The basic composition of control valves, basic knowledge of control valves, and pneumatic control valves

The basic components of a control valve are: The actuator generates thrust torque → adjusts valve core displacement → changes flow area → changes flow rate → also known as a regulating valve.
The role of control valves in the system:



Basic classification of control valves:



Valve body type:



Executing Agency:



Drive device for control valve:



According to energy supply, it can be divided into pneumatic, electric, and electro-hydraulic.



Pneumatic membrane actuator:



Pneumatic piston actuator:



Electric actuator:



Electric actuator: an actuator powered by electricity, with electric control and electromagnetic control. (See the electronic control valve related courseware published in the South Society Encyclopedia).



According to displacement, it can be divided into straight stroke, angular stroke, and multi turn.



It has the characteristics of fast action, fast response, convenient power supply, and long transmission distance.



Hydraulic (electro-hydraulic) actuator:



Hydraulic, electro-hydraulic or gas-liquid actuators: mainly used for matching devices and valves that require high thrust, high torque, fast response, safety and reliability.



Electro hydraulic actuators are widely used due to the need for electro-hydraulic execution.



enclosure:



Control valves can also be equipped with various accessories to achieve various control functions of the product itself;



The attachments can meet various special requirements of the control system for valves;



The purpose of the attachment is to make the function of the control valve more complete, reasonable, and perfect.



positioner:



Valve positioner: used to improve the regulating performance of control valves and achieve correct positioning. Valve positioner; Electric valve positioner; Intelligent valve locator.



Working principle of locator:



Air filter pressure reducer: purifies the gas source and has automatic pressure stabilization function.



Handwheel mechanism: When the air or power signal fails or the actuator parts are damaged, the handwheel can continue to manually operate the control valve.



There are also valve position transmitters, solenoid valves, limit switches, holding valves, pneumatic amplifiers, gas storage tanks, etc.



Characteristics of regulating valve:



One is the flow coefficient.



Flow coefficient cv: The number of US gallons per minute of water flowing through a regulating valve at a temperature of 60 ° F under a pressure drop of 1 PSI.



Flow coefficient Kv: The cubic meter of water passing through a regulating valve at a temperature of 5 ℃ -40 ℃ under a pressure drop of 100KPa within 1 hour.



Cv is the flow coefficient in British units (in US gallons per minute). (US gallon=3.785 liters)



Kv is the flow coefficient in international units (m3/h).



Kv=0.8569Cv，Cv=1.167Kv。



Kv or Cv represents the circulation capacity of the valve, which is related to the valve form, valve size, valve seat size, and valve stroke.



Calculation of KV value:



Attention: Mark the KV value on the nameplate of the regulating valve (rated flow coefficient when the valve is fully open).



The second is adjustable ratio.



The ratio of the maximum flow rate to the minimum flow rate that the control valve can control



R=Qmax/Qmin。



Reflects the ability of the control valve to regulate flow.



Usually, the straight through valve R is 30-50. The rotary valve can reach 300.



3. Flow characteristics of regulating valve - regulating quality.



Definition: The functional relationship between the relative flow rate and relative stroke of a fluid passing through a control valve: Q/Qmax=f (L/Lmax)



Comparison between Linear Flow Characteristics and Equal Percentage Flow Characteristics:



For example, a flow change of 10% in the opening of the regulating valve.



Linear: 22.67-13/13=74.4% 90.33-80.67/80.67=12%



Equal ratio: 6.58-4.68/4.68=40.6% 71.17-50.65/50.65=40.6%



The linear flow characteristic has a large change in flow rate and high sensitivity within a small opening. When the valve works back and forth, it will cause severe erosion of the valve core and generate valve vibration.



When the opening is large, the valve flow rate changes little, the sensitivity is low, and the regulating function is lost.



The feature of waiting for percentage adjustment has improved these aspects.



Valve core and flow characteristics:



The shape of the valve core determines different flow characteristics.



Sleeve valves, plunger valves, and V-opening valves can change the shape and flow characteristics of the valve core.



4. Leakage:



When the valve is in the fully closed position under the specified closing thrust, the amount of fluid flowing through the valve at the specified pressure difference and temperature can be measured.



The leakage rate is usually expressed as a percentage of the flow capacity at the rated stroke, or as the cumulative amount of flow over a specified period of time.



Usually expressed in terms of leakage level:



I: Negotiate between the user and the manufacturer.



II: 0.5% * kv.



3： 0.1% * Kv



4： 0.01% * Kv



v: 3 * 10-4 milliliters per minute.



V: Foam grade, soft seal.



Note: The difference between leakage and minimum flow rate.



5. Positive and negative effects.



The entire control valve has air opening and air closing (fault opening, fault closing).



Select the required parameters for the valve:



The selection of regulating valves requires knowledge in various aspects such as mechanical, electrical, and technical aspects. Provide detailed technical parameters as much as possible to help choose the valve with the highest cost-effectiveness and the most accurate control. The following are the technical parameters required for selecting a regulating valve.



The material, size, and specifications of the pipeline.



Process medium name: Chinese and English.



Media characteristics: state, density, viscosity, concentration, do they contain particles? Is there any corrosiveness, etc?



Maximum, normal, and minimum operating temperatures;



Maximum, normal, and minimum flow rates;



Maximum, normal, and minimum inlet pressures;



Maximum, normal, and minimum outlet pressure (or differential pressure);



Maximum allowable closing pressure difference;



Requirements for the state of the regulating valve during gas loss, such as a faulty switch



Instrument control signal;



Connection method: flange, thread or welding.



Selection of regulating valve:



Selection steps for regulating valves:



Calculate Kv value, diameter, and valve size;



Determine traffic characteristics;



Determine the nominal pressure and material;



Determine the type of control valve;



Select valve accessories as needed.



1. Calculate Kv value and diameter.



*The diameter of the selected valve shall not be less than half of the pipe size.



Opening verification: It is usually desirable to control the valve to operate at 20% -80% of the opening.



The above calculation is usually completed by the calculation software program of the regulating valve.



2. Determine the flow characteristics of the control valve.



Purpose: To ensure the stable operation of the control system.



Due to the complexity of calculations, empirical standards are often used for selecting flow characteristics.



The situation of selecting a linear flow characteristic valve is generally: ① The differential pressure changes are small and almost constant ② The changes in the main parameters of the process system are linear ③ Most of the system pressure loss is allocated to the regulating valve (changing the opening degree results in relatively small changes in valve differential pressure) ④ The external interference is small, the given value changes are small, and the adjustment range requirements are small.



When choosing valves with equal percentage characteristics, ① the actual adjustable range is large ② the opening changes, and the differential pressure on the valve changes relatively large ③ the pressure loss in the piping system is large ④ the process system load changes significantly ⑤ the regulating valve often operates at a small opening.



3. Determine the nominal pressure and material of the valve.



Selection of materials for regulating valves:



1) Pressure bearing components such as valve body, upper valve cover, lower flange, and connecting parts; There are cast iron, carbon steel, stainless steel, alloy steel, etc.



Valve internal component materials: valve core, valve seat, sleeve, etc.



Stainless steel, stainless steel, alloy steel, overlay welding with Stellite alloy, etc;



3) Execution mechanism materials: thin film materials, cylinders, springs, etc.; Selection of main regulating valve manufacturers.



4. Determine the type of control valve.



Valve body (single seat and sleeve); Valve cover (medium temperature);



Packing structure (steam, superheated steam, natural gas, thermal oil, toxic and harmful media).



Executing Agency:



Output force (torque) (maximum closing pressure difference and friction force);



Pneumatic, electric, electro-hydraulic (price, reliability, explosion-proof);



Functional mode (fault on, fault off).



5. Attachment (Process Requirements)



Experience parameters for selecting valves



If the maximum allowable closing pressure difference is not provided, the maximum allowable closing pressure difference shall be based on the maximum pressure at the valve inlet.



The standard flow characteristic of a straight through valve is proportional. The standard flow characteristic of a three-way valve is linear.



The electric valve remains in the position before power loss. If there are special requirements, please specify.



High viscosity fluids must provide viscosity, otherwise it will affect the calculation of valve flow coefficient.



The importance of selecting valves:



How to choose a regulating valve, especially the valve diameter and actuator thrust? Running the regulating valve at a high level will be a key issue.



Inaccurate selection can easily cause system instability, poor regulation performance, and short service life.



(1) Correct selection - System designer (accurate technical parameters, process drawings, and sufficient communication with manufacturers);



(2) Product Quality - Manufacturer (using good materials and processing techniques);



(3) Proper installation, use, and maintenance - users.



Pneumatic control valve:



Pneumatic control valves are mainly composed of pneumatic actuators, valve bodies, and accessories. The actuator is powered by clean compressed air and receives 4-20mA electrical signals or 20-100KPa gas signals to drive the valve body to move, changing the flow area between the valve core and valve seat, thereby achieving the effect of regulating flow rate. In order to improve the linearity of the valve, overcome the friction of the valve stem and the influence of changes in the working conditions of the regulated medium (temperature and pressure), the valve positioner is matched with the regulating valve, so that the valve position can be accurately positioned according to the regulating signal.



To ensure the safe operation of the unit, some important valves are designed with accessories such as solenoid valves, holding valves, and quick relief valves to ensure that the regulating valve can quickly open (close) or hold (three break self-locking protection) in the event of power loss, signal loss, or gas loss, meeting the requirements for safe operation of the process system.



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