Sect VIII Div 1 Vessel PSV Discharge via Flex Braided Hose? 5

[Zylinderkopf]

Greetings to all:

Thank you in-advance for considering my enquiry – please know that I have performed a routine “search” of this Forum and reviewed applicable API 520 and 521 Code information prior to starting this “thread”…

I have recently made a transition from refining to chemical processing and have noticed that on some of the Sect VIII Div 1 vessels, and that have agitators installed, there exists PSV installations that discharge to a Header via a metallic braided hose that has bolting flange connections at both ends.

Please, would you as a PSV-knowledgeable Engineer comment on the subject installation practice?

The API Figures seem to convey that the “spirit and intent” API Codes consider the use of only “hard-piped” discharge connections, whether to a header or to the atmosphere. However, I can not locate any language that would prohibit the use of a metallic braided hose at the PSV discharge (maybe I did not search well-enough?).

It would seem to me that an Area Equipment Inspector would have some heartburn with this because of the challenge of performing a customary wall-thickness inspection at the braided hose.

Additionally, I found an “accident investigation report” (attached) where the Operator reports a loss of containment with a subject type installation.

As such, I “think” that installing a metallic braided hose at the discharge of a PSV would not be a “best practice”, but, I can not find any Code prohibitions or any other text language that I could confirm my suspicions…

Thank you, again, in-advance for “keeping me between the ditches”.

 

[don1980]

This is a level of detail that's not addressed in codes and standards. The user is responsible for proper selection of pipe materials, pipe class, etc. And, this is a detail similar to those.

When flex lines are used in critical or hazardous service, the owner should have a high-integrity maintenance system that ensures the flex line is always in good mechanical condition. Note that that same comment applies to hard-piping too. Flex hoses are more likely to experience catastrophic failure as compared to hard-pipe, and that's one reason it's generally unacceptable from a risk perspective to simply run-until-failure. it's very difficult to implement any type of preventative maintenance testing that will accurately predict the life expectancy of a flex line. Consequently, good maintenance integrity programs typically call for an initial hydro-test of new hoses, and a fixed time period at which the hose is replaced.

Flex hoses are seldom used in relief piping. It's best to avoid them in relief services, but possibly you have an application in which a flex line is required. If so, engineer the installation so that it's safe and reliable, and replace the hose at a defined safe interval.

 

[Zylinderkopf]

Greetings, don1980 - thank you for taking the time from your busy schedule to respond to my enquiry. Your words of advice are greatly appreciated...

 

[moltenmetal]

If the tanks are on a weighscale, for instance, what other sort of detail would you suggest for the connection of the relief valve discharge to the relief header?

Properly specified, designed and maintained, there is nothing wrong with using hoses for pressure containment services including relief. The design needs to be properly done, with the reaction/inertial forces properly considered and appropriate piping anchors/supports installed etc., which is not a trivial exercise.

 

[Latexman]

In a batch or semi-batch plants, considerable tanks on load cells have flex hoses in relief piping. In my career, I've seen hundreds of them.

Good luck,
Latexman

 

[SNORGY]

To me the answer comes down to whether or not the computed reaction forces arising from the relief valve lifting can safely be accommodated by how the valve is supported in the absence of benefit from rigid piping on the outlet side of the valve.

 

[Zylinderkopf]

Thank you all for providing me with excellent guidance - please know that I do not take your time for granted.