Correct selection skills of the hottest regulating

2022-10-22
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Skills of correct type selection of regulating valve

skills of correct type selection of regulating valve:

Abstract: this lecture focuses on summarizing and analyzing the matters needing attention in the correct type selection of regulating valve, and summarizes the experience and matters needing attention in the type selection of regulating valve according to the valve type, actuator, material, flow characteristics, action mode, spring range, flow direction, packing and accessories of regulating valve, It is helpful for the practitioners of regulating valve to understand the knowledge of regulating valve selection and improve the level of model selection. Special topic of valve selection

I. selection of valve type:

(1) to determine the nominal pressure, it is not to use Pmax to set PN, but to find out the corresponding PN from the table according to the three conditions of temperature, pressure and material, and meet the PN value of the selected valve

(2) the leakage of the determined valve type meets the process requirements

(3) the working differential pressure of the determined valve type should be less than the allowable differential pressure of the valve. If not, it must be considered from a special point of view or another valve

(4) the temperature of the medium is within the working temperature range of the valve, and the ambient temperature meets the requirements

(5) consider the anti blocking problem of the valve according to the unclean condition of the medium

(6) consider the corrosion resistance of the valve according to the chemical properties of the medium

(7) consider the erosion and wear resistance of the valve according to the pressure difference and the medium containing hard objects

(8) performance and price ratio considering comprehensive economic effect. Three issues need to be considered:

a. simple structure (the simpler the higher the reliability), convenient maintenance, and the source of spare parts

b. service life

c. price

(9) preferred order

butterfly valve - single seat valve - double seat valve - sleeve valve - angle valve - three-way valve - ball valve - eccentric rotary valve - diaphragm valve

II. Selection of actuator:

(1) the simplest is pneumatic diaphragm type, followed by piston type, and finally electric type

(2) the main advantage of electric actuator is that the driving source (power supply) is convenient, but the price is high, and the reliability, waterproof and explosion-proof are not as good as pneumatic actuator, so pneumatic actuator should be preferred

(3) the old electric actuator is cumbersome, and we have electronic fine, small and highly reliable electric actuators (the price is correspondingly high)

(4) the old ZMA and ZMB thin film actuators can be eliminated and replaced by multi spring light actuators (the performance is improved, and the weight and height are reduced by about 30%)

(5) there are many varieties and specifications of piston actuator. It is suggested to choose a new light structure instead of the old, big and stupid one

III. selection of materials:

(1) the pressure resistance grade, 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

(2) cast iron valves should not be used for steam or wet gas with more water and flammable and explosive media

(3) when the ambient temperature is lower than - 20 ℃ (especially in the North), cast iron valves should not be selected

(4) for the rectangular coordinates composed of medium temperature and differential pressure with serious cavitation and erosion, when the temperature is 300 ℃ and the differential pressure is 1.5MPa, wear-resistant materials should be selected for the throttling sealing surface, such as cobalt based alloy or surface surfacing stellite alloy, etc

(5) for highly corrosive media, the selection of corrosion-resistant alloys must be based on the type, concentration, temperature and pressure of the media, and the selection of appropriate corrosion-resistant materials

(6) treat the valve body and throttling parts separately. The throttling speed of the inner wall of the valve body is small and certain corrosion is allowed. The corrosion rate can be about 1mm/year; Throttling parts are scoured by high speed Corrosion will cause bow [leakage and increase, and its corrosion rate should be less than 0.1mm/year.

(7) 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 consider the physical and mechanical damage (such as shear damage) to it when the valve acts.

(8) the vacuum valve should be re welded; if the performance of components and parts becomes poor, the rubber and plastic structure of the valve body lining should not be selected.

(9) Rubber lining material should not be selected for the two position shut-off valve of the water treatment system

(10) selection of typical corrosion-resistant alloy materials for typical media:

a. sulfuric acid: 316L, Hastelloy, alloy 20

b. nitric acid: aluminum, C4 steel, C6 steel

c. hydrochloric acid: Hastelloy B

d. hydrofluoric acid: Monel

e. acetic acid, formic acid: 316L, Hastelloy

f. phosphoric acid: Inconel, Hastelloy

g. urea: 316L

h. caustic soda: Monel

i. chlorine: Hastelloy C

j. seawater: Inconel, 316L

(11) by far, the most versatile corrosion-resistant material is tetrafluoro, known as the "king of corrosion resistance". Therefore, all tetrafluoro corrosion-resistant valve (patented by Hualin Institute) should be selected first, and alloy should be selected only when there is no alternative (such as temperature>180 ℃, PN > 1.6)

IV. selection of flow characteristics:

the following is the preliminary selection. See the special data for the detailed selection:

(1) select the logarithmic characteristic when s> 0.6

(2) select logarithmic characteristics when working with small opening and large change of unbalanced force

(3) the required adjusted parameters reflect that when the speed is fast, the straight line is selected, and when the speed is slow, the logarithm is selected

(4) the pressure regulating system can choose linear characteristics

(5) the liquid level regulation system can choose linear characteristics

v. selection of action mode:

(1) it is commonly expressed by opening or closing under fault in foreign countries, that is, fault opening and fault closing, which are just opposite to the air opening and air closing in China. Fault opening corresponds to air closing valve, and fault closing corresponds to air opening valve

avoid damaging the jaw seat (2). The new light valve and fine small valve do not emphasize the positive and negative effects of the actuator, so it must be marked on the endnote

b (air close) K (air open)

VI. selection of spring range:

(1) first, select the spring range and determine the working spring range

(2) determining the working spring range involves calculating the output force to overcome the unbalanced force. If there are difficulties, the conditions (mainly the pressure difference when the valve is closed) should be told to the manufacturer to help calculate and adjust the spring and working range before leaving the factory (at present, many manufacturers do not calculate at all)

VII. Selection of flow direction:

(1) at the throttle port, the medium flows towards the opening direction of the valve core. Each batch shall be composed of steel bars with the same brand, furnace and tank number, specification and delivery state as flow open, and the flow towards the closing direction is flow closed

(2) the choice of flow direction is mainly single seal regulating valve, including single seat valve, angle valve and single seal sleeve valve. It is based on the specified flow direction (such as double seat valve, V ball) and arbitrary flow (such as o ball)

(3) when DG> 15, the flow is usually selected to open. When DG ≤ 15, the small diameter valve, especially the high-pressure valve, can be selected to close to improve its service life

(4) flow closing is optional for two position on-off valve

(5) if the flow closed valve produces oscillation, change it over and the flow open valve can be eliminated

VIII. Selection of packing:

(1) PTFE "V" packing and graphite "O" packing are commonly used in regulating valves

(2) tetrafluoro packing has small friction, but temperature difference resistance and short service life; Graphite filler has large friction, but good temperature resistance and long service life; Graphite packing is recommended for valves with positioner under high temperature

(3) if tetrafluoro packing is often replaced, graphite packing can be considered

IX. selection of accessories:

Changxing UV curing resin

(1) the accessories of the control valve mainly include: positioner, converter, relay, booster valve, position keeping valve, pressure reducing valve, filter, oil mist, travel switch, position transmitter, solenoid valve, handwheel mechanism

(2) accessories play a supplementary role and ensure the operation of the valve. If necessary, increase; if unnecessary, do not increase. Adding accessories when unnecessary will increase the price and reduce the reliability

(3) the main function of the positioner is to improve the output force and action speed. When these functions are not required, they can be omitted, not with the positioner

(4) for the rapid response system, do not move the valve fast, and choose the converter

(5) strictly explosion-proof occasions, optional: electrical converter + pneumatic positioner

(6) the solenoid valve should choose a reliable product to prevent it from not acting when it is required to act

(7) it is recommended not to use the hand wheel mechanism in important occasions to prevent human misoperation

(8) it is best to be provided by the manufacturer and supplied on the valve assembly to ensure the reliability of the connection between the system and the assembly

(9) when ordering, the name, model, specification, input signal, output signal, etc. of the accessories shall be provided

(10) reiterate: please pay attention to the importance of these "little things", especially the reliability. If necessary, warringham can match Japanese pneumatic components, such as solenoid valves

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