Precautions for selection of regulating valves

Butterfly valves - single-seat valves - double-seat valves - sleeve valves - angle valves - three-way valves - ball valves - eccentric rotary valves - Diaphragm valves.

1. The simplest is pneumatic thin-film type, followed by piston type, and finally electric type.

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

3. The old electric actuator is bulky, we have electronic compact and highly reliable electric actuator to provide (price is correspondingly high).

4. The old ZMA, ZMB thin-film actuators can be eliminated, by the multi-spring lightweight actuators instead (improved performance, weight, and height down about 30%).

5. Piston actuator varieties of specifications more, the old, big and bulky recommendations are no longer selected, and the choice of light new structure.

Material selection:

1. Regulating valve body pressure rating, use of temperature and corrosion resistance should be no less than the requirements of the process connection pipeline, and should give priority to the use of manufacturing plant stereotypes.

2. Water vapour or wet gas containing more water and flammable and explosive media, should not be selected cast iron valve.

3. Ambient temperature below -20 ℃ (especially in the north), not suitable for cast iron valves.

4. For vapour corrosion, erosion is more serious medium temperature and pressure difference constitutes a right-angle coordinates, its temperature is 300 ℃, the pressure difference is 1.5MPa two points outside the line of the region, the throttle sealing surface should be selected wear-resistant materials, such as cobalt-based alloys or surface surfacing Stryker alloys, etc..

5. For strong corrosive media, the selection of corrosion-resistant alloys must be based on the type of media, concentration, temperature, pressure, select the appropriate corrosion-resistant materials.

6. The valve body and throttle parts are treated separately, the valve body wall throttling speed is small and allows a certain degree of corrosion, the corrosion rate can be lmm / year or so; throttle parts are subject to high-speed scouring, corrosion will be bowed [up the leakage increase, the corrosion rate should be less than 0.1mm / year.

7. On the lining material (rubber, plastic) the choice of the working medium temperature, pressure, concentration must meet the use of the material range, and consider the valve action on its physical and mechanical damage (such as shear damage).

8. Vacuum valve is not suitable for selection of the valve body lined with rubber, plastic structure.

9. Water treatment system of the two cut-off valve is not suitable for rubber-lined materials.

10. Typical media typical corrosion-resistant alloy material selection:

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

b. Nitric acid: aluminium, 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) So far, the most versatile corrosion-resistant material is fluorine, known as the "king of corrosion resistance". Therefore, should be the first choice of all PTFE corrosion-resistant valves, as a last resort (such as temperature > 180 ℃, PN> 1.6) before the choice of alloy.

Selection of flow characteristics:

Provided below is a preliminary selection, detailed selection see special information:

1. Select logarithmic characteristics when S>0.6.

2. Small opening work, unbalance force changes when the logarithmic characteristic is selected.

3. Requirements of the regulated parameters reflect the speed of fast selection of linear, slow selection of logarithmic.

4. Pressure regulation system can be selected linear characteristics.

5. Level adjustment system can be selected linear characteristics.

Flow direction selection:

1. In the throttle port, the medium to the spool open direction flow for flow open, to close the direction of flow for flow closed.

2. The choice of flow direction is mainly a single seal class control valve, there are single seat valve class, angle valve class, single seal sleeve valve three categories. Base it for the specified flow direction (such as double seat valve, V ball) and arbitrary flow (such as O ball).

3. When dg>15, usually selected flow open, when dg≤15 small diameter valves, especially high-pressure valves can be selected flow closed to improve life.

4. For two-position switching valve can be selected flow closed.

5. If the flow-closed type valve produces oscillation, change over, flow-open type can be eliminated.

The choice of packing:

1. Regulating valve commonly used is fluorine "V" shaped packing and graphite "O" shaped packing.

2. PTFE packing friction is small, but poor temperature resistance, short life; graphite packing friction, but good temperature resistance, long life; high temperature and with the locator valve is recommended to choose graphite packing.

3. If the PTFE packing is often changed, graphite packing can be considered.

Selection of accessories:

1. The accessories of regulating valve mainly include: positioner, converter, relay, booster valve, holding valve, pressure reducing valve, filter, oil mist, travel switch, position transmitter, solenoid valve, handwheel mechanism.

2. Accessories play a supplementary function and ensure the operation of the valve. Increase if necessary, do not increase if unnecessary. Adding accessories unnecessarily increases the price and reduces reliability.

3. The main function of the positioner is to improve the output force and speed of action, do not need these functions, may not bring, not with the positioner is good.

4. For fast response system, don't want the valve to move fast, optional converter.

5. Strict explosion-proof occasions, optional: electrical converter + pneumatic positioner.

6. Solenoid valve should choose reliable products to prevent to it when the action is not action.

7. Important occasions are not recommended to use the handwheel mechanism to prevent human error.