Control valves automatically regulate pressure and/or flow rate, and are available for any pressure. If different plant systems operate up to, and at pressure/temperature combinations that require Class 300 valves, sometimes (where the design permits), all control valves chosen will be Class 300 for interchange-ability. However, if none of the systems exceeds the ratings for Class 150 valves, this is not necessary.
Globe valves are normally used for control, and their ends are usually flanged for ease of maintenance. Depending on their type of supply, the disk is moved by a hydraulic, pneumatic, electrical or mechanical actuator. The valve modulates flow through movement of a valve plug in relation to the port(s) located within the valve body. The valve plug is attached to a valve stem, which, in turn, is connected to the actuator.
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 keep some important process variable such as pressure, flow, level, temperature, etc. within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and 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 about the process variable and its relationship to some desired set point. A controller then processes this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring, comparing, and calculating are done, some type of final control element must implement the strategy selected by the controller.
Types of Control Valves :
Not all type of valves can be used as control valves. Following is the guide for use of different type of valves as control valves based on application as well as services and caution where not to use it
1. Ball valve:
Use of ball valve as control valve is suitable for service where precise control is not required. Typical use of ball valve as control valve is for valve diameter 4 inch (100 mm) and above. Ball valve has advantage of lower pressure drop when full open, this also makes it suitable where lower pressure drop are required. Ball valves are light weight and compact. Ball valve gives high capacity and good range ability.
Ball valves are suitable for use for flow reversal and direction change is required. Direction change helps the remove the accumulations in the valve body. If ball valve is used as control device for slurry service then it should have online cleaning mechanism.
Ball valves are not recommended for high pressure drop service where choke flow or cavitation is likely. For high pressure and high temperatures services, ball valves are not recommended. Even for small sizes up to 3 inch (DN 80mm), butterfly valve are preferred as the cost plays important role.
2. Gate valve:
Usually, gate valves are not preferred choice of control valves but it can be used services where use of ball valve or butterfly valve is not recommended and precise control is not required. Gate valve has advantage of lower pressure drop; this also makes it suitable where lower pressure drop applications. Gate valves are suitable for liquid with solid content.
Typical examples of gate valve are for bypass valve or equalization valves.
Gate valves shall not be used in application of frequent opening and closing is required.
3. Butterfly valve:
Use of butterfly valve as control valve is recommended where service is clean and no solids or crystallization is expected. Butterfly valves gives lower pressure drop among control valves as well as the lower price makes it preferred choice compared to other control valves for sizes 4 inch (100 mm) and above. Butterfly valves are typically recommended for low pressure services.
Most suitable service for butterfly valve as control valve is clean service like clean liquid or gaseous service. Typical examples of butterfly valve as control are cooling water distribution, gas distribution, etc. In some cases butterfly valve is only option as no other control valve able to cater to big size.
In terms of limitations, butter valve has uneven dynamics and flow control is not proper at partial opening conditions. There are other issues like solids particles get accumulated and closing might be impaired due to this. Other limitations about butterfly valve is, due to its nature is introduces large turbulence is in the downstream system and prone to vibrations, leaks and damage. Butterfly valve also has limitations in terms of closing and opening action, sudden closing might lead water hammer kind of situation and for opening it needs larger torque to open and in some applications it becomes quite difficult.
4. Plug Valve:
Plug valves are similar to ball valve but shape of plug what makes it different and that allows it to handle fluid with solids. With its ability to handle the solids in the fluid as well as there are numerous ports like 3 or even 4 port makes it good choice for use in various applications. These valves are also suitable for on/off operations only.
Plug valves are relatively costlier and it is not recommended for throttling action.
5. Diaphragm Valve:
Diaphragm valves are well suited for the abrasive services as it does not have direct contact of fluid it is handling. Diaphragm material can be selected based on service.
Diaphragm valves are good control and with proper sealing these valves can be used for throttling service without any major issues. Typical applications where Diaphragm valves can be used are slurry service, sanitary services as well as microbiological liquid. In terms of limitations, the diaphragm valves are expensive and sealing and diaphragm is exposed to slurry and fluids with solids and frequent change-out might be required due to shorter lifespan.
6. Globe valve:
Usually when someone say control valve, then first valve comes in any Engineer’s mind is globe valves. They are very good in terms of control and coupled with actuator and characteristic of valve type, globe valve can be used for wide range.
Most of the industrial applications in process industry uses globe valve as control valves. With proper trim arrangements these valves can be used for multi phase services as well as flashing applications where other control valves are not recommended.
In terms of limitations, the globe valve mechanism is the fluid changes its direction and passes through the opening which is controlled by disc. This mechanism will lead to higher pressure drop in the control valve. The size of valve also becomes limitations for bigger sizes.
7. Choke valve:
Typically choke valves are used at oil and gas production wellhead and typically angular type of nature.
With multiple bean size design, the valves can handle wide range of flow-rates and makes it good choice for high pressure drop services. These valves can handle multi phase as well as flashing service.
Typical limitations of choke valve are the area it occupies and cost of the valves.