Control Valves: Types and Positioners

The final element of the loop

In most process loops the actuator is a control valve: it changes the size of the fluid passage according to the controller signal and, with it, the flow, and consequently the pressure, temperature or level. A loop can have the best transmitter and the best tuning, but if the valve is unsuitable, the result will be poor. That is why at commissioning the valve is often the cause of more control problems than the controller itself. To set the PID gains, the Ziegler-Nichols method and its alternatives are explained in Ziegler-Nichols tuning and alternatives.

A complete control valve has three sets of elements:

1. Body and trim. The body carries the fluid; the trim (plug, seat, cage) forms the variable restriction.
2. Actuator. Converts the signal into movement.
3. Accessories. Positioner, I/P converter, filter regulator, limit switches, handwheel.

Valve types

The general classification by motion (sliding or rotary) and by pressure recovery (high in butterfly, ball and plug; low in globe and angle) is the usual one. The assignment of applications in the table is an engineering criterion and does not replace the manufacturer’s recommendation for a specific fluid and conditions.

Actuators

The most common actuator is the pneumatic one, because it only needs compressed air, is fast and, if pressure is lost, a spring can take the valve to a safe position. Electric ones need additional wiring, and hydraulic ones need high-pressure lines.

Traditional pneumatic signals run from 3 to 15 psi (0.2 to 1.0 bar); in industry today 4 to 20 mA signals are mostly used, and in HVAC 0 to 10 V is common. To bring a current signal to a pneumatic actuator, a current-to-pressure (I/P) converter is used, today built into most positioners.

The positioner

The positioner is a closed position loop around the actuator. It compares the actual stem position with the command and regulates the air until they match. Its reason for being is to overcome packing friction and fluid forces on the plug, which without it would keep the valve from moving until the signal exceeded some threshold.

There are three types:

• Pneumatic: lever, bellows and nozzle with mechanical feedback.
• Analog: receives 4 to 20 mA and converts it to pressure through an I/P converter.
• Digital: with a microprocessor, adds diagnostics and two-way communication through HART, FOUNDATION Fieldbus or PROFIBUS.

A digital positioner also allows recording the valve signature, measuring friction, detecting sticking and issuing maintenance alerts, which is useful in a plant because the condition of a valve cannot be seen from outside. It also allows adjusting the travel limit and the flow characteristic by software.

Fail-safe and direction of action

Two concepts are often confused:

• Actuator action: “air to open” (the restriction falls as the signal rises) or “air to close”.
• Fail position: where the valve goes if air or signal is lost, which depends on the spring and the actuator action.

The fail-safe position is chosen by what happens in the process: the cooling water valve of a reactor fails open so as not to lose cooling; the valve of a chemical reagent fails closed so as not to keep feeding. That decision belongs to process engineering and is documented, because it changes the controller action and the interlocks. For valves without a spring, such as some with a double-acting actuator, a reserve air receiver or a design that defines behavior on loss of air is needed.

Flow characteristics

The inherent characteristic relates travel to flow at constant pressure drop. The three most common:

In a real installation the pressure drop across the valve is not constant: it changes with flow because the other piping losses also vary. So the installed characteristic differs from the inherent one. The selection criterion is that the total loop gain be approximately constant across the whole operating range.

Selection criteria

1. Fluid and conditions: liquid, gas or steam, pressure, temperature, viscosity, solids, corrosion.
2. Maximum, normal and minimum flow and the associated pressure drops.
3. Capacity: required flow coefficient (see the sizing guide).
4. Rangeability: ratio between maximum and minimum controllable flow.
5. Shutoff: allowable leakage class.
6. Cavitation and flashing: risk of trim damage with liquids when local pressure falls too far.
7. Fail-safe position and actuator action.
8. Materials compatible with the fluid and with the applicable standard.
9. Response speed required by the loop.
10. Accessories and communications.

Example (illustrative)

A tank level loop, with an outlet valve. The data are made up.

• The valve must fail open so the tank does not overflow if air is lost: spring actuator, “air to close”.
• If the level rises, the controller must open the outlet further: the controller output increases with level (direct action), but since the valve is “air to close”, the 4 to 20 mA signal must be inverted in the controller or in the positioner.
• If that combination is not documented, a technician who replaces the positioner with another without configuring the direction of action will leave the loop with positive feedback.

The example shows why the direction of action must be recorded for each valve on its datasheet.

Common mistakes

• Oversized valve. It works almost closed, with little resolution and seat wear.
• No positioner where there is appreciable friction. The loop oscillates in a limit cycle.
• Fail-safe not analyzed. The valve ends up in a hazardous position when air is lost.
• Dirty or wet instrument air. The most frequent cause of positioner failures.
• Confusing actuator action with controller action.
• Flow characteristic chosen from the catalog alone. Without considering the installed pressure drop.
• Ignoring cavitation and noise. Over time they erode the trim.
• Not doing a stroke test. At loop handover, nobody checks that the valve travels its whole range.

Checklist

1. Datasheet with fluid, conditions, capacity, shutoff class and materials.
2. Actuator action and fail position defined and documented.
3. Positioner configured: direction, range, characteristic, limits.
4. Instrument air quality verified: dry, clean and regulated.
5. Full stroke test with position-versus-signal record.
6. Opening and closing times measured.
7. Friction and hysteresis within what is acceptable for the loop.
8. Fail-safe tested by simulating loss of air and loss of signal.
9. Positioner alarms and diagnostics integrated into the control system.

Engineering support

Valve selection and its integration with control is part of plant design; its checking is part of commissioning. If you need support in defining instrumentation and strategy, see engineering design and studies, delivered remotely and worldwide. If you need to verify the stroke, fail-safe and response of each loop before handover, see testing and commissioning, in Colombia or elsewhere on request. Write to us through contact.

Related guides

• Motor control center (MCC)
• Siemens SINAMICS drive
• ABB ACS880 drive
• Induction motor and its protections
• Valve sizing: Cv and Kv
• Industrial pneumatics basics: components and control
• Cascade, ratio and feedforward control

References

• Wikipedia, Control valve (actuators, positioners, signals, fail-safe, classification).
• Kuphaldt, T. R., Lessons in Industrial Instrumentation, chapter on control valves (CC BY 4.0).
• Woolf, P., Chemical Process Dynamics and Controls, chapter on sensors and actuators (CC BY 3.0).
• IEC 60534 family, industrial-process control valves (general reference, no figures).

Frequently asked questions