Control Valves

Fundamentals

Absolute Zero Pressure

A perfect vacuum which would be obtained if all molecules were removed from an enclosed space. In reality, this is impossible to achieve, but it does serve as a convenient reference for pressure measurement

Atmospheric Pressure

The amount of pressure exerted by the atmosphere above absolute zero pressure. The "standard" atmospheric pressure (at 0 feet altitude/sea level) is 14.696 psia or 760 millimeters of mercury at 0°C

It is important to realize that atmospheric pressure at any one location varies day to day with weather conditions. More important, the atmospheric pressure changes rapidly with elevation above sea level

Gauge Pressure

Gauge pressure is always measured above atmospheric pressure. To obtain absolute pressure, the atmospheric pressure must be added to the gauge pressure

Vacuum Pressure

Usually expressed in inches of mercury below atmospheric pressure

Absolute Zero Pressure

A perfect vacuum which would be obtained if all molecules were removed from an enclosed space. In reality, this is impossible to achieve, but it does serve as a convenient reference for pressure measurement

Delta P Shutoff

Maximum differential pressure across the valve when closed. Unless otherwise specified by the user, Delta P shutoff is normally considered equal to maximum inlet pressure

Differential Pressure

Also known as DP. For a control valve, the difference between inlet and outlet pressure

Pressure

The force exerted by a fluid per unit area.

Density

The mass of a substance per unit volume. Usually expressed in pounds-mass per cubic foot (lbm/ft3 or kgm/m3).

Specific Gravity

The ratio of the density of one substance to the density of a second or reference substance. The reference substance depends on whether the flowing media is liquid or gas.For liquids, water at 60°F is used as the reference substance. The density of distilled water at 60°F is 62.3707 lb/ft3.For gases, air is used as the reference fluid. However, instead of a ratio of densities, the ideal specific gravity of a gas is defined as the ratio of the molecular weight of the gas to the molecular weight of air. The molecular weight of air is 28.9644.

Specific Weight

The weight, due to the gravitational pull of the earth, of a substance per unit volume. Usually expressed in pounds-force per cubic foot (lbf/ft3). Only at locations where the local acceleration of gravity is equal to the standard acceleration of gravity (gc = 32.1740 ft/sec2) does the numerical value of the specific weight equal that of the numerical value of density

Weight Fraction

Portion by fluid phase of the total mass of a two-phase flow.

Flow

Often used as a substitute to flow rate. Normally expressed as volumetric or mass flow

Flow Rate

The rate of flow of process fluid expressed in unit of volume or weight per unit of time

Cv

A capacity index known as the valve flow coefficient. It is dependent on the geometry of the valve and the position of the closure member. It can be used to calculate the flow rate through a valve for specific system conditions at the valve.

It is a dimensionless figure defined as "the number of U.S. gallons per minute of water at 60°F which will pass through a given flow restriction with a pressure drop of 1 psi under stated conditions

Rated Cv

The flow Cv of the selected valve at full travel

Temperature

A measure of the degree of hotness or coldness (heat energy) of a substance, most commonly expressed in degrees Fahrenheit (°F), Centigrade (°C), Rankine (°R), or Kelvin (°K).

°F = (9/5) °C + 32

°C = 5/9 (°F-32)

°R = °F + 460

°K = °C + 273

Viscosity

The internal resistance of a fluid to flow. Water has low viscosity, while syrup has high viscosity. Viscosity is highly temperature dependent.

There are two ways to measure the viscosity of a fluid. One is density independent, known as the "absolute viscosity" usually expressed in centipoise. The second is density dependent, known as the "kinematic viscosity" usually expressed in centistokes.

Valve Sizing

Fluid flow rate can be expressed in units of volume or weight per unit of time

A method of predicting the flow rate of a compressible and/or incompressible fluid through a control valve when all the factors, including those related to the fluid and its flowing condition, are known. Sizing can be done using a computer program or by hand calculation. Masoneilan's sizing program is based on Industry Standard ISA-S75.01 and International Standard IEC 534-2 equations.

FL

The liquid pressure recovery factor of a valve.

Cavitation

A two-stage phenomenon of liquid flow. The first stage is the formation of cavities (vapor bubbles) within the liquid stream; the second stage is the collapse or implosion of those cavities (beyond the vena contracta) back into an all liquid state. The energy released by cavitating liquids can, under certain circumstances, cause physical damage of valve or piping components

Sonic Diameter

The diameter at which the process fluid will reach sonic velocity at flowing conditions

Vena Contracta

The location where the cross-sectional area of the flow stream is at its minimum. The vena contracta normally occurs just downstream of the actual physical restriction in a control valve

Flashing

The formation of voids or cavities (vapor bubbles) in a liquid flow stream during rapid acceleration of the fluid within the valve orifice. Downstream of the orifice, the system pressure remains at or below the vapor pressure of the liquid and the vapor formed at the orifice will remain in the flow stream. Under some conditions, flashing can cause damage to internal valve components

Dead Band

The range through which input can be varied without initiating an observable output response.

Rated Travel

The amount of movement of the valve closure member from the closed position to the rated full open position

Travel

Also called stroke (linear) or degrees of rotation (rotary), the amount of movement of the closure member from the closed position to an intermediate position or to the rated full open position

Throttling

The act of regulating fluid flow through a valve by restricting its orifice or opening

Ball Valve

A valve that modifies flow rates through rotation of the closure member, which is either a sphere with an internal passage or a segment of a sphere, notched or contoured to obtain various flow characteristics.

Butterfly Valve

A valve that controls fluid flow through rotary motion of its closure member, which is either a disc or a vane pivotally supported by a shaft

Globe Valve

A valve with a linear motion closure member, one or more ports, and a body distinguished by a globular shaped cavity around the port region.

Body

The part of a valve that is the main pressure boundary, providing pipe connecting ends, the fluid flow passageway, and in some cases, the part that supports the seating surfaces and closure member

Bonnet

The portion of the valve pressure-containing boundary that provides guiding to the stem, contains the packing box and stem seal, and provides a means to attach the actuator. In welded, threaded, or bolted bonnet valves, the bonnet is the principal opening to the body cavity for assembly and disassembly of the internal parts of the valve.

A bonnet also may be integral to the body of the valve. An extension bonnet may be required to compensate for temperature (either high or low) by allowing the packing box to be located away from the fluid flow.

Accessory

An electronic, pneumatic, or mechanical device connected to and affecting the operation of a control valve.

Valve

A valve is a device used for the control of fluid flow. It consists of a fluid containing assembly (body and bonnet), one or more ports (seat(s)) between end openings (inlet and outlet), and a moveable flow-controlling element (closure member) which opens, restricts, or closes the port(s).

Control Valve

A power operated device that modifies the fluid flow rate in a process control system. It consists of a valve connected to an actuator mechanism that is capable of changing the position of a flow-controlling element within the valve in response to a signal from the controlling system.

Valve Sub-Assembly

The body, bonnet, and trim parts assembled. One of the two components of a control valve.

Configuration

The specific information that defines the valve, actuation, and accessory package specified as a result of control valve sizing and selection

Dimensional Drawing

A representation of the size, shape, and orientation of the valve, actuator, and accessories specified

Schematic Drawing

A representation of the pneumatic piping and/or electrical wiring required to operate the control valve actuator and accessories as specified

Actuator

A fluid-powered or electrically-powered device that supplies force and motion to components of a control valve.

Actuator Sub-Assembly

The actuator and yoke assembled. One of the two components of a control valve.

Yoke

A structure by which the actuator assembly is attached rigidly on the bonnet

Hand wheel

A manual means of operating a valve (opening or closing the flow control orifice). May be the sole means of operation or an optional override to an actuator

Cage

A trim part in a valve sub-assembly that surrounds the closure member, provides alignment or guiding, and facilitates assembly/disassembly of the trim. The cage may provide flow characterization through the geometry of its flow passages. The cage also may include the seating surface or may retain a separate seat ring within the valve body cavity

Plug

A moveable closure member which provides a variable restriction to the fluid flow

Seat Ring

A trim part assembled in the valve body that may provide part or all of the flow control orifice and the contact surface for the closure member

Closure Member

The flow-controlling element of a valve. It is a moveable part positioned in the flow path to modify the rate of flow through the valve. Closure members are commonly known as plug, disc, vane, or ball. It moves through the flow path in either a linear or rotary motion

Seat

The portion of a seat ring, cage, or valve body that a valve closure member (plug, ball, disc, etc.) contacts for closure

Port

A flow control orifice of a control valve. A valve can be single ported or multi-ported

Soft-Seated Trim

The use of an elastomeric, plastic, or other readily deformable material in the valve plug or seat ring to provide tight shutoff with reduced actuator forces

Reduced Area Trim

Based on port area, full area and reduced area trims are available. Reduced trim may be achieved with a seat ring (35002 series rotary) or cage (41000 series cage guided) change alone. In other designs, it may be necessary to change the plug and seat ring.

Bushing

Also called guide bushing, a fixed member which guides the closure member, valve stem, and/or actuator stem. The bushing supports the non-axial loads and is subject to relative motion of parts

Bottom Flange

A part that closes the valve body opening opposite the bonnet opening as in a double seated globe valve. This flange may be used to retain an additional guide bushing

Lantern Ring

A rigid spacer assembled within the packing box with the packing normally above and below the ring. It is designed to allow lubrication of the packing or to provide a leak-off connection.

Stem

A rod, shaft, or spindle that connects the valve actuator with the closure member

Trim

The internal parts of a valve sub-assembly, which are in contact with the controlled fluid, such as the plug, seat ring, seat ring retainer, cage, and stem.

Anti-Cavitation Trim.

A combination of plug and seat ring or plug and cage that by its geometry provides non-cavitating operation or reduces the tendency of the flowing liquid to cavitate, thereby minimizing damage to the valve parts and downstream piping

Balanced Trim

An arrangement of ports and plug or a combination of plug, cage, seals, and ports that tends to equalize the pressure above and below the valve plug to minimize the net static and dynamic fluid flow forces acting along the axis of the stem.

Lo-dB® Trim

A combination of plug and seat ring or plug and cage that by its geometry reduces the noise generated by fluid flowing through the valve

Bubble Tight

A non-standard term commonly used in reference to seal leakage. Refer to ANSI/FCI 70-2 for the specification of leakage classifications. The use of an elastomeric, plastic, or other readily deformable material in the valve plug or seat ring to provide tight shutoff with minimal actuator forces

Leakage

The quantity of fluid passing through a valve when the valve is in the fully closed position under stated closure forces, with the pressure differential and temperature as specified in ANSI/FCI 70-2. Leakage is usually expressed as a percentage of the valve capacity at full rated travel. Refer to ANSI/FCI 70-2 for the specification of leakage quantity.

Clearance Flow

The flow volume below the minimum controllable flow rate with the closure member not seated

Capacity

The rate of flow through a valve under stated test conditions

Quick Opening Characteristic

An inherent flow characteristic in which the greatest rate of increase in flow occurs at the lowest travel positions

Equal Percentage

The inherent flow characteristic, which, for equal increments of rated travel, will ideally give equal percentage changes of the existing flow Cv at constant Delta P.

Modified Equal Percentage

An inherent flow characteristic that provides fine throttling action at low valve plug travel and is approximately linear in characteristic for upper portions of valve travel

Linear Characteristic

An inherent flow characteristic, which can be represented by a straight line on a rectangular plot of flow Cv versus percent rated travel. Therefore, equal increments of travel provide equal increments of flow Cv at constant pressure drop

Cam Characterized

The use of a mechanical or electronic adjustment to modify the inherent relationship of signal to stem travel

Characteristic

The relationship between flow coefficient (Cv) or valve flow output and valve travel or valve physical input. Linear, equal percentage, modified percentage, and quick opening are the most common control valve characteristics

Inherent Flow Characteristic

The relationship between the flow rate through a valve and the travel of the closure member as the closure member is moved from the closed position to the rated travel with a constant pressure drop across the valve

Inherent Rangeability

The ratio of the largest controllable flow Cv to the smallest controllable flow Cv within which the deviation from the specified inherent flow characteristic does not exceed the stated limits

Characteristic

The relationship between flow coefficient (Cv) or valve flow output and valve travel or valve physical input. Linear, equal percentage, modified percentage, and quick opening are the most common control valve characteristics

Conventional Butterfly valves

The advantages of Butterfly valves

Economy compactnessHigh capacityTight shutoff. CBVs are available either elastomeric lined or unlined versions.

Brief Design details

The design is simplistic with a relatively few number of parts.

The shaft is supported by three bushings while the disc is rigidly affixed to the shaft by use of precision taper pins. The phenolic backed seat is held firmly within the inner diameter of the body.

Conventional butterfly valve offers an equal percentage characteristic over a range approximating a Cv ratio of 100:1. The lined versions provide bubble-tight shutoff while the unlined version, typically used in damper service, yields seat leakage of 1% of the rated capacity

Contd…BVConventional butterfly valve is available in a variety of materials as shown in the chart below. Ductile iron body constructions provide economy while stainless constructions are suitable for corrosive service. A wide selection of optional liner materials are available including food grade Buna, Hypalon, High-Temperature Viton and Black Neoprene.

High Performance Butterfly valves

Sizes:2" through 36"End Conn:Wafer or Single FlangeRating:ANSI Class 150, 300, 600Materials:Carbon Steel316 Stainless SteelTrim Type:Metal SealSoft SealLeakage:per ANSI B16.104Class VI, PTFE, RTFEClass IV metal sealsCat

Stem guided control valves

Top guided control valves

Top guided globe valve design featuring noise attenuation and anti-cavitation trim options. It is the most versatile single ported control valve on the market today, well suited to handle a wide variety of process control applications. Most generally used and it is rugged post-guided stem design and an unbalanced trim. It is very versatile and can be used in many types of process applications. The design uses a simple, open flow path with the guide bushing located away from the flow stream. This is more suitable than a cage-guided design for applications with sticky or corrosive media or for process fluids containing particulates. This valve generally equipped with either a threaded or a quick Additionally, top guided valves are available with angle bodies, high pressure construction, low emissions LE® packing, and bellows seal options.

Top Guided Control valves

Cage guided valves

Cage guided valves:Control valve is a cage-guided control valve design that utilizes a balanced plug construction. The standard valve trim materials are hardened steel

components which give the valve longer life. 

Double seated valve

Double seated valves are generally used in refinery applications such as crude or bottom service. The double ported valve has two seating surfaces and utilizes the process pressure to assist in the opening and closing of the valve. The valve plug is post guided at both the top and the bottom of the throttling section. This results in a very rugged and stable design that performs well in high pressure applications. The double ported design has full area and reduced capacity trims using both V-ported and contoured trim configurations. The valve has low pressure recovery and provides high capacities.

Double seated valve

3 WAY valve

I/P converter

A device that receives an electrical input signal and outputs a pressure proportional to the input signal The electrical input signal is usually a 4-20mA current signal (10-50mA is also available upon request) Outputs are 3-15 psi or 6-30 psi, however: Output scan be calibrated to 1-17psi, 0-20 psi, 3-27psi, or 0-35 psi Normally these devices are used with a pneumatic valve positioner; however, in rare cases the I/P pressure output is used directly in a pneumatic actuator

What is an I/P transducer?

What is valve Positioner

A device that compares the valve travel to a control input signal and then outputs a pneumatic signals to accurately adjust the valve’s position

Why valve positioner is needed?

A valve positioner will: Decrease the valve’s hysteresis and deadband (i.e., friction) Increase repeatability of valve movement Increase sensitivity to the control signal Improve process loop performance