Optimization of an Elevated Staged Flare for a Large Gas Plant

In recent years, the newly designed and constructed gas plants have become increasingly larger in size, with a consequent huge increase in the required flaring capacity. At the same time there is the necessity to have smokeless flaring, also at very low flow rates when compared with the total design flow rate capacity. This in conjunction with increasingly more stringent pollution regulations, present new design problems. This article will describe how starting from a process requirements to flare more than 3.5 MMkg/h, the solution of an elevated staged flare arrangement was born, and how all process and design problems with standard flare systems were faced and resolved.

Hamworthy Combustion Engineering Srl lllllllllllll Societa a Socio Unico ViaA. Gramsci, 11- 20035 Lissone (MI) Italy Tel: [phone] Fax: [phone] Email: [e-mail] Website: www.hamworthy-combustion.com O P T IM IZ A T IO N F L A R E F O R A HAM W ORTHY C O M B U S T I O N O F A N L A R G E E L E V A T E D G A S S T A G E D P L A N T W h e n an e le v a te d s ta g e d fla re s y s te m a rra n g e m e n t ca n be th e s o lu tio n w ith la rg e fla rin g c a p a c itie s a n d s trin g e n t a m b ie n t re g u la tio n s □ Introduction In recent years, the new ly designed and constructe d gas plants have beco m e increasingly larger in size, w ith a con se q u e n t huge increase in the required flaring capacity. A t the sam e tim e there is the nece ssity to have sm o kele ss flaring, also at very low flow rates w h en co m pared w ith the total design flo w rate capacity. This in conjunction w ith increa singly m ore stringent pollution regulations, present new design problem s. T his article will describ e how starting from a process requirem ents to flare m ore than 3.5 M M kg/h, the solution of an elevated staged fla re arra n g e m e n t w a s born, and how all process and design problem s w ith standard flare system s w e re faced and resolved. □ S tandard solutions A ll the typical and standard design solutions w e re review ed and investigated by H am w orthy C o m bu stion w ith the follo w in g conclusions: a) T he m ost traditional config ura tion fo r a flare system , i.e. a single elevated pipe flare, due to the ve ry large flaring capa city w o uld require a large dia m e te r riser and a tip s iz e 1 w hich involve m any technical conce rns both at low and high flow rate. This arrangem ent, com bined w ith the sm okele ss requirem ents fo r very low flo w rates com pared w ith the total capacity, leads to a design of a flare th a t is not fu lly com pliant w ith the expe cted process conditions. a.1) T o prevent sm oke form ation, an e ne rgy is nece ssary to create tu rb u le n ce and im prove m ixing o f the entrained air w ith in the flare g a s2. U sing D u e to the large d ia m e te r required, a steam assisted flare tip gen era tes fuel rich zone s w h e re sm oke Incorporating: P E A B O D YE N G IN EER IN G AIRO IL-FLAREG AS CHENTRONICS Registered in Italy:: Trib. di Monza C.S. n° 47020 Capitale Sociale €28.080- C.C.I.A.A. 731.279 Mecc. N° MI129086 - C.F. 00716120159 - P.IVA 02145420960 Brazil • Canada • China • France • Germany • Holland • India• Italy • Japan • South Korea • Mexico • Poland • Spain • UK• USA Mod. S/3 Format rev. 5 2 Illllllllllll HAMWORTHY COMBUSTI ON occurs because o f bad penetration o f steam inside the core of the flaring gas, w ith a resulting reduction in the effective sm o kele ss process. a.2) Air, and ste a m /a ir assisted flare tips w e re analyzed but rejected b eca use of huge co m bustion air requirem ent, w ith con se q u e n t m echanical design re q uirem ent for installation o f slots or internal pipes that w o uld have lead to a flare tip size that is not referenced in operation, and w ith a w e ig h t that m ay be da n g e ro u s if supported by a single stack. T h e unusual pipe fla re tip diam eter, com bin ed w ith the w id e range o f flaring rates, present problem s w ith the fla m e stability, bu rn b a ck (internal burning w ith in the tip shell, resulting from air inside the fla re tip at standard purge or low flaring rates), and fla sh b a ck (th a t occurs in a fla m m a b le m ixture of air and gas w h e n the local ve lo city o f the com bustible m ixture becom es less than the fla m e velocity, causing the fla m e to travel b ack tow a rd s the stack). In the stan d b y case o f only purge gas flaring, the purge gas consu m ptio n required to sustain a uniform fla m e and to avoid b urnba ck and fla sh b a ck is ve ry high, w h ich increase d ram atically the carbon dioxide em ission. T h e calculated flare height o f single pipe flare w o uld be very high, due to high em issivity value. P a ram eters influencing this fa c to r include the com position of the gas, the exit ve lo city of the gas and the g eo m etry o f the burner. W ith a single fla re the exit velocity along the range of operatio n o f the gas is a verage ly low, hence the em issivity is high3. C alculation indicated a fla re o f 2 00 m high. b) T he ene rg y to prom ote uniform air distribution th ro u g h o u t the fla m e m ay also be present in the gas in the form o f p ressure2, h o w e ve r this gas pressure, fo r a single flare, w o uld be availab le at high flo w rates only, not at sm o kele ss rates. M oreover, if a sonic fla re tip is designed, it cann ot be equipped w ith nece ssary ancillaries like steam assem blies fo r sm okele ss flaring at low flow s due to vib ratio ns occurring as a result of a too high gas exit velocity at m axim um design flow. c) O pen m ulti burners ground fla re s offe r m any advantages, particularly w h e re strict enviro nm e nta l issues have to be considered. T h e y consist o f a large n u m b e r o f sm all staged burners installed at ground level in a very large flaring area surro unded by protective fencing. Staging allow s operation of each group o f burners (stage) to be at set pressures w h e re sm oking will not occur. T he split o f flo w rate in m any burners ena bles high sm okele ss capa cities along w ith reduced visual im pact. M u lti-burner flare system s utilize the available pressure ene rg y o f the gas to entrain additional air, w h ich im proves co m bustion e fficien cy as fla re gas is better m ixed w ith the air. A sm okele ss fla m e is obtained only through the high exit gas ve lo citie s4, and fo r low flaring rates this is achieved by providing a large n u m ber o f stages th a t require higher pressure at the inlet m anifold to m anage the operatio n of the staging valves. This solution w a s not fe a sib le due to low a vailab le pressure and the presence o f acid gases also flared inside the sam e availab le sterile area, preventing accep tab le e m issions in respect o f site regulations. Even though the exit ve lo city of gases is ave ra g e ly high, the source elevation is near grade and this could result in the non m atching o f pollution lim its in presence o f sour products. Mod. S/3 Format rev. 5 3 Illllllllllll HAMWORTHY COMBUSTI ON d) E nclosed ch a m b e r ground fla re ena bles flaring w ith o u t the fla m e being visib le from outside the cham ber. T he capa city o f this flare is lim ited and cann ot m atch w ith the e xtre m ely large flare capa city required. e) Burn pits w e re not considered because o f stringent pollution limits. □ S taged elevated flare In o rd e r to solve all the technical difficulties and problem s listed above, safety, process, operability, environm ental, m echanical, and fabrica tio n investigation studies have been perform ed w ith m ost m odern softw are and technologies. T h e best fla re config ura tion is considered to be an elevated staged flare system , the first stage being open directly to a tm o sphere and the oth e r stages fitted w ith staging valves and dedicated safety e m e rg e n cy device s (ru p tu re discs-pins). The first stage, fitted w ith a d yn a m ic seal "G a se e l" type, is dedicated to ensu re a sm o kele ss fla m e so it is equipped w ith the steam assisted flare tip, w h ile the o th e r stages consist of m ulti-branch fla re tips. S taged flares are fed from a m anifold, as show n in FIG. 1, that distribu tes the flow, by m ean of auto m a tic valves, to individual flare stacks, each one equipped w ith a flare tip. T he staging valves are located at the base of the fla re stacks, and are operated by pressure tran sm itters located on the m ain fla re header. The n um ber o f flares in operation is proportional to the relief gas flow. The solution to segrega te the total stream into d ifferent flares allow s a reduction in the d ia m e te r o f the each fla re tip. T he reduced d ia m e te r o f fla re tips ensu re the p roper sm okele ss operation since the flare tip of the first stage can be designed fo r the m axim um contin uous ope rational flaring rate, and can be fitted w ith steam facilities to e nsu re sm o kele ss flam e. See FIG. 2 fo r sonic and su b -so n ic flares in sim ultan eous operation. Steam flo w control is perform ed using the H a m w orthy " F larscan" system , a w ell established proprietary system consisting o f no. 4 therm o -e le m e n ts installed on rear side of black bodies placed on top of flare. T h e sensors w o rk in parallel in order to provide an a verage signal o f th e ir m easures. T he resulting signal is then corrected w ith the signal o f a fu rth e r co m pensa ting therm oe lem ent, installed at low er elevation w h e re it is not influenced by the flam e. T he final signal is converted into a flo w range that can directly control the co rre sp o n d e n t steam control valve. This system , show n in FIG. 3.1, has the adva n ta g e of not being affected by wind, by adverse w e a th e r cond itions like rain, fog, and reflective sunlight. T he staging valves located below the flare stacks are o n /o ff butterfly type. T h e ir set pressures are defined based on the h ydraulic calculatio ns o f the system and have to g u a ran tee a su fficien t gas velo city to avoid fo rm a tio n of possible explosive m ixture in the staged stack risers. T h e opening set pressure of staging valves is increased from the 2nd stage to the follow ing stages in o rd e r to ensu re that the stages open in the corre ct sequence. C losing set pressures of the staging valves are decrease d from last stage to 2 nd stage fo r the sam e Mod. S/3 Format rev. 5 4 Illllllllllll HAMWORTHY COMBUSTI ON reason. W ith the proposed configuration, the last stage has been d esigned to be operated only in case o f ve ry large flaring rates resulting from an overall plant em ergency. For safety and m odularization reasons, each stage, and relevant flare tip has been designed to co ve r the extra rate caused by the abnorm al situation o f one staging valve failure. Flare system safety co nsiderations require that staging valves w ill be provided w ith valve by-passes, each o f them containing a fa il­ safe device like rupture d isc or rupture pin. Flare staging curves and set pressures to corre ctly ope rate each stage are show n in the FIG. 4. T h e given curves are typical and m ay be adjusted based upon specific need like operating cases of the flare system , tole ra n ce o f the set point o f selected rupture discs etc.. T h e size o f the tip on the first stage has been designed to lim it the gas exit velo city to 0.2 M ach at sm okele ss flo w rate, and to 0.5 M ach at m axim um flo w in ord e r to avoid vibrations. Exit sections o f the oth e r stages have been designed fo r exit velo city abo ve 0.7 M ach. W ith a staged flare the exit velocity o f the gas is a verage ly hig h e r than a single flare, and the tip stages can be designed w ith a sonic approach. T he sonic flare tip, th a t is show n in FIG. 3.2, has a design adva n ta g e o f low flam e e m issivity due to m ore efficient com bustion o f fla re gas and o f sh o rte r flam e. A s a co n se q u e n ce the heat radiation at grade results lower, and flare height has resulted in being 130 m. In addition, the dispersion at grade has been reduced due to the gas exit ve lo cty being sustained across the com ple te range o f operation. W hen the flare gas is discharged at high pressure, so n ic flare tips also allow the d e sig n e r to reduce header, sub-headers, risers and valves size, and hence cost. T h e m ain flare sub-h e a d e rs and fla re risers have been sized fo r a m axim um velocity of 0.5 M ach to lim it the pressure drop across the valves and to atte nuate the noise o f the w h ole system . T he lim itation o f the d ia m e te r o f fla re tips avoids flam e fro n t instability, and m inim izes the potential fo r bu rn b a ck and flashback. T h e capa city o f each stage is larger than th a t of each previous stage. M inim um gas velocities in each stage have been designed in ord e r to avoid fo rm ation o f explosive m ixtures inside the risers. T h e staging valves are pne u m a tica lly actuated and each a ctu a to r is designed to allow the fa st opening o f the valve T his cond ition is essential to avoid pressure build-up in m ain header, and thus in the plant. T he valves are airfa ilu re open, i.e. in case the air supp ly fails the valves open, it being prohibited to block the sub-headers, since this could cause an explosion due to inability to d ischarge the em erg e n cy flow. W hen one or m ore rupture discs break there is a sudden u n b alan cing o f the force s causing a force o f inertia induced by the d yn a m ic load o f the d ischarged gas. T his force m oves from the breaking area o f the rupture disc up to the top o f the flare, m oving at sonic speed and inducing a fu rth e r d yn a m ic load on the inlet elbow, due to the chan ge of direction o f the flow. T he sam e p he nom enon also app ears in low er intensity w h e n the valves open, d e p e n d e n t on the tim e needed fo r full opening. Mod. S/3 Format rev. 5 5 Illllllllllll HAMWORTHY COMBUSTI ON T his calculation has been perform ed by a team o f m echanical e n g inee rs d esigning the m anifold to m axim ise the linearity o f the flo w pattern and optim ise the system fluid-dynam ics. In a case o f valve opening failure the pressure increase cause s the bursting d isk to break, and the resulting shock w a ve d yn a m ic load on inlet o f the flare is higher than the above case. A d dition al co u n te rm e a su re s are taken by installation o f a d a m pen ing system dow n stream the rupture disk. This has the aim: a) to reduce the w idth o f the unb alan ced force s acting on s u b ­ hea d e r w h e n d isk breaks; b) to increase the tim e o f the tran sition period ju s t afte r disk bursting; c) to contain the horizontal shift of the piping. T he e ne rgy of the d yn a m ic loads is constant, but its distribu tion varies and its peaks are lower. T he set pressure o f the bursting disk is higher than opening set pressure o f staging valves, so to avoid the bursting before the valve open; ho w e ve r it cann ot be too high o therw ise the pressure in the m ain hea d e r can rise very q uickly w ith con se q u e n t d a n g e r o f overpressure. D u e to the large dim en sion of the valves they can take som e secon ds to fu lly open and close, so that the tim ing o f th e ir opening and closing m ay be anticipated in respect to th e ir theoretical actuation pressure. □ C o nclusion T he analysis to identify the best config ura tion fo r a flare a d e quate to m anage the largest flaring rate of the w orld, com ing from a single hea d e r confirm s th a t the solution of a single large fla re is n e ith e r practical nor con ve n ie n t as it presents technical d ifficulties to secure the stability o f the flam e, the safety o f the system and to g u a ran tee the sm o kele ss requirem ents. T he proposed alternative consisting o f a elevated staged flare has show n the a d va nta ges of a) lim iting size o f the flare tips w hich do not introduce fla m e stability issues; b) distributing properly the steam to the first stage, w h ich will govern oth e r e m e rg e n cy scenarios characterized by low er flo w rate; c) g u a ran tee ing a longer life cycle fo r the flare tips being norm ally closed and needing no contin uous purging. This data, in conjunction w ith the fact that the design flaring rates are related to e m ergency scenarios th a t are ve ry unlikely to o ccu r have allow ed to the d esigners to set the height o f the flares at a reasonable, reliable, and referenced value. Captions FIG. 1 P ressure distribution along staging system located at stack base as result o f sim ulation softw are; by-pass lines are fed by m anifold in depen dently from m ain sub-headers. FIG. 2 Photo o f so n ic and su b -so n ic fla re s in operatio n sim ultan eous operation. FIG. 3 Photo o f steam assisted (FU S ) fla re tip equ ippe d w ith H a m w orthy " F larsca n" system , and m ulti-branch (FSM M KIII) fla re tip. FIG. 4 D iagram o f staging system show ing the relationship betw een flo w rate and pressure. Lite ra tu re cited 1API RP521 "Guidefor Pressure-Relievingand Depressuring Systems" 2API RP537"Flare Detailsfor General Refineryand Petrochemical Service" 3DEPSHELL80.45.10.10"Pressure relief, emergencydepressuring, flareandvent systems". 4"AnAlbumof FluidMotion" - Van Dykepictures 169/170/171 Mod. S/3 Format rev. 5