In fact, the spectacle blind is applied for positive isolation purposes; then depending on the conditions it can be put before or after a valve. The main point is that it should be located where would be in line with maintenance procedures and local line/equipment isolation i.e. its application would have minimum impact on the plant operation and leading to minimum loss of inventory...
so ... To isolate the downstream pressure:
Shut the valve.
Break flange on downstream flange of valve -> CANNOT use a wafer valve for this! ... Note: Breaking this flange is a one-valve isolation - but you have to break the flange to establish a two-point isolation.
Insert spectacle flange, rotate the spectacle, make up the spectacle flange to clamp the spectacle against pressure loss and leakage past the valve.
Write work permit to release work.
Neither upstream or downstream of the valve- but on the side with the least isolations required to swing the spectacle blind- normally the vessel side.
i.e. if the flow is going into the vessel the blind goes downstream of the valve. If the flow is going out of the vessel the blind goes upstream of the valve. In both cases it's on the vessel side.
Putting the valve closer to the vessel than the blind means you have to isolate everything else in the process piping that feeds into the location to swing the blind.
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
There might be some exceptions; for example: in the case of a maintenance drain valve even though the flow direction is out of the vessel, the blind should go downstream of the valve.
Correct europipe- I'm assuming the spectacle blind is in place to allow positive isolation of the vessel for maintenance or for workers to get inside for inspection.
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
I would agree with you europipe that most of the problems are elsewhere- but in my industry we use valves to isolate the exchangers, pumps etc for maintenance and the spectacle blinds to isolate equipment that requires a human to enter (vessels).
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
Every equipment for which maintenance has been foreseen to be done every now and then, should be capable to be isolated with spectacle blinds either requires a human to enter such as a vessel or without such possibility/requirement e.g. pumps, exchangers...
In the PIDs i have seen/reviewed the spectacle blind is placed on the nozzle of the vessel. if there is a spectacle blind for positive isolation somewhere in the PIDs, then depends on the equipment/section/process that you intend to isolate.
