Required temperature capabilities are also a foregone conclusion, but one that is likely to narrow valve selection possibilities. The considerations include the strength or ductility of
the body material, as well as relative thermal expansion of various parts.
Temperature limits also may be imposed due to disintegration of soft parts at high temperatures or loss of resiliency at low temperatures. The soft materials under consideration include various elastomers, plastics, and PTFE. They may be found in parts such as seat rings, seal or piston rings, packing, rotary shaft bearings and butterfly valve liners. Typical upper temperature limits for elastomers are in the 200 – 350°F range, and the general limit for PTFE is 450°F.
Temperature affects valve selection by excluding certain valves that do not have high or low
temperature options. It also may have some affect on the valve’s performance. For instance, going from PTFE to metal seals for high temperatures generally increases the shutoff leakage flow.Similarly, high temperature metal bearing sleeves in rotary valves impose more friction upon the shaft than do PTFE bearings, so that the shaft cannot withstand as high a pressure-drop load at shutoff. Selection of the valve packing is also based largely upon service temperature.
Most control valve manufacturers can provide valves with reduced- or restricted- capacity trim parts. The reduced flow rate might be desirable for any of the following reasons:
1.Restricted capacity trim may make it possible to select a valve body large enough for increased future flow requirements, but with trim capacity properly sized for present needs .
2.Valves can be selected for adequate structural strength, yet retain reasonable
3. Large bodies with restricted capacity trim can be used to reduce inlet and outlet fluid velocities.
4. Purchase of expensive pipeline reducers can be avoided
5. Over-sizing errors can be corrected by use of restricted capacity trim parts.
Conventional globe-style valve bodies can be fitted with seat rings with smaller port size than normal and valve plugs sized to fit those smaller ports.Valves with cage-guided trim often achieve the reduced capacity effect by utilizing valve plug, cage, and seat ring parts from a smaller valve size of similar construction and adapter pieces above the cage and below the seat ring to mate those smaller parts with the valve body (figure 1-28).Because reduced capacity service is not unusual,leading manufacturers provide readily available trim part combinations to perform the required function. Many restricted capacity trim combinations are designed to furnish approximately 40% of full-size trim capacity
1.Suitable for high temperature nuclear service or where low chloride content is desirable
2.Provides leak-free operation, high thermal conductivity, and long service life, but produces high stem friction and resultant hysteresis.
3. Impervious to most hard-to-handle fluids and high radiation
4.Suitable temperature range: Cryogenic temperatures to 1200°F (649°C)
5. Lubrication not required, but an extension bonnet or steel yoke should be used when packing box temperature exceeds 800°F (427°C).
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
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. build site
Many styles of control valve bodies have been developed. Some can be used effectively in a
number of applications while others meet specific service demands or conditions and are used less frequently. The subsequent text describes popular
control valve body styles utilized today.
2—–0.02 3–0.08 4–0.12 5–0.2 6– 0.32 7—-0.5 8—0.8 9—1.2
10—–eq:1.6 linear:1.8 12—–eq:2.5 linear—-2.8
Process plants consist of hundreds, or even thousands, of control loops all networked together
to produce a product to be offered for sale. Each of these control loops is designed to control a
critical process variable such as pressure, flow,level, temperature, etc., within a required operating range to ensure the quality of the end-product.
These loops receive, and internally create,disturbances that detrimentally affect the process
variable. Interaction from other loops in the network provides disturbances that influence the
process variable. To reduce the effect of these load disturbances, sensors and transmitters collect information regarding the process variable and its relationship to a desired set point. A controller then processes this information and decides what must occur in order to get the process variable back to where it should be after a load disturbance occurs.
When all measuring, comparing, and calculating are complete, the strategy selected by the
controller is implemented via some type of final control element. The most common final control element in the process control industries is the control valve.
A control valve manipulates a flowing fluid such as gas, steam, water, or chemical compounds to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point. Many people who speak of “control valves” are actually referring to “control valve assemblies.”
The control valve assembly typically consists of the valve body, the internal trim parts, an actuator to provide the motive power to operate the valve, and a variety of additional valve accessories, which may include positioners, transducers, supply pressure regulators, manual operators, snubbers, or limit switches.
It is best to think of a control loop as an instrumentation chain. Like any other chain, the
entire chain is only as good as its weakest link. It is important to ensure that the control valve is not the weakest link.
Often there are references to valve-caused problems or difficulties. The list of problems include valve erosion from process media,stickiness caused by excessive friction (stiction),excessive play in valve to actuator linkages (typically found in rotary valves) that causes deadband, excessive valve stem packing leakage, and valve materials that are incompatible with the flowing medium. Any one,or a combination of these difficulties, may affect process quality and throughput with a resulting negative impact on mill profitability.
Many of these problems can be avoided or minimized through proper valve selection. Consideration should be given to valve style and size, actuator capabilities, analog versus digital instrumentation, materials of construction, etc. Although not being all-inclusive, the information found in this sourcebook should facilitate the valve selection process.
Most of the considerations that guide the selection of control valve type and brand are rather basic. However,there are some matters that may be overlooked by users whose familiarity is mainly limited to just one or a few valve types. Follow provides a checklist of important criteria.
Suggested General Criteria for Selecting Type and Brand of Control Valve
1.Body pressure rating
2.High and low temperature limits
3.Material compatibility and durability
4.Inherent flow characteristic and rangeability
5.Maximum pressure drop (shut off and flowing)
6.Noise and cavitations
7.Shut off leakage
8.Capacity versus cost
9.Nature of flowing media
Accurate guiding of the valve plug is necessary for
proper alignment with the seat ring and efficient
control of the process fluid. The common methods
used are listed below.