ports are the most common, while valves with multiple ports (up to 20) are used in special applications. Nearly all valves are built with some means of connection at the ports. These include Threads (male or female); BSP or NPT are most common. Compression fittings, to suit tube s/s or copper. Glue or cement application (especially for plastic) almost always a socket type connection (not a butt) Flanges ANSI, BS, DIN, or JIS. (US, British, European, Japanese standards) Welding either Socket type or Butt type welds.

Disc / Rotor / Valve Member

Inside the valve body, flow through the valve may be partly or fully blocked by an object called a disc or valve member. Although valve discs of some kinds of valves are traditionally disc-shaped, discs can come in various shapes. Although the valve body remains stationary within the fluid system, the disc in the valve is movable so it can control flow. A round type of disc with fluid pathway(s) inside which can be rotated to direct flow between certain ports is usually called a ball. Ball valves are valves which use spherical rotors, except for the interior fluid passageways. Plug valves use cylindrical or conically tapered rotors called plugs. Other round shapes for rotors are possible too in rotor valves, as long as the rotor can be turned inside the valve body. However not all round or spherical discs are rotors; for example, a ball check valve uses the ball to block reverse flow, but is not a rotor because operating the valve does not involve rotation of the ball.

Seat
The valve seat is the interior surface in the body which contacts or could contact the disc to form a seal which should be leak-tight when the valve is shut. If the disc moves linearly as the valve is controlled, the disc comes into contact with the seat when the valve is shut. When the valve has a rotor, the seat is always in contact with the rotor, but the surface area of contact on the rotor changes as the rotor is turned. If the disc swings on a hinge, as in a swing check valve, it contacts the seat to shut the valve and stop flow. In all the above cases, the seat remains stationary while the disc or rotor moves. The body and the seat could both come in one piece of solid material, or the seat could be a separate piece attached or fixed to the inside of the valve body, depending on the valve design.

Seats can be integral to the valve body, that is "hard" metal or plastic. Nearly all metal seated valves leak, even though some leaks are extremely small.

"Soft" seats can be fitted to the valve body and made of materials such as PTFE or various elastomers such as NBR, EPDM, FKM. Each of these soft materials is limited by temperature (rough maximum temperatures are listed below)

NBR 80 °C

EPDM 120 °C

FKM 170 °C

PTFE 200 °C

The advantage of soft seats is that they are more likely to offer 100% tight shutoff when valve is closed.

There are advantages of Hard seated Valves as well in applications where there is heavy erosion due to the material flowing from the pipes then the metal seated valves are preferred over soft seated valves.

Metal seated Valves have longer life as well.

Gate Valves, Globe Valves, Check Valves are usually hard seated Valves and Butterfly Valves, Ball Valves, Plug Valves, Diapharagm Valves are Usually soft seated Valves.

Though there are some special cases where we do have hard seated Butterfly Valves and Hard seated Ball Valves as well.

Stem
The stem is a rod or similar piece spanning the inside and the outside of the valve, transmitting motion to control the internal disc or rotor from outside the valve. Inside the valve, the rod is joined to or contacts the disc/rotor. Outside the valve the stem is attached to a handle or another controlling device. Between inside and outside, the stem typically goes through a valve bonnet if there is one. In some cases, the stem and the disc can be combined in one piece, or the stem and the handle are combined in one piece.

The motion transmitted by the stem can be a linear push or pull motion, a rotating motion, or some combination of these. A valve with a rotor would be controlled by turning the stem. The valve and stem can be threaded such that the stem can be screwed into or out of the valve by turning it in one direction or the other, thus moving the disc back or forth inside the body. Packing is often used between the stem and the bonnet to seal fluid inside the valve in spite of turning of the stem. Some valves have no external control and do not need a stem; for example, most check valves. Check valves are valves which allow flow in one direction, but block flow in the opposite direction. Some refer to them as one-way valves.

Valves whose disc is between the seat and the stem and where the stem moves in a direction into the valve to shut it are normally-seated (also called 'front seated'). Valves whose seat is between the disc and the stem and where the stem moves in a direction out of the valve to shut it are reverse-seated (also called 'back seated'). These terms do not apply to valves with no stem nor to valves using rotors.

Bonnet
A bonnet acts as a cover on the valve body. It is commonly semi-permanently screwed into the valve body. During manufacture of the valve, the internal parts are put into the body and then the bonnet is attached to hold everything together inside. To access internal parts of a valve, a user would take off the bonnet, usually for maintenance. Many valves do not have bonnets; for example, plug valves usually do not have bonnets.

Spring

Many valves have a spring for spring-loading, to normally shift the disc into some position by default but allow control to reposition the disc. Relief valves commonly use a spring to keep the valve shut, but allow excessive pressure to force the valve open against the spring-loading. Typical spring materials include carbon steel (often cad plated), 304 Series stainless steels and for high temperature applications Inconel X750. Springs can be typical 'coil' types or 'bellville" washer stacks or even bimetallic elements which exert a spring force on temperature change.