SPECIAL TASK VALVES PART 4: PNEUMATICALLY ACTUATED FLOWRATE CONTROL VALVES

Smooth operatorSmooth ...operatorSuch a smooth operator

("Smooth Operator," by Sade and R. St. John, 1984)

REVISITING A FAMILIAR EXAMPLE OF A PNUEMATICALLY ACTUATED CONTROL VALVE

This Pneumatically Actuated Control Valve is supplied Instrument Air in the range of 3 to 15 psig) to move the Valve from being "totally open" to "totally closed."

The Pneumatically Actuated Control Valve is the most popular type of actuated valve because of its smooth movement over a range of operation ... from totally open to totally closed... whilst actuated by an Instrument Air signal supplied between 3 to 15 psig.

The above statement means the Pnuematic Actuated Control Valve does not move in a choppy, step-stair manner over its range of operation.

The $10 dollar word that can be efficiently substituted for the phrase "smoothly moves over a range of operation that corresponds to an input signal instead of moving in a choppy, stair-step manner" is the word analog.

The smooth, analog operation of a Pneumatically Actuated Valve results in correspondence between the Instrument Air input signal and the position of the Valve over its range of operation.

Hence, maintaining the Valve's position where it will achieve the Set Point Value of the controlled Process Variable (Temperature, Pressure, Flowrate, or Level) is easier.

Shell and Tube Heat Exchanger E-1004 exchanges heat between hot Reactor Effluent (Tube Side) and cold Reactor Feed (Shell Side).

The most brilliant PTOA Readers and Students are those who have been slogging along ... reading the PTOA Segments in the intended, sequential order.

Way back in PTOA Segment #16, a typical PV Temperature Control Loop 10045 was featured in the nearby P&ID snippet.

STOP HERE and refresh need be on the role each of the hardware elements plays in Temperature Control Loop 10045.

Go ahead. Your Mentor can wait.

Alrighty!

Every PTOA Reader and Student is "knowed-up" on the form and function of each hardware component which appear in Temperature Control Loop 10045.

Pneumatically Actuated Butterfly Valve.

TV10045 is a great example of a Pneumatically Actuated Temperature Control Valve.

The actual valve might look like the valve shown in the nearby photo of a Pneumatically Actuated Butterfly Valve (Butterfly Valves were featured in PTOA Segment #253). The gif at the top of this PTOA Segment featured a Pneumatically Actuated Plug Valve.

Any valve that is actuated by Instrument Air is a Pneumatically Actuated Valve.

Left: Pneumatically Actuated Control Valve. Middle: Solenoid rapid On-Off Valve Rightmost: Inching and On/Off MOVs.

PTOA Segment #258 described the process service niches which would warrant selection of Electrically Actuated and Hydraulically Actuated Valves.

Pneumatically Actuated Valves are so much more popular than either Electrically Actuated or Hydraulically Actuated Valves that they are typically referred to as the Final Control Element (FCE) or as a Final Control Valve (FCV) and very often as simply "the Control Valve."

Otherwise stated:

A casual reference to an "FCE" or "FCV" or simply "the Control Valve" infers a Pneumatically Actuated Control Valve unless otherwise stated.

A row of Pneumatically Actuated Valves. Air Regulators can be seen on each Valve. Each Air Regulator reduces the Air Header Pressure to the workable range for the valve.

The Pneumatically Actuated Valve is most popular because:

Air is free. Electricity must be generated.
Air compressors and their associated distribution hardware (air filters, headers, tubing for instrument air supplied to the Valve) are low tech, reliable, and inexpensive.
Electric Valve Actuators cannot be located in spark free process services.
With an incorporated Valve Positioner, the Pneumatically Actuated Valve can achieve accurate and continuous "analog" control of a Process Variable (PV Pressure, PV Temperature, and mostly PV Flowrate) throughout the specified range.

FUNCTION OF THE PNUEMATICALLY ACTUATED CONTROL VALVE

Fred

The function of a typical Pneumatically Actuated Control Valve can best be explained by dissecting the PV Temperature Control Loop 10045 P&ID snippet again.

A great animated graphic that shows how a Shell and Tube Heat Exchanger works.

The last time PTOA Readers and Students like Fred focused on Temperature Control Loop 10045, they did not know what Shell and Tube Heat Exchangers were or how Conduction Heat Transfer and Convection Heat Transfer work.

Shell and Tube Heat Exchangers were featured in PTOA Segments #30 through #35. Conduction Heat Transfer was featured in PTOA Segments #62 and #63. Convection Heat Transfer was featured in PTOA Segments #64 and #65.

Temperature Control Loop T 10045. The PV Temperature of the E-1004 Shell Side effluent process stream is the PV Temperature that must be controlled. The effluent stream is heated up by heat exchange with hot process fluid flowing through the Tube Side of E-1004.

Brilliant PTOA Readers and Students are now prepared to decode how Temperature Control Loop 10045 controls the temperature of the process stream exiting the Shell Side of E-1004.

Note that ...

although the controlled process variable is the PV Temperature detected at TE 10045 (aka, the "Shell Side Effluent" flow from the Exchanger) ...

it is the amount of hot fluid bypassed around E1001 ... aka a "flowrate"... which will determine how hot or cold the PV Temperature measured by TE 10045 will be.

Note! Varying a Flowrate to control the PV Temperature is very common.

Guess what, Fred?

The Pneumatically Actuated Temperature Control Valve tagged TV 10045 is the only hardware component in the Temperature Control Loop that performs a physical action which encourages the PV Temperature sensed and measured at TE 10045 to move closer to the Set Point Temperature Value.

The job of the Pneumatic Actuator is to convert instrument air in the range of 3-15 psi into the mechanical action that makes it possible to open, close, or just smoothly regulate the PV Flowrate of the process fluid flowing through the valve.

The mechanical action could be rotary action (for example, a Ball Valve or Butterfly Valve) or up-and-down action for more precise flow control elements (for example, a Plug Valve).

Exactly how does a Pnuematic Actuator perform its job function?

The mechanics regarding how a Pneumatically Actuated Temperature Control Valve like TV 10045 moves up and down or rotates is explained in the next section. The rest of this section gives some context regarding how the movement of a Butterfly Valve like TV 10045 helps control the Process Variable Temperature.

Butterfly Valves and Ball Valves must be rotated. They do not move up-and-down.

First, the Control Board Operator enters the desired Set Point Temperature into Temperature Indicating Controller TIC 10045. The Set Point Temperature is the target temperature that will be sensed at TE 10045.

When the PV Temperature sensed at TE 10045 is lower than the Set Point Temperature Value, TV 10045 will be automatically actuated to smoothly close a little which causes more hot process fluid to flow through the tube side of E1001.

Exchange 1001 works like this: YELLOW ARROWS: Hot Tube Side Process Flow that needs to be cooled down. RED ARROW: Cold Shell Side Process Flow that needs to be heated up.

The increase in convection heat transfer will eventually raise the PV Temperature sensed at TE 10045 to the desired Set Point Temperature.

And vice versa ...when the PV Temperature sensed at TE 10045 is higher than the Set Point Value, TV 10045 will be actuated to increase flow through the E1001 bypass process stream piping. The decrease in convection heat transfer will eventually lower the PV Temperature sensed at TE 10045.

When the PV Temperature sensed at TE 10045 is at the desired Set Point Temperature, TV 10045 will not be automatically actuated to move from the position it is in and will hold its position steady.

FORM OF PNUEMATICALLY ACTUATED VALVES

Up-and-Down Pneumatic Actuators

Thanks to BCSTVALVE.COM for the great gif that shows how Instrument Air is used to Pneumatically Actuate this Trim Valve. Up-and-Down valve movement is used when precision PV Flowrate control is needed.

Pneumatic Actuators that generate up-and-down Valve movement will have a Diaphragm.

This Pneumatically Actuated Plug Valve includes an example of a Direct Acting, up-and-down movement Actuator. Instrument Air is supplied at the external top of the Valve Actuator section of the Valve. The force balance relationship between the force factor of the Instrument Air pressure and the force of the Spring on either side of Diaphragm will determine whether the connected Stems and Plug will move up (Open) or down (Closed).

The nearby labelled graphic shows a Pneumatic Actuator with a Diaphragm.

Note the red labelling that delineates the hardware components within the upper 'Valve actuator' section from the hardware components in the lower 'Valve body' section.

Note that the Actuator Stem in the top Valve actuator section is attached to the Valve Stem in the lower Valve body section by a Connecting ring.

The architecture of the Pneumatic Actuator will determine if the Valve is Direct Acting (increasing air pushes Valve Stem downward) or Reverse Acting (increasing air pushes Valve Stem upward). The Valve architecture determines if the Valve will "Fail Open" or "Fail Closed" upon loss of Instrument Air.

The Diaphragm splits the Valve Actuator section into a (not labelled) upper chamber and lower chamber. A Spring(s) is/are in the lower chamber.

In the nearby graphic, Instrument Air enters the upper chamber at the external center top of the (Pneumatic) Valve Actuator.

The downward force created by the force factor of the Instrument Air pressure is exerted upon the upper surface of the Diaphragm. This force from Instrument Air pressure is simultaneously offset by the upward, opposing force of a Spring impacting the bottom side of the Diaphragm.

Thank you BCSTVALVE.COM for illustrating how the force balance relationship between the Instrument Air and Spring makes the connected Stems move on the Stroke Scale.

At minimum Instrument Air pressure, the Spring tension is set to hold the Diaphragm at the end of the travel path, at the top of the Stroke Scale.

A quick glance at the nearby animated Pneumatically Actuated Valve gif will better illustrate how the connected Stems move up-and-down on the Stroke Scale while the force balance relationship between the Spring and the force factor of the supplied Instrument Air changes.

A "Direct Acting Control Valve" means more air lowers the Valve Trim, less air lifts up the Valve Trim. Valves can be "Direct Acting" and "Indirect Acting."

The Pneumatically Actuated Valve gif also illustrates how the connected Stems and the Plug would move 100% upward at minimum Instrument Air pressure, resulting in a completely open Valve.

Likewise, the connected Stems and Plug would move downward as the force factor of the Instrument Air pressure exceeds the Spring's force.

At maximum Instrument Air Pressure, the Plug settles into the Valve Seat and the process flow through the valve stops.

The tension in the Spring is specified so that when maximum Instrument Air Pressure is exerted on the top of the Diaphragm, the connected Valve Stems and Plug Valve will move to seat the Plug in the Valve Seat. The Plug Valve will shut off all flow. The PV Flowrate is 0.

Between the maximum and zero flowrates described above, the PV Flowrate can be regulated by Instrument Air to a desired Set Point value.

Guess what? Not all Pneumatic Actuators have Springs.

BEWARE! The Fail Open/Fail Closed architecture of a Valve depends upon the design and selection of the Valve Actuator and Valve Body.

Springless Pneumatic Actuators may use a Four-Way Solenoid Valve to switch air pressure between the upper chamber and lower chamber of the Valve Actuator section. Solenoid Valves were recently featured in PTOA Segment #258. Springless Pneumatic Actuators are more flexible, but they require more air pressure controls.

Rotary-action Pneumatic Actuators

The ISA Symbol for TV 10045 indicates that this valve is a Butterfly Valve.

Temperature Control Valve TV 10045 in the nearby P&ID snippet is depicted as a Butterfly Valve. Butterfly Valves are not up-and-down action Valves.

The Butterfly Flow Control Valve body of TCV 10045 would be attached to a Rotary Piston Pneumatic Actuator,

Pneumatic Actuators used for rotary action will have Pistons, not Diaphragms.

Thanks to Engineering Concepts for a You Tube that tries to explain the Working Principle of a Rotary Pneumatic Actuator HERE.

Sincere thanks to Engineering Concepts albeit the You Tube is robotic and choppy. Furthermore, the example shown is a Ball Valve, not a Butterfly Valve as repeatedly stated. Lastly, PTOA Readers and Students can cut off the video at 1 min 39 seconds; the remaining part of the video is not related to the subject matter.

VALVE POSITIONERS

The input to a Positioner is a pneumatic or electric control signal. The output of a Positioner is a proportional Instrument Air pressure signal delivered to the Pneumatic Actuator's Diaphragm.

A Valve Positioner will provide tighter flow control than a solo Pnuematically Actuated Valve can.

The input to the Valve Positioner is Instrument Air. The output of the Valve Positioner is proportional air pressure supplied to the Pneumatic Actuator.

The Valve Positioner amplifies the pneumatic signal sent to the Pneumatic Actuator which makes it possible to more precisely move the connected Actuator Stem-Valve Stem-Flow Element assembly.

An electro-pneumatic Valve Positioner,

An electro-pneumatic Valve Positioner has a 1-5 mA, 4-20 mA, or 10-50 mA current supplied to it. The pneumatic-pneumatic Valve Positioner typically has a 3-15 psig instrument Air signal supplied to it.

Detailed knowledge regarding the operating mechanics of a Valve Positioner is not required of a Process Operator.

Briefly ...

A force balance relationship is the operating principle for both the Pneumatic Actuator and the Valve Positioner.

Just like the Pneumatic Actuators described above, the Valve Positioner operates according to a force balance relationship.

The force balance relationship within the Valve Positioner involves yet another Spring (or Bellows) and the back pressure created from Instrument Air flowing through a flapper-nozzle.

The outcome of the force balance relationship is an amplified Instrument Air signal sent to the Pneumatic Valve's Actuator.

The internals of a Valve Positioner include a Spring or Bellows in a force balance relationship with a Flapper-Nozzle through which Instrument Air flows. The Instrument Air Pressure signal is amplified via a Booster Diaphragm located in the Booster Relay. The amplified air signal is sent to the Actuator's Diaphragm. Feedback from a Lever that moves up and down with the Stem constantly re-establishes the force balance relationship between the Spring/Bellows and the Flapper-Nozzle.

Simultaneously, a Lever that moves with the Pneumatic Valve's Stroke Scaler provides continuous positioning feedback to the pivoting flapper. The distance between the flapper and the nozzle is constantly re-adjusted, hence the force balance relationship between the flapper-nozzle and the Spring (or Bellows) is constantly updated.

In summary ...

The Valve Positioner is yet another example of low-tech yet impressive process technology that PTOA Readers can just be astonished was invented in the first place.

The Instrument Tech installs a Pneumatic-Pneumatic Valve Positioner.

This low-tech device greatly fine tunes the up-and-down movement of the Pneumatically Actuated Valve's Stem and connected flow element. The greater sensitivity of the Valve's movement significantly improves the efficiency of the Control Loop regarding maintaining the desired Set Point Value of the controlled Process Variable.

TAKE HOME MESSAGES: Pneumatically Actuated Valves are the most popular type of actuated valve. A casual reference to a "Control Valve," a "Final Control Element (FCE)," or a Final Control Valve (FCV)" typically infers a Pneumatically Actuated Control Valve.

Any valve that is actuated with a gas is a Pneumatically Actuated Valve. The gas is typically Instrument Air supplied at 3-15 psig because air is free.

The smooth, analog operation of a Pneumatically Actuated Valve results in correspondence between the Instrument Air input signal and the position of the Valve over its range of operation.

The job of the Pneumatic Actuator is to convert the supplied Instrument Air into the mechanical action that makes it possible to open, close, or just smoothly regulate the PV Flowrate of whatever process fluid is flowing through whatever type of Valve the Pneumatic Actuator is attached to.

The mechanical action created by the Pneumatic Actuator could be rotary action (for example, a Ball Valve or Butterfly Valve) or up-and-down action for more precise flow control elements (for example, a Plug Valve).

Pneumatic Actuators work by force balancing the force factor of the supplied Instrument Air pressure and the opposing force of a Spring. These two forces are applied to either side of a Diaphragm.

Springless Pneumatic Actuators exist; they require more Instrument Air controls.

Valve Positioners are used to amplify the Instrument Air signal to the Valve Actuator. Signal amplification makes it possible to more precisely move the connected Actuator Stem-Valve Stem-Flow Element assembly. Just like a Pneumatic Actuator, the operation of a Valve Positioner depends upon a force balance relationship.

The flowrate through a Pneumatically Actuated Valve is commonly used to control PV Temperature, PV Pressure, and PV Level even though the tag name of the Valve will not be labelled as "F" for "Flowrate."

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

You need to login or register to bookmark/favorite this content.