Selection of control valve

Release time:

2024-01-02 09:00

The control valve is the actuator in the automation control system. Its application quality is reflected in the adjustment quality of the system. The improvement of the industrial automatic control level has penetrated into every corner of the production. It is in stable production, optimized control, maintenance and Maintenance cost control and other aspects play a pivotal role.

Why do you need to select the control valve?

There is a rhetoric in the automation industry: ten valves and nine leaks.

Regarding the selection of the control valve, it is more difficult in the selection process of the instrument, especially in the process control with complex process and complicated medium, the selection problem is more prominent. The application of the control valve is related to the following aspects:

Correct parameters---Design stage

The correct selection---design bidding stage

Correct installation ----Engineering construction

Correct use and maintenance-on-site service

From this we can see that when we talk about the quality of the control valve, it is not a problem for the manufacturer. It should be a broad concept, including designers, manufacturers, construction personnel, and maintenance personnel. To sum up, it is: choose and install, use and maintain,

Common selection problems of regulating valve

The common selection problems are listed below. During the selection process, if these problems can be handled well, it can be said that there will be no major quality accidents in the valve selection.

How to choose a control valve

selection principle

Meet the requirements of process conditions.

Meet the requirements of the automatic control system.

Meet the requirements of economy.

Select the control valve in twelve steps

The first step is to accurately grasp the process parameters and system requirements

It is a critical issue to select the regulating valve so that it can operate at a high level. When selecting a regulating valve, first collect the complete physical and chemical characteristic parameters of the process fluid and the working conditions of the regulating valve.

mainly includes fluid composition, temperature, density, viscosity, normal flow, maximum flow, minimum flow, inlet and outlet pressures under maximum and minimum flow, maximum cut-off pressure difference, etc.

Before the specific selection of the regulating valve is determined, it is necessary to fully grasp and determine the characteristics of the structure, form, material and other aspects of the regulating valve body itself.

Technical aspects mainly consider issues such as flow characteristics, pressure drop, flashing, cavitation, and noise.

design phase

The parameters to be prepared when designing the combination

Process parameters: temperature, pressure, pressure difference at normal flow and pressure difference at cut-off.

Fluid characteristics: the influence of corrosiveness, viscosity, and temperature changes on fluid characteristics.

System requirements: leakage, adjustable ratio, action speed and frequency, linearity and noise.

The chemical nature of the medium, detailed composition.

The second step of the calculation of the flow coefficient Cv and caliber of the regulating valve

The flow coefficient Cv (flow capacity) is defined as: the pressure difference before and after the regulating valve is 1Kg/cm2, and the volumetric flow rate (m3/h) through the valve per hour for the heavy fluid of 1g/cm2.

There are many calculation methods for the flow coefficient Cv of the control valve, and it is also more complicated. But this calculation is very important. If you can't calculate it yourself, you must provide detailed parameters for the professional selection personnel of the valve factory to calculate.

The third step caliber calculation

This is the first consideration in the selection of regulating valves. The size of the valve should be determined according to the process parameters. If the size is small and cannot meet Qmax, if the size is too large, it will often work at a small opening, resulting in poor regulation performance and short valve life.

To sum up, when the size of the valve core and valve seat is changed, but the requirements are still not met, the size of the valve must be changed.

According to the production capacity, equipment load, and the working conditions of the controlled medium, determine the data required for the calculation of the flow capacity, and obtain the Cvmax and Cvmin at the maximum and minimum flow. Through years of practice and theoretical discussion, when determining the diameter of the throttle valve, 1.2~1.4 times the flow coefficient Cv value required by the working condition is used as the flow coefficient value of the valve. The approximate range of the regulating valve opening is as follows:

Maximum opening: 70%~90%

Common opening degree: 40%~70%

Minimum opening: 10%

Verify the adjustable range based on the calculated Cvmin and the adjustable ratio of the used control valve, and verify that it is appropriate. The Cv value can be used to determine the diameter of the control valve.

In principle, the diameter of the valve is determined according to the pipe size, but after the valve with the same pipe size is adopted, the flow cannot meet the adjustment requirements, then the reduced diameter of the reduced diameter and the expanded diameter of the expanded diameter.

Calculation of valve diameter

How to determine the diameter of the regulating valve?

Determine the calculated flow Qmax, Qmin

Determine the calculated pressure difference, select the resistance ratio S value according to the characteristics of the system, and then determine the calculated pressure difference (when the valve is fully opened);

Calculate the flow coefficient, select the appropriate calculation formula chart or software to obtain KV;

KV value selection, according to the KV max value in the selected product series that is closest to the KV of the first gear, the primary selection caliber is obtained;

Opening degree check calculation, when Qmax is required, ≯90% valve opening; when Qmin is ≮10% valve opening;

The actual adjustable ratio check calculation, the general requirement should be ≮10; R actual>R requirement

The caliber is determined, if it is unqualified, reselect the KV value and verify again.

Fourth step selection of flow characteristics

The flow characteristic of the regulating valve is the relationship between the relative flow of the medium flowing through the regulating valve and its opening under the condition that the pressure difference between the two ends of the valve remains constant. The flow characteristics of the regulating valve include linear characteristics, equal percentage characteristics and parabolic characteristics.

How to choose flow characteristics

The principle of flow characteristics selection:

The logarithmic characteristic is selected when the opening is small and the unbalanced force changes greatly.

The required adjusted parameter reflection speed is a straight line when the speed is fast, and a logarithm when it is slow.

The pressure regulation system can choose the linear characteristic.

The liquid level adjustment system has optional linear characteristics.

The fifth step is to calculate the pressure difference when closing

Close pressure difference involves two problems

causes the valve to not be closed when it should be closed, and cannot be opened when it should be opened.

will affect the accuracy of the valve's Cv value selection.

The sixth step is to select the adjustable ratio

Adjustable ratio R is an important parameter of the valve. The accuracy of the selection directly affects the quality of the adjustment.

The ratio of the maximum flow rate and the minimum flow rate that can be controlled by the regulating valve is called the adjustable ratio R. When the pressure difference between both ends of the valve remains constant, the ratio of the maximum flow rate to the minimum flow rate is called the ideal adjustable ratio. In actual use, the pressure difference between the two ends of the valve changes, and the adjustable ratio at this time is called the actual adjustable ratio. Note: R is too small to meet the range of flow change

The seventh step is to determine the spring range of the actuator

It must be considered comprehensively from factors such as starting working pressure, output force, stability, and whether it can be adjusted. Generally speaking, as long as the source pressure provided is accurate and the media parameters are accurate, there is no major problem.

The eighth step material selection

For the company, the process is very complicated, and the control medium is also diverse. Therefore, the choice of material is the most knowledgeable. During the selection process, if the process and process conditions, and the physical and chemical properties of the medium are thoroughly mastered, the selected valve structure And the materials are more scientific.

The valve structure and material selected by the excellent sizing personnel should be the simplest structure and the lowest price (relatively speaking) under the premise of actually satisfying the use function. On the contrary, it may have spent a lot of money, and the valve still cannot be selected well. , This cost can be 20-30 times worse. For production, the impact of bad use is even greater, starting and stopping, causing economic losses and lowering the quality of product output, and even the production cannot run normally.

The pressure rating, service 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 products should be preferred. Cast iron valves are not suitable for water vapor or wet gas with a lot of water and flammable and explosive media.

If the company is located in the north, it is not advisable to choose cast iron valves for places where cavitation and erosion are serious or where the environment temperature is lower than -20℃. Wear-resistant materials, such as cobalt-based, should be used for the throttle sealing surface. For highly corrosive media such as alloy or surface surfacing Stellite alloy, 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. The valve body and the throttle material should be treated separately. Generally speaking, the ratio of the corrosion rate of the valve body to the corrosion rate of the valve trim should be better than 1:8. When selecting the lining material, the temperature, pressure and concentration of the working medium are all It must meet the scope of use of the material, and consider the fluid's wear on the lining. Vacuum valves should not be lined with rubber or plastic in the valve body. It is not recommended to use rubber lining for valves in domestic sewage treatment systems, especially industrial sewage treatment systems and oily media.

Valve trim material selection

High and low temperature material selection

Typical corrosion-resistant alloy material selection for typical media

Sulfuric acid: 316L, Hastelloy, No. 20 alloy. Hydrochloric acid: Hastelloy B. Acetic acid, formic acid: 316L, Hastelloy. Phosphoric acid: Inconel, Hastelloy. Nitric acid: aluminum, C4 steel, C6 steel. Hydrofluoric acid: Monel. Caustic soda: Monel. Chlorine (water content greater than 1%): Hastelloy C. Salt water: titanium, 316L. Nickel Sulfate: Titanium

non-metallic corrosion-resistant materials

So far, the most versatile corrosion-resistant material is tetrafluoroethylene, which is called the king of corrosion resistance. Therefore, the full PTFE corrosion-resistant valve should be selected first. However, it is not recommended to choose in the following situations: temperature>160℃PN>1.6 severely worn occasions.

Excellent selection and design personnel know the importance of the site, and they will closely integrate with the process personnel during the design integration stage, and pay attention to the correct selection of valve materials.

The ninth step packing and bonnet type selection

(1) Under normal circumstances, when the medium temperature is less than 200℃, select "V"-shaped tetrafluoroethylene packing and common type upper valve cover; when the medium temperature is less than 450℃, use "V"-shaped tetrafluoroethylene packing, but it must be a heat dissipation valve. cover.

(2) For straight-stroke valves, if there is a positioner accessory, for high temperature valves with a medium temperature of ≤450°C, ordinary bonnets can still be used, but graphite packing must be used.

(3) When the temperature of the medium is >400℃, a heat-dissipating valve cover and graphite packing are required.

(4) In order to increase the reliability of the valve stem seal, a double-layer packing structure can be selected. If the medium is highly toxic or flammable and explosive, the medium may have a hydrolysis reaction with air, or the leakage rate of the process requirements is very low, it is recommended to use the bellows sealing method.

Tenth step selection of action mode

In the production process, the choice of the air-opening and air-closing modes of the regulating valve is mainly considered from the safety of the process production. For example: steam heater, chlorine gas leaching process chlorine gas regulating valve chooses air-opening valve; boiler water inlet regulating valve adopts gas-closing type.

The choice of air opening and air closing of the pneumatic control valve is not a simple design and selection problem of automatic control. supply.

For some special cases, you can also consider keeping the regulating valve in place when the air supply is interrupted. For example, in the discharging operation in the pressurization kettle, it is not desired to suddenly cut off the high-pressure medium or empty it completely. In this case, the regulating valve should remain in place

Eleventh step selection of valve accessories

This is relative to the theme-the valve, to ensure the normal operation of the valve. Valve accessories: 1) Valve positioner-used to improve the working characteristics of the regulating valve to achieve correct positioning; travel switch-display on the regulating valve, Working position of the lower limit of the stroke;

Pneumatic retaining valve-keep the valve at the current position when the air source fails; solenoid valve-realize automatic switching of the air circuit. Two-position three for single air control; two-position five-way for double air control;

Manual mechanism-manual operation can be switched when the system fails;

Pneumatic relay-to speed up the action of the pneumatic film actuator. Air filter pressure reducer-used for air source purification and pressure regulation. Gas storage tank-When the gas source fails, the valve can continue to work for a period of time, generally requiring three-stage protection. The selection of valve accessories should be practical, and try not to use them if they are not necessary, but the key accessories should still be selected and reliable.

The twelfth step valve structure selection

Classification of valves

Classified by purpose and function

Two-position valve: mainly used to close or connect the medium;

Regulating valve: Mainly used for regulating the system. When selecting a valve, it is necessary to determine the flow characteristics of the regulating valve;

Split valve: used to distribute or mix media;

Shut-off valve: usually refers to a valve with a leakage rate of less than one hundred thousandths.

Classified by main parameters

1 Classified by pressure

Vacuum valve: the working pressure is lower than the standard atmospheric pressure;

low pressure valve: nominal pressure PN≤1.6MPa;

Medium pressure valve: PN2.5~6.4MPa;

High pressure valve: PN10~80.OMPa, usually PN22, PN32;

Ultra high pressure valve: PN≥100MPa.

2 Classified by medium working temperature

High temperature valve: t>450℃;

Medium temperature valve: 220℃≤t≤450℃;

Normal temperature valve: -40℃≤t≤220℃;

Low temperature valve: -200℃≤t≤-40℃.

According to the main special purpose (ie special, special valve) soft-sealed shut-off valve; hard-sealed shut-off valve; wear-resistant regulating valve and corrosion-resistant regulating valve; Vent valve; anti-blocking regulating valve; corrosion-resistant anti-blocking shut-off valve insulation jacket valve; small flow regulating valve; large adjustable ratio regulating valve; fine and small regulating valve; bellows sealing valve and various special valves (such as chlorine gas For lye) etc.

Classified by driving energy

Electro-pneumatic hydraulic

Commonly used classification

This classification method is divided according to the principle, function and structure, and is currently the most commonly used classification method both domestically and internationally. Generally divided into nine categories:

Through single-seat control valve

Single seat control valve structure

Through two-seat control valve

Sleeve Valve

angle valve

Three-way valve:

Diaphragm valve:

Butterfly Valve

ball valve

Type selection tips

Under the premise of meeting the requirements of process control, the selected valve should be as simple as possible, reliable, inexpensive, long-lived, convenient for maintenance, and timely and reliable for the source of spare parts. Try to avoid simply pursuing good structure, good materials, and multiple accessories, while ignoring the considerations of reliability and economy.

Adjust the valve priority order

①Full-function ultra-light regulating valve→②Butterfly valve→③Sleeve valve→④Single-seat valve→⑤Double-seat valve→⑥Eccentric rotary valve→⑦Ball valve→⑧Angle valve→⑨Three-way valve→⑩Diaphragm valve.

Equipment selection stage (tender stage)

Develop a detailed equipment plan

If it is a mature process or a control valve with application experience, it should be selected according to the original valve operation, advantages and disadvantages, and application effects.

The detailed process parameters should be checked carefully in the engineering project. If in doubt, the parameters must be checked in conjunction with other professions.

Before submitting the equipment plan or bidding plan, the relevant technical personnel should be organized to review the plan.

If you are not fully aware of the material and structure of the valve, it is recommended to conduct technical exchanges and equipment inspections with the equipment manufacturer.

The pre-selected manufacturers should choose the ones with good service quality and good product quality.

Remember: the selection of valves is a complicated process, and the technical parameters you provide will directly lead to application effects and control quality. The parameters proposed in the equipment plan must be confirmed by process technicians before they can be reported.

Don’t trust the design institute too much, if If there is any doubt about the parameters provided by the design institute, the parameters shall be confirmed through proper channels.

Issues noticed in the bidding process

During the bidding process, it is necessary to carefully check whether the product specifications, parameters, materials, and accessories provided in the bidding document of the equipment manufacturer can correspond to or better than the bidding document. If there is any doubt, it must be clarified.

To compare key parameters and materials together. Really shop around.

When the requirements are not met or the key parameters deviate from the bidding document, the principle must be adhered to, and you must not use it or not. Don’t force it.

Control valve signal piping and wiring

The performance index of the regulating valve

(1) Basic error: increase or decrease the 20~100kPa signal steadily into the air chamber (or positioner), measure the stroke value corresponding to each point, and calculate the relationship between the "signal-stroke" relationship and the theoretical value The maximum value of the error of each point is the basic error. The test points should be carried out at 5 points of 0%, 25%, 50%, 75%, and 100% of the signal range, and the basic error of the measuring instrument should be limited to 25% of the basic error limit of the tested valve (2) Hysteresis: the test method is the same as above . The maximum difference between the positive and negative travel measured on the same input signal is the hysteresis.

What should be paid attention to during the selection process?

Drive mode selection?

Electric actuator selection?

Ordinary or electronic?

Electric actuator linear stroke or angular stroke?

Is the electric actuator directly connected or crank connected?

Selection of Servo Amplifier

Is it integrated or servo amplifier?

Explosion-proof requirements

According to the process design requirements, the explosion-proof level is proposed

Is valve position feedback required?

Do you need overload protection?

Do you need a handheld communicator?

Pneumatic actuator selection

Choose pneumatic film or cylinder?

Position or proportion?

Straight stroke or angular stroke?

Spring or double piston?

Single spring or multi-spring small structure?

Is it positive or negative?

Selection of spring range (must be calculated)?

Do you need a stand?

Thrust calculation?

What is the pressure of the air source?

Stroke and angle size?

Motion speed requirement?

Environmental conditions?

What is the weight requirement?

What is the color requirement?

General principles of model selection

General requirements for installation

(1) The regulating valve should be installed vertically and upright on the horizontal pipeline, and the regulating valve with a nominal diameter of Dg≥50 should have permanent supports on the pipelines before and after the valve.

(2) The installation position of the regulating valve should be convenient for operation and maintenance, so that personnel can carry out maintenance and operation. If necessary, a platform should be installed.

(3) Sufficient space should be left in the upper and lower parts of the regulating valve to remove the actuator, valve trim, and the lower flange and plug of the valve during maintenance.

(4) When the regulating valve is installed in a vibration environment, anti-vibration measures should be considered.

(5) For the regulating valve without a valve positioner, it is better to install a small pressure gauge indicating the control signal on the membrane head.

(6) The regulating valve should be checked first and installed after the pipeline is purged.