Dig into transfer bulkhead design

February 2, 2023 By    

Sometimes technology settles in and not much changes for a while. Some people feel that sticking with a tried, comfortable solution is their best choice, even though tweaks and a higher-technology solution may meet the same design requirements.

Such seems to be the case with transfer bulkhead design. Changes to this design seem to come mostly from new requirements based on dealing with incidents.

A question about transfer bulkhead design arises

We were recently challenged by a builder about the riser length on the transfer bulkhead.

He said that we couldn’t require a specific minimum length since there was no such specification in code, law or regulation.

He’s right that the length is not legally specified anywhere. The code states a performance requirement but does not specify how to do it.

We have accepted risers of at least 12 in. long, rejecting anything shorter. If they want to use something shorter, we ask them to show us it works. They usually go with 12- or 16-in. risers.

The challenge made me curious about bulkhead design.

I knew my predecessor left a file about bulkhead design, and I had used parts of it throughout my career. I delved into the folder and found the following gems.

What research shows about riser length standards

Significant changes seem to have started when the 1976 edition of NFPA 58 first required emergency shutoff valves (ESVs) at bulkheads. In the folder was a Corken sales bulletin titled “LP Gas Plant Riser Design,” dated March 1979. Corken manufactured ESVs then.

The bulletin largely considered the length of risers needed to exert enough force to pull out/break the riser without damaging the hose or the valves and fittings on the bobtail. It also delved into the design of the concrete or steel stanchion used to secure the piping to prevent forces from damaging plant piping. That was the performance criteria.

The bulletin didn’t settle on a riser length, but most discussion was centered on 12- to 24-in. risers. The surprising finding of the testing was that the pipe-nipple and fitting threads showed practically no damage when the risers were pulled out.

I also found a reprint of an article from LP-Gas, dated November 1979, describing some testing in Texas of bulkhead designs by pulling away with a bobtail. It was conducted by RegO and the Railroad Commission of Texas. The riser for this testing was 18 in. This study was largely aimed at testing the ESVs and closing systems for different configurations. They tested thermal devices, cable pulls attached to risers, pneumatic systems and cables attached to hoses.

How an incident led to technological development

An industry-defining incident occurred in late 1992 in Mount Airy, North Carolina. A bobtail pull-away resulted in a large, ignited release and a huge traffic pileup.

It also led to the passage of a new regulation in North Carolina that required automatic closure of the bulkhead ESVs when a pull-away was detected by the breaking or displacement of the riser.

It took until the 2014 edition for the NFPA 58 LP gas code to implement a similar requirement.

Some new technology soon followed the new rule. Both RegO and Squibb Taylor demonstrated breakaway devices that come apart without damage, releasing extremely little product. They could be reconnected in minutes for nearly immediate return to service.

I saw some of these products used in gasoline tank farms where they blend in butane. They have not appeared in many propane dealers’ plants.

Going back to riser design, nearly all sites use a vertical riser. Therefore, a pull in any horizontal direction will be a pull perpendicular to the riser, creating a moment arm that will result in a torque at the riser/coupling interface.

The Corken study showed a clean riser separation with no damage beyond the bulkhead. It even described an unexpected field test in a real pull-away. Repairs were completed in hours, and the site was back in service.

Our inspectors have come across similar situations. All damaged sites we saw had risers at least 12 in. long.


So how did we settle on a minimum riser length of 12 in.? I don’t conclusively know.

It may have had something to do with my predecessor’s request to bulk plant builders to submit their designs for consideration. All of the surviving designs submitted showed risers from 12 in. to 30 in. long. Since they appear to agree with the designs for the Texas and Corken studies, he probably felt there was a consensus that these lengths would meet the performance criteria of protecting plant-side piping and shutting off flow in the case of a pull-away as had been demonstrated in the studies.

There is no indication that tests were performed.

Simple, low-cost designs using basic materials can sometimes be as useful in certain applications as higher-technology solutions.

However, in the interest of innovation and better solutions, laws and codes should be written so as not to restrict new technology.

Richard Fredenburg is an LP gas engineer at the North Carolina Department of Agriculture and Consumer Services, Standards Division. He is also a member of NFPA’s Technical Committee on LP Gases. Contact him at richard.fredenburg@ncagr.gov or 984-236-4752.

NOTE: The opinions and viewpoints expressed herein are solely the author’s and should in no way be interpreted as those of LP Gas magazine or any of its staff members.

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