A Simple Guide to the Selection of Control Valves

Importance of selection

In order to have a prolonged service life on site, correct calculation and selection of control valves are very important in addition to the quality of valves and the way the user correctly installs, uses, and maintains. Due to errors in calculation and selection, the entire system will be affected, and it will even fail to be put into use or cause huge economic losses. Therefore, user procurement managers and piping system designers must pay attention to the selection of control valves according to correct procedures.

Selection of actuator

1.    Select the type of mechanical drive according to the site conditions.

When the site can only provide air supply and control signals, select pneumatic actuators; when the site can only provide electric power and control signals, select electric actuators; In case of a lack of the air supply, power supply or control signal, self-operated control valves are selected..

2.    Electric actuator VS Pneumatic actuator

	Strength	Shortcoming
Electric actuator	(1) the power supply is convenient; (2) control circuit is simpler .	(1) the reliability, waterproof and explosion-proof performance, stability, and life is not good as pneumatic actuator (2) the cost is higher
Pneumatic actuator	(1) stable performance; (2) diversified explosion protection; (3) significant cost savings, (4) light weight; (5) long life	(1) the control circuit is relatively complicated; (2) it need deploy air source pipelines and signal lines.

Choice of structure type

Commonly used structural types of control valves are:

1.    straight-stroke single-seat valve;

2.    sleeve valve;

3.    double-seat valve;

4.    three-way valve (merging, diverging)

5.    right angle valve;

6.    rotary stroke O ball valve;

7.    Rotary V ball valves;

8.    Rotary butterfly valves.

According to the on-site process parameter conditions and requirements, combined with the characteristics of the different structures of the control valve in the following table:

Structure type	characteristics	application occasions
Single seat valve	The valve seat has a small leakage and a small allowable pressure difference	general fluids that require good sealing or both shut-off functions.
Double seat valve	the unbalance force is small, the allowable pressure difference is larger than that of the single seat valve, but the flow path is complicated, and the valve seat leakage is larger than that of the single seat valve and the sleeve valve	It is suitable for occasions where the pressure difference is large and the sealing performance is not required.
Sleeve valve	good stability, large allowable pressure difference, easy to replace and maintain internal parts.	general fluids, with low pressure drop loss, large flow rate, wide adjustable range, good dynamic stability, low noise, and low cavitation corrosion. It combines the advantages of single-seat and double-seat.
Angle valve	There are two types of structure, single seat and sleeve. The flow path is simple. The inlet and outlet are at a right angle of 90 degrees, which is convenient for self-cleaning and cleaning.	It is suitable for the occasion of right-angle connection and the medium with high viscosity and particles.
Three-way control valve	The straight-stroke three-way control valve is divided into two types: converging valve and diverting valve. Confluence is to mix two different media into one medium according to different proportions or requirements, and split is to mix one medium according to different The ratio or requirement is divided into two outflows.	These two kinds of valves are often used in heating or cooling systems and heat exchangers.
O-type ball valve	large flow coefficient, good sealing performance, compact structure, but single flow characteristics, it is only an approximate percentage flow characteristic like butterfly valves, which is worse than its structure.	It is suitable for sewage, fluids containing a small amount of particles, etc., and the regulation performance is not particularly strict and requires cutting off at the same time.
V-type ball valve	better regulation performance than O-type ball valve, large flow coefficient, large allowable pressure difference, good sealing, compact structure,	suitable for pulp, sewage, fluids containing fiber, particles and other fluids.
Butterfly valve	large flow coefficient, compact structure, large diameter range, light weight, low cost, but single flow characteristics, it is just an approximate percentage flow characteristic like O-type ball valve, which is worse than its structure.	It is suitable for the initial adjustment of sewage and air conditioning systems, and the adjustment performance is not particularly strict.

Specification and torque selection of pneumatic actuator

When using pneumatic actuators, first determine the torque of the valve.

①Water vapor or non-lubricated medium increases the safety value by 25%; ②Non-lubricated dry gas medium increases the safety value by 60%;

③Non-lubricated gas conveyed granular powder medium Increase the safety value by 100%;

④ Increase the safety value by 20% for clean, frictionless lubricating media, and then look up the torque table of the pneumatic actuator according to the working pressure of the air source to obtain the accurate actuator model. (Note: This safety value is recommended.)

In case of double-acting type: air source pressure is only 5bar, control a ball valve that requires torque of 200N.m, and the medium is non-lubricated water vapor. Considering safety factors, increase by 25% equal to 250N.m, first look up the air source pressure of 5bar according to the table, and then Find the torque data equal to or similar in the pneumatic actuator torque table, select 272N.m, and then select the pneumatic actuator model with similar torque in the table.

In case of single-acting (spring return) type: the air source pressure is only 4bar, and a butterfly valve that requires a torque of 100N.m is controlled. The medium is non-lubricated dry gas. Taking into account safety factors, an increase of 60% is equal to 160N.m. First, look for the spring according to the table. Return the end point to get a similar torque of 166N.m, and then look up the end point torque of 4bar in the table for 196N.m of air source pressure, the air source pressure torque should be greater than the spring return torque, and it just happens that the air source pressure torque is greater than the spring return torque, and choose a pneumatic actuator model with similar torque and the number of spring configurations.

Selection of valve body material and sealing material

1. The pressure rating, use temperature and corrosion resistance of the valve body should not be lower than the requirements of the process connection pipeline, and the manufacturer's finalized product should be preferred

2. Cast iron valve is not suitable for water vapor or wet gas with high water content and flammable and explosive medium.

3. When the ambient temperature is lower than -20℃ (especially in the north), cast iron valve is not suitable.

4. For strong corrosive media, the selection of corrosion-resistant alloy must be based on the type, concentration, temperature, and pressure of the media, and appropriate corrosion-resistant materials must be selected.

5. When selecting lining materials (fluororubber, plastic, PTFE), the temperature, pressure, and concentration of the working medium must meet the range of use of the material, and consider the physical and mechanical damage to it when the valve is actuated (such as shear damage).

6. Typical corrosion-resistant alloy materials should be selected for typical special media.

Selection of flow characteristics

1. Equal percentage flow characteristics.

2. Linear flow characteristics are divided into linearity and parabolic linearity.

3. Approximate equal percentage flow characteristics.

Action mode and fault protection selection

Electric control valve, action mode selection: electric open type, electric close type, fault protection (need to maintain the power supply), when the signal is interrupted: fully closed, maintained, fully open.

Pneumatic control valve, selection of action modes: air-opening, air-closed, fault protection (when the air source signal is interrupted): fully closed (air-opened), fully open (air-closed).

Self-operated pressure control valves, selection of action mode: before the control valve, it plays a role of pressure relief; after the control valve, it plays a role of pressure reduction and stabilization.

Types	action mode selection	fault protection
Electric control valve	Electric-open type, electric- close type	fully closed, maintained, fully open
Pneumatic control valve	air-opening, air-closed	fully closed (air-opened), fully open (air-closed).
Self-operated pressure control valves	It plays a role of pressure relief for upstream; it plays a role of pressure reduction and stabilization for downstream

Choice of accessories

When the control system puts forward various special requirements for the control valves, the following various accessories must be selected to achieve these requirements:

1. EP positioner: it receives the control signal and is used to improve the working characteristics of the control valve adjustment performance and realize the correct positioning.

2. Limit switch: it is used for the valve position switch display or remote feedback of the travel and working position when the valve is running.

3. The air filter pressure reducer: it is used to purify the air source and adjust the air pressure.

4. The solenoid valve: it realizes the switching of the control air source to ensure that the valve is in the desired safe opening position when the power supply fails.

5. Handwheel: when the controller of the control system fails, the valve can be operated manually.

6. Valve position transmitter: it gives feedback the movement position of the control valves to the remote control room or receiving equipment in the form of signal in a timely manner.

7. Pneumatic retaining valve, also called air lock valve, when the air source of the control valves fails, keep the valve at the opening position before the air source fails.