Problems needing attention in the selection and in

2022-07-28
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Problems needing attention in the selection and installation of control valve Author: xieyanhong Abstract: through the elaboration of the selection and installation of control valve, it is proposed that the design and application personnel should consider not only the requirements of process and environment, but also various characteristics of the valve itself when selecting and installing the type; Only by measuring the sample size and considering the organic combination, can the expected effect be achieved. 0 Introduction

in engineering design, as a technician engaged in automatic instrument design, he needs to deal with various regulating valves frequently. For an automatic control system, whether the regulating valve is properly selected directly affects the regulation quality of the control system. If the control valve is improperly selected, the system will not work normally, or the valve will be damaged quickly, or even cause production accidents. A considerable number of people tend to pay more attention to the caliber calculation of the valve when selecting the regulating valve, but do not pay attention to the selection of valve type and other aspects. According to my years of work experience and the investigation of relevant departments, more than 95% of the accidents caused by the regulating valve that the control system does not work normally are caused by improper selection, while less than 5% are caused by calculation errors. This paper will focus on the selection of control valve and other related issues

1 classification and characteristics

control valve base products are divided into several categories of common products according to the structural characteristics of the base type. They are: straight through single seat valve, straight through double seat valve, sleeve valve, angle valve, three-way valve, diaphragm valve, butterfly valve, ball valve and eccentric rotary valve. Among them, the first six are straight stroke control valves and the last three are angle stroke control valves. As the users of control valves, they must first understand the characteristics of the base type products Precautions for use, various variant products and improved products

(1) straight through single seat regulating valve

this valve has the characteristics of small leakage, small allowable differential pressure, complex flow path and simple structure. It is suitable for clean media occasions with strict leakage requirements and small working differential pressure. However, small size valves (such as DN15, 20 and 25) can also be used in occasions with large differential pressure. It is one of the most widely used valves

(2) the straight through two seat regulating valve

is opposite to the single seat valve. It has the characteristics of large leakage and large allowable pressure difference. It is suitable for clean media occasions with lax leakage requirements and large working pressure difference. It is one of the most widely used valves

(3) sleeve valve

sleeve valve has two structures: single seal and double seal. The former is equivalent to a single seat valve, and the latter is equivalent to a double seat valve. It is suitable for double seat valve occasions. In addition, sleeve valve also has the characteristics of good stability and convenient loading and unloading, but the price is 50%~200% higher than that of a single double seat valve. It also needs special winding gasket. Second only to single and double seat valves, it is widely used, "said Michel glotin, director of materials science of akoma

(4) the throttling type of angle valve

is equivalent to a single seat valve, but the valve body has a simple flow path, which is suitable for dirty media with small leakage requirements and small pressure difference, as well as occasions requiring right angle piping

(5) the three-way valve

has three channels, which can replace the two through single seat valves for the occasions where the temperature difference between the two-phase flow and the two-phase flow is ≤ 150 ℃. When dn300mm, the butterfly valve is usually used

(8) ball valve

"O" type ball valve has the best self-cleaning performance when it is fully open. It is suitable for two position cutting off occasions with particularly dirty and fibrous media. The "V" shaped ball valve has approximately equal percentage adjustment characteristics, and is suitable for the adjustment occasions with unclean and large fiber containing media. Ball valves are expensive

(9) eccentric rotary valve

this valve is between butterfly valve and ball valve. It has good self-cleaning performance, good regulation performance, and can also be cut off. It is suitable for regulating occasions with dirty media and small leakage requirements, but the valve is expensive

2 precautions for model selection

2.1 selection of materials

2.1.1 selection of valve body materials

the pressure resistance grade, service temperature and corrosion resistance of the valve body shall not be lower than the requirements of the process connection pipeline, and the manufacturer's finalized products shall be preferred. In case of water vapor or wet gas with poor water content and flammable and explosive media, or when the environmental temperature is lower than -30 ℃, cast iron valves shall not be selected

2.1.2 selection of valve trim materials

(1) 1Cr18Ni9Ti or other stainless steel is generally selected as non corrosive medium

(2) in the case of serious cavitation and erosion, and in the rectangular coordinates composed of medium temperature and pressure difference. Wear resistant materials, such as drill base alloy or surface surfacing stellite alloy, shall be selected for areas outside the two-point connecting line with a temperature of 300 ℃ and a pressure difference of 1.5MPa

(3) if the leakage is required to be very small, soft seals should be selected, such as tetrafluoroethylene and rubber

2.1.3 selection of corrosion-resistant materials

(1) for corrosive media, appropriate corrosion-resistant materials shall be selected according to the type, concentration, temperature and pressure of the media, and the economy of the valve shall also be considered

(2) when selecting the lining material (rubber and plastic), the temperature, pressure and concentration of the working medium must meet the application range of the material, and the physical and mechanical damage caused by the valve action must be considered

(3) it is not suitable to use rubber and plastic lined valve body for vacuum valve

(4) the two position shut-off valve of the water treatment system should not be made of rubber lined materials, such as rubber lined butterfly valve and diaphragm valve

2.1.4 selection of high and low temperature materials

(1) -200~-60 ℃, copper or 1Cr18Ni9Ti shall be selected

(2) -450~+600 ℃, choose titanium molybdenum stainless steel

2.2 other precautions

on the premise of meeting the necessary functions for use, the selected valve should be as simple, reliable and cheap as possible, with long service life and convenient maintenance. There are sources of spare parts

the main factors to be considered are as follows:

(1) the most basic condition is to meet the requirements of process pressure, flow and temperature

Andre, a physicist at the University of Manchester, UK

(2) sealing, reliability and economy of the valve

(3) the working differential pressure of the valve is less than the allowable differential pressure of the valve

(4) consider the unclean medium, easy scaling medium and strong corrosive medium passing through the valve

(5) consideration of valve action speed, valve flow characteristics, action mode and flow direction

(6) consideration of actuator form, output force, stiffness and spring range

(7) consideration of accessories and their completeness and special performance and requirements of special valves

3 installation precautions

(1) the regulating valve with nominal diameter dn>50mm should be installed vertically and upright on the horizontal pipeline. It is better to have permanent supports on the front and rear pipelines of the valve

(2) the regulating valve piping shall be compact, with sufficient space reserved for the upper and lower parts. If necessary, a platform shall be set to facilitate operation, maintenance and liquid drainage. The pipe diameter shall be consistent with the valve diameter as far as possible. The continuous curing process is reversible, and the connection form shall comply with the provisions of the manufacturer's product manual

(3) when the regulating valve is used for high viscosity, easy crystallization, easy vaporization and low temperature fluid, thermal insulation and anti freezing measures shall be taken. The operating ambient temperature is generally not higher than 60 ℃ and not lower than -30 ℃. For the control valve without valve positioner, anti vibration measures shall be considered when the valve is installed in the place with vibration

(4) for the regulating valve without a valve positioner, it is better to install a small pressure gauge indicating the control signal on the diaphragm head

(5) check and verify the valve before installation, and install it after the pipeline is purged. For suspended solids and high viscosity media, flushing pipeline shall be equipped

(6) the length of the inlet pipe section shall be as long as possible. For small diameter valves, it shall be greater than 10 times the pipe diameter. The straight pipe section at the outlet of the control valve shall be 3~5 times the pipe diameter. The pressure tapping points at the inlet and outlet of the regulating valve are located at 2 times the pipe diameter in front of the valve and 3 times the pipe diameter behind the valve

(7) filter device shall be installed in front of small flow regulating valve

(8) be sure to install according to the flow direction arrow, and avoid excessive stress during installation. When there is much dust in the environment, measures shall be taken to protect the valve rod to avoid wear or seizure

4 conclusion

the use effect of the regulating valve is reflected in the device. The quality is not only the quality problem of the product itself, but also related to the Design Institute, users and manufacturers. Any problem must be analyzed and found out from these three aspects. The design institute shall do a good job in caliber calculation, allowable differential pressure verification, actuator overcoming unbalanced torque and other problems related to model selection; The user shall ensure that the process conditions and environment are consistent with the design; the manufacturer shall ensure the quality. In this way, the three parties can achieve the desired results

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