Spectacle blind in PSV inlet line

[rogim]

Can someone help to find a reference regarding spectacle blind in PSV inlet lines?

In general, I don't like to have a spectacle blind in a PSV line from a pressure vessel. For maintenance purposes, the nozzles on the vessel shall be equipped with spectacle blinds. However, for PSV lines I would like to have a removable spool installed as this reduce the risk to have the PSV line blocked during start-up (as the blind can be left in close position.

So, does it exist a reference to a API standard or similar regarding this topic?

 

[TD2K]

I'm not aware of one. Personally, I think a spectacle blind is preferable since it's obvious if it's in the correct position. With a spacer, the question can be asked "is this a spacer or a blind left in?". Yes, you can have holes in handles to indicate what you have but human error can still occur.

The only time I've had to provide spectacle blinds on PSV inlet when we installed a new pair of PSVs routed to a flare header in addition to the original pair routed to atmosphere. The ones routed to flare were added because of an ongoing issue with the PSVs to atmosphere lifting when the compressors tripped and relieving the process to atmosphere on a regular basis wasn't acceptable to the client. The PSVs routed to atmosphere were left because during regeneration, the process was full of air and you couldn't relieve that to the flare. As a result of this, the client's environmental group deemed the atmospheric PSVs during normal operation became a fugitive emission point and as we only had a single block valve on the inlet, we had to install the spectacle blnds (due to access, that was preferable due to installing another inlet block valve).

 

[don1980]

rogim - How is a spectacle blind utilized on the inlet side of the valve? I don't understand how it could be of any value in that location because one would have to de-pressure (shutdown) the vessel in order to rotate the blind. Did you mean to say outlet side instead of inlet side?

 

[rogim]

I am a part of a engineering team designing a platform in the north sea. It is com on to have positive isolation on all nozles on a pressure vessel. With positive isolation I mean spectacle blind. However, I do not find it reasonable at nozles for psv inlet lines. I.e. Upstream the psv. I think a removable spool is better. However, written procedures does exist and it should be easy to see if the blind is open or close. But my basic question is: does the standards( API, etc) say something about this?

Thanks for your response

 

[don1980]

There's no standard that says you can't do it - I just didn't understand why anyone would want to because changing the spectacle blind is a line-opening operation. Seems like locking open a valve would be better, but I'm not knowledgeable about practices on offshore platforms.

Spectacle blinds have fallen out of favor in some companies due to problems with leakage.

 

[rogim]

Don1980, thanks. I think I will continue to use removable spools instead of spectacle blinds then. Use of locking device on a valve for physical separation of the vessel is not allowed. I agree that it seems to be different philosophies on this topic for different oil companies.

 

[Duwe6]

ASME / NBIC require that the device be 'locked' open. This can be a frangible as a plastic car-seal. Personally, I prefer carseals on my valves. Having a serial number, it is easy to walk around the unit and see if the operators have been doing things they are not allowed to do.