Sealing Threaded Joints for Chemical Handling 3

[Subystud]

Hello -

What are the recommended methods of sealing threaded joints for materials like PVC and Steel that are handling chemicals?

Is there a standard outlining required materials or method, like using teflon tape versus pipe dope.

Any insight into plumbing standards is appreciated.

Thanks -

Rob

 

[moltenmetal]

We use Teflon tape (1 s.g. or higher) PLUS good quality anaerobic pipethread sealant (Loktite 592, 567 etc.) as a top-dressing to the tape. This is a system that is proven to work, long term, and is resistant to poor installation practice due to poor pipefitter skill. The system originates from the threaded piping specifications of a major chemical manufacturer who shall remain nameless.

 

[MJCronin]

subystud....

Threaded pipe joints are not a good choice for all chemicals.

If the chemical in question is extremely aggressive or lethal, socket welded joints are a better choice.

 

[moltenmetal]

The sealant system above is for an absolute maximum of 350 F operating temperature.

Threaded joints should be limited to 2" NPS maximum, and are to be avoided for:

- lethal service, or any service where a very small leak represents a disproportionately large hazard
- high purity service (socket weld is out for those services too)
- vibrating service
- buried or otherwise inaccessible piping
- services with very frequent and rapid thermal cycling
- high pressure hydrogen
- any service beyond 400 F (due to the ineffectiveness of thread sealants beyond this temperature)
- services where there is a risk of crevice corrosion (you shouldn't use socket welded for those either)

Threaded joints, properly designed, are suitable for normal fluid service per B31.3, which encompasses all services that are not Category M (lethal), cyclic service as defined by that code, or high pressure (beyond 2500# class). Some owners will be (much!) more restrictive than this, permitting threads only for low pressure utilities etc.