PLUGS, CAPS, AND BLIND FLANGES 1

[martin888888]

We are putting together some demolition plans on a good size industrial site. I wanted to see what comments I would get on the capping of both pressurized and non pressurized utility systems.

With Pressurized ductile iron pipe systems such as water and fire, it seems as if a blind flange with mechanical restraint is always the best option when cutting a capping the pipe. Is a plug and restraining gland ever an option? Is there an advantage of either? If the pipe is cut on the other side of a valve could just a plug be used?

With gravity flow storm and sanitary lines of HDPE, PVC, etc pipe type it would seem like a simple plug would always work fine. Even under surchagred conditions I dont see a plug being pushed out of place.

With pressurized PVC pipe I beleive a blind flange and restraint would always be a good option.

 

[rconner]

All of the items you list have of course been used to close off pipelines at different locations and for various purposes. To use a blind flange on an existing flange, they generally must have the same bolt circle and general dimensions. To use a plug, you must have a socket that will accept same, unless the plug is one that fits suitably up inside a pipe or inflates to accommodate different dimensions etc.
If you go to plug, cap or blank off live/pressurized lines, resulting in differential pressure at the bulkhead, there can be substantial thrust force to deal with.
As with any other Engineering structure, when there are forces there must be suitable resistance to same provided, in the case of pipelines provided integrally with suitable restrained joints in the pipeline, or external buttressing as with concrete thrust blocks.
Critical to such provisions will be knowledge of the actual sizes and pressure of the pipelines involved, as well as exactly what kinds of joints etc are actually present in the existing lines. To get an idea of the size of forces you may potentially be dealing with, see e.g. Table 17-6 of Table 17-6 in the file at

http://www.american-usa.com/system/assets/151/original/section 17_RB.pdf

 

[rconner]

(Oops, meant page 17-13 of that .pdf)