Pressure vessels, tanks, large specialty heat exchangers, and other similar equipment generally require large openings for personnel entry (for inspection, cleaning, maintenance, or repairs) or even for certain process additions. A “manway” is the industry term most commonly used to describe these large, pressure-rated openings in large static equipment. In pressure vessels, the manway opening is typically the largest and therefore often the limiting factor with pressure ratings and design details.

Once the manway itself is designed, the next consideration is the manway closure/cover and there are a few primary options to choose from. And, sometimes you can start with the cover type wanted for the application and try to design the vessel around that.

In most applications, the manway cover is simply a rated flat blind flange, designed to handle the pressure rating of the vessel. Since flat surfaces don’t hold pressure as well as similar thickness curved surfaces, these covers are often very thick and therefore heavy and, with a full array of standard bolting hardware (studs & nuts), it is rather time-consuming to remove & reinstall, not to mention the cost of the gasket, which in most cases needs to be replaced each time the cover is removed. Weight and installation/removal time aside, the flat manway cover is the most commonly-used and least-expensive method of manway closure.

Many boilers utilize an ID-mounted, elliptical cover that uses the pressure inside the unit to help seal the gasket. These can be installed in regular pressure vessels as well, but they are not as common in that application. These type closures are typically both inexpensive and very quick to open/remove/reinstall, but they have the downside of not being nearly as large as “standard” vessel manways and therefore the type and frequency of planned interior work must be taken into consideration before committing to an elliptical manway. They also cannot be rated for vacuum pressure, since internal pressure is used to help make the seal.

Another common cover/closure is the hinged (sometimes called quick-release) type. There are several varieties of this type of closure. Some are very similar to a flat blind cover, but are hinged. This type requires all the studs to be removed to open, just like a flat blind cover. Generally speaking, these have lower pressure ratings. Others have more of a domed head design to increase the pressure rating of the closure for higher-pressure vessels. Some of the hinged designs, whether flat or domed, have slotted tabs and toggle/swing bolts, that can cut time out of the opening/closing process. Lastly, the units designed for the quickest access will have special “winged” nuts that allow the user to knock loose the nuts with a hammer, then swing the stud aside to free the cover.

Our last mention in this article is the manway clamp fastener, which allows very quick release and installation and does not require bolt holes to be drilled in the manway flange nor the (specially-designed) flat cover. Relatively high-pressure ratings can be handled with this type of design, as it is possible to use many of these clamps in a close pitch to better spread out the forces generated by pressure on a flat cover.

Before deciding on the best option for your manway and its cover, talk to the operators of the equipment, the maintenance department at your plant, and discuss the ASME code requirements for access openings with your fabricator. Hopefully, the design can incorporate your first choice.