Optimale feil-posisjoner for reguleringsventiler i et kjølemedium-system
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- Institutt for elkraftteknikk 
SummaryAs part of a modification project for one of the fields in the North Sea, the optimal failure position of three new globe valves has been studied. In a gas compression train, the gas needs to be cooled to a set temperature, and the globe valves control the amount of cooling medium required through the heat exchangers to reach this set temperature. A wrong choice of failure position could potentially cause expensive and important process equipment to get damaged.The four failure positions that have been studied are fail open, fail close, fail close with travel stop and fail in position. Results were obtained using the simulation tools HYSYS and PIPENET, and calculations in Excel.Fail close is not an optimal failure position, but it has been considered the most optimal as of today. It is very likely that this failure position will lead to a process shut down, which will give reduced production. The reason why it is still considered the most optimal solution is that no equipment will get damaged, and the actuator solution is well proven and trustable.The results show that fail in position is the failure position which is best from a process point of view. There are however several questions connected to the actuator solution of fail in position. It is a relatively new failure position solution, which is why AKSO have not been able to get any feedback on how it works. It is very likely that fail in position will be used in similar installations in the future, when the actuator solution has been verified and improved if necessary.It has been found that fail open is the least optimal failure solution. As the heat exchanger cooling medium demand will increase significantly when the valves are left fully open, there will be deficit of cooling medium to the other cooling medium consumers. A possible consequence of this is a process shutdown to avoid overheating of important process equipment. Another possible consequence of fail open as the failure position is hydrate formation, which in worst case can lead to damages to the compressors.