Control valves and actuators: Basic principles

Goals

We now review some basic types and principle of operation of process control valves and their associated actuator and positioner systems.

Learning goals include:

• List the common types of process control valves, and briefly describe their design and basic construction

• Explain the meanings of valve characteristics, rangeability and sizing

• Describe the types of actuators commonly found in process control systems, and list their applications.

Basic types of control valves: Overview

In most process control systems the final control element, driven by the output of the process controller, is usually some form of valve. This section serves to introduce the student to eight of the most common types of control valves, flow throttling devices and the basic range of actuators used to control them.

--- describe the various types of valves in question, starting with an overview and general description, the types and variances within their manufactured ranges, sizes, design pressures and temperature ranges and their rangeability. Any special attributes or uses a valve may have are also described. --- introduces the reader to some of the more unusual types of valves, their design and usages.

Ball valves

Overview: The rotary ball valve, which used to be considered as an on-off shut-off valve is now used quite extensively as a flow control device. Some of the advantages include lower cost and weight, high flow capacity, tight shut-off and fire-safe designs. The ball valve contains a spherical plug that controls the flow of fluid through the valve body. Ball and cage valves are close to linear in terms of percent of flow or CV to percent of stem or ball rotation. The three basic types of ball valve are listed below.

Types of ball valves:

• Conventional: 1/4 turn pierced ball type

• Characterized: V and U notched along with a parabolic ball type

• Cage: Positioning a ball by means of a cage in relation to a seat ring and discharge port is used for control.

++++ Cross-sectional views of conventional and characterized ball valves

Size and design pressure:

• 0.5-42 in. (12.5 mm-1.06 m) in ANSI class 150 to 12 in. (300 mm) in ANSI class 2500

• Segmented ball - 1-24 in. (50-600 mm) in ANSI class 150-16 in. (400 mm) in ANSI class 300

• Pressure up to 2500 psig (17 MPa).

Design temperature:

• Varies with design and material but typically -160 to +310 °C

• Special designs extend this range from - 180 to > +1000 °C.

Rangeability: Generally claimed to be about 50:1.

++++ The characterized ball valve with a parabolic-notch is nearly equal percentage, while the ball and cage characteristics are closer to linear, when used on a water service. On gas services at critical velocities, the characterized ball valve lines move closer to linear. -

Characterized ball valves. Ball and cage, V-notch, U-notch Parabolic notch, Conventional

Butterfly valves

Overview: This is one of the oldest types of valves still in use, dating back from the 1920s. It acts as a damper or as a throttle valve in a pipe and consists of a disk turning on a diametral axis.

Like the ball valve its actuation rotation from fully closed to fully open is 90°. Due to the fact that the disk can act like an airfoil in the main stream flow it’s controlling, care must be exercised to ensure that any resultant increase in torque can be absorbed by the control actuator being used.

++++ Vane positions of butterfly valve. Closed (Damper perpendicular to flow), Throttling Open (Damper parallel to flow)

Types of butterfly valves:

• General purpose, aligned shaft, where the vane, disk, louver or flapper is rotated via the shaft to which it’s attached.

• High-performance butterfly valve (HPBV), offset (eccentric) shaft; This design combines tight shut-off, reduced operating torque, and good throttling capabilities of the swing-through special disk shapes.

Size and design pressure:

• To 48 in. (51 mm-1.22 m) are typical

• Units have been made from 0.75 to 200 in. (19 mm-5 m).

Design temperature:

• Typically - 260 to +540 °C

• Special designs extend this range up to > +1200 °C.

Rangeability: Generally claimed to be about 50:1.

++++ The flow characteristics of butterfly valves are affected by the position of the shaft (aligned or eccentric) and the relative size of the shaft compared to the valve size. For throttling purposes the valve is usually limited to rotate from 0° to 60° positions . 24 in. (0.61m) High-performance 2 in. (50mm) High-performance Typical general purpose; % Rotation (of 90°)

Digital valves

Overview: Digital valves comprise a group of valve elements, or ports, assembled into a common manifold. Each element has a binary relationship with its neighbor which means that starting with the smallest port, the next port is twice the size of the previous one. The main advantages of this type of valve are their high speed, high precision and practically unlimited rangeability. Their biggest disadvantage is their high cost.

Size 3/4-10 in. (19-250 mm) in both line and angle patterns.

Design temperature: Cryogenic to +670 °C.

Design pressure limits: Up to 10 000 psig (690 bars).

++++ In a digital valve, each valve element is twice the size of its smaller neighbor

Rangeability: ' ' 8 10 12 14 16; 255:1 1023:1 4095:1 16, 383:1 65, 535:1 No. of bits Resolution

Applications:

Where high speed (25-100 ms), accuracy, large rangeability and tight shut-off is needed.

Globe valves

Overview: Twenty or so years ago the majority of throttling control valves were of the globe type, characterized by linear plug movements and actuated by spring/diaphragm operators. The main advantages of the globe valve include the simplicity of the spring/diaphragm actuator, a wide range of characteristics, low cavitation and noise, a wide range of designs for corrosive, abrasive, high temperature and high pressure applications, a linear relationship between the control signal and valve stem movements and relative small amounts of dead band and hysteresis values.

Types of globe valves:

• Single ported with characterized plug

• Single ported, cage guided

• Single ported, split body

• Double ported, top-bottom or skirt-guided plug

• Eccentric disk, rotary globe

• Angle

• Three way or Y type.

++++ Cross section of a single ported globe valve; Flow characteristics

These characteristics are determined by the valve plug profile:

• Equal percentage

• Linear

• Quick opening.

Size and design pressure:

• Generally 1/2-14 in. (20 mm-356 m)

• Maximum size for type C is 6 in. (152 mm)

• Maximum size for type E is 12 in. (305 mm)

• Maximum size for type D is 16 in. (406 mm)

• Type F (the angle type) has been made in sizes up to 42 in. (1.05 m). Typically all pressure ratings are available up to ANSI class 1500, with types B and D available through ANSI class 2500 and Types C and E are limited to ANSI class 600.

Design temperature:

• Generally from - 200 to +540 °C

• Type B is limited to a maximum temperature of +400 °C

• Type C can operate down to - 260 °C.

Rangeability: If it’s defined as the region within which the valve gain remains within 25% of the theoretical, it seldom exceeds 20:1. Manufacturers using other definitions claim 35:1.

++++ The theoretical valve characteristics shift as a function of installation. The dotted lines reflect such a shift in a mostly friction process where 100% flow, 20% pressure drop was assigned to the control valve.

Pinch valves

Overview: These type of valves are called either pinch or clamp valves depending on the configuration of the flexible tube and the means used for tube compression. They are also manufactured from a large range of materials such as Teflon, PVC, neoprene, and polyurethane, each type of elastomer or plastic having its own particular application use. Open position; Throttling position; Closed position

++++Pinch valve with 'accurate closure'

This type of control valve, if carefully selected, has many advantages like high abrasion and corrosion resistance, packless construction, reasonable flow control rangeability, smooth flow, low replacement costs and a longer life than metal valves where abrasion and corrosion are present.

As in all things, though this valve has limitations such as pressure and temperature restraints due to the nature of the material used for the sleeves, and the number of operations, or flexing, that a particular type of liner can cope with, although a life span of >50 000 opening and closing cycles should be considered as the minimum.

Size:

• 1-24 in. (25-610 mm)

• Special units from 0.1 to 72 in. (2.5 mm-1.8 m).

Design pressure:

Generally up to ANSI class 150 with special units up to class 300.

Design temperature Varies with design and material but typically -30 to +200 °C.

Rangeability:

Generally claimed to be between 5:1 and 10:1.

++++By reducing the port size or by making the sleeve 'cone' shaped, the characteristics are made more linear.

The addition of a variable orifice within the sleeve provides flow throttling in the upper half of the stroke. Reduced port sleeve; Teflon tube clamp

Typical sectional view of a plug valve -- Lubricant grooves Lubricant check valves Packing Tapered plug Thrust bearing with adjustment screw; Flow

Plug valves

Overview: Plug valves are probably the oldest type of valve in existence, being used in water distribution systems in ancient Rome and they probably pre-date the butterfly valve. Consisting of a tapered vertical cylinder with a horizontal opening or flow-way inserted into the cavity of the valve body and due to the taper and lubricating system they use they are virtually leak proof to both gases and liquids.

A very common use for this type of valve is in the tapping of beer barrels. They afford quick opening and closing action with tight leakproof closures under working pressures from vacuum to as high as 10 000 psig (70 MPa). They can be used for liquids, gases and non-abrasive slurries, and eccentric and can be styled with lift plugs for use with sticky fluids. Again, like the butterfly and ball valves, they operate through an actuator having angular motion of 90°.

Size: 1/2-36 in. (12.5-960 mm).

Types:

• V-ported: This style is used for both On-off and throttling control, utilizing a V-shaped plug and a V-shaped notched body. This is ideal for fibrous or viscous materials

• Three, four and five way or multi-ported designs are available

• Fire-sealed.

Design pressure Typically from ANSI class 125 to ANSI class 300 ratings and up to 720 psig (5 MPa) pressure, with special units available for ANSI class 2500.

Design temperature:

• Typically -70 to +200 °C

• Special units are available -160 to +315 °C.

Rangeability; Generally claimed to be between 20:1. Plug valve characteristics are a function of the shape of the throttling plate or V-Port

Saunders diaphragm valves

Overview: The Saunders or diaphragm valve is sometimes also referred to as a weir valve. This valve operates by moving a flexible diaphragm toward or away from a weir. This valve can be considered as a half pinch valve as only one diaphragm is used, moving relative to a fixed weir; because of this however their flow characteristics are similar. The normal Saunders valve has a body with side section in the form of an inverted U shape, with the diaphragm closing the orifice at the top. A full-bore type is also available that has, when fully open, a fully rounded bore which is an important feature for ball-brush cleaning as required in applications like the food industry. It should be noted that mechanical damage can occur when opening this type of valve against a process vacuum.

Main positions of weir-type; Saunders control valve. Streamline flow in open position; Flow control in throttling position; Leak tightness in closed position.

Size:

• 1/2-12 in. (12.5-300 mm)

• Special units manufactured up to 20 in. (500 mm).

Types:

• Weir

• Full bore

• Straight-through

• Dual range.

Design pressure:

• Sizes <= 4 in. (100 mm) 150 psig (10.3 bar)

• 6 in. (150 mm) 125 psig (8.6 bar)

• 8 in. (200 mm) 100 psig (6.9 bar)

• 10-12 in. (250-300 mm) 65 psig (4.5 bar).

Design temperature:

• With most elastomer diaphragms -12 to +65 °C

• With PTFE diaphragms -34 to +175 °C.

Rangeability:

Generally claimed to 10:1.

The characteristics of conventional Saunders valves are near to quick-opening, while the characteristics of the dual range design is closer to linear

Sliding gate valves

Overview: In this type of valve, the flow rate is controlled by sliding a plate with a hole in it past a stationary plate, usually placed at 90° to the line of flow, with a corresponding hole in it.

These holes can be round, or shaped to profile the flow characteristic of the valve. This valve is sometimes used in automatic control but is not really considered a control valve.

However, this type of valve can operate with pressures up to 10 000 psig (70 MPa). The accuracy of these valves, particularly in proportional control, depends solely on the accuracy of the chosen actuator.

++++ Typical sliding gate valve with 'V' Insert -- Fully open Throttling Fully closed

Types:

• Knife gate

• V-insert

• Plate and disk (multi-orifice)

• Positioned disk.

Size:

• On-off: 2-120 in. (50 mm to 3.0 m)

• Throttling: 1/2-24 in. (12-600 mm)

• Throttling: 1/2-6 in. (12-150 mm)

• Throttling: 1 in. and 2 in. (25 mm and 50 mm).

Design pressure:

• Types A and B: Up to ANSI class 150

• Type C: Up to ANSI class 300

• Type D: Up to 10 000 psig (70 MPa).

Design temperature:

• Types A and B: Cryogenic to 260 °C

• Type C and D: - 30 to +600 °C.

Rangeability:

• Types A: 10:1

• Type B: 20:1

• Type C: Up to 50:1 is claimed.

++++Characteristics of various sliding gate valve types -- % Lift or rotation

Special valve designs

This section is included to expose the student to some of the more uncommon types of valves that are currently being used. The reason for this is that with the current technical advancements and stringent requirements of accuracy, etc. that are now being made of process control systems, these valves are becoming more common. These valves are neither linear nor rotary in operation, but use other methods such as fluidics or static pressure of the process fluid in throttling the valve.

Dynamically balanced plug valves:

This family of valves is used where there is no external power available to operate the valve, and therefore the static pressure of the process fluid is used to achieve throttling.

The upstream, or back, pressure is used to move a plug against the force of a return spring. Variances in supply pressure affect the position of the plug relative to the spring tension. Control is achieved with a pilot valve poppet assembly.

Sectional view showing the operation of the dynamically balanced plug valves -- Closing port, Opening port, Plug Piston Cylinder

Diaphragm-operated cylinder in-line valves:

This valve is used for high pressure gas services due to its low level of vibration, turbulence and noise. It consists of a low convolution diaphragm for positive sealing.

Inlet to outlet pressures of 1400 and 600 MPa respectively are possible in the 2 in. (50 mm) size.

Sectional view of a diaphragm-operated cylinder in-line valve -- Inlet pressure Outlet pressure External control; Loading pressure

Diaphragm, Spring

Expandable element in-line valves:

Streamlined flow of gas occurs in a valve where a solid rubber cylinder is expanded or contracted to change the area of an annular space. Control occurs via a hydraulic actuator or piston that is used to vary the rubber cylinders expansion. Pressures up to 1200 psid can be controlled with this valve. An expandable element or diaphragm is stretched over a perforated dome shutting off the flow of the valve when the pressure above the diaphragm is greater than the line pressure. By externally varying the pressure to the exterior of the element control of the mainstream flow can be achieved.

++++General view of an expandable element in-line valves

Fluid interaction valve:

The Coanda effect, the basics of Fluidics, is used in this type of diverting valve which comes in sizes from 0.5 to 4 in. (12.5-100 mm). This valve has a flip-flop type of action used to divert a discharge from one port to another in a Y configuration by use of lateral control ports located at the base of the V intersection of the Y. This type of valve has numerous uses particularly in the chemical industry; the ability to divert a flow rapidly, usually in less than 100 ms, makes it an important member of the control valve family.

Splitter Control ports closed PR

+++ Operation of fluid interaction valves

NEXT: Control valve gain, characteristics, distortion, rangeability

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