Control Valves

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Control Valves

Table of Contents

CONTROL VALVES................................................................................................................2GLOBE VALVES..................................................................................................................2

CAGE GUIDED VALVES................................................................................................3STEM AND POST-GUIDED VALVES...........................................................................3

BALL VALVES.....................................................................................................................4V-NOTCH BALL VALVES..............................................................................................5ECCENTRIC ROTARY PLUG VALVES........................................................................6

BUTTERFLY VALVES........................................................................................................6HEAVY DUTY BUTTERFLY VALVES.........................................................................6HIGH PERFORMANCE BUTTERFLY VALVES..........................................................7

SELECTION PARAMETERS...............................................................................................7PRESSURE RATINGS......................................................................................................8PRESSURE DROP RATINGS..........................................................................................8TEMPERATURE RATINGS............................................................................................9MATERIAL SELECTION................................................................................................9CAPACITY......................................................................................................................10FLOW CHARACTERISTICS.........................................................................................11

SHUTOFF ANSI CLASS SEAT LEAKAGE.....................................................................11ANSI SEAT LEAKAGE CLASSIFICATIONS..............................................................12CLASS V1.......................................................................................................................13

NOISE CONSIDERATIONS..............................................................................................14END CONNECTIONS........................................................................................................14GLOSSARY.........................................................................................................................15

GLOBE VALVE NOMENCLATURE............................................................................15ROTARY-SHAFT VALVE NOMENCLATURE...........................................................17

Berry’s Commissioning Handbook

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CONTROL VALVESTo define a control valve, we will apply a standard definition from the Instrument Society of America; a society that endorses standards, procedures, and definitions that are commonly recognized in the process industries. ISA defines a control valve as "A power operated device which 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 in the valve in response to a signal from the controlling system".

In other words, a control valve automatically responds to some feedback from sensing element that measures a variable in the process loop. The variable can be temperature, pressure, flow, or level of the process fluid. The sensing element sends a signal, either directly or through intermediate instrumentation, to the actuator. The actuator, which is often pneumatically operated, responds to the signal and adjusts the position of the valve.

The ISA definition implies major differences between control valves and non-control valves. Non-control valves are generally on-off devices, manually operated, and designed for light or intermittent service. On the other hand, a control valve that automatically responds to frequent changes in the control loop may throttle or cycle very frequently - from several times a day to several times a minute. Consequently, control valves must be ruggedly built to provide consistent and reliable operation day after day in high cycle service. As compared to on-off valves, control valves generally use higher-grade materials, heavier internal parts, and more precise tolerances. Another basic design difference between on-off and control valves stems from specific purposes of each.

An on-off valve is generally used to either open a pipeline to direct flow, or to close off a pipeline for shutdown or emergencies. Therefore, the on-off valve should offer little restriction when open and tight shutoff when closed. This is the function it is designed for.

In contrast, a control valve is used to absorb energy from an imperfect process. Therefore, even when fully opened, the control valve absorbs a fractional amount of energy so that it can provide an immediate system effect in the first increment of closure. On-off valves may move 15 to 20 percent of the available travel before a significant system impact occurs. Therefore, a control valve provides an immediate response, while a non-control, or on-off valve, would likely introduce an unacceptable lag if used in a throttling application.

GLOBE VALVES

Globe valves are so named because of the globular shaped cavity through which flow passes. These rugged valves use a plug to shut off flow at the valve seat and to control flow through the body passages. The plug is attached to a valve stem that protrudes through the bonnet. The bonnet retains pressure in the valve body and contains the packing that prevents leakage of process media along the valve stem. Because the valve plug is positioned by linear motion of the plug stem, these valves are also referred to as sliding-stem, valves. Most globe or sliding stem valves fall into one of the general categories, categories, cage-guided valves and stem or post-guided valves.


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CAGE GUIDED VALVES

The cage serves two major functions. First, it provides positive alignment and control of the plug. Second, because the flow is directed through openings or “windows” in the wall of the cage, the cage can modify or “ characterize” the way in which media flows through the valve body. Different flow characteristics are attained by varying the shape of the windows. In many designs, the cage is also used to hold the seat ring in place in the valve body.

The primary design feature of cage-guided valves is the massive guiding the cage provides. In service, the dynamic forces of flow on internal valve parts can cause vibration and bending forces that may result in instability of the valve plug a premature wear. By guiding the plug on the inside diameter of the cage through the full length of plug travel cage guiding minimizes the negative effects of vibration and instability. No other valve type gives as great a guiding surface area as the cage-guided valve.

Interchangeability of trim is another important design feature. Because the plug, cage and seat or trim set can be replaced with trim sets for different service conditions, the cage-guided valve is extremely versatile.

The top-entry design allows replacement of all trim parts through the bonnet opening with the valve remaining in the pipeline

With unbalanced trim, all the forces associated with the flow of the process fluid are applied directly to the bottom of the plug. This results in a pressure differential between the top and the bottom of the plug. If the flow direction is such that the effect of flow is to push the plug upward (flow up), very large actuators may be required to seat the valve in high pressure applications. The primary advantage of unbalanced trim is very tight plug seating - or shutoff.

Balanced trim uses a port through the vertical axis of the valve plug to allow process media into the cavity above the plug. This balances the forces applied by the process fluid above and below the plug and results in a reduction of actuator force required to stroke the valve.

Because of its ruggedness, excellent throttling control, and easily interchangeable trim, adaptations of cage guided valves are found in virtually every industry. Though, suitable for a broad range of general services with clean fluids, they are also used in difficult services involving corrosive fluids, high pressure, and temperature extremes. The only applications where they are not commonly used, involve viscous or erosive process fluids where closely guided parts are not desirable.

The tortuous flow path that is common to all globe valves results in less capacity per valve size compared to most rotary shaft valves. The close clearances that accompany cage guiding may exclude its use with slurries or with extremely gritty, or dirty process fluid. In sizes over six inches, they are more costly than rotary valves. Because they are ruggedly made, they are large and heavy and may require hoists or cranes to move or install. Unbalanced designs may require large actuators.

STEM AND POST-GUIDED VALVES

Like cage-guided valves, stem or post-guided valves also use a plug for controlling the flow of process fluids. In these designs, the valve plug is guided by bushings around the stem or post.

These valves provide a more open flow path than cage-guided valves so they are often used in applications with slurries or with viscous or sticky media that may tend to clog a cage-guided valve. They are also commonly applied to corrosive services.


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A design feature of the stem or post-guided valve is that the guide bushings are generally out of the flow stream. Because the guiding surfaces are protected from the effects of flow, these designs are often used in viscous and-erosive applications or in coking service; that is, with gritty media that would tend to cake up and interfere with cage guiding.

Like cage-guided valves, trim and other internal parts can be easily replaced maintained while the valve remains in the pipeline.

Stem and post-guided valves are found in a number of applications. The following basic design distinctions can be made:

First, for easy general service applications requiring smaller valves, economy constructions of this design can provide a cost effective control solution.

Secondly, for specialty chemical services where exotic alloys must be used, this valve type can be fabricated from bar stock at a considerable savings over a cast or forged valve body.

Third, adaptations of heavy-duty designs have been developed for:

services involving slurries and viscous process fluids, and

corrosive services encountered in chemical and other industry applications

Finally, special constructions are available to handle difficult services with pressures ranging up to 50,000 psi.

Economy valves are found in a variety of services generally involving clean fluids, air or water at moderate temperatures and pressures. All the other configurations are applied in more difficult services involving erosive, corrosive, gritty or viscous process fluids. For instance, standard heavy duty designs are commonly used with slurries over a broad range of pressure and temperature conditions. Bar stock and chemical service designs are generally used in chemical and specialty chemical production. High pressure designs are used in services such as oil and gas production, plastics manufacturing, and letdown applications.

Compared to cage-guided valves, stem and post-guided valves have fewer trim options available for special applications such as services involving noise or cavitation. In some unbalanced designs, process pressure is fully applied to the bottom of the plug so the tendency of flow is to open the valve. Therefore, high pressure applications may require considerable actuator force to seat the valve plug. The economy valves do not provide a large guiding surface so plug instability may, limit pressure handling capability. Like cage guided valves, they are large, heavy, and costly compared to many rotary valves, in a given valve size, they have less capacity than rotary shaft valves.

BALL VALVES

Ball valves use a full sphere or a portion of an spherically shaped component to control flow through a body passage. The three common categories are reduced bore ball, V notch partial ball segment, and rotary plug.

Reduced Bore Ball Valves

Reduced bore ball valves control flow with a complete, rotating ball. The bore of the ball is reduced from line size so that the valve can begin to control flow as soon as it is stroked away from the fully open position. This is in keeping with the definition of a control valve given


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earlier. Because of this feature, these valves are also referred to as "venturi ball" valves to distinguish them from full bore , non-control valve designs.

A chief design feature is that there is minimal obstruction to the port or ball bore. As a result, these valves are commonly used in high capacity, higher pressure, applications where minimum pressure loss in the valve is desired.

Because they are subject to considerable pressure and flow, they are very ruggedly, built. Shafts, seals, and actuator connections are all oversized to satisfy the requirements of high pressure throttling service.

Because these valves combine tight shutoff and minimal pressure loss with the ability to quickly respond to system upsets, reduced bore ball valves are commonly used for throttling pressure and flow control required in gas transmission lines, gas distribution service, and liquid pipelines.

Compared to other valve types, reduced bore ball valves tend to be costly and have limited temperature capabilities. They are generally applied in services with temperatures under 300*F.

V-NOTCH BALL VALVES

Ball segment V-notch valves use only a portion of a spherically shaped component (partial ball) to control flow. The spherical segment has a contoured notch that provides a flow characteristic that is associated with precise control in a broad range of applications. The segment remains in contact with the sealing surfaces throughout its rotation.

A major benefit of this design is wide rangeability - or the ability to control both very low flow rates and very high flow rates. When the V-notch ball segment is fully open, there is minimal obstruction so a high maximum flow is obtained. As the segment approaches the closed position, the V-notch accurately controls flow to a very low rate before seating. Rangeability - expressed as a ratio of maximum to minimum controllable flow coefficient- is approximately 300:1 for this valve type.

When compared to globe valves providing equivalent flow capacity, these valves are smaller, lighter, and require less material and machining. Especially in sizes above six inches, this results in excellent throttling control with great economy compared to globe valves.

Because the ball segment is in constant contact with the seals, the V-notch design provides a shearing effect as the ball segment seats. This feature makes it particularly suitable for applications involving fibrous slurries and fluids with entrained solids. This wiping action also provides a self-cleaning effect when used with fluids that might tend to cake up on internal valve parts.

Wide rangeability, straight through flow, and shear-on-closing action has made this valve popular in the pulp and paper industry where slurries of wood fiber tend to clog other valve designs. Minimal pressure loss and wide rangeability have also made this a popular valve for use in the natural gas industry. Excellent throttling control and a favorable cost position have led to increased application in chemical and other industries. V-notch valves represent an excellent value for controlling flow through large pipelines.

Compared to globe valves, V-notch ball valves have reduced pressure handling capability and cannot deal with as broad a range of corrosive and erosive service. Standard maintenance procedures generally involve a shimming procedure to establish proper seal contact with the ball segment. Because they are high recovery valves, there is a potential for cavitation under certain service conditions.


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ECCENTRIC ROTARY PLUG VALVES

Eccentric rotary plug valves use a partial ball segment as the controlling component when the valve is closed, the segment seats against a metal seat ring. In some designs, the seat ring is a "floating" element which self- centers without the use of shims.

This basic design differs from reduced bore ball valves and V-notch ball valves in that the ball segment - or plug - operates on an eccentric path so that it is not in contact with its sealing surfaces (seat ring) during throttling. This design helps prevent seat wear and requires less operating torque.

Metal-to-metal seating surfaces provide reliable shutoff in erosive, high temperature and high pressure applications. The seating method provides a self-lapping effect so that shutoff actually tends to improve over time.

The flow path is designed so that the damaging effects of high velocity flow and impingement of gritty media is directed away from the body and toward hardened and easily replaceable parts. When manufactured with hard trim materials such as stellite or tungsten carbide, this valve type is well suited to extremely erosive service conditions.

This valve type is commonly used in mining, mineral production, refining, and other extremely corrosive processes where extended valve trim life is difficult to maintain. Because of successes in these difficult services, eccentric plug valves have been increasingly used in non-erosive applications where economy (compared to globe valves) and long trim life is a requirement.

Pressure drop capability decreases rapidly with increased size. They are generally available in a limited range of sizes - typically through eight inches. Because they are high recovery valves, there is a potential for cavitation under some service conditions.

BUTTERFLY VALVES

Butterfly valves are available in two basic styles, heavy-duty butterfly valves and high performance butterfly valves.

HEAVY DUTY BUTTERFLY VALVES

Heavy duty butterfly valves use a rotating disk to control flow through the pipeline.

Disks are generally operable through 90 degrees of rotation so these valves are also referred to as "quarter-turn" valves.

Standard butterfly valves may be referred to as "swing-through" valves because there are no seals at the seating surfaces. The lack of seals results in only moderately tight shutoff.

Some variations of butterfly valves use soft, elastomer liners and/or coated disks. These are referred to as "lined" valves and may be used to provide either tight shutoff or resistance to some corrosive process fluids.

Because the valve disk imposes a relatively small obstruction to the flow path, the valves provide very high capacity when compared to many other valve types.

The small number of parts and straightforward design provide the most capacity per investment dollar compared to all other valve types. This cost advantage increases


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dramatically in sizes over twelve inches.

Standard butterfly valves are used in a variety of services where economy is a consideration and tight shutoff is not a requirement. They may be used with a variety of liquids and gases including those that are gritty (erosive), sticky, or tend to produce solid particle buildup on internal valve parts. In services where tight shutoff is desired or corrosion resistance is required, the lined products may be used.

Because the large surface area of the disk acts like a lever in applying the dynamic forces off flowing media to the drive shaft, butterfly valves are generally limited in pressure handling capability compared to other valve types. Because they are high recovery valves, there is a potential for cavitation under certain service conditions. Standard designs are limited in their shutoff capabilities. Lined valves are generally limited to services with clean process fluids and require more operating torque for seating and unseating the disk.

HIGH PERFORMANCE BUTTERFLY VALVES

High performance butterfly valves improve upon the basic butterfly design by incorporating an eccentric disk path and a sealing method. This combination results in economical throttling control and tight shutoff.

The eccentric motion created by the double offset disk removes the disk seating surfaces from the seals soon after the valve is unseated. This eccentric action reduces wear on the sealing surfaces and reduces the torque required to seat and unseat the valve.

Combining the economy of a standard butterfly valve with a tight sealing method results in good shutoff and throttling control in a compact and lightweight package.

Like other butter-fly valves, eccentric disk valves have a favorable cost position especially when compared to globe valves in sizes above four to six inches.

Eccentric disk valves are commonly applied in non-severe services that require economy, moderate pressure drop capability, and good shutoff. Typical applications include services involving clean fluids and gases, low pressure steam, and some corrosive media.

Throttling control and stability is not as great as with globe valves or V-notch ball segment valves. In terms of pressure handling capability, eccentric disk valves are less capable than globe valves but more capable than standard butterfly valves. Like other high recovery valves, there is a potential for cavitation under certain service conditions.

SELECTION PARAMETERS

When selecting a valve for a specific service, several parameters of valve design and performance must be evaluated. This discussion will focus on the more important specifications and attributes to be considered when selecting a control valve. They a pressure / temperature ratings, material compatibility, capacity, flow characteristics, shutoff, cavitation and flashing, noise, end connections, economy, and maintenance.


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PRESSURE RATINGS

When selecting a valve for a given service we must consider both the inlet pressure rating and the maximum pressure drop rating of the control valve.

Inlet pressure ratings refer to the capabilities of the pressure retaining components such as bodies and bonnets. This capability is commonly expressed in terms of ANSI Class ratings. ANSI stands for the American National Standards Institute. ANSI Class ratings of 150, 300, 600, 900, 1500, or 2500 pounds are commonly used. Generally speaking, valves with higher ANSI Class ratings have heavier or thicker pressure retaining components.

Other ANSI ratings define requirements for classes over 2500 and other special categories. While ANSI standards are commonly referenced, other standards such those published by the API (American Petroleum Institute) are also used to define pressure/temperature capabilities.

A valve with a given ANSI Class rating is likely to be capable of withstanding inlet pressures much greater than the nominal ANSI class. For example, an ANSI Class rating of 150 does not limit the valve to 150 psig. In fact, ANSI Class 150 carbon steel valves bodies are typically rated at 285 psig at lower temperatures.

Within the same ANSI Class, different materials have different pressure-temperature capabilities. Therefore, when a valve is selected that does not quite meet the service pressure requirements, it may be possible to select the same valve in the same ANSI class but in a different material that will meet pressure/temperature requirements. In, many instances, this can be more economical than selecting a heavier body with a higher ANSI Class rating.

Increased temperatures limit the maximum inlet pressure rating, the higher the temperature, the lower the maximum inlet pressure rating.

PRESSURE DROP RATINGS

Sometimes more limiting than the ANSI Class rating is the pressure drop (∆P) rating. Pressure drop is defined as the difference between the inlet and the outlet pressures of the valve. While control valve inlet pressure ratings just discussed are determined by the strength of pressure retaining parts such as bodies and bonnets, control valve pressure drop capabilities are more a function of the valve's internal parts; closure members, stems, shafts, bearings, and so forth.

Flowing pressure drop refers to the difference between downstream and upstream pressure, while the valve is throttling. Flowing pressure drops are highest when the valve closure member is positioned near its seat. In this position, downstream pressure is greatly reduced, velocity in the valve is high and the forces of fluid flow acting on the closure member (plug, ball, or disk) are increased.

As a general rule of thumb, shutoff pressure drop is equivalent to upstream pressure. When the valve is closed, downstream pressure is essentially zero so the full inlet pressure is the pressure drop. Exceptions to this guideline are when downstream back-pressure acts to reduce pressure drop or, where a downstream vacuum increases pressure drop.

Pressure containment is sometimes accompanied by a change in energy states; a change from static pressure - or potential energy - to a form of kinetic energy such as heat, vibration, or noise. Any of these forces can produce damaging effects oil control valve components.

Depending on how the forces associated with pressure drop are applied to closure members and controlling components, either flowing or shutoff pressure drop may become a limiting


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factor in control valve selection. In rotary shaft valves, fluid forces acting on the closure member may stress or destabilize the member itself. In addition, these forces are ultimately converted to torque that applies stress to shafts, bearings, pins, and other components connected to the closure member or shaft. In globe or sliding stem valves, forces associated with pressure drop result in compressive and tensile forces on the stem, stress on the stem-to-plug connection, and instability of the valve plug. When applying a control valve, it must be determined that the valve selected has maximum flowing and shutoff pressure drop ratings that match the service conditions. Control valve manufacturers determine and publish both flowing and shutoff pressure drop capabilities.

The control valve actuator must be sized to effectively oppose the forces of flow o the valve closure member. Therefore, unbalanced globe valves and many rotary-shaft valves may require extreme amounts of actuator force under certain conditions.

TEMPERATURE RATINGS

Temperature capabilities are essentially a function of the materials used. However, as we saw in the ANSI pressure-temperature ratings charts, temperature a pressure ratings are interactive; that is, the higher the temperature, the lower the pressure capability.

Each component of the valve has a temperature rating; the body, trim, packing, seals, gaskets, and so forth. Therefore, the temperature capability of the valve assembly is defined by the component with the most restrictive rating. In the workbook example, the bonnet bolts limit the maximum service temperature to 450 degrees. Since all other materials are capable of higher temperatures, the bolting specification is the limiting factor. Most manufacturers have checking systems in place to prevent this inconsistency in material selection.

In some instances, the effects of thermal expansion and contraction may be more limiting than the nominal temperature rating of the construction material used. This is especially true when large or long valve components are used in extremely high or low temperature service. In such instances, the valve assembly may have a more limited temperature capability than the materials from which it was manufactured. Manufacturers generally consider these factors and rate their products accordingly.

MATERIAL SELECTION

Material specification is an important step in control valve selection. Material selection has great impact on the suitability of a valve for a given medium, the life of the valve, and the cost of the valve. Factor's which govern material selection are:

Chemical compatibility with a given process fluid

Physical ability to withstand the affects or wear, pressure drop, and erosive fluids

Trim materials are often made of more durable materials than the body, because they are in the flowstream and subject to more wear. Heat treated or hard-faced stainless steels are commonly used for valve trim.

Carbon steel valve bodies are popular because of their economy and broad range of availability. Stainless steel valve bodies may be specified when exceptionally high or low temperature capabilities are required or for resistance to some corrosive process fluids.

Soft valve components such as seats, seals, and packing are available in a broad range of natural and synthetic materials. Application requirements such as temperature and chemical


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compatibility dictate the materials used.

In erosive applications, trim parts can be hard-faced or manufactured from very durable materials such as Alloy 6, tungsten carbide, ceramics, and other new specialty materials.

For extremely corrosive applications, bodies and trim may be made from one of a broad range of nickel based alloys such as the Hastelloys, Inconels, and Monels.

As a word of caution, many different standards exist for describing materials. Materials can be described with common names such as "stainless steel", or with proprietary names, such as Inconnel, which is a trademark of the International Nickel Company. These rather general terms could apply to any one of a number of specific materials within that family. For instance, a general reference to Inconel could be interpreted as Inconel 718 or Inconel 600. The 718 material has a yield strength of 180,000 psi while the 600 material has a yield strength of only 25,000 PSI. Therefore, with this wide range of capability in the same family of material, it is important to specify the exact material desired.

CAPACITY

Capacity is defined as the rate of flow through a valve under stated conditions as, expressed as a "flow coefficient". The flow coefficient is supplied by the valve manufacturer and is useful for determining the size of a given control valve needed for a given flow requirement. Different flow coefficients are used for each of the physical states - liquid, gas, and steam.

Liquid capacity is expressed as the coefficient Cv. Cv is defined as the number of gallons of water at 60 degrees F. that will flow through the valve in one minute at a pressure drop of 1 psi at stated conditions. The stated conditions will include percent of rated travel.

Gas and steam capacities are expressed with different flow coefficients because they behave differently than liquids. In arriving at these coefficients, allowances are made for the effects of compressibility and other important factors. The coefficients Cg, - gases, and Cs - for steam, are commonly used.

Specific ANSI standards define commonly accepted procedures for determining flow coefficients. These standards define procedures for determining flow coefficients by actual pipeline tests and by calculation. The procedures used by different manufacturers to determine flow coefficients can vary so it is a good practice to know if the manufacturer typically rates their valves conservatively or optimistically. Tested results may be preferred as they may be more accurate.

Rotary-shaft valves, with the exception of the eccentric rotary-plug design, provide the highest liquid flow coefficients. This is expected as these valves have a straight through flow path with minimal obstruction. While it might be expected that the full ball valve would have the highest flow coefficient, recall that the full ball valve discussed earlier had a reduced port to provide accurate throttling control. Because, globe valves have a more tortuous flow path than rotary-shaft valves, we would expect them to have somewhat diminished capacity compared to rotary valves.

When flowing gasses, the difference in capacity between the valve families is less pronounced. This is because liquid flow coefficients are dependant on how tortuous the flow path is, while gas flow coefficients are more a function of the minimum cross-sectional flow area of the control valve.

Closely related to capacity, recovery is a relative term used to describe how downstream pressure is affected by the design and geometry of the control valve. For example, globe valves tend to be low recovery valves. That is, they have a relatively tortuous flow path,


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which limits capacity. On the other hand, most rotary (ball and butterfly) valves have a "line of sight" flow path, which provides high capacity so they are referred to as "high recovery" valves. Though high recovery valves have the benefit of being high capacity valves, they are also more susceptible to troublesome and damaging phenomena such as cavitation.

FLOW CHARACTERISTICS

Flow characteristics determine how the flow coefficient changes in response change in the valve position; in other words, how the rate of flow will be changed when the valve receives a signal from the sensing instrument. Selection of the best flow characteristic for a process is a relatively involved process, so for the purposes of this discussion, we will simply identify the more common characteristics.

Quick Opening - An inherent flow characteristic in which there is maximum flow coefficient with minimum travel.

Linear - A inherent characteristic which can be represented ideally by a straight line plot of flow coefficient versus percent of rated travel.

(Equal increments of travel produce equal increments of flow at a constant pressure drop)

Equal percentage - An inherent flow characteristic in which a given percentage change of valve travel will produce and equal percentage change in the exiting flow coefficient.

Flow characteristics of cage guided valves can be determined by the shape of the cage windows. Stem or post guided valves can be characterized by the shape of the plug, and most rotary valves have a flow characteristic inherent to their basic design.

The flow characteristics discussed are "inherent" characteristics. That is, they describe the flow characteristics under static conditions of constant pressure and pressure drop. In actual service conditions, changes in pressure and pressure drop will change the flow characteristic dramatically. Therefore, a distinction is made between "inherent" and "installed" characteristics. The installed characteristic takes into account changes in pressure drop and other conditions in the system.

SHUTOFF ANSI CLASS SEAT LEAKAGE

Shutoff is ordinarily stated in terms of classes of seat leakage defined in the American National Standard for Control Valve Seat Leakage. In actual service, shutoff leakage depends on many factors including pressure drop, temperature, the condition of the sealing surfaces, and the force load on the seat -, which is a function of actuator force available. Since shutoff ratings are based on standard test conditions, which may be very different from actual service conditions, service leakage cannot be absolutely predicted. However, the ANSI shutoff classes provide a good basis for comparison among valves of similar configuration.

ANSI Classes Compared

As we identify the different seat leakage standards, we can roughly calculate the seat leakage of a typical 3-inch globe body that would conform to each of the leak classes. First, because ANSI Class two, three, and four leakage is expressed as a percentage of rated capacity, we'll have to calculate the normal wide open flow of our three inch valve under test conditions. The basic formula for flow is C, times the square-root of AP so we'll have to know the C, of the valve and the pressure drop of our setup. The maximum rated C, of 140 comes from the manufacturers literature and the pressure drop of 50 psi is one of the test conditions in the ANSI Standard. Solving the equation, we find that our 3-inch valve will produce a maximum flow of approximately 1,000 gallons per minute under test conditions.


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ANSI SEAT LEAKAGE CLASSIFICATIONS

Leakage

Class

Maximum Leakage Allowable

Test Medium

Test Pressures Testing Procedures

Required for Establishing Rating

I - - - No test required provided user and supplier so agree.

II 0.5 % of rated capacity

Air or Water

50-125*F

10-52 *C

45 to 60 psig or max operating differential whichever is lower

Pressure applied to valve inlet, with outlet open to atmosphere or connected to a low head loss measuring device, full normal closing thrust provided by actuator.

III 0. 1% of rated capacity

As above As above As above

IV 0.0 1% of rated capacity

As above As above As above

V 0.0005 ml per minute of water per inch of port diameter per psi differential

As above Max. service pressure drop across valve plug, not to exceed ANSI body rating

(100 psi pressure drop minimum)

Pressure applied to valve inlet after filling entire body cavity and connected piping with water and stroking valve plug closed. Use net specified max. actuator thrust, but no more, even if available during test. Allow time for leakage flow to stabilizer

VI Not to exceed amounts shown in following table based on port diameter

Air or Nitrogen at 50-125*F

10-52*C

50 psig or max. rated differential pressure across valve plug, whichever is lower.

Actuator should be adjusted to operating conditions specified with full normal closing applied to valve plug seat thrust. Allow time for leakage flow to stabilize and use suitable measuring device.


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CLASS V1

NOMINAL PORT DIAMETER LEAK RATE

Inches Millimeters ml Per Minute

Bubbles Per Minute'

1 25 0.15 1

1-1/2 38 0.30 2

2 51 0.45 3

2-1/2 64 0.60 4

3 76 0.90 6

4 102 1.70 11

6 152 4.00 27

8 203 6.75 45

Class I shutoff does not require testing but is mutually defined by and agreed to by the user and supplier. It is a special classification that might apply to a valve with a higher leakage class rating which has been modified for some purpose.

Class II shutoff allows leakage of up to one-half of one percent (0.5%) of the rated capacity of the valve using air or water as the test medium at a pressure drop of 50 psid. In our example, leakage of the specified valve is five gallons per minute.

Class III shutoff allows leakage of up to one-tenth of one percent (0.1 %) of the rated capacity of the valve again using air or water as the test medium with a 50 psid pressure drop. This is one-fifth the leakage of Class II and as an the example, is one gallon per minute.

Class IV shutoff allows leakage of one one-hundredth of one percent (.01 %) of the rated capacity of the valve under the same test conditions as above. Leakage is slightly less than one pint per minute.

Class V standards become more stringent and allow only.0005 milliliters of water per minute per inch of port diameter at a minimum test pressure drop of 100 psid. In the example, wide open flow should be increased to about 1400 gallons per minute because of the increased test pressure. However, because of the more demanding requirements, an eyedropper could be used to measure leakage accumulated in one minute.

Class VI standards are very demanding. Instead of water as a test medium, air or nitrogen is used with a pressure drop of 50 psid. The allowable leakage for different nominal port diameters is expressed in both milliliters per minute and bubbles per minute. Allowable leakage does not follow a linear scale but is identified for port diameters through 8-inches. Obviously, this leak class provides very tight shutoff.


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In actual application, not all throttling valves need to provide tight shutoff. Block valves placed around the control valve provide the tight shutoff function. Over specifying control valve shutoff has been identified as one of the greatest unnecessary costs incurred during control valve selection. What typically happens is that every specifier in the chain - the designer, plant manager, engineer, and so on -adds a safety margin to the specifications; each person who reviews the plans and specifications increases the shutoff requirement. The result is good - though not always necessary - shutoff, at a progressive penalty in cost.

In some instances, tight shutoff should be specified even though it not a specific process requirement. For instance, when flowing toxic or flammable fluid, tight shutoff is often specified for safety reasons. In severe services involving erosive fluids, high pressure drops, or cavitation (discussed later), tight shutoff may be specified to reduce wear or erosion of closure members and seats.

NOISE CONSIDERATIONS

Aerodynamic noise is generated by the turbulence associated with control of gas, steam, or vapors. While generally thought of as accompanying high capacity, high pressure systems, damaging noise levels can be produced in a two-inch line with as little as a 200 psi pressure drop.

Concern for high levels of noise is a twofold issue. First, noise control is an environmental issue as OSHA (the Office for Safety and Health Administration) has determined safe and acceptable limits for control valve noise as it affects the working environment. Secondly, research indicates that noise levels above the recommended limit of approximately 110 decibels can result in mechanical damage to control valves, their associated parts and piping.

During the valve sizing process, noise can be predicted by using well established mathematical techniques. When unacceptable noise levels are predicted, noise abatement equipment and techniques are available to reduce noise to acceptable levels. Noise control can be provided by source treatment at the origin (generally the control valve), by path treatment downstream of the origin, or by both.

Source treatment strategies generally involve special valve trims or accessories designed to reduce turbulence of the process fluid within the valve. Source treatments actually limit the amount of noise generated by a control valve. For cage guided valves, special cage designs are used. For other types of valves, diffusing components are inserted downstream to reduce noise generation within the valve.

Path treatments such as pipeline insulation or the use of heavy wall pipe do not eliminate the noise but rather reduce the intensity, of the sound transmitted into the working environment. Silencers, on the other hand, do absorb part of the noise energy and therefore reduce the sound intensity in both the flowstream and the working environment.

END CONNECTIONS

End connection selection is generally a simple question of whether the desired end connection style is available for the type of valve being considered. In some instances, however, end connection style is dictated by safety standards or plant policies.


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Screwed connections can be used in sizes up to 2-inch in general service applications.

Flanged connections are available in several styles.

Flat face flanges provide full face contact. They are commonly used in low pressure service with cast iron or brass valves.

Raised face flanges are the most popular of all end connections. They have a grooved gasketing surface to provide a tight gasket seal and to help prevent gasket blowout. These flanges can be used with temperatures through 1500*F and pressures to 6000 psig.

The ring type joint flange is similar to the raised face flange except that a compressible metal ring is wedged into two mating grooves when the flanges are tightened. These are used for high pressure service to 15,000 psi but are not used for high temperature service.

Separable flanges are manufactured independently of the valve body and are secured to the body with split rings. They are economically beneficial when bodies must be made of costly alloys for corrosion, and erosive services. Because the process fluid does not normally contact the flanges, flanges can be manufactured from materials that are more economical.

Welded end connections have the advantage of being leak tight at all pressures and temperatures. Obviously, they are only used with weldable materials and make it difficult to remove the valve from the pipeline

Socket weld ends are dimensionally the same regardless of pipe schedule and are generally used in sizes through two inches.

Butt weld ends are prepared by beveling, each end of the valve to match a similar bevel on the pipe. End preparation is different for different schedules of pipe. This end connection is typically used with valve sizes over two-inches.

GLOSSARYGLOBE VALVE NOMENCLATUREBonnetA valve pressure retaining boundary which may guide the stem and contain the packing box and seal. The major part of the bonnet assembly, excluding the sealing means. (This term is often used in referring to the bonnet and its included packing parts. More properly, this group of component parts should be called the Bonnet Assembly.)

Bonnet AssemblyAn assembly including the part through which a valve plug stem moves and a means for sealing against leakage along the stem. It usually provides a means for mounting the actuator.

CageA hollow cylindrical trim element that is a guide to align the movement of' a valve plug with a seat ring. The cage may also retain the seat ring in the valve body. (The walls of the cage have openings that usually determine the flow characteristic of the control valve.)

Cage Guided ValveA type of valve that uses a cage for plug guiding and alignment. See Cage.

Extension BonnetA bonnet with an extension between the packing box and bonnet flange for hot or cold service.


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Globe ValveA valve construction style with a linear motion flow controlling member with one or more ports, normally distinguished by a globular-shaped cavity around the port region. Two categories are commonly recognized depending on the method of plug guiding; cage guided and stem or plug guided.

Guide BushingA bushing in a bonnet, bottom flange, or body to align the movement of a valve plug with a seat ring.

Isolating ValveA hand-operated valve between the packing lubricator and the packing box to shut off the fluid pressure from the lubricator.

Packing Box (Assembly)The part of the bonnet assembly used to seal against leakage around the valve plug stem. Included in the complete packing box assembly are various combinations of some or all of the following component parts: Packing, Packing Follower, Packing Nut, Lantern Ring, Packing Spring, Packing Flange, Packing Flange Studs or Bolts, Packing Flange Nuts, Packing Ring, Packing Wiper Ring, Felt Wiper Ring.

Packing LubricatorAn optional part of the bonnet assembly used to inject lubricant into the packing box.

PortA fixed opening, normally the inside diameter of a seat ring, through which fluid passes.

Retaining RingA split ring that is used to retain a separable flange on a valve body.

SeatThat portion of the seat ring or valve body which a valve plug contacts for closure.

Seat RingA separate piece inserted in a valve body to form a valve body port. It generally provides a seating surface for the closure member.

Separable FlangeA flange which fits over a valve body flow connection. It is generally held in place by means of a retaining ring.

Stern ConnectorA two piece clamp which connects the actuator stem to the valve plug stem.

Stem or Plug-Guided ValveA valve whose plug is guided by a bushing surrounding the plug or the stem (as opposed to cage guiding).


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TrimThe internal parts of a valve which are in flowing contact with the controlled fluid. (in a globe valve body, trim would typically include valve plug, seat ring, cage, stem, and stem pin.)

ROTARY-SHAFT VALVE NOMENCLATUREBall, FullThe flow-controlling member of rotary-shaft control valves utilizing a complete sphere with a flow passage through it.

Ball, V-notchThe flow-controlling member for a popular style of throttling ball valve. The V-notch ball includes a polished or plated partial-sphere surface that rotates against the seal ring throughout the travel range. The V-shaped notch in the ball permits wide rangeability and produces an equal percentage flow characteristic.

Ball Segment, EccentricThe flow controlling member of the eccentric rotary plug valve. Because of its eccentric action, it clears its seat soon after opening. This results in longer life, especially in erosive services, and reduces the actuator force required to operate the valve.

NoteThe balls mentioned above, and the disks that follow, perform a function comparable to the valve plug in a globe-style control valve. That is, as they rotate they vary the size and shape of the flowstream by opening more or less the seal area to the flowing fluid.

Disk, ConventionalThe flow-controlling member used in the most common varieties of butterfly rotary valves. High dynamic torques normally limits conventional disks to 60 degrees maximum rotating in throttling service.

Disk, Dynamically DesignedA butterfly valve disk contoured to reduce dynamic torque at large increments of rotation, thereby making it suitable for throttling service with up to 90 degrees of disk rotation.

Disk, EccentricCommon name for valve design in which the positioning of the valve shaft/disk connections causes the disk to take a slightly eccentric path on opening. (This allows the disk to be swung out of contact with the seal as soon as it is opened, thereby reducing friction and wear.) This design is also commonly referred to as a high performance butterfly valve (HPBV).

Flangeless BodyBody style common to rotary-shaft control valves. Flangeless bodies are, held between ANSI-class flanges by long through-bolts. (Sometimes also called wafer-style valve bodies.)

Flow RingHeavy-duty ring used in place of ball seal ring for V-notch rotary valves in severe service, applications where some leakage can be tolerated.


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High Performance Butterfly Valve (HPBV): See Disk, Eccentric.

Plug, Eccentric: See Ball, Eccentric Segment

Reverse FlowFlow of a fluid in the opposite direction from that normally considered the standard direction. (Some rotary-shaft control valves, such as conventional-disk butterfly valves, are capable of handling flow equally well in either direction. Other rotary designs may require modification of actuator linkage to handle reverse flow. Capacity and allowable working pressures are often lowered to maintain allowable leakage limits with flow in the reverse direction.)

Rotary-Shaft Control ValveA valve style in which the flow closure member (full ball, partial ball, or disk) is rotated in the flowstream to modify the amount of fluid passing through the valve.

Seal RingThe portion of a rotary-shaft control valve assembly corresponding to the seat ring of a globe valve. Positioning of the disk or ball relative to the seal ring determines the flow area and capacity of the unit at that particular increment of rotational travel. As indicated above, some seal ring designs permit bi-directional flow.

ShaftThe portion of a rotary-shaft control valve assembly corresponding to the valve stem of a globe valve. Rotation of the shaft positions the disk or ball in the flowstream and thereby controls the amount of fluid that can pass through the valve.


Control Valves

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