(49b) Lessons Learned with Different Transfer Line Valve Designs

Olefins cracking furnace Transfer Line Valves (TLV's) operate in very demanding service handling high temperature, fouling conditions, high tightness requirements, high reliability requirements and they are critical from a safety point of view. These valves are expected to operate trouble-free between turn-a-round cycles and require minimum maintenance during the turn-a-round. Operating problems with TLV on single furnace, such as, a stuck valve during opening/closing could cause, in the worst case, a complete shutdown of the whole plant. Closing problems during radiant coil failure can result in serious safety risk and extensive damage due to fire caused by backflow from the process. Borealis and its affiliate plants have experienced during their long operating life many different problems. For example, poor design consideration can result in stresses on the TLV from the transfer lines when not including proper stress/flexibility calculations of all operation conditions, such as, decoking and S/D effects in combination with all other furnaces that are connected to the same cracked gas header. Furthermore problems result from, various fouling cases which cause incomplete closing of TLV, or operating errors, and not following manufacturer instructions for purging etc. The paper is a summary of experience and lessons learned from the Borealis and its affiliates with various TLV designs. This paper addresses mechanical failures, difficulties with maintenance, issues with piping and structure stress and flexibility calculations cases, piping and support structure design, cases of foul operation and in some cases the difficulties resulting from valve design.