“Split Code” design approach to pig trap quick actuating closures

By Rolf Gunnar Lie, T.D. Williamson, Singapore and Neil McKnight, T.D. Williamson, U.K.

Delivering quality product at required volumes means leaving nothing to chance, including asset maintenance, which is essential to pipeline integrity. Pigging activities, including in-line inspection (ILI) and cleaning, help keep pipelines in prime operating condition and extend useful life, but it takes proper equipment, planning and operator training to ensure safety, especially during pig launch and retrieval. Without the right technology for safe and easy access, opening the pig trap closure door can present unacceptable risk to both personnel and infrastructure. In fact, opening the closure is considered one of the most critical operations in pipeline pigging.

Why? The key concern is that when the pig trap door opens it will allow air to mix with hydrocarbons, creating an explosive atmosphere. But that’s not the only potential hazard. Sudden releases of pressure and projectiles can also jeopardize the operators’ safety, and valves must be opened and closed in the appropriate sequence to avoid damaging the pigs and the pipeline system itself.

One way to mitigate incidents is to minimize the time and effort involved in opening and closing the pig trap, which is generally a function of the type of pig trap closure in use — some are simply more straightforward and safer to operate than others.

For example, it’s easier to open and close a Quick Actuating Closure (QAC) or Quick Opening Closure (QOC) than it is to install or remove a blind flange.

The QAC or QOC can be opened by one person, typically in a single motion, without tools. In fact, the ability to swing the door open with one movement is part of how ASME defines the equipment: ASME Boilers and Pressure Vessel Code (BPVC) Section VIII standards note that quick actuating means “all elements loosening in a single actuation.” This feature enables the rapid introduction and removal of pipeline pigs without compromising the safety of field personnel, damaging equipment or releasing hydrocarbons into the environment. By being the main access point into the pipeline system the QAC is a critical component in minimizing the volume of methane emissions that could escape into the atmosphere. Given the world’s net zero ambitions and the industry’s drive toward sustainability — not to mention regulatory compliance — this is an increasingly important factor. Although it’s difficult to find industry-wide or agency statistics about the amount of methane released during pig trap operation, the U.K.’s National Transmission System found that each time they averted a failure by replacing pig trap seals, they saved 8.5 tons of carbon dioxide equivalent (CO2e).

By contrast, installing or removing a blind flange requires dozens of bolts and nuts to be loosened and torqued. Not only does this process call for its own set of tools, but it also adds hours of labor and additional risk for field technicians.

Compliance Without Overdesign

Like any other oilfield equipment, QACs must comply with codes that vary by country and product. Although older pig traps were designed to meet pressure vessel codes, that’s no longer the case, and with good reason: the function of the closure is to provide internal access to the pipeline system not to the inside of a vessel.

Instead, pipeline operators now typically require pig traps to be designed according to the same code(s) as the pipeline on which they are installed — for example, ASME B31.8 - Gas Transmission and Distribution Piping Systems or ASME B31.4 - Pipeline Transportation Systems for Liquids and Slurries. This is possible because pig traps are considered pipeline assemblies, meaning the code that covers the pipeline itself also applies to the pig trap.

Of course, different codes have different qualifications — and different ways a pig trap closure design can satisfy them. For a closure to be installed on an ASME B31.4 or ASME B31.8 pipeline, for example, the interface between the closure assembly and the trap must reflect an intentional methodology that meets safety considerations — like keeping the technician out of the “line of fire” in case of a pressure release. ASME BPVC Section VIII says the closure design must incorporate a safety locking mechanism so the closure cannot be opened while it is under pressure.

Trying to understand and meet all the applicable codes is no easy task. In an effort to cover all the bases, equipment providers may be tempted to overdesign their pipeline traps and closures, which can be a waste of resources.

Let’s say a pig trap is built entirely to pressure vessel code. Not only is this unnecessary, but it’s also more expensive to manufacture. That’s because pipeline code allows the use of thinner (and less costly) high-yield strength API pipe for the barrel and nominal section instead of the lower yield strength, thicker materials limited by ASME BPVC Section VIII. The codes provide flexibility with regards to raw material selection, which reduces overall cost. Why not take advantage of it?

That’s exactly what the split code approach does.

Figure 1: D2000 QAC main components

Figure 2: Operation of QAC outside "Line of Fire."