Selection of valve type:
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1. Determine the nominal pressure, not with Pmax to set PN, but by the temperature, pressure, and material three conditions from the table to find out the corresponding PN and meet the PN value of the selected valve.
2. Determine the valve type, and its leakage to meet the process requirements.
3. Determine the valve type, its working differential pressure should be less than the valve's allowable differential pressure, such as not, it is necessary to consider from a special point of view or choose another valve.
4. The temperature of the medium in the valve's operating temperature range, the ambient temperature, meets the requirements.
5. According to the medium of the unclean situation consider the valve's anti-blocking problem.
6. According to the chemical properties of the medium to consider the corrosion resistance of the valve.
7. According to the differential pressure and containing complex media, consider the valve erosion and wear resistance.
8. The comprehensive economic effect of the performance and price ratio. Need to consider three issues:
a. Simple structure (the simpler the higher reliability), easy maintenance, spare parts have a source;
b. Service life;
c. Price.
9. Preferred order.
Butterfly valves - single-seat valves - double-seat valves - sleeve valves - angle valves - three-way valves - ball valves - eccentric rotary valves - Diaphragm valves.
Actuator selection:
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.
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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.
Block 6 of The Steam and Condensate Loop considers the practical aspects of control, putting the basic control theory discussed in Block 5 into practice.
A basic control system would normally consist of the following components:
All of these terms are generic and each can include many variations and characteristics. With the advance of technology, the dividing line between individual items of equipment and their definitions are becoming less clear. For example, the positioner, which traditionally adjusted the valve to a particular position within its range of travel, can now:
However, for the sake of clarity at this point, each item of equipment will be considered separately.
Whilst a wide variety of valve types exist, this document will concentrate on those which are most widely used in the automatic control of steam and other industrial fluids. These include:
The first choice to be made is between two-port and three-port valves.
Globe valves
Globe valves are frequently used for control applications because of their suitability for throttling flow and the ease with which they can be given a specific &#;characteristic&#;, relating valve opening to flow.
Two typical globe valve types are shown in Figure 6.1.1. An actuator coupled to the valve spindle would provide valve movement.
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