Pneumatic Mixing Valve

Introduction

This is a 3D model of a Pneumatic Mixing Valve.

3D Model Annotations

Pneumatic Mixing Valve

This valve is a type of three-way globe valve often used in boiler feedwater systems for regulating the feedwater temperature.

Position Indicator

Valves are often fitted with a position indicator. Green or yellow is used to symbolise open, whilst red symbolises closed; the letters O and S are also used to indicate open and closed respectively. For a mixing valve, numbers such as 25, 50, 75 and 100, indicate the percentage flow rate through the valve.

Gearbox

It is possible to mount a gearbox between the actuator and stem; this makes it easier to change the position of large valves without applying significant torque.

Actuator

An actuator is used to actuate (change the position) of a valve. All valves require some means of actuation to move from the open to close position, or vice versa. Valve actuation is usually achieved using mechanical (hand lever, handwheel, spring etc.), pneumatic (piston, diaphragm etc.), hydraulic (piston), or electrical means (electric motor).

Body

The body forms the main pressure boundary of all valves and must be constructed from suitable materials to withstand the service pressure to which it will be subjected. It is often cast as a single piece, although it is possible to construct the body from several pieces.

Disc

Valves are often named after the type of disc they employ e.g. ball valve, plug valve etc. Discs may be linearly actuated (gate, and globe valves etc.), or rotary actuated (ball and plug valves etc.); this valve is linearly actuated. Note that this disc has grooves, this allows greater flow control than would be possible with a normal globe valve disc.

Seat

The valve disc presses against the valve seat. It is imperative that the disc and seat surfaces remain clean. If the seat or disc surfaces are damaged, or not clean, it will not be possible to obtain a seal between the seat and disc; this will lead to the valve passing (leaking) when in the closed position.

Stem

The stem connects the actuator to the disc. Stems must be strong enough to withstand the mechanical actuation stresses they are subjected to during operation, this is particularly true for large rotary operated (1/4 turn) valves, where the torsional stresses encountered during actuation are significant.

Bonnet

Many valves require a bonnet. A valve bonnet allows personnel to access a valve’s internals (known as ‘valve trim’) without needing to dismount the valve. The bonnet is attached to the valve body using nuts and bolts (or studs). Gaskets are used to seal the space between the bonnet and body.

Flange

Flanges are attached to the valve body; they allow for associated piping to be attached.

Packing

Packing is installed between the stem and bonnet to ensure the valve does not leak. Packing can be periodically adjusted so that a constant pressure is maintained on the packing gland, which reduces the likelihood of leakage. Overtightening the packing makes the valve difficult to operate and may also lead to damage of the valve stem.

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