How to se­lect the right so­le­noid valve

The cor­rect se­lec­tion of the valves be­ing used within any process con­trol sys­tem is key to en­sur­ing the per­for­mance and re­li­a­bil­ity of that sys­tem. Bürk­ert ex­am­ines the se­lec­tion process, with a fo­cus on the chemical re­sis­tance of the var­i­ous com­po­nents.

DEMM Engineering & Manufacturing - - NEWS -­eren­quiry #M140542

Se­lec­tion of valves for any given ap­pli­ca­tion is no triv­ial task. A valve in a brew­ery can be very dif­fer­ent to one used in a phar­ma­ceu­ti­cal ap­pli­ca­tion for ex­am­ple; one of the crit­i­cal as­pects of valve de­sign is the chemical re­sis­tance prop­er­ties of the valve hous­ing and the gas­ket ma­te­ri­als in re­la­tion to the sub­stances which it is ex­pected to con­trol.

A long and trou­ble-free ser­vice can only be ex­pected if the cor­rect ma­te­ri­als have been selected in the de­sign process.

In the case of a sim­ple so­le­noid valve, many of the com­po­nents, such as the plunger, re­turn spring and seals are all ex­posed to the me­dia. There­fore, in­for­ma­tion on the chemical com­pounds, tem­per­a­ture and pres­sure are all nec­es­sary to make an in­formed ma­te­rial se­lec­tion. If the pro­duc­tion process in­volves liq­uid food prod­ucts, the plas­tics and elas­tomers used should also con­form to the lo­cal food and hy­giene reg­u­la­tions. In ad­di­tion, some pro­cesses re­quire a clean­ing cy­cle to be per­formed in-be­tween pro­duc­tion pro­cesses, which re­quires data that fur­ther­more en­sures no un­due ma­te­rial degra­da­tion.

Fac­tors such as me­chan­i­cal load­ing, in­creased tem­per­a­tures and chemical con­cen­tra­tions all have a bear­ing on the per­for­mance of in­di­vid­ual com­po­nents, which can be man­u­fac­tured from a range of ma­te­ri­als. In gen­eral the valve bod­ies are made from ei­ther brass or stain­less steel with some be­ing con­structed from Polyamide.

The type of valve most ap­pro­pri­ate for each ap­pli­ca­tion is gov­erned by a num­ber of fac­tors, but in gen­eral, plunger-type di­rect act­ing valves are best suited to neu­tral and clean f lu­ids while piv­oted ar­ma­ture valves of­fer im­proved re­li­a­bil­ity by em­ploy­ing a me­dia sep­a­rat­ing mem­brane to con­trol cor­ro­sive, con­tam­i­nated or ag­gres­sive f lu­ids.

The range of op­tions for seal­ing and sep­a­rat­ing mem­branes is quite ex­ten­sive with each hav­ing ben­e­fits and draw­backs. The com­bi­na­tion of chemical com­pound, tem­per­a­ture, pres­sure and cost all have in­flu­ence in this de­sign de­ci­sion. Some of the more com­mon com­pounds in use with brief de­scrip­tions of their prop­er­ties in­clude:

• PTFE (Poly­te­traflu­o­roethy­lene) is al­most to­tally in­sol­u­ble and chem­i­cally in­ert. It has a high tem­per­a­ture re­sis­tance and PTFE ball valve seats, be­cause of their nat­u­ral lu­bric­ity, re­quire no lubri­ca­tion. PTFE di­aphragms and f lange gas­kets are used in the most se­vere chemical re­sis­tance ap­pli­ca­tions.

• EPDM (Eth­yl­ene Propy­lene Ter­poly­mer) is a syn­thetic rub­ber used as the stan­dard seal ma­te­rial for many valves. It is the most eco­nom­i­cal choice of elas­tomer and has ex­cel­lent chemical re­sis­tance to acids, al­ka­lis, salts and many oth­ers at tem­per­a­tures up to 90°C. How­ever, it is not suit­able for use with oils, petro­chem­i­cals and con­cen­trated acids. NBR (Nitrile rub­ber) has a high chemical re­sis­tance to oil and petroleum but is weak on ox­i­dis­ing me­dia such as acids. Nitrile also has ex­cel­lent abra­sion re­sis­tance and is less ex­pen­sive than FKM and FFKM.

• FKM (Flu­o­ri­nated elas­tomer) and per­flu­o­roe­las­tomers (FFKM) are more ex­pen­sive than neo­prene and nitrile elas­tomers be­cause they pro­vide additional tem­per­a­ture and chemical re­sis­tance. Cer­tain grades of FFKM can per­form con­tin­u­ously at tem­per­a­tures above 200°C.

• PPS (Polyphenyl sul­phide) is a high per­for­mance ther­mo­plas­tic used in many en­gi­neer­ing ap­pli­ca­tions. With the abil­ity to per­form above 200°C and with re­sis­tance to acids and al­ka­lis as well as abra­sion, PPS can be selected as a valve body ma­te­rial

• PVDF (Polyvinyli­dene f lu­o­ride) is re­sis­tant to sol­vents, acids and bases, mak­ing it an ideal ma­te­rial for the valve body where high tem­per­a­ture re­sis­tance is not re­quired. A very f lex­i­ble prod­uct, it can be in­jected, moulded and welded, mak­ing for eas­ier man­u­fac­tur­ing of the valve bod­ies.

• PEEK (Polyether ether ke­tone) has ex­cel­lent me­chan­i­cal and chemical prop­er­ties but is sus­cep­ti­ble to high con­cen­tra­tions of sul­phuric and ni­tric acid. This im­proved per­for­mance ac­counts for a higher price but pro­vides su­pe­rior qual­i­ties for valve bod­ies.

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