An Introduction To Control Valves Types , Applications And Sizing

An Introduction To Control Valves Types , Applications And Sizing

Introduction 
A control valve is defined as a mechanical device that fits in a pipeline creating an externally adjustable variable restriction.This throttles the flow for any given pressure drop or it raises the pressure drop for any given flow. Typical process applications can be made based on this ability to change pressure drop or flow capacity as will be seen in the next section. However, we must firstly understand how a typical control valve actually creates a pressure drop by looking at the fundamentals of flow in a pipeline and through a restricted area.

Control Valve Classification
A variety of types of control valves are used in all sectors of the process industries, depending on the suitability of a valve for a process. Two general types of control valves are based on their motion:
1- linear-motion valves
2- rotarymotion valves.

FIG 1

Control Vavles Apllications
Different types of Control Valves may be added to a system:
a. FCV - to Control the flow rate through a particular pipe.
b. PRV - to reduce the pressure at the end of a pipe by introducing a pressure loss in the pipe.
c. BPV - to maintain the pressure at the start of a pipe by introducing a pressure loss in the pipe.
Flow Control Valve (FCV)
A flow control valve is used to control the flow rate through a particular pipe.
Pressure Reducing Valve (PRV)
A pressure reducing valve is used to control the pressure at the end of a particular pipe.
Back Pressure Valve (BPV)
A back pressure valve is used to control the pressure at the start of a particular pipe.
1- Linear-motion valves 
Linear-motion valves have a tortuous flow and low recovery. They can be offered in a variety of special trim designs and can throttle small flow rates. Most linear motion valves are suitable for high-pressure applications.
They are usually flanged or threaded and have separable bonnets. The most common valve types in flow control industries include:
            - Gate valves
            - Globe valves
            - Pinch valves
            - Diaphragm valves
            - Needle valves

1- Gate Control Valves 

Gate valves are generally used when a straight-line flow of fluid and minimum restriction

are desired. They are so named because the part that either stops or allows flow through the valve acts somewhat like the opening and closing of a gate. When the valve is wide open, it is fully drawn up into the valve, leaving an opening for flow through the valve of the same size as the pipe in which the valve is installed. Therefore, there is little pressure drop or flow restriction through the valve.

Gate valves are not usually suitable for throttling purposes because flow control would be difficult, due to the valve design, and the flow of fluid slapping against a partially open gate can cause serious damage to the valve.

FIG 2

Gate valves used in steam systems always have flexible gates .The reason is to prevent binding of the gate within the valve when the valve is in the closed position. When steam lines are heated, they will expand, causing some distortion of valve bodies. If a solid gate fits snugly between the seat

of the valve in a cold steam system, when the system is heated and pipes elongate, the seats will compress against the gate, wedging the gate between them and clamping the valve shut. This problem is overcome by the use of a flexible gate. This allows the gate to flex as the valve seat compresses it, thus preventing clamping

2- Globe  Control Valves 

These are probably the most common valves in existence. The globe valve derives its name from the globular shape of the valve body. However, positive identification of a globe valve must be made

internally because other valve types may also have globular bodies . Globe valve inlet and outlet openings are used extensively throughout the engineering plant and other parts of the ship in a variety of systems. In this type of valve, fluid passes through a restricted opening and changes direction

several times. It is used extensively for the regulation of flow.

FIG 3

They are ideal for high-pressure drop applications, are available with either single- or double-seated construction, and may have pressure balanced trim.

A-Double-Ported control valves 

the double-potted (double-sealed) balanced valve (Figure 4 ) was  one  of  the  first  glube  valves  developed  during  the  early 20th century. It b still available today, but has been replaced in most  applications  by  single-seated  globe  valves.  Size  for  size, it is much Larger and heavier than its single-seated counterpart. Shut-off is  poor because it is not practical to have both plugs in  tight  contact  with  the  seats  at  the  same time,  but  the  valve was  intended  for  throttling  control  rather  than  for  tight  shut­ off,  Some  special  seat  designs  have  been  developed  to  help covercome this, but application is limited

FIG 4

The double-seated valve requires fewer actuator forces and is top and bottom guided. However, it is more expensive; more difficult to service, maintain, and adjust; and does not provide tight shutoff. The double seated valve is not commonly used. Three-way valves (figure 5) are an extension of the

typical double-seated globe valve.

FIG 5

B- single-seated control valves 

single-seated valves  atre  the  most  widely  used  of  the  globe body  patterns.  There  are  good  reasons  for  this.  They  are available  in  a  wide  variety  of  configurations,  including special purpose  trims,  They  have  good  seating  shut-off  capa­bility,  are  less  subject  to  vibration  due  to  reduced  plug  mass, and  are  generally  easy  to  maintain.  Them  are  three  general types of seat construction :

The single-seated valve ( figure 6) is typically used on all 1-in. (25 mm) and smaller valves and is usually top guided.

FIG 6

C- Angle Control Valve

Another type of globe valve is the angle valve ( figure 7 ). This is considered a singleseated valve, and its streamlined interior, with its self-draining construction, tends to prevent solid buildup inside the body. These valves are also used for erosive fluids and in situations

where the piping arrangement restricts the use of globe valves. Angle valves are typically offered in 1- to 6-in. (25 to 150 mm) sizes, but they are not available in jacketed construction.

They are generally installed with the flow coming in on the side and exiting at the bottom. This configuration minimizes body erosion but will create a flow-to-close valve

FIG 7

D- Cage Control Valves

Another type of globe valve that is very popular is the cage-guided balanced trim valve ( figure 8)This design uses the cage as a plug guide. The plug is grooved along its sides, which equalizes the pressure in the valve body. The cage-guided balanced trim valve provides a balanced valve plug, valve plug guiding, and excellent shutoff capabilities.

FIG 8

3- Diaphragm Control Valves

Diaphragm valves, also known as Saunders valves, are operated by forcing a flexible diaphragm

against a bridge or weir to stop the flow. The weir-type design ( figure 9 ) lasts longer than the straight-through type, but has less flow capacity. The straight-through valve, sometimes called a “pinch valve” ( figure 10 ), is best suited for slurries but has a lower differential-pressure rating than the weir design.

Diaphragm valves are excellent for sanitary and slurry service as well as for liquids that contain

solids or dirt. They are made of a packless construction (because fluid contacts only the liner) and are available as tight shutoff. Diaphragm valves are low-cost devices and their maintenance is simple. However, they have poor flow characteristics and are inadequate as modulating control valves. In addition, the diaphragm materials available are limited, and due to their application and construction, diaphragm valves tend to be a high-maintenance item.

FIG 9

4- Pinch Control Valves

A cost-effective flow control valve, pinch valves are ideal for applications of slurries or liquids containing significant amounts of suspended solids. Pinch valves seal using one or more flexible elements like rubber tubes that become pinched to turn off the flow. These rubber sleeves are the valve’s only wetted part, and their flexibility allows pinch valves to close tightly around entrapped solids. Air or hydraulic pressure is placed directly on the elastomer sleeve to actuate pinch valves. A pinch valve’s body acts as a built-in actuator, which eliminates expensive hydraulic, pneumatic, or electric operators and results in the cost-effectiveness of this type of flow control valve

FIG 10

5- Needle Control Valve
The need to remotely control process flow at high pressure used to require both pneumatic and electric systems combined, which is cumbersome and expensive to operate

2- Rotary-Motion Control Valves
Rotary-motion control valves have a streamlined flow path and high recovery in nature. They have more capacity than that of linear-motion valves. This type of valve has an advantage in handling slurries and abrasives. They are easy to handle because they are flangeless and have an integral bonnet. Rotary-motion valves are designed to have high rangeability. Examples of this type of valve are butterfly valves, ball valves, and plug valves.
1- Butterfly Control Valve
The butterfly valve is used in a variety of systems aboard vessels. These valves can be used
effectively in saltwater, lube oil, and freshwater systems . Butterfly valves are light in weight, relatively small, quick acting, provide positive shutoff, and can be used in throttling. This valve has a body, a resilient seat, a butterfly disk, a stem, packing, a notched positioning plate, and a handle. The resilient seat is under compression when it is mounted in the valve body, thus making a seal around the periphery of the disk and both upper and lower points where the stem passes through the seat. Packing is provided to form a positive seal around the stem for added protection in case the seal formed by the seat should become damaged.
Butterfly valves are easy to maintain . The resilient seat is held in place by mechanical means, and neither bonding nor cementing is necessary. Because the seat is replaceable, the valve seat does not require lapping, grinding, or machine work.

FIG 11

2- Ball Control Valves
Ball Valves These are stop valves that use a ball
to stop or start the flow of fluid . When the valve handle is operated to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet. When the valve is shut, which requires only a 90◦ rotation of the hand wheel
for most valves, the ball is rotated so that the hole is perpendicular to the flow openings of the valve body, and flow is stopped. Most ball valves are of the quick-acting type, but many are planetary gear operated .

FIG 12

This type of gearing allows the use of a relatively small hand wheel and operating force to operate a fairly large valve but increases the valve operating time. Ball valves are normally found in the following systems: desalination, trim and drain, air, hydraulic, and oil transfer. They are used for general service, high-temperature conditions, and slurries.
3- Plug Control Valves
Plug Valves These are quarter-turn valves that controls flow by means of a cylindrical or tapered plug
with a hole through the center which can be positioned from open to close by a 90◦ turn. They are used for general services slurries, liquids, vapors, gases, and corrosives

FIG 13

Types of Control Valves based on Action

Control valves operated through pneumatic actuators can be either
(i) air to open.
(ii) air to close.

FIG 14

Types of Control Valves based on flow characteristics

From the flow characteristics, there are three primary types of control valves:

1. Equal percentage. Equal increments of valve travel produce an equal percentage in flow change.

2. Linear. Valve travel is directly proportional to the valve stoke.

3. Quick opening. In this type, a large increase in flow is coupled with a small change in valve stroke.

So how do you decide which control valve to use? Here are some rules of thumb for each:

FIG 15

1. Equal percentage (the most commonly used valve control)

         a. Used in processes where large changes in pressure drop are expected

         b. Used in processes where a small percentage of the total pressure drop is permitted by the                      valve

         c. Used in temperature and pressure control loops

2. Linear

         a. Used in liquid level or flow loops

         b. Used in systems where the pressure drop across the valve is expected to remain fairly                            constant (i.e., steady-state systems)

3. Quick opening

         a. Used for frequent on–off service

         b. Used for processes where an “instantly” large flow is needed (i.e., safety systems or cooling                 water systems)

Control Valves Selection Considerations 

The control valve regulates the rate of fluid flow as the position of the valve plug or disk is changed by force from the actuator. To do this, the valve must:


- Contain the fluid without external leakage;
- Have adequate capacity for the intended service;
- Be capable of withstanding the erosive, corrosive, and temperature influences of the process; 
- Incorporate appropriate end connections to mate with adjacent pipelines and actuator attachment means to permit transmission of actuator thrust to the valve plug stem or rotary shaft. Many styles of control valve bodies have been developed through the years. Some have found wide application; others meet specific service conditions and are used less frequently. The following summary describes some popular control valve body styles in use today. 

 Flow Characteristics  And Selection

The relationship between control valve capacity and valve stem travel is known as the Flow Characteristic of the Control Valve

Trim design of the valve affects how the control valve capacity changes as the valve moves through its complete travel. Because of the variation in trim design, many valves are not linear in nature. Valve trims are instead designed, or characterized, in order to meet the large variety of control application needs. Many control loops have inherent non linearity's, which may be possible to compensate selecting the control valve trim.

FIG 16

Inherent Control Valve Flow Characteristics 

The most common characteristics are shown in the figure above. The percent of flow through the valve is plotted against valve stem position. The curves shown are typical of those available from valve manufacturers. These curves are based on constant pressure drop across the valve and are called inherent flow characteristics. 

Linear -            flow capacity increases linearly with valve travel. 

Equal percentage - flow capacity increases exponentially with valve trim travel. Equal                                                       increments of valve travel produce equal percentage changes in the                                                        existing Cv. 

A modified parabolic characteristic 
                              is approximately midway between linear and     equal-percentage characteristics.                               It provides fine throttling at low flow capacity and   approximately linear                                                characteristics at higher flow capacity. 

Quick opening
                              provides large changes in flow for very small changes in lift. It usually has                                           too  high a valve gain for use in modulating control. So it is limited to on-off                                         service, such as   sequential operation in either batch or semi-continuous                                             processes. 

Hyperbolic  Square Root 

The majority of control applications are with valves with linear, equal-percentage, or modified-flow characteristics. 

Installed Control Valve Flow Characteristics 

When valves are installed with pumps, piping and fittings, and other process equipment, the pressure drop across the valve will vary as the plug moves through its travel. 

When the actual flow in a system is plotted against valve opening, the curve is called the Installed Flow Characteristic. 

In most applications, when the valve opens, and the resistance due to fluids flow decreases the pressure drop across the valve. This moves the inherent characteristic: 

      A linear inherent curve will in general resemble a quick opening characteristic 

      An equal percentage curve will in general resemble a linear curve 

Valve Standardization

Standardization activities for control valve sizing can be traced back to the early 1960’s when a trade association, the Fluids Control Institute, published sizing equations for use with both compressible and incompressible  fluids. The range of service conditions that could be accommodated accurately by these equations was quite narrow, and the standard did not achieve a high degree of acceptance. In 1967, the ISA established a committee to develop and publish standard equations. The efforts of this committee culminated in a valve sizing procedure that has achieved the status of American National Standard. Later, a committee of the International Electrotechnical Commission (IEC) used the ISA works as a basis to formulate international standards for sizing control valves. (Some information in this introductory material has been extracted from ANSI/ISA S75.01 standard with the permission of the publisher, the ISA.) Except for some slight differences in nomenclature and procedures, the ISA and IEC standards have been harmonized. ANSI/ISA Standard S75.01 is harmonized with IEC Standards 534-2-1 and 534-2-2. (IEC Publications 534-2, Sections One and Two for incompressible and compressible fluids, respectively.)

Flow coefficient and proper design of control valves - Imperial Units 

With the flow coefficients capacities of valves at different sizes, types and manufacturers can be compared. The flow coefficients are in general determined experimentally and express the flowcapacity in imperial units - GPM (US gallons per minute) that a valve will pass for a pressure drop of 1 lb/in2 (psi) 

The flow factor - Kv - is also commonly used with capacity in SI-units. 

The flow coefficient - Cv - required for a specific application can be estimated by using specific formulas for the different fluids or gases. With an estimated Cv value - the correct size of control valve can be selected from the manufacturers catalogs. 

Note that an oversized control valve may hurt process variability by putting too much gain in the valve leaving less flexibility for the controller. An oversized valve operates more frequently at lower openings with increased dead band as result. 

Valve Sizing Calculations (Traditional Method)


Flow Coefficient - Cv - for Liquids 


For liquids the flow coefficient - Cv -is expressed with water flow capacity in gallons per minute (GPM) of 60oF with pressure drop 1 psi (lb/in²).

Flow expressed by volume 

Cv = q (SG / dp)^₁∕ ²

where 

q = water flow (US gallons per minute) 

SG = specific gravity (1 for water) 

dp = pressure drop (psi) 

or alternatively in metric units: 

Cv = 11.6 q (SG / dp)^1/2 

where 

q = water flow (m3/hr) 

SG = specific gravity (1 for water) 

dp = pressure drop (kPa) 

Flow expressed by weight 

Cv = w / (500 (dp SG)^1/2) 

where 

w = water flow (lb/hr) 

SG = specific gravity (1 for water) 

dp = pressure drop (psia) 

or alternatively in SI units: 

Cv = 5.8 w / (500 (dp SG)^1/2)

where 

w = water flow (kg/hr) 

SG = specific gravity (1 for water) 

dp = pressure drop (kPa) 

Flow Coefficient - Cv - for Saturated Steam 

Since steam and gases are compressible fluids, the formula must be altered to accommodate changes in density. 

Critical (Choked) Pressure Drop With choked flow and critical pressure drop, the outlet pressure - po - after the control valve is aprox. 58% of the inlet pressure - pi - before the control valve. The flow coefficient at choked - or critical - flow can be expressed as: 

Cv = m / 1.61 pi 

where 

m = steam flow (lb/hr) 

pi = inlet steam absolute pressure (psia) 

po = outlet steam absolute pressure (psia) 

Flow Coefficient - Cv - Super-heated Steam 

The flow coefficient for superheated steam should be multiplied with a correction factor: 

Cv = Cv_saturated (1 + 0.00065 dt)  

where 

dt = steam temperature above saturation temperature at the actual pressure (oF) 

Flow Coefficient - Cv - Saturated Wet Steam 

Saturated wet steam includes non evaporated water particles reducing the "steam quality" and a flow coefficient for very wet saturated steam should be multiplied with a correction factor: 

Cv = Cv_saturated ζ^1/2 

where 

ζ = dryness fraction 

Flow Coefficient - Cv - Air and other Gases 

Note! - there is a difference between critical and non critical pressure drops. 

For sub critical pressure drop - chocked flow, where the outlet pressure - po - from the control valve is less than 53%of the inlet pressure - pi, the flow coefficient can be expressed as: 

Cv = q [SG (T + 460)]⅟²/ (FL 834) pi 

where 

q = free gas per hour, standard cubic feet per hour (Cu.ft/h) 

SG = upstream specific gravity of flowing gas gas relative to air at 14.7 psia and 60oF 

T = flowing air or gas temperature (oF) 

FL = pressure recovery factor 

pi = inlet gas absolute pressure (psia) 

The flow coefficient - Cv - or the flow factor - Kv - are commonly used to specify capacities of control valves. 

Flow Coefficient - Cv 

It is often convenient to express the capacity and flow characteristic of a control valve in terms of the 

--Flow Coefficient - Cv 

          The flow coefficient - Cv - is based on the imperial units system and is defined as: 

 -- the flow of water through a valve at 60 oF in US gallon/minute at a pressure drop of 1 lb/in2 

    The flow coefficient is commonly used in the U.S. 

Flow Factor - Kv 

The metric equivalent of the flow coefficient - Cv - is based on the SI-system and is called the 

--Flow Factor - Kv 

          The flow factor - Kv - is defined as 

--the flow of water with temperature ranging 5 - 30 oC through a valve in cubic meters per hour                   (m3/h) with a pressure drop of 1 bar 

The flow factor is commonly used outside U.S. 

Converting between Flow Coefficient Cv and Flow Factor Kv 

The relationship between Cv and Kv can be expressed as: 

Cv = 1.16 Kv (1) 

Kv = 0.862 Cv (2) 

Control Valves in Series

FIG 17

The resulting Kv or Cv for two control valves installed in series can be calculated as

1 / (Kvt)² = 1 / (Kv1)² + 1 / (Kv2)²

where

Kvt = resulting Kv

Kv1 = Kv valve 1

Kv2 = Kv valve 2

Control Valves in Parallel

FIG 18

The resulting Kv or Cv for two control valves installed in parallel can be calculated as

Kvt = Kv1 + Kv2