Control Valves, Actuators & Positioners : Operation & Design

Describe common valve parts and functions in the process industry, including body, bonnet, stem, actuator, packing, seat, and desk, and describe four flow control elements and stem leakage prevention.

Learn how control valves regulate fluid flow and pressure by stopping, starting, and regulating flow, direction, and downstream pressure, using basic parts like body, bonnet, trim, packing, and actuator.

The valve body, or shell, forms the primary pressure boundary and framework, receiving inlet and outlet piping through bolted flanges, while partitions support the throttling seat and balance manufacturing considerations.

Explore the valve bonnet, the second pressure boundary, and how bolted joints attach the bonnet to the body, with design variations affecting cost and leakage.

Explore valve trim components—disc, seat, stem, and sleeve—and how their interface and disc position control flow in rotational and linear designs.

Understand how a bonneted valve uses the disc as the third primary pressure boundary to permit or prohibit flow, with a finely finished seating surface for reliable sealing.

Connect the stem to the actuator to position the disc, and note rising stems rise on opening while non rising stems stay stationary.

Describe the main principles and operation of valve actuators, featuring hand wheel, motor, solenoid, nomadic and hydraulic operators, and how bonnets or yokes support them outside the pressure boundary.

Describe how packing seals the space between the valve stem and bonnet and demonstrate packing replacement via the packing gland, using Teflon, emphasizing compression to prevent leaks and stem damage.

Analyze the internals of a pneumatic control valve, including the actuator, diaphragm, spring, and trim, and how signal pressure and spring balance throttle flow to control process variables.

Compare single board trim and double board trim, showing how process pressure creates unbalanced force on the plug and how double trim reduces unbalance but increases wear and seating issues.

Select the direction of action for control valves: air to close (direct acting) or air to open (reverse acting), guiding safety during instrument air failure.

Explore a 3d model showing the dismantling of a standard plug valve with a diaphragm actuator, detailing steps from unscrewing the packing adjuster to removing the plug valve body.

Dismantle the same control valve using a2d model, sequentially removing packing adjuster, gland and packing, pressure chamber, diaphragm and plate, spring assembly, bonnet, yoke, stem, plug, and valve body.

Explore valve types for process control, including linear globe and gate valves and rotary bowl valves, butterfly valves, and plug valves, and compare two-port and three-port configurations.

Understand shut-off tightness by examining double seat leakage at class three (0.1% of full flow) and single seat classifications (class four to five) per ANSI B16.104, noting cost.

Globe valves enable throttling control by linking valve opening to flow, with single-seat and double-seat two-port configurations driven by an actuator for closure under differential pressure.

Specify a single seat valve to prevent leakage with tight shut-off, and employ a piston balanced mechanism that uses a balancing chamber to provide downforce and aid closure.

Slide valves, wedge gate and parallel slide, isolate fluid with tight shutoff and low pressure drop. Parallel slide uses a wedging mechanism to open on CCW and close on CW.

Rotary valves, including ball, butterfly, and plug types, use a rotating element to open or close flow with full bore and low pressure drop, often fitted with actuators.

Explore globe valve options for control valves, including gland packing materials and configurations, friction effects on actuators, and guide methods such as double guided, guided plug, and perforated plugs.

Examine the operation of two port valves for steam, liquids, and thermal oils, highlighting easy activation, versatility, and balancing flows; note that three port valves divert or mix when needed.

Explore three-port valves' ability to mix or divert flows using ports A, B, and the constant volume port AB; piston, globe plug, and rotating shoe types.

Examine three-port valves in closed circuits, using a mixing valve to route or divert hot water between the heat exchanger and bypass, controlled by ports A and B.

Control valves influence processes by adjusting flow rate and differential pressure, including two-port and three-port configurations, delta P, and the flow coefficients kvs, kv, and cv.

Explore Cv and CV as the valve flow coefficient and its conversion, and calculate pressure drop from flow rate in gallons per minute at 1 psi.

Learn to use the flow coefficient CV to evaluate hydraulic performance of valves and components, calculating equivalent CV and predicting flow or pressure changes.

Explore how valve trim shape determines flow characteristics, and how valve lift relates to the orifice area, with fast opening, linear, and equal percentage curves in globe and rotary valves.

Explore fast opening valve plugs that produce large flow changes with small valve lifts, often used for on-off control, while noting that these curves are not standardized and can vary.

Shows how a linear valve plug makes flow rate directly proportional to valve lift at constant differential pressure through a linear lift-to-pass area relationship.

Equal percentage valves use a plug shape so lift increments yield proportional flow increases, with a logarithmic lift–flow relationship defined by e, theta, H, and Vmax.

Analyze how an equal percentage control valve with a 50 range and constant differential pressure yields flow increases with lift, showing 40%, 50%, and 60% cases.

Match the valve characteristic to the process, ensuring a valve opening-to-flow relationship. Compare linear valves for water and equal percentage valves for steam, with pressure differentials and pump curves.

Demonstrates a water circulating heating system using a three-port valve to maintain balance by keeping valve pressure loss stable, favoring a linear valve characteristic for matched operation.

Explore a boiler water level control system using a two-port feed water valve, and analyze how flow rate, differential pressure, and head loss affect valve sizing and installation curves.

Compare linear and equal percentage valves; linear valves require less lift for the same flow, while equal percentage valves demand higher lift, e.g., 7 m3/h: 35% vs 75%.

Explore temperature control of a steam-based heat exchanger by varying steam flow through a two-port control valve, linking pressure, temperature, and heat transfer to valve sizing.

Analyze how choosing a larger control valve shifts lift–flow curves for linear and equal percentage valves, revealing fast acting behavior and why equal percentage offers better control when oversized.

Explore how a control valve actuator positions the plug against the seat using a control signal, with nomadic and electric actuation for open, closed positions via on/off or continuous control.

Piston actuators handle higher pressures and faster actuation than diaphragms, driving globe valves by air-to-open or air-to-close action with components like adjusting screw, pistons, cylinders, stems, and o-rings.

Diaphragm actuators use compressed air on a flexible diaphragm to move the spindle and open or close valves, in direct or reverse acting configurations.

Choose actuator and valve combinations based on application and safety requirements, and analyze fail-open and fail-closed behaviors across direct and reverse acting configurations when air supply fails.

Differential pressure adds lifting force to the valve plug, altering valve lift for a given air signal, and the effect varies with actuator type.

Valve positioners relate input signals to valve position, providing actuator pressure to overcome friction and differential pressure. They can be numeric or digital, fitting to the actuator yoke with feedback.

In force balance positioners, input pressure is opposed by a spring; valve stem position is adjusted via bellows and a flapper-nozzle to restore balance and produce the desired valve travel.

Demonstrates how a positioner amplifies a control signal to modulate air pressure in a valve actuator, overcoming spring resistance to precisely position the valve plug.

Positioners link input pressure to valve position, maintaining it despite differential pressure. They amplify signals and convert 4–20 mA inputs via electro pneumatic or I to p converters.

Fit a positioner when precise positioning or faster response is needed; it uses higher pressure and airflow to adjust valve, amplifies actuator force, and reduces hysteresis under varying differential pressures.

A p to p pneumatic positioner accepts a pneumatic signal from the control system and outputs a numeric signal to move the actuator, closing a valve against high differential pressure.

Show how the I to p converter allows electronic control systems to drive a numeric valve and actuator by converting 4 to 20 milliamps to 0.2 to 1 bar.

I to p transducers convert electronic current signals (4–20 mA) to pneumatic signals (3–15 psi) to drive pneumatic valve actuators in process control systems.

Digital valve positioners use a microprocessor to sum input and feedback and adjust valve position, enabling calibration and communication via hart, foundation fieldbus, and profibus.

Select a valve using process data and match its characteristics to the process. Determine valve action on power failure, decide on a positioner, and specify numeric or electric signal converter.

Explore pneumatic actuators powered by gas pressure for linear and quarter-turn valves, including single and double acting designs, fail open/close modes, and easy mode conversions illustrated with 3D animation.

Examine a single acting spring-return actuator, its components, and how gas pressure on pistons creates torque to operate a quartered turn butterfly valve, with springs closing valve when pressure drops.

Examine a single acting actuator with spring, piston, piston rod, position indicator, and yoke, where gas pressure opens or closes the valve and the spring returns it when pressure drops.

Explore a double acting actuator with two cylinders and two pistons, featuring four gas inlets for opening and closing positions, and observe its fail-safe behavior during gas pressure loss.

See a double acting actuator with two cylinders and two pistons, four gas inlets for opening and closing positions, and the valve keeps its position when gas pressure is lost.

Convert a single acting spring return actuator to a double acting design by removing the spring and using gas pressure from both sides to open and close the valve.

Convert a fail-close, single-acting spring-return actuator to fail-open by rotating Scotch Yoke actuator 180 degrees anticlockwise, reversing the spring action so that in the absence of gas the valve opens.

Convert a failed closed single acting rack and pinion actuator to fail open by reversing piston orientation and reassembling springs, enabling air pressure to open the valve when supply fails.

Compare rack and pinion and Scotch yoke pneumatic actuators, analyzing torque profiles, cycle times, throttling performance, and cost to select the right design for your valve application.

Explain remote valve position indication for field and control room operators using position detectors and indicator lights, including mechanical limit switches, rising stem position, and non rising stem pointers.

Select actuator type by power mode and match to control accuracy and speed; for steam use two-port valves, for liquids two-port or three-port valves, ensuring sizing and differential pressure considerations.

Ensure correct valve installation with proper size, pressure rating, materials, and end connections. Install square flange joints in horizontal pipelines with vertical spindles, upstream strainers, maintenance access, and bypass.

Explore actuator installation and compatibility considerations, including vertical mounting, exposure limits to heat and moisture, built-in heaters and enclosure ratings, and sensor immersion strategies to balance response time.

Review essential concepts in control valves, actuators and positioners to ensure you complete the course with a solid understanding of operation and design.