Control valve

Generally fall under two categories.

Sliding stem valves

Rotating stem valves

Sliding Stem Valves

Valve plug and stem move in a linear motion.

Common examples are globe and gate valves.

Available in straight and angle bodies.

Gives better control ability compared to rotary stem valve designs.

More expensive than equivalent rotary stem valve designs.

Rotary Stem Valves

Valve plug and stem move in a rotary motion.

Also called quarter turn valves.

Common examples are ball and butterfly valves.

Gives larger flow capacities and less pressure drops compared to

sliding stem valve designs of the same body sizes.

The large capacity is at the expense of control ability.

Relatively cheaper for the same sizes.

Components

i. Valve body

The mechanical portion that contains the process fluid.

Available in different sizes to suit process requirements.

Normally made from cast or forged steel, although superior material can be ordered to suit process fluids.

End connections are normally flanged.

Typical standard is ANSI.

Also available in welded connections (high pressure valves), threaded connections (small valves).

Internals of valve body is called valve trim, consisting of plug and seat.

Material of trim is carefully selected based on process fluid compatibility and process conditions.

The relative position of plug to seat determines the flow capacity through the valve, thus the process resistance provided by the valve.

The trim size does not necessarily match the body size of the valve.

Trims smaller in capacity than the body maximum capacity is called reduced trim.

ii. Actuator

The device that provides the force to adjust the valve condition, which in turn adjusts the process flow.

Typically a pneumatically operated spring-balanced device. Also available as hydraulic operated or electrically operated devices. Typical designs are diaphragm and cylinder types.

Mounted directly on the valve body by a yoke or bracket.

iii. Stem/shaft

The physical link that transfers force between the actuator and the valve body internal components.

Consist of actuator stem, valve plug stem/shaft, with a link mechanism in between.

The link mechanism is typically threaded with lock, cam mechanism, or rack-and-pinion mechanism.

The size and type of stem and link is determined by the actuator force, process requirements, and manufacturers standard designs.

iv. Positioner

The device that controls the force or applied by the actuator. Only applicable to regulating valves that takes CO from a controller. For pneumatic positioners, the mechanism is similar to pneumatic controllers, with

the flapper valve as the primary device. Typically mounted on the yoke or actuator with a mechanical feedback linkage connected to

the actuator stem.

v. Accessoriesa. Current-to-Pneumatic (I/P) converters

- Converts current signals to pneumatic signals.- Used to support older pneumatic-to-pneumatic (P/P) positioners with

electronic or microprocessor based controllersb. Solenoid valves

- Routes supply pressure to actuators or drains pressure from actuators depending on energizing position.

- Used in-lieu of positioners for valves in on-off services.- Used to trip regulating valves into its fail-safe position when required

by safety systems.- Used to route supply pressures and lock pressures for hydraulic piston

actuators.c. Limit switches

- Switches installed at valve travel limits and sometimes intermediate travel positions.

- Used to feedback valve travel positions to controllers for further action.

- Typically applied to on-off service valves and sequencing valves.d. Regulators

- Regulates instrument air pressure from supply header (typical 7.6 barg) to control valve supply pressure (typical < 4.0 barg).

- Used to protect positioners and actuators due to over-pressure damage.

- Proper sizing is required to ensure valve stroking time meets design requirements

e. Boosters- Takes the output of positioners as its Controller Output (CO) and

multiplies the volume, and sometimes pressure, applied to the actuator.

- Typically used for large volume actuators to ensure valve stroking time meets design requirements.

- Also used to multiply positioner output pressure to meet actuator spring force

f. Lock-up valves- Locks pressure inside actuator chamber when instrument air header

pressure drops below a prescribed setpoint.- Used to maintain a valve at its last travel position in the event of

instrument air failure.- Typically specified to protect process equipment from damage caused

by a sudden change in process flows.g. Pneumatic speed controllers

- Regulates the rate of supply to actuators or exhaust from actuators thus the valve stroking speed.

- Used to protect the valve or its associated process equipment from harmful sudden changes in process flows.

- Used exclusively on valves without positioners.

Flow Characteristic

Defines the flow variation through the valve with respect to percentage of valve opening with a constant pressure drop.

Three common characteristic types.

Linear

- A flow characteristic in which the valve relative opening is directly proportional to the percentage flow eg. 50% open valve gives 50% of maximum flow, with a constant pressure drop across the valve

- Valve gain is the same at all flows.- Commonly specified for liquid level control and some flow control.

Equal Percentage

- a flow characteristic in which equal increment in the valve opening cause a constant percentage increase in flow with a constant pressure drop across the valve.

- Valve gain increases with increasing flow.- The most typical valve characteristic used.- Process gain normally decreases with increasing flow.

Quick Opening

- Quick opening characteristic provides a majority of its flow capacity at small valve opening.

- Flow close to a maximum flow is reached with a very small opening- Valve gain is high at smaller opening but decreases quickly afterwards.- Typically used for on-off services and with liquids with high density

and viscosity-

Cavitation and Flashing

(Cavitation: the formation of an empty space within a solid object or body.)

(Flashing : pieces of sheet metal or the like used to cover and protect certain joints and angles)

Two physical phenomena in liquid streams that causes damage to control valve bodies and trim.

These can also limit the flow capacity of a valve.

Caused by a phase change of the process fluid as it goes through a valve due to pressure drop.

As flow velocity increases at the valve due to the flow restriction, pressure decreases. Pressure is recovered, but not fully, downstream of the restriction.

Problem when pressure drops below the process fluid vapor pressure.

Cavitation

Cavitation is a two stage liquid flow phenomena. The first stage is the formation of vapor bubbles in the liquid as the fluid passes through the trim and the pressure is reduced below the fluid's vapor pressure. The second stage is the collapse of the vapor bubbles back to a liquid as the fluid passes the vena contracta and the pressure recovers and increases above the vapor pressure. The collapsing bubbles are very destructive when they contact metal parts and the bubble collapse may produce high noise levels.

Pressure drop below process fluid vapor pressure causing bubbles to form in the fluid. The bubbles collapse as pressure recovers above process fluid vapor pressure downstream.

Recognizable by whistling or gravel-like noises at the valve and downstream piping.

Collapsing of the bubbles causes the mechanical damage.

Most damage occurs in downstream half of valve body and downstream piping.

Special valve designs are available to prevent cavitation.

Most manufacturers use multi-stage designs to control pressure drop within the valve above the process fluid vapor pressure.

Most designs are cage type, although some use multi-stage globe designs.

a. Single Stage Trim Anti-Cavitation Containment

Fluid enters through the drilled cage. Bubbles form and collide into each other. As the bubbles collide they implode against each other instead of the valve trim.

Flashing

Flashing is similar to cavitation except the vapor bubbles do not collapse, as the downstream pressure remains less than the vapor pressure. The flow will remain a mixture of vapor and liquid.

Pressure drop below process fluid vapor pressure and does not recover, causing the fluid to completely flash to vapor downstream of the valve.

The high velocity of the flow causes the mechanical damage.

Most damage occurs at the point of highest velocity, typically at the valve plug, seat, and cage.

Sometimes flashing is a required service of a control valve.

Example in cold box services.

AGRU process

Flashing is tackled by valve design.

Remove the flashing point away from physical components and pipe walls e.g. use venturi seat and pipe expanders.

Use hardened material to resist damage due to flashing.