Selection notes
三. Selection notes
Selection
(1) Ensure nominal pressure, not mean putting Pmax to PN, it needs to find the corresponded PN from temperature, pressure, material in tables and satisfy the selected valve PN value.
(2) Decided valve type, its leakage meets to technical requirement.
(3) Decided valve type, its working pressure differential is less than allowed pressure differential, if not, we need to consider from special angle or selecting other valve.
(4) Medium temperature within the valve working temperature scope, environmental temperature is fit for requirement.
(5) Consider valve anti-block problem according to medium squalor condition.
(6) Consider anti-corrosion problem according to medium chemical properties.
(7) Consider valve erosion and anti-wear problem according to pressure differential and medium containing hard object.
(8) Comprehensive economic effects considering performance, cost. To consider three questions:
a. simple structure (more easier more reliability), convenient maintenance, spare parts with sources.
b. service life.
c. price.
(9) Prefer selecting sequence
Butterfly valves - single seat valve - two-seat valves - sleeve valves - angle valves - three-way valve - control valves- eccentric rotate valve - control valve.
Actuator selection
(1) the simplest is pneumatic diaphragm type, followed by the piston type, and finally is the electric type.
(2) electric actuators main advantage is convenient drive source (power), but the price is high, reliability, waterproof explosion-proof is inferior to pneumatic actuators, so we should first choose pneumatic type.
(3) old electric actuator is bulky, we have electronic pure small high reliability electric actuators provided (price is corresponding high).
(4) old ZMA, ZMB diaphragm actuator can be eliminated, replaced by multi-spring light actuators (increased performance, weight, height drop about 30%).
(5) piston actuators have many varieties and specifications, old, big and silly advice will not choose, while selecting light new structure.
Selection
(1) body pressure resistant levels, use temperature and corrosion resistance etc shall not be lower than the process connecting pipeline requirement and should first choose factory approved product.
(2) steam or wet gas containing more water and inflammable and explosive medium, unfavorable to choose cast iron valves.
(3) environmental temperature below - 20 ℃ (especially the north), unfavorable to choose cast iron valves.
(4) to serious cavitation and erosion medium temperature and pressure differential constitute the rectangular coordinate, when its temperature is 300 ℃, pressure differential is 1.5 MPa area besides two-point ligature, should choosing wear-resistant materials to throttling seal face, such as cobalt-based alloy or surface weled stellite alloy, etc.
(5) to strong corrosive medium, choose corrosion resistant alloy must, according to media types, concentration, temperature and pressure, choose appropriate corrosion resistant materials.
(6) body and throttling parts respectively treated, body wall throttling speed is small and allow a certain corrosion, its corrosion rates can be in 1mm/year, Throttling pieces high-speed scour and corrosion will cause leak increased, its corrosive rate should be less than 0.1 mm/year.
(7) to lining material (rubber, plastic) selection, the working medium temperature, pressure, concentration must meet the material use scope, and consider the valve movement to its physical, mechanical damage (e.g., shear failure).
(8) vacuum valve is unfavorable to choose body lining rubber, plastic structure.
(9) water treatment system two cut-off valve is unfavorable to choose lining rubber materials.
(10) typical medium typical corrosion resistant alloy material selection:
A. vitriol: 316L, hastelloy alloy, 20 alloy.
B. nitric acid: aluminum, C4 steel, steel C6.
C. hydrochloric acid: hastelloy B.
D. hydrofluoric acid: monel.
E. acetic acid, formic acid: 316L, hastelloy alloy.
F. phosphate: nickel and hastelloy alloy.
G. urea: 316L.
H. caustic soda: monel.
I. chlorine: hastelloy C.
J. seawater: nickel, 316L .
(11) So far, the most universal anti-corrosion materials is F4, called "the corrosion resistance king". Therefore, we should first selecting F4 anti-corrosion resistant valve, extreme circumstances (such as temperature > 180℃, PN > 1.6) just choose alloy.
Flow characteristics choice:
We provided are preliminary selection, for detailed selection, see special material:
(1) S > 0.6 choosing a logarithmic characteristics.
(2) small open degree work, unbalance force change a lot, choosing logarithmic characteristics.
(3) Required adjustable parameters reflect speed, choosing straight line, slow choosing logarithm.
(4) Pressure adjust system can choose linear characteristic.
(5) Liquid level regulative system can choose linear characteristic.
acting mode selection
(1) foreign common fault represents open or close, namely fault open, fault close, is opposite to our country gas open, gas close, fault open corresponds gas close valve, fault close corresponds gas open valve.
(2) new light valve, refined small valve has not emphasized execute mechanical positive role, reaction, therefore it must be marked on the note.
B(gas close)K(gas open )
spring scope selection
(1) first is selecting spring range, but make sure working spring range.
(2) determine working spring range related to calculating output force to overcome unbalance force. If there is difficult, should inform conditions (mainly is pressure differential when valve closed) to factory, assist calculation and adjusted the spring and working range to deliver factory (currently, many manufacturers doesn't do calculations).
flow direction selection:
(1) In throttle port, media to the throttle valve core direction flow is flow open, to close direction flow is flow close.
(2) the flow choice is mainly single sealing regulator, with single seat valves, angle valves, signle sealing seleeve three catagories. Others are stipulate flow (such as two seat valve, V ball) and arbitrary flow (such as O-ball).
(3) when dg>15, we usually choose flow open, when dg≤15 small valve, especially high-pressure valves optional flow closed, in order to improve life.
(4) To two-way on-off valves optional flow shut.
(5) if flow closed type valve produce oscillations, change it, flow open type can eliminate it.
packing selection
(1) regulator commonly used is F4 "v-shaped" packing and graphite 7
O" form packing.
(2) F4 packing friction is small, but with poor anti-temperature, short life, graphite packing friction is large, but anti-temperature is good, long life. High temperatures and with locator valve suggested to choose graphite packing.
(3) if F4 packing is often changed, can consider to use graphite packing.
accessory selection
(1) Regulator accessories mainly has: locator, converters, relay, pressurized valve, protect position valve, pressure reducing valve, filter, oil mist device, travel switch, position transmission device, solenoid valve, handwheel.
(2) Accessories act additional functions and ensure valve operation function. The necessary increases, unnecessary not increase. Unnecessary add accessories will raise prices and reduces reliability.
(3) The locator's main function is to improve output force and speed of action, without need these functions, can without locator, not with locator is good.
(4) To rapid response system, not needing valve hurry, optional converters.
(5) Strict explosion-proof occasions, optional: electrical converter + pneumatic locator.
(6) Solenoid valve should choose reliable product, prevent needing its movements while it does not act.
(7) Important occasions suggested without handwheel, prevent artificial misoperation.
(8) It is better to provide by manufacturers and assembly in valve supply, to guarantee the reliability of the system and assembly connection.
(9) When ordering, should offer accessory names, types, specifications, input signal, output signal, etc.
(10) Repeat: please note that these "dot" importance, especially reliability, if necessary, can equip with pneumatic components, such as: solenoid valves.
