The Weekly Reflektion 18/2022

Some of the routines that are carried out on a process facility may seem banal and a waste of time. They are often established as the result of a serious incident or the potential for a serious incident. Sometimes the reason is lost in time and the routine then becomes the target for the latest efficiency drive.

Interlock system for a PSV configuration of one PSV on line and one offline

Are some of your operational routines considered a waste of time?

Critical valves are often controlled by interlocks. The interlock system is designed such that the valve(s) have to be in the correct position in order for a key to be extracted from the interlock. This key can then be hung in a prominent place thereby verifying the valve(s) position(s). In some systems there may be multiple valves and multiple keys. The interlock system in the above figure is part of the overpressureprotection for a vessel. The system has two pressure safety valves (PSVs) with one PSV on line at all times. This PSV has sufficient capacity to protect the vessel from overpressure. The other PSV is offline, however can be put in service if the online PSV needs to be inspected and testing. The interlock system ensures that one of the upstream valves (1 or 2) is open and the other is closed, and that both the downstream valves (3 and 4) are open. In this case PSV A is online and key A is in the control room. To change over the valves key A is inserted to allow opening of valve 2, key B is released. Key B is used to close valve 1 and this releases key C. Key C is then placed in the control room.

On one of the platforms I worked on the production technicians had a monthly routine to check the interlock system. This entailed noting the status of the keys in the interlock cabinet and then physically checking that the associated valves were in the correct position. This routine took about 2 hours and some thought the routine was a waste of time since the status of the valves rarely changed. Another justification for eliminating the routine was that the interlock system was reliable and difficult to defeat or undermine. The interlock system was manufactured in high quality stainless steel and there were few problems with the operation. The rotuine was however retained.

On another platform I worked on a cost reduction campaign had rationalized away, the monthly interlock routine. The interlock system on the PSVs on one of the separators, a similar system to the above figure, was made of carbon steel and the salty atmosphere led to corrosion. Some keys were rusted in place, some were difficult to operate and the integrity of the system was compromised. The interlock system was replaced. Unfortunately the interlock system was installed incorrectly and the new configuration allowed the key to be retrieved with both isolation valves (1 and 2) closed. The valve positions were not checked at this time. The overpressure protection system was therefore not availableuntil the situation was discovered a year later when the PSVswere due to be tested. 

Does everyone understand the reasons for the operational routines that are carried out? Are these reasons mentioned during the discussions on how to reduce operating costs?

Reflekt AS