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Selection of Control Valves
Release time:
2026-04-07 15:22
Control valves are the actuators in automated control systems, and their performance directly affects the control quality of the system. With the continuous advancement of industrial automation, control valves have become integral to every aspect of production, playing a pivotal role in ensuring stable operations, optimizing control strategies, and effectively managing maintenance and overhaul costs.
Why is it important to properly select control valves?
There’s a popular saying in the automation industry: “Nine out of ten valves leak.”
The selection of control valves is one of the most challenging aspects of instrument selection, particularly in process control applications involving complex processes and media, where the selection issues are even more pronounced. The performance of a control valve depends on the following factors:
Correct Parameters——Design Phase
Correct Selection — Design Tender Phase
Correct Installation — Construction Engineering
Proper Use and Maintenance – On-Site Service
From this, we can see that when we talk about the quality of control valves, it is by no means solely the responsibility of the manufacturer; rather, it is a broad concept that encompasses designers, manufacturers, installation personnel, and operators and maintenance staff. In summary: select the right valve, install it properly, operate it effectively, and maintain it well.
Common Selection Issues for Control Valves
The following lists common selection issues; if these issues are properly addressed during the selection process, major quality problems with the valve can be effectively avoided.
How to Select Control Valves
Selection Principles
Meet the requirements of the process conditions.
Meet the requirements of the automatic control system.
Meet the requirements of cost-effectiveness.
Twelve Steps to Select the Right Control Valve
Step 1: Accurately determine process parameters and system requirements.
Selecting a control valve that operates at a high performance level is a critical consideration. When choosing a control valve, the first step is to gather comprehensive data on the physical and chemical properties of the process fluid as well as the operating conditions for the valve.
The key parameters include the fluid’s composition, temperature, density, viscosity, normal flow rate, maximum flow rate, minimum flow rate, inlet and outlet pressures at both the maximum and minimum flow rates, and the maximum shut-off pressure differential.
Before finalizing the specific selection of a control valve, it is essential to thoroughly understand and determine the characteristics of the valve body itself, including its structure, configuration, and materials.
From a technical standpoint, the primary considerations include flow characteristics, pressure drop, flash vaporization, cavitation, and noise.
Design Phase
Parameters to Prepare for Design Integration
Process parameters: temperature, pressure, differential pressure at normal flow rate, and differential pressure at shutoff.
Fluid properties: corrosivity, viscosity, and the impact of temperature variations on fluid properties.
System requirements: leakage rate, turndown ratio, actuation speed and frequency, linearity, and noise.
The chemical properties of the medium and its detailed composition.
Step 2: Calculation of the control valve flow coefficient Cv and nominal diameter
The flow coefficient Cv (flow capacity) is defined as the volumetric flow rate (m³/h) of a fluid with a specific gravity of 1 g/cm³ passing through the control valve when the pressure differential across the valve is 1 kg/cm².
There are numerous and rather complex methods for calculating the flow coefficient Cv of a control valve. However, this calculation is critically important; if you are unable to perform it yourself, you must provide detailed process parameters so that the valve manufacturer’s application engineers can carry out the selection and calculation.
Calculation of the third-step caliber
This is the primary consideration in selecting control valves: the valve size must be determined based on the process parameters. If the valve is undersized, it will fail to meet the maximum flow rate (Qmax); if it is oversized, it will often operate at small openings, resulting in poor control performance and a shortened service life.
In summary, if, after adjusting the spool and seat dimensions, the requirements still cannot be met, the valve size must be changed.
Based on production capacity, equipment load, and the operating conditions of the controlled medium, the data required for calculating flow capacity are determined, from which the maximum and minimum flow rates—corresponding to Cvmax and Cvmin—are derived. Through years of practical experience and theoretical analysis, it has been established that, when sizing control valves, the valve’s flow coefficient should be set at 1.2 to 1.4 times the flow coefficient value (Cv) required by the operating conditions. The approximate range of control valve opening is as follows:
Maximum opening: 70%–90%
Common opening degree: 40%–70%
Minimum opening: 10%
Based on the calculated Cvmin and the adjustable ratio of the selected control valve, the adjustable range is verified to be adequate, allowing the valve size to be determined directly from the Cv value.
In principle, the valve nominal size should be determined based on the pipe size; however, if using a valve with a bore matching the pipe size results in flow rates that fail to meet control requirements, then the pipe should be reduced where reduction is necessary and enlarged where enlargement is required.
Calculation of Valve Port Diameter
How to determine the size of a control valve?
Determine the calculated flow rates Qmax and Qmin.
Determine the differential pressure for calculation, select the resistance ratio S value based on the system characteristics, and then determine the differential pressure for calculation (with the valve fully open).
Calculate the flow coefficient and select the appropriate calculation formula, chart, or software to determine KV.
Select the KV value by choosing the KV rating in the selected product series that is closest to the maximum KV value, thereby determining the preliminary nominal diameter.
Opening-degree verification: at Qmax, the valve opening shall not exceed 90%; at Qmin, the valve opening shall not be less than 10%.
Verification of the actual adjustable ratio: generally, it shall be ≥10; R_actual > R_required.
Once the caliber is determined, if it fails to meet the requirements, reselect the KV value and verify again.
Step 4: Selection of Flow Characteristics
The flow characteristic of a control valve refers to the relationship between the relative flow rate of the medium passing through the valve and the valve opening, under the condition that the pressure differential across the valve remains constant. There are three types of flow characteristics for control valves: linear, equal-percentage, and parabolic.
How to Select Flow Characteristics
Principles for selecting flow characteristics:
Select logarithmic characteristics when operating at small openings and when unbalanced forces vary significantly.
Select a linear response when the required controlled parameter responds quickly, and a logarithmic response when the response is slow.
The pressure regulation system offers an optional linear characteristic.
The liquid level control system offers a linear characteristic as an option.
Step 5: Calculate the pressure differential at closure.
Closing the pressure differential involves two issues.
This results in the valve failing to close when it should and failing to open when it should.
This will affect the accuracy of Cv value selection for the valve.
Step 6: Select the turndown ratio
The adjustable ratio R is an important parameter for valves, and the accuracy of its selection directly affects the quality of regulation.
The ratio of the maximum to the minimum flow rate that a control valve can regulate is called the turndown ratio R. When the pressure differential across the valve remains constant, the ratio of the maximum to the minimum flow rate is referred to as the ideal turndown ratio. In practical applications, however, the pressure differential across the valve varies; in such cases, the resulting turndown ratio is known as the actual turndown ratio. Note: A small value of R indicates that the valve cannot accommodate the full range of flow variations.
Step 7: Determine the spring range of the actuator.
A comprehensive assessment should be made based on factors such as starting working pressure, output force, stability, and adjustability. In general, as long as the supplied source pressure and medium parameters are accurate, there should be no major issues.
Step 8: Material Selection
For companies, the process is highly complex, and the service media are equally diverse; therefore, material selection is of paramount importance. During the valve selection process, the more thoroughly one understands the process flow, operating conditions, and the physicochemical properties of the service medium, the more scientifically sound the chosen valve design and materials will be.
An excellent selection engineer should choose valve designs and materials that not only meet the functional requirements but also feature the simplest possible design and the lowest relative cost. Conversely, even if substantial funds are spent, a poorly chosen valve can still fail to deliver satisfactory performance—such misselection can result in costs that differ by a factor of 20 to 30. For manufacturing operations, the adverse consequences are even more severe: frequent start–stop cycles lead to economic losses, reduced product output and quality, and may even bring production to a standstill.
The valve body shall meet or exceed the pressure rating, service temperature, and corrosion resistance requirements of the process piping to which it is connected, and factory-standardized products should be given priority. Cast-iron valves are not suitable for use with steam, highly moist gases, or flammable and explosive media.
When the company is located in northern regions, cast-iron valves are not suitable for outdoor applications or environments where ambient temperatures fall below –20°C, nor for service conditions involving severe cavitation and erosion, or for handling slurry media. For throttling sealing surfaces, wear-resistant materials such as cobalt-based alloys or Stellite surfacing alloys should be selected. For highly corrosive media, corrosion-resistant alloys must be chosen based on the specific characteristics of the medium—namely its type, concentration, temperature, and pressure—in order to ensure optimal material selection. Valve body materials and throttling-element materials should be evaluated separately; generally, the corrosion rate of the valve body should be no more than one-eighth that of the internal components. When selecting lining materials, the operating temperature, pressure, and concentration of the process fluid must all remain within the material’s specified service limits, and consideration must also be given to the erosive wear imposed by the fluid on the lining. Vacuum valves should not employ rubber or plastic linings inside the valve body. In domestic wastewater treatment systems, particularly in industrial wastewater treatment systems and in applications involving oil-containing media, valves with rubber linings are not recommended.
Selection of Valve Internal Component Materials
High- and Low-Temperature Material Selection
Selection of Typical Corrosion-Resistant Alloy Materials for Typical Media
Sulfuric acid: 316L, Hastelloy, Alloy 20. Hydrochloric acid: Hastelloy B. Acetic acid and formic acid: 316L, Hastelloy. Phosphoric acid: Incoloy, Hastelloy. Nitric acid: Aluminum, C4 steel, C6 steel. Hydrofluoric acid: Monel. Caustic soda: Monel. Chlorine (with water content greater than 1%): Hastelloy C. Brine: Titanium, 316L. Nickel sulfate: Titanium.
Non-metallic corrosion-resistant materials
To date, the most versatile corrosion-resistant material is polytetrafluoroethylene, often referred to as the “King of Corrosion Resistance.” Therefore, fully PTFE-lined corrosion-resistant valves should be selected first. However, the following situations are not recommended for such selection: operating temperatures exceeding 160°C, nominal pressures greater than 1.6 MPa, and applications involving severe abrasion.
Excellent selection and design engineers recognize the importance of the field and, during the design integration phase, work closely with process engineers to ensure the proper selection of valve materials.
Step 9: Selection of Packing and Bonnet Type
(1) Under normal conditions, when the medium temperature is below 200°C, a “V”-shaped PTFE packing and a standard valve bonnet shall be used; when the medium temperature is below 450°C, a “V”-shaped PTFE packing shall be used, but a heat-dissipating valve bonnet must be employed.
(2) For straight-stroke valves equipped with a positioner, for high-temperature valves with a medium temperature ≤450°C, a standard valve bonnet may still be used; however, graphite packing must be specified.
(3) When the medium temperature exceeds 400°C, a heat-dissipating valve bonnet and graphite packing shall be used.
(4) To enhance the reliability of the valve stem seal, a double-packed gland design may be employed. When the process medium is highly toxic or flammable and explosive, or when it is prone to hydrolysis upon contact with air, or when the process requires an extremely low external leakage rate, a bellows seal is recommended.
Step 10: Selection of the Mechanism of Action
In the production process, the selection of air-to-open or air-to-close actuation for control valves is primarily based on process safety considerations. For example, steam heaters and chlorine-leaching processes typically use air-to-open valves for chlorine control, while boiler feedwater control valves are generally configured as air-to-close.
The selection of air-to-open or air-to-close for a pneumatic control valve is not merely a design and selection issue specific to the instrumentation and control discipline; rather, it is a matter of coordination between two disciplines—process engineering and instrumentation and control—and the actuation mode is determined by the process engineers.
In certain special circumstances, it is also advisable to maintain the control valve in its original position when the gas supply is interrupted. For example, during material discharge operations in a pressurized reactor, it is undesirable for the high-pressure medium to be abruptly shut off or completely vented; in such cases, the control valve should remain in its original position.
Step 11: Selection of Valve Accessories
This is in relation to the subject—valves—and is intended to ensure their normal operation. Valve accessories include: 1) valve positioners—used to improve the operating characteristics of control valves and achieve accurate positioning; limit switches—used to indicate the travel positions at the upper and lower limits of the control valve;
Pneumatic position-holding valve—maintains the valve in its current position in the event of a pneumatic supply failure; solenoid valve—automatically switches the air circuit. For single-air control, use a 2/3-way valve; for dual-air control, use a 2/5-way valve.
Manual mechanism – allows switching to manual operation in the event of a system failure;
Pneumatic relay—used to accelerate the actuation of pneumatic diaphragm actuators. Air filter-regulator—used for purifying and regulating the air supply. Air receiver tank—provides a buffer to allow the valve to continue operating for a period of time in the event of an air-supply failure; typically required when three-level protection is specified. When selecting valve accessories, prioritize practicality: avoid unnecessary items whenever possible, but ensure that critical accessories are reliable.
Step 12: Selection of Valve Configuration
Classification of Valves
Classification by Purpose and Function
Two valves: primarily used to shut off or connect the medium;
Control valves: primarily used for system regulation. When selecting a valve, it is necessary to determine the flow characteristics of the control valve;
Diverting valve: used for distributing or mixing media;
Shut-off valve: Typically refers to a valve with a leakage rate of less than one part in 100,000.
Classification by Main Parameters
1 Classification by Pressure
Vacuum valve: operating pressure below standard atmospheric pressure;
Low-pressure valve: nominal pressure PN ≤ 1.6 MPa;
Medium-pressure valves: PN 2.5–6.4 MPa;
High-pressure valves: PN 10.0–80.0 MPa, typically PN 22 and PN 32;
Ultra-high-pressure valve: PN ≥ 100 MPa.
2 Classification by Medium Operating Temperature
High-temperature valve: t > 450°C;
Medium-temperature valve: 220°C ≤ t ≤ 450°C;
Ambient-temperature valve: –40°C ≤ t ≤ 220°C;
Low-temperature valve: –200°C ≤ t ≤ –40°C.
Classified according to their primary special applications (i.e., special and dedicated valves): soft-seal shut-off valves; hard-seal shut-off valves; wear-resistant control valves and corrosion-resistant control valves; all-fluoroplastic corrosion-resistant control valves and all-corrosion-resistant alloy control valves; emergency-action shut-off or vent valves; anti-clogging control valves; corrosion-resistant, anti-clogging shut-off valves with thermal-insulated jacketed bodies; small-flow control valves; large-range-ratio control valves; compact, high-precision control valves; bellows-sealed valves; and various specialized valves (such as those for chlorine gas and caustic solutions), among others.
Classification by Drive Energy
Electric, Pneumatic, Hydraulic
Common Classification Methods
This classification method, which categorizes devices based on both their operating principles and functions as well as their structural characteristics, is currently the most widely used classification approach both domestically and internationally. It is generally divided into nine major categories:
Straight-through single-seat control valve
Structural Types of Single-Seat Control Valves
Direct-acting double-seat control valve
Sleeve valve
Angle valve
Three-way valve:
Diaphragm valve:
Butterfly valve
Ball valve
Selection Tips
Subject to meeting process-control requirements, the selected valve should be as simple, reliable, cost-effective, long-lived, easy to maintain, and supported by timely and dependable spare-parts supply as possible. Every effort should be made to avoid prioritizing superior design, high-quality materials, or an abundance of accessories at the expense of reliability and cost-effectiveness.
Preferred Sequence for Valve Adjustment
① Full-function ultra-light regulating valve → ② Butterfly valve → ③ Sleeve valve → ④ Single-seat valve → ⑤ Double-seat valve → ⑥ Eccentric rotary valve → ⑦ Ball valve → ⑧ Angle valve → ⑨ Three-way valve → ⑩ Diaphragm valve.
Equipment Selection Phase (Tendering Phase)
Develop a detailed equipment plan
For mature technologies or control valves with proven application experience, the selection should be based on the original valve’s operating performance, its strengths and weaknesses, and the desired application outcomes.
In engineering projects, detailed process parameters provided must be carefully verified. If any doubts arise, these parameters must be cross-checked in conjunction with other disciplines.
Prior to submitting the equipment procurement plan or the tendering plan, relevant technical personnel shall be organized to review the plan.
If you are not fully confident about the valve’s materials and design, it is recommended to conduct technical exchanges and on-site equipment inspections with the equipment manufacturer.
When pre-selecting manufacturers, prioritize those that deliver high-quality service and reliable product quality.
Remember: valve selection is a complex process, and the technical parameters you provide directly affect the performance of the application and the quality of control. Any parameters proposed in the equipment plan must be verified by process engineering personnel before they can be submitted for approval.
Do not place excessive trust in design institutes; if you have any doubts about the parameters they provide, you must verify them through proper channels.
Points to Note During the Bidding Process
During the tendering process, it is essential to carefully verify whether the product specifications, technical parameters, materials, and accessories provided in the equipment manufacturer’s bid are consistent with or superior to those specified in the tender documents; any ambiguities must be clarified.
Key parameters and materials should be compared side by side to ensure thorough, competitive benchmarking.
When requirements are not met or key parameters deviate from the tender documents, we must adhere to principle: if it’s not acceptable, it’s not acceptable—under no circumstances should we force the issue.
Piping and wiring for control valve signals
Performance Specifications of Control Valves
(1) Basic error: Gradually increase or decrease the 20–100 kPa signal applied to the input chamber (or positioner), measure the corresponding stroke values at each point, and calculate the deviation of the “signal–stroke” relationship from the theoretical value at each point. The maximum of these deviations is defined as the basic error. Test points shall be selected at 0%, 25%, 50%, 75%, and 100% of the signal range. The basic error of the measuring instrument shall not exceed 25% of the basic error limit of the valve under test. (2) Hysteresis: The test procedure is the same as above. Hysteresis is defined as the maximum difference between the forward and reverse stroke measurements obtained for the same input signal.
What issues should be considered during the selection process?
Drive mode selection?
Selection of electric actuators?
Regular or electronic?
Is the electric actuator linear or angular?
Should the electric actuator be directly connected or via a crank linkage?
Selection of Servo Amplifiers
Integrated or servo amplifier?
Explosion-proof requirements
Propose the explosion-proof rating in accordance with process design requirements.
Is valve position feedback required?
Is overload protection required?
Is a manual controller required?
Selection of Pneumatic Actuators
Should you choose a pneumatic diaphragm actuator or a pneumatic cylinder?
Positional or proportional?
Straight travel or angular travel?
Spring or dual-piston?
Single-spring or multi-spring compact design?
Positive effect or negative effect?
Selection of the spring range (must be calculated)?
Is a stent needed?
Thrust calculation?
What is the air supply pressure?
Travel distance and angle magnitude?
What are the requirements for movement speed?
Environmental conditions?
What are the weight requirements?
What are the color requirements?
General Principles for Selection
General Installation Requirements
(1) Control valves shall be installed vertically and in the upright position on horizontal pipelines. For control valves with a nominal diameter of Dg ≥ 50, it is recommended to provide permanent supports on the piping both upstream and downstream of the valve.
(2) The installation location of control valves shall be convenient for operation and maintenance, enabling personnel to perform maintenance and operation; a platform shall be provided where necessary.
(3) Sufficient clearance shall be provided above and below the control valve to allow removal of the actuator, internal valve components, the lower flange, and the blind flange during maintenance.
(4) When a control valve is installed in a vibrating environment, vibration-proof measures shall be considered.
(5) For control valves without a valve positioner installed, it is recommended to mount a small pressure gauge on the diaphragm chamber to indicate the control signal.
(6) Control valves shall be inspected and calibrated prior to installation, and installed after pipeline purging.
2020-01-19
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