Large Spectacle Blinds (Figures 8) 1

[admt63]

My client wants to use spectacle blinds (Figures 8) for all sizes of diameters. In my projects it means use spectacle blind up to 36 inches and 600 pounds of rating.

I totally disagree with that, and I do not recommended them for sizes greater than 12”, I am explaining to my client that from ergonomic point of view and the possible interferences that it will occur, this large spectacle blinds are not recommended. But my client is still making this a point of honor.

I would like your opinions and trying to establish more inconveniences based in your experiences for not to use these large spectacle blind.

Thanks and regards

Admt63

 

[ASA20]

Admt63
Much information on the project will be required to really find a good reason to convince your client, also i will want to know the reason for his choice of figure 8 spectacle blind, but Weight limitations and the associated difficulty of handling heavy pieces in the field are the primary considerations in specifying a Figure 8 Blind over separate components.
If it can be established that these constrains "especially weight" will affect the integrity of the project,you can drive home your point.

adeboy

 

[rzrbk]

His own maintenance personnel will probably back up the limitation on spectacle blind size, as they will not want to swing a large diameter 600# blind.

As to how to bring that up without seeming to try and go around him, I don't have a good suggestion.

 

[zdas04]

What is your alternative? If the project requires that level of positive isolation the alternatives are all very expensive (e.g., put a spool between two valves that you can shut to drop the spool and replace it with standard blind flanges) then maybe there is a way to make it happen in such a way that it is safe and the maintenance guys don't burn your house down. Off the top of my head I would look at orienting the blind verticaly and mount a davit over the blind to jack it up and down.

Instead of everyone falling on their swords over this, why don't you talk to them (I know, its a novel concept) and see the issue that they are trying to address. If the only thing you get out of that is "company policy" or some other blank wall, then install the spectacle blinds with davits to support and switch them and jacking bolts to get the flanges far enough apart to affect the change.

To paraphrase a cliche "the client is always right, especially when he's wrong". I always try to avoid forcing people into decisions based on "points of honor".

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[rzrbk]

Since overly large spec blinds would be difficult or dangerous to lift or turn by hand, the common alternative would be to install a spacer only, with separate blind,on large flange sets.
This way the spacer and blind have to be rigged in and out of place, and there is no temptation to 'spin' a piece of steel that is potentially the size of a desktop, and 2 or 3" thick, on a bolt. I would say its more of a safety aspect, due to dropping and pinching potential when manhandling heavy and/or large spec blinds.

 

[FIF99]

Hi,

Your client should indeed be notified that Figure 8 in these sizes will result in inconvenient operation.

Besides that, I don't know the environment, the figure 8 in these sizes may catch a lot of wind if installed in the open position, which may result in damages at the rotation handle. (in case of welded design)

I would recommend spades and spacers above 16".

Regards,
FKE

 

[evdbroek]

Maybe you can look at

http://www.onislineblind.com

They have a smart solution to quickly change spectacle blinds.

 

[jte]

admt63-

I agree that in many cases the weight of the spec blinds will be too much. On the other hand, some isolation will be required, and FKE's recommendation to use spades and spacers makes a lot of sense. One approach you could take to at least reduce the size of piping for which spec blinds are required is to point out that ASME B16.48 Steel Line Blanks which provides dimensions for spec blinds ("figure 8 blanks") maxes out at a 24" NPS.

jt

 

[jonesey]

Like jte states, B16:48 (and the old API590) only cover up to 24", so anything over this will have to be calculated. We have no specific company guidelines for when to use a spectacle blind and when to use a spade/spacer. As noted in other comments there is a point when trying to rotate a large spectacle blind becomes an unsafe operation and can result in injury to personnel. We generally say that when you are over 25kg that we would rather use a spade/spacer arrangement. Don't know how much a 36" 600# spectacle blind would weigh but we are talking a few hundred kilograms. I wouldn't like to rotate it. The other disadvantage with using large diameter spectacle blinds is that they require a lot of free space to be rotated. Where we are pushed for space we always use a spade/spacer arrangement. The only disadvantage we accept with use of spade/spacers is that the part that is not installed in the line can sometimes go missing (someone has borrowed it). But this can be countered by fixing a chain/wire to the item. Like rzrbk says, their own operations personnel will probably not be too happy at the prospect of swinging a large diameter spectacle blind.