Fail Safe Position of Flare Connected Control Valves

Fail Safe Position of Flare Connected Control Valves Mojtaba HabibiProcess HabibiProcess Engineer at Wood GroupTop Contributor Dears, As you know fare connected control valves are widely used by designers to get rid o accumulated pressure at some sections o a plant. I want to discuss about the ail sae position o this fare connected control control valve. My points are as ollows: . !he control valve should be designed as "ail sae". #ne good de$nition or "ail sae" is the position that route the system energy to minimum state. %ollowing this de$nition the ail sae position o fare connected control valves can be selected as %#. &. Most o time these control valves are considered in parallel with 'D( which always is %#. )o the concern is that i both o the *+( and 'D( are speci$ed as %# then the fare network may be overloaded in case o instrument air ailure. ailure. . Most o the time or the e-uipment which is provided with 'D( and *+(, )D( is considered at inlet and outlet sides o the process lines. )o in case o instrument air ailure the )D(s will be ailed to closed position and the peak fow rate resultant rom simultaneous opening o 'D( and *+( may not be as high as raised as a concern at previous item o my points. Is that true /usti$cation0 1. #ne proposed solution is to provide separate air volume bottles or 'D( and *+(. In this case can we ensure fare will not be overloaded0 or these separate air volume bottle /ust make make some time dalays0 dalays0 2. Another solution can be to speciy *+( as %+. !his can be a good option but at the same time is di3cult to prove because because most o the operation guys and clients belive on the concept that I have e4palined at item  o my points. 5. I heard & other drawbacks o *+( which is speci$ed as %#. %irst is that philosophy o permissive blowdown may be blemished. )econd is that plant can not be kept at pressuri6ed shutdown state 7plant will be depressuri6ed and all o the start8up activities should be repeated rom 6ero to 99 )o at the end which one you preer: %# *+( or %+ *+(0 ;et me know your idea and e4periences. e4periences. All !he 'est, Mo/taba omments

S M umar Process !esign Consultant

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Mo/taba: eep *+( as %#. As long as air is available. it will remain shut to maintain upstream pressure. 7& I you want, add air bottles only to selected 'D(s •

Saeid "a#imi Mofrad Senior Specialt$ Process Engineer at Fluor Top Contributor Co ntributor . In line with your de$nition 7which I do agree with, all control valves which are source o pressure and energy should be ail closed. 'ut it may not work or all services, or e4ample i li-uid level control valve is %#, it may cause gas break through and pushing downstream system in ma4imum energy state. In other hand, using %+ ;+( may cause li-uid over$lling o upstream vessel and risk o overpressure, li-uid release to fare, etc. so we should search or a solution with minimum drawbacks. In this particular case, though %# seems to be #k with respect to system protection, it causes ma/or process upset because o depressuri6ing not8isolated 7running process system. Most probably you won=t be able to operate the plant anymore, unless route route to fare is manually closed or unit shutdown initiated. It causes loss o hydrocarbon and unnecessary unnecessar y faring as well. I believe, %+ is the right selection because: ? *lant can continue normal operation as valve is re-uired when system pressure goes beyond the capacity o process control valve 7*+( to fare is normally closed ? In case o plant overall air or power ailure@ all *+(s will not open at the same time. ? *rocess system is anyway protected by *) 7most probably and relie valve 7or sure &. I fare is able to handle the fow rate rom all 'D(s and *+(s at the same time, *+( can be retained as %#. Apart rom instrument air ailure, power ailure can also cause simultaneous wide opening o all %# *+(s and 'D(s. )ee below comment to see how to prevent these scenarios. . It is not completely true because: ? In case o local ailure o instrument air supply to *+( and 'D( o single e-uipment, they will open at the same time when e-uipment inlet and outlet )D(s are open. *+( and 'D( will depressuri6e depressuri6e the system with much higher fow rate than designed depressuring rate. ? Bhen plantCunit wide instrument air ailure takes place, most probably )D(s are closed when all %# *+(s and 'D(s o that section o plantCunit open. +onsidering number o *+( and 'D( in each plantCunit, the resultant fow rate can be considerably high. Bithout calculation it is hard to make any conclusion because I remember an oil separation plant where simultaneous simultaneous opening o all 'D(s did not overload the fare. 1. Adding air volume bottle will help to keep 'D( unctional but not or *+( as it consumes





too. 5. Eormally closure o )D( is considered as permissive or blowdown not *+( position. *+( may temporally open when pressuri6ed shutdown is initiated because o pressure fuctuation 7sudden closure o )D(s and all but it will close when pressure is reduced below its set point. +omplete depressuri6ation is not e4pected. Eormally *+( set pressure is higher than system pressure at pressuri6ed shutdown condition. •

Saeid "a#imi Mofrad Senior Specialt$ Process Engineer at Fluor Top Contributor Co ntributor  )imultaneous opening o ail open *+(s and all 'D(s can happen i instrument air ails. !o ! o prevent this scenario scenario ollowings ollowings can be done: done: ? *rovide separate air volume bottle or each 'D( ? Make fare *+(s ail closed otherwise check the fare capacity or simultaneous opening o all %# *+(s Eote: !otal IA ailure can be considered a remote 7less probable scenario due to ollowing: ? Eumber o IA compressors ? IA buFer vessel ? Instrument Air compressor is supplied rom emergency diesel generator which will cater the IA re-uirement during )D ? IA )ystem is able to cut oF non8essential users users during low pressure pressure in IA header

& )imultaneous opening o ail open *+(s and all normally energi6ed energi6ed 7E 'D(s can happen i electrical power ails@ to prevent this scenario ollowings can be done: ? E 'D( can be supplied rom two independent G*) systems 7even location o G*) should be diFerent so that ailure o both supplies at the same time is not envisaged. ? Eormally de8energi6ed 7ED 'D(s can be added parallel to E one. 'oth will be supplied rom common G*) and utili6ed or $re and adiabatic depressuri6ation purposes. In case o total power ailure 7main power supply and G*) due to any reason E 'D(s will open only. %lare system capacity shall be checked or all E 'D(s. ? ED valve can be used with two independent G*) supply systems 7even location o G*) should be diFerent so that ailure o both supplies at the same time is not envisaged. Eote: with ED valves, local $re 7around valve can cause power 7cable ailure or instrument 7tube air which $nally ends up with non8operational 'D(. It is where 'D( is more re-uired to depressuri6e the system. !hereore ade-uate saety measures shall be taken or reliable unctioning o 'D( during $re 7which may be di3cult to achieve.





Mo/taba

Mojtaba Habibi Process Engineer at Wood Group Top Contributor Co ntributor Dear )aeid, About this point o you "Adding air volume bottle will help to keep 'D( unctional but not or *+( as it consumes air continuously continuously or operation": . Do you mean 'D( does not consume the instrument air continuously0 &. !he provision o air volume bottles or 'D(, make some delay or totally resolve instrument air ailure concern or the 'D(0 . Bhat is the si6ing criteria 7or speci$cation or volume air bottles to be provided or 'D(0 •

S M umar Process !esign Consultant )aeid: )econd point in your &nd comment. ")imultaneous opening o ail open *+(s and all normally energi6ed energi6ed 7E 'D(s can happen i electrical power ails@" I am unable to understand why electrical ailure will pen %# *+(s. +ould you elaborate. I thought and as you mention reserves in Inst Air Heceiver  mergency Air +ompressor should keep the *+( shut. •

%earn S$an %ee Process !esign Engineer Mo/taba: Hegarding your third comment to )aeid, please $nd below relevant section rom D*: . !he design re-uirements o air volume bottles 7a.k.a. secure instrument air buFer vessel or 'D( is mentioned in section .&.& o D* &.12.9.9 &.12.9.98Jen 8Jen %eb &9, which states that "Gpon instrument air supply ailure, the )ecure Instrument Air 7)IA system )A;; K*)L maintain su3cient pressure pressure in the buFer vessel to allow or at least three valve strokes, based on an initial pressure in the )IA vessel e-ual to the minimum Instrument Air eader pressure." %urther, it is mentioned that "!he )IA vessel shall be supplied rom the Instrument Air eader via a $lter. !wo non8return valves shall be installed to prevent back fow rom the )IA system in the case o loss o Instrument Air eader pressure. !he non8return valves shall be o the ball or poppet type and mounted in the vertical line with fow up and marked as +lass  on the *rocess ngineering %low )cheme. 'leeding devices such as regulators shall not be used downstream o the non8return valves. A low pressure alarm shall be provided to indicate low air pressure in the )IA, set at the minimum pressure that





•

Saeid "a#imi Mofrad Senior Specialt$ Process Engineer at Fluor Top Contributor Co ntributor Mo/taba, 'D( needs instrument air /ust to open and close on demand@ it does not consume air continuously. *roviding air volume bottle si6ed or  stokes o 'D(with conditions C arrangement e4plained by ;ee with prevent 'D( rom opening in IA ailure. >umar, In case o power ailure, signal rom controller to valve 7I to * convertor approaches Nero. Direct acting IC* convertor which is receiving 6ero input signal will send opening command to control valve. *roviding )IA will help in case o IA ailure not power 7G*) ailure. •

S M umar Process !esign Consultant G*) ailure0 !hanks. >umar •

V&M'%ESH 'G(&H)T"& Senior Process Engineer at Engineers &ndia %imited Dear >umar, )aeid, Mo/taba and lee, %irst o all thanks or such good and elaborated discussion. Eow i want to know about the conclusion o discussion0 whether we have to kept *+( as %# or %+0 Bhat is standard practice worldwide or such kind o *+(s0 •

S M umar Process !esign Consultant 'ased on my e4perience: %# •

Mojtaba Habibi Process Engineer at Wood Group Top Contributor Co ntributor Dear (imalesh, !he conclusion conclusion depends on your engineering engineering /udgments /udgments based on on above mentioned points.





Saeid "a#imi Mofrad Senior Specialt$ Process Engineer at Fluor Top Contributor Co ntributor I have used both %+ and %# or control valve to fare. %+ is my $rst option which needs least e4tra provisions. %# i it is mandatory by client can be made sae as e4plained in earlier posts. !o answer >umar=s !o >umar=s -uestion -uestion about possibility possibility o ailure ailure o G*), G*), it happened during start start up o one o our plant when power ailure was ollowed by G*) ailure 7G*) did not take up. !hat is why why in my company total total power 7main and G*) supply supply ailure ailure is the basis basis or designing control valve to fare and 'D(.
'$rat *urgula+ov Process Commissioning Engineer )aeid, is there written recommendation on fare *+( ail8sae positions in A*I guidelines or other standards0 I have situation where the fare *+(s were added based on AN#* recommendations, recommend ations, but ail8sae positions were not speci$ed in the same report and have been chosen later by engineer alone. •

,ur$ Papitsic# ,ur$ Papitsic# Senior 'utomation and Control Engineer . In this case use only %# %+(C'D( or fare subsystem. . ;ack o IA is a "classic" plant )D. . !o prevent accidental activation and )D 7instrument ailure or loss o power or "critical" instruments instruments use "voting", based on the data rom & or  transmitters. It is normal practice. Bell, you can use )I;&C. 1. 4tra IA line 8 O!HA problemsCcost with controlling her condition, logic etc. •

Saeid "a#imi Mofrad Senior Specialt$ Process Engineer at Fluor Top Contributor Co ntributor Ayrat, A*I82& does not have any speci$c guideline in this regard and as I mentioned beore both %# and %+ is acceptable in this application i the system is suitably designed or that. •







)aeid, As you know we might have staged 7se-uenced fare and blow8down schedule schedule or avoiding oversi6ing a fare network especially in brown$eld pro/ects. As the status o the 'D(s are %.# 7both in terms o instrument air supply and solenoid valves located on the I.A supply line, i the G*) ailure is a credible scenario to consider, then the staging is not possible in any plant and we always have to si6e the fare or the ull release load rom all the 'D(s. I we consider the solenoid valves as ED, then in the case o $re, the cables may be damaged and since the solenoid valves have been selected as ED in this case, they will not cut out the I.A supply to the actuator and so the 'D(s will not open. !hen whatPs your recommendation to cover the G*) ailure case0 Is two completely independent independent G*) enough to cross8out the loss o &1 ( ailure rom the faring scenarios0

Fail Safe Position of Flare Connected Control Valves

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