For the majority of the next month I am going to be tied up with two significant pieces of work. One is a continuing plant modification that I have been working on since I began my time here. The other is updating the locked-open/locked-closed (LO/LC) valve register and P&IDs.
I imagine that for the vast quantity of you that the notion of LO/LC valves is a foreign thing; it certainly was for me until I worked on an operational site. The concept literally means that during normal operation certain valves will be locked open or closed using a physical device preventing them from being put in the wrong position.
The premise for leaving valves locked in position is down to safety and environmental considerations. Take for instance the case of a relief system on a distillation column. Imagine that the unit is operating at 30 barg and a process upset realises a vapour pressure at the top of the column of 34 barg. A PSV (pressure safety valve) then lifts and relieves the pressure into the flare system. To enable that this safety critical device can function properly, any isolation valves upstream of the valve (between the column and PSV) and downstream of the valve (between the PSV and flare header) need to be open to relieve the pressure; if they are closed then the system will continue to pressure with potentially devastating consequences.
To avoids such incidents, especially where human error may play a part (i.e. if after replacing a valve an operator forgets to re-open the valve) the valve is physically locked open. To compliment this a register is kept in the control room explaining the position the valves are supposed to be in, and the positions they currently hold.
An example of a locked-closed valve may be on a vessel containing hydrocarbons operating at 2 barg. During a maintenance period the vessel is taken out of commission and isolated. The pressure is first relieved and the hydrocarbons are then drained by opening a valve flowing to a closed-drains system. This valve to the closed-drains will be locked-closed in normal operation to prevent higher pressure hydrocarbons entering an atmospheric system. This is known as a HP/LP (high pressure/low pressure) interface.
This means that in the next few weeks I have to review around 300 P&IDs and ensure that both the diagrams and register reflect the true nature of our asset. Lots to keep me busy.....!
If you read my last post you will remember that I have been away on a 2 week course. My plan on the back of this was to go through some of the things that I learnt; at least, outside of the normal process engineering remit.
Unfortunately, that will take slightly more time than I have at the moment so I will save it for another day. The reason for my lack of time is that I work on an operational asset. This means that the majority of my time is spent dealing with the daily running of the plant, and much more importantly: keeping the oil in the pipeline!
On a site such as the one I work at there are monthly, quarterly, yearly inspections of equipment etc. This will be to assess the integrity of the unit, operability and availability. Before these inspections can happen/or on the back of them, there is much work to do. It just so happens that my return after 2 weeks coincides with a increased workload attributable to such inspections.
So, until next time.......
