Tank patrol: keeping manways safe

Some applications demand frequent vessel access, while others may only need someone to climb inside once in a blue moon.

Whatever the case, it’s pretty obvious that tank and vessel manways need to be properly specified to ensure that they’re safe. What may be less apparent is the impact that the right manway can have on operating efficiency and downtime.

“So why would you consider the manway? A manway is a manway, right?” says Neil Alger, director and managing director of Regal Tanks, which supplies a range of low-pressure tanks.

Time sensitive

The trouble is that the most basic solution – a bolted flange – can also be extremely time consuming, “So set aside 20 to 60 minutes of your valuable time, go and get a spanner or socket, spray a bit of lubricant on the bolts, give it five to 10 minutes to soak and now try and undo all 24 to 36 bolts, being extra careful not to lose any. And 60 minutes later you’re in,” says Alger.

Once you’ve finished inside the tank, you’re then going to have to do it all again in reverse, typically taking a further 20 minutes. That’s a total of 80 minutes per manway. “Perhaps those manways should be something to consider when you next hire a storage tank,” says Alger.

Regal instead favours quick-release manways that rely on just eight hinged wingnuts. Alger says they can be opened without special tools in around a minute.

When it comes to high-pressure, hazardous applications, the stakes are even higher. Not only do manways have to maintain an effective seal, but they also need to ensure that they can only be opened when the system has been safely depressurised.

There are varying pressure vessel access designs in the market place, which have their own place for varying applications

Carla Connelly, sales manager, GD Engineering

GD Engineering, a division of SPX Flow, manufactures the Bandlock 2 closure system. This is the go-to solution for many companies in oil and gas looking for manways to suit high-pressure pigging launcher and receiver vessels, as well as the filtration vessels that help clear gas flow lines of so-called black powder (a mixture of iron sulphides and other contaminants).

The same type of locking band-based design is also the starting point for quick opening closures (QOCs) in a range of other applications. Sales manager Carla Connelly, acknowledges that there are several alternative approaches for providing manways in pressurised applications: “There are varying pressure vessel access designs in the market place, which have their own place for varying applications.”

A bolted flange is again the simplest option, but also the slowest. “[These are] generally specified when zero or minimal access to the vessel is required… Large flanges can take over two hours to remove, resulting in high opex costs,” she says.

She also highlights several QOC alternatives. For instance, swingbolt systems and threaded closures are both well-proven solutions, although they are limited in terms of pressure ratings and size, according to Connelly. Clamp-type closures offer an option across all pressure classes and sizes, but they’re not inherently safe (so if one part of the system fails, they could in theory fail to maintain the seal).

High pressure

“The majority of other QOC classes have limitation in regards to size, pressure and temperature variables. However, the GD Bandlock 2 can be manufactured from six to 100-inches (pressure dependant) and is used at up to 2500lb pressure class, and higher pressure in custom designs,” says Connelly.

It gets increasingly difficult to manufacture manways of any design in bigger sizes the higher up the pressure range you go, since they become progressively heftier and harder to handle, she says.

The Bandlock relies on a duplex locking band that is pressed between the door and the hub. This carries the load uniformly around the circumference and, even in the event of a failure, the manway will remain firmly closed. “Locking bands are loaded in compression,” explains Connelly. “This is the safest and most efficient method of transmitting load.”

Sealing is another key consideration in pressure vessel manways. For instance, the Bandlock 2 system features a seal built into a groove that’s specially machined onto the face of the door. However, Dave Burgess, gasketing applications specialist with Garlock Sealing Technologies, says that the biggest gasket challenges typically arise from internal manways, in which the manway doors press out against the inside of the vessel. These often rely on the internal pressure of the vessel to seat the sealing gasket properly.

The compressive load at gasket installation is far higher and far better for the gasket with external manways, since these flanges have bigger bolts and more bolts

Dave Burgess, gasketing applications specialist, Garlock Sealing Technologies

“The compressive load at gasket installation is far higher and far better for the gasket with external manways, since these flanges have bigger bolts and more bolts,” he says. “Most internal manways do not adequately compress gaskets until the vessel is pressurised (some never achieve proper loading). Gaskets compress further when the internal manways are pressurised, leaving bolts loose. Many customers tighten the bolts while the vessel is pressurised. We consider this very dangerous.”

The other main difference is that external manways are usually round, while internal manways are more often oval or obround, so they can be turned and passed out through the vessel opening when necessary. While the outward, radial pressure is distributed evenly around a circular or oval gasket, it will be unevenly spread around an obround, leading to potential problems. The correct gasket construction will depend on the application, says Burgess.

“As the internal pressure increases, so does the stress on the gaskets, a factor that must be taken into account to optimally match gaskets to vessel design pressures. Lower-pressure systems call for softer, more easily deformed gaskets, but these are subject to blowout on the ends where the compression is low. Conversely, higher-pressure vessels require more rigid, higher-density seals such as metal reinforced gaskets, which can be difficult to seal.”