A stainless steel manway is the lid that serves as access point to the inside of a vessel, usually consisting of a flanged nozzle with a cover plate. Also called a “manhole,” the size, style, and number used may vary from application to application. Stainless steel manways might be selected as part of a custom design or may be specified as an addition to an existing vessel.
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A stainless steel manway does not need additional materials to protect its surface-its longevity is a result of alloying composition and a natural resistance to corrosion. The most common grades used for stainless steel manways are 304 and 316, which are both austenitic. These grades offer a reduced carbon content that increases corrosion-resistance, which helps to protect the material from high temperatures. Low-carbon version of 316 stainless steel, referred to as 316L, is often used because it offers higher creep, stress to rupture and tensile strength at elevated temperatures. The manways in industrial applications often meet API (American Petroleum Institute) and ASME (American Society of Mechanical Engineers) standards. API code sanitary stainless steel manways are a specialty design that meet food-hygiene grade and other industry regulations and are ideal for pharmaceutical and food, and beverage applications. The various types of manways include those specified by shape; oval, rectangular, dome, conical, flat, bubble and those specified by design; spring, slide, shadowless and hatch.
The ease of modern stainless steel fabrication techniques means that stainless steel can be cut and welded as readily as traditional steels. Manufacturing stainless steel manways begins with the raw steel melted in an electric furnace for up to 12 hours. Once it reaches recrystallization temperature, the molten steel is cast into a semi-finished form and then undergoes a process called hot rolling to solidify the shape. This is followed by heat treating through annealing, where the steel is heated and then cooled through quenching or air hardening. The stainless steel is descaled after it has cooled and this removes buildup by electrocleaning or pickling. The final size and shape of the manway is commonly obtained by plasma cutting, during which gas is blown out of a plasma torch at high speed causing the hot plasma to melt the metal away from the cut. The stainless steel manway is finished with welding. Manways come in two types: pressure manways (used on pressure vessels holding gases, chemicals, food, and beverages) and non-pressure manways (used for inspection and access applications such as for ducting systems, bin entry, cleanout ports, and fill openings). In addition, there are elliptical yoke and bolt manways are manufactured in accordance with ASME code specifications serve as access to pressure vessels. Standard or bolt-style stainless steel manways have a basic configuration suitable for most uses, and all three of these types of stainless steel manways have top and side entry, pressure or atmospheric service. Quick-open stainless steel manways are a fast access mechanism suitable for critical process tanks and storage vessels that require frequent access.
