Com­pres­sors’ Suc­tion Knock­out Drum

S Raghava Chari, Con­sul­tant

Chemical Industry Digest - - What’s In? - S. Raghava Chari

Com­pres­sors, depend­ing on the type are used by al­most all in­dus­tries. In­ad­e­quate suc­tion Knock­out Drum (KOD) is a com­mon cause of com­pres­sor fail­ure. Ways to pre­vent com­pres­sor wrecks, as­so­ci­ated pro­duc­tion loss, failed de­liv­er­ies, etc are dis­cussed in this ar­ti­cle.

Com­pres­sors - pis­ton type, cen­trifu­gal and screw - serve al­most all in­dus­tries. In­ad­e­quate suc­tion Knock­out Drum (KOD) is a com­mon cause of com­pres­sor fail­ure. This ar­ti­cle dis­cusses ways to pre­vent com­pres­sor wrecks, as­so­ci­ated pro­duc­tion loss, failed de­liv­er­ies and cus­tomer wrath. In­tro­duc­tion

Prac­ti­cally, any gas com­pres­sor in petroleum re­finer­ies, chem­i­cal process plants, power plants and for that mat­ter, any plant han­dled gases, is wet with aque­ous or or­ganic liq­uids. Even traces of th­ese es­cap­ing the KOD and silently en­ter­ing can dam­age the com­pres­sor. From the pit­ting cor­ro­sion, salt de­posits to di­luted lu­bri­cants, the liq­uid ingress tell­tale signs point out the cause, viz. failed KOD. Only ad­dress­ing the symp­toms does not solve the prob­lems; they per­sist. Hence, find­ing the root cause(s) and re­mov­ing it (them) is the only way to get rid of de­bil­i­tat­ing com­pres­sors fail­ures. Of­ten, prob­lems stem from KODs, es­pe­cially their Mist Elim­i­na­tor ME (fig 1) fail­ing to trap the gas en­trained liq­uids and let­ting th­ese into the com­pres­sors – see case study 1 at the end. Usual prob­lems are: im­prop­erly spec­i­fied ME, KOD over­load­ing, un­even ve­loc­ity pro­files, in­cor­rect in­stal­la­tions, high liq­uid vis­cos­ity, waxy de­posits, liq­uid slugs, foam- ing, and sev­eral oth­ers. Of­ten, KODs come with no mist elim­i­na­tor. Even in feed gases of no free liq­uid par­ti­cles, fine mist droplets of­ten ex­ist. Th­ese co­a­lesce into large drops over the pipe wall in­side and even­tu­ally reach the com­pres­sor in­side. Hence, pro­tect­ing the com­pres­sors with in­let ME is nec­es­sary in case of highly dry gases. For­tu­nately, present day eco­nom­i­cal high-ca­pac­ity and high-ef­fi­ciency ME tech­nolo­gies pay off, as even averted one shut­down is worth nu­mer­ous times the KOD in­vest­ment. Ob­vi­ously, as with any process equip­ment, com­pres­sor KODs too, must be prop­erly de­signed and sized for ad­e­quate flow mar­gins. In ad­di­tion, its mist elim­i­na­tor (ME) el­e­ments shall be se­lected for op­ti­mum mois­ture sep­a­ra­tion and re­sult­ing long com­pres­sors on stream hours. Re­mem­ber, in­di­vid­u­ally engi­neer­ing multi-stage com­pres­sors’ each stage in KOD is es­sen­tial as each stage in­let con­di­tions dif­fer con­sid­er­ably. In ad­di­tion, in case of op­er­a­tive con­di­tions, e.g. through­puts in­crease or de­crease, gas

com­po­si­tion and mois­ture con­tent changes and op­er­a­tions and con­trol pro­ce­dure re­vi­sions, KOD ad­e­quacy needs check­ing and up­grad­ing as needed. Pe­ri­od­i­cally check­ing the mist elim­i­na­tor el­e­ments, es­pe­cially af­ter any ma­jor process up­sets and mak­ing sure they are in­tact and de­posit free is nec­es­sary for their con­tin­ued good per­for­mance.

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