Tuesday, September 23, 2008
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Locked / Car Sealed Close close (LC / CSC) pressure relief valve (PSV) isolation valve ???
For those who follow the earlier discussion "Isolation Valve Position when Spare PSV present", you may have aware of the reasoning why SPARE PSV is recommended to be in CLOSE position. Besides, API RP 520 part 2, section 6.3.3 recommendation is inline with above discussion. This post is a follow-up discussion on the same subject, however it extends to the requirement of Locking or Car-Sealed Closed an inlet isolation valve for a SPARE PSV.
One point shall be clear up-front is that the main subject here is the locked or car-sealed close a SPARE PSV. For DUTY PSVs, all inlet and outlet isolation valves SHALL be in OPEN position to ensure clear relief flow path.Why Locked or Car-Sealed Close a SPARE PSV inlet isolation valve ?
A duty PSV inlet isolation valve shall be locked or car-sealed to avoid inadvertently closure of inlet isolation valve. However, the locking or car-sealing of spare PSV inlet isolation valve which should be in close position is arguable. Some engineers argued that the inlet should not be locked or car-sealed. It is quicker to unlocked a LC or unsealed a CSC isolation valve when it is required or on demand during emergency.
Above argument could be right. However, inadvertent opening followed by overpressure relieving scenario could lead to potential catastrophe. As explained in "Isolation Valve Position when Spare PSV present", PSV chattering is likely to occur when spare PSV is relieving simultaneously with duty PSV. PSV chattering not only lead to seat damage, severe vibration lead to mechanical damage (which is difficult to be studied) could occur due to PSV chattering. The consequence of mechanical damage may lead to catastrophe.
"A recently gas leak occurred on an offshore gas Central Processing Facility as a consequence of the failure of the bellows in balanced bellows Pressure Safety Valves (PSVs). All three PSVs in service on the inlet Slug Catcher from a remote field were simultaneously affected by the failure of their bellows due to chattering during relief of pressure from the vessel. Each of the three ¾ inch PSV bonnet vents exhausted directly to atmosphere and the incident resulted in approximately 235kg of gas being released in the process area, an Emergency Shutdown (ESD), pressure blowdown and a General Platform Alarm."
Above is one incident reported in one the offshore platform.
PSV Chattering lead to bellow damage and gas leak in to atmosphere via bonnet vent. This gas leak would potential form gas cloud and lead to vapor cloud explosion. So do not under-estimate PSV chattering.
Besides the catastrophe, disc and seat damage during chattering cause improper closure and permanent continuous fluid leaks. Plant is forced to shutdown and depressurise. Inventory loss, cost for PSV repair or replacement of PSV and production loss during shutdown period would be much much higher than a simple locking or car-sealing device on a spare PSV inlet isolation valve.
Inadvertent opening followed by overpressure relieving scenario... Is this double jeopardy ?
Operator inadvertently open the spare PSV inlet isolation valve and overpressure of the system lead to relieving are two non-related event. In simple term, open spare PSV inlet isolation valve event will NOT cause the happening of overpressure of the system event. They are non-related. However, they are possibly occur in sequence. Morning shift operator may open the spare PSV inlet isolation valve and leave it open. Nothing happen during morning shift as no overpressure event occur. During night shift, controller failure of outlet pressure control loop lead to blocked outlet event (control valve close position) and overpressure occur. Both duty and spare PSV open and chattering occur. Thus, both events are non-related but in sequence.