Control valve actuators, positioners, sizing

Control valve actuators

An actuator is the part of a valve assembly that responds to the output signal of the process controller, causing a mechanical motion to occur which, in turn, results in modification of fluid motion through the valve. An actuator has to be able to perform two basic functions:

1. To respond to an external signal and cause a valve to move accordingly and with correct selection, other functions can be integrated into this assembly, such as desired fail-safe actions.

2. To provide support (if required) for accessories such as positioners, limit switches, solenoid valves and local controllers.

There are five basic forms of valve actuator, as listed below, and a description of each follows:

1. Digital

2. Electric

3. Hydraulic

4. Solenoid

5. Pneumatic.

The first four have a totally different method of operation and application use as compared to the last one, the pneumatic actuator.

Digital, electric, hydraulic and solenoid actuators

This section describes the common factors of these valves:

1. Digital

2. Electromechanical

- Stepping motors in smaller size valves

- Reversible motors and gearboxes for larger size valves

3. Electrohydraulic (the pump being driven by stepping or servo motors)

4. Solenoid operation.

Energy sources

Electrical or electrohydraulic.

Speed reduction techniques; Worm gear, spur gear or gearless.

Torque ranges:

• 0.5-30 ft lb (0.6-40 Nm) for type 2a above

• 1-75 000 ft lb (1.3-100 000 Nm) for type 2b.

Speeds of rotation:

From 5 to 300s for a complete opening or closing cycle.

Linear thrust ranges

• Maximum of 500 lb (225 kg) output force from type 2a actuators

• 100-10 000 lb (45-4500 kg) output force from type 2a actuators

• 100 000 lb (45 000 kg) output force from type 3 actuators.

Speeds of full stroke:

• Small solenoids can close within 8-12 ms

• Throttling solenoids can stroke in about 1sec

• Electromechanical motor-driven valves stroke in 5-300sec

• Electrohydraulic actuators usually move at 0.25 in./s (6 mm/s).

In essence, actuators have to be able to exert some form of higher torque to overcome the resistance of some types of valve opening, usually described as a high breakaway force. Operation in the opposite direction, that is closing a valve, also sometimes requires extra torque to ensure firm seating is obtained; however, some form of spring-retentive clutch is also needed in case a foreign object is trapped in the valve body.

Limit switches, of many types, ranging from reed, IR, to microswitches can also be mounted within the mechanical assembly of these actuators to signal, and thereby prevent, overrun and excessive movements occurring.

Pneumatic actuators

Pneumatic actuators respond to an air signal by moving the valve trim into a corresponding throttling position. There are two basic types, linear and rotary, the specifications of both being listed.

Types and applications of pneumatic actuators:

(A) Linear

a1. Spring diaphragm

a2. Piston

(B) Rotary

b1. Cylinder with scotch yoke

b2. Cylinder with rack and pinion

b3. Dual cylinder

b4. Spline or helix

b5. Vane

b6. Pneumo-hydraulic

b7. Air motor

b8. Electro-pneumatic.

The above actuators are applicable to the following valve size:

Type a1: 0.5-8 in. (12.5-200 mm)

Type a2: 0.5-16 in. (12.5-400 mm)

Type B: 2-30 in. (50-750 mm).

Maximum actuator pressure rating:

Type a1: 60 PSI (414 kPa); some higher

Type a2: 150 PSI (1035 kPa)

Type B: 250 PSI (1725 kPa).

Actuator areas:

Type a1: 25-500 sq.in. (0.016-0.323 sq.m)

Type a2 and B: 10-600 sq. in. (0.006-0.38 sq.m)

Bore diameters from 2 to 44 in. (50mm to 1.1 m)

Strokes up to 24 in. (0.61 m).

Linear thrust (stem force ranges)

Type a1: 200 to 45 000 lbf (100-20 400 kgf)

Type a2: 200 to 32 000 lbf (100-14 500 kgf)

Specials up to 186 000 lbf (84 000 kgf)

Speeds of full stroke Type a1: 15 s Type a2: 0.33-6.0 s (8-150 mm/s).

The steady-state equation

In pneumatic spring and diaphragm actuators, valve stem positioning is achieved by a balance of forces on the stem. (Forces on a spring-and diaphragm valve) the following equation can be derived from a summation of the forces involved, adopting a positive direction downward (closing), and flow is left-to-right:

VV =0 PA KX P A -- With a reverse flow, right-to-left: VV +=0 PA KX P A - The inverse of this, where the stem is moving in a negative direction upwards (opening), and flow is left-to-right: VV +=0 PA KX P A -- With a reverse flow, right-to-left: VV ++ =0 PA KX P A – Where:

---is the effective diaphragm area is the effective inner valve area

---is the spring rate is the diaphragm pressure is the valve pressure drop

---is the stem travel.

++++ Forces on a spring-and-diaphragm forward and reverse acting valve

These equations are simplified because they don’t consider friction occurring in the valve stem packing, in the actuator guide and in the valve plug guide(s) or inertia.

++++ serves to illustrate the relationship between pressure on a diaphragm and the amount of travel of the valve stem, showing yet another area that generates non linearity and distortion.

Stem travel (% lift)

Diaphragm pressure: Ideal and actual relationship between diaphragm pressure and valve stem pressure indicates the advantages/disadvantages and application for the four most common types of actuator.

========

Type of Actuator | Advantage| Disadvantage Application| Linear spring/diaphragm Low cost Slow speed Linear valves

Mechanical fail-safe; Lack of stiffness 0.5-8 in.

• Body size
• Moderate thrust Instability
• Small package
• Simple design
• Excellent control (with or without control devices)

Linear piston; Low cost; No mechanical fail-safe spring; Linear valves

Moderate thrust; Slow speed: 0.5-16 in. Body size

Small package; Lack of stiffness

Simple design; Instability; Excellent control with a control device

Long stroke; Rotary spring/diaphragm; Low cost; Low thrust in spring cycle

Rotary valves 1-6 in. body style

Mechanical fail-safe, Slow speed

Small package; Instability

Simple design

Easily reversible

Excellent control with a control device

Rotary pistons, Low cost, Slow speed, Rotary valves 1-24 in. body style

Moderate thrust, Large spring compression

Small-large package

Mechanical fail-safe

Good control with a control device

=+++= Features of pneumatic actuators

========

Control valve positioners

Probably the most significant accessory that can be used for valve control is the positioner, sometimes referred to as 'smart valve electronics' many of which are microprocessor controlled. A positioner is a high gain proportional controller which measures the stem position, to within 0.1 mm, compares this position to a setpoint, which should be considered as the output of the main process controller, and performs correction on any resultant error signal. The open loop gain of these positioners ranges from 10 to 200 giving a proportional band between 10 and 0.5% and their periods of oscillation ranges from 0.3 to 10 s, a frequency response of 3 - 0.1 Hz. In other words it’s a very sensitive tuned proportional only controller.

The SUNPACK system shows what could be considered a full-house positioner.

Microprocessing unit CW range - span, etc. linearization; Measure Control Industrial interfaces (RS232) (4-20 mA) Hart-fieldbus, etc.

Temperature; Pressure; Pressure; Flow measurement and control

++++ Smart valve packages can be provided with local display and sensors for temperature, flow, pressures, pressure differentials and stem position.

Not only will it control and measure the flow through the valve, but also measure up and downstream pressures and as such the pressure differential, stem position and temperatures. It has the advantage of being able to store valve 'profiles' to enable software correction or modifications to flow characteristics.

When NOT to use positioners

Remembering that a positioner becomes an intrinsic part of the full control loop very much like the secondary controller in a cascaded system, care must be exercised in their uses. A rule of thumb is that the time constant of the slave should be 10 times shorter (open loop gain 10 times higher), and the period of oscillation of the slave 3 times shorter (frequency response 3 times higher) than that of the primary or master controller.

Valve sizing

The methods that can be used for the calculations of valve size are many and varied and sometimes very complicated and as such are beyond the scope of this publication. As a rule though, the minimum and maximum CV requirements for the valve should be determined, and taken into account.

Requirements like 'Process start-up'; 'Any abnormal process functions required' and, very importantly 'Reactions required to any Emergency conditions occurring' must first be taken into account and the valve should be selected to operate adequately over the range of 0.8CV min. to 1.2CV max. If this results in a rangeability which exceeds the capabilities of one valve, then two or more valves should be used. Control valves should not be used outside their rangeability specification. Also, care should be exercised that in summing up all the pressure drops that can occur in a constant pumping speed application, that the result be not applied to the valve for correction, as this always results in Over sizing of the valve and as such having it operate for most of its time in a nearly closed position.

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