There are many types of flanged openings that serve as manways and handholes, providing access into tanks and other types of closed vessels. Some use standard ASME-type flanges and are bolted much like flanged pipe connections. Internal manways and handholes have covers or plates that fit inside a vessel and are drawn back against a gasket and external flange.
These designs pose a number of sealing challenges. For example, flange surfaces are often less than perfect. Moreover, a round cover will not fit through a round opening since it must be larger than the opening to serve its function. Therefore, gaskets for these flanges are usually oval or obround so the covers can be turned sideways to fit through the openings.
Oval gaskets maintain a smooth curvature, providing sufficient hoop strength for manufacturing and installation. By contrast, obround gaskets include a straight section that limits their radial strength. Manufactured to OEM specifications in a wide range of sizes, these gaskets are typically dimensioned from the inside diameter (ID). Both the major and minor axes are delineated, along with the flange width of the gasket. In gaskets incorporating an inner ring, the width of the ring must also be specified.
Figure 1. The single stud bolt on this manway cover will apply low, uneven compressive force on the gasket, sealing the flanged access to the vessel.
Internal manway covers sealed by these gaskets typically have just one or two stud bolts welded to the center of the cover (Figure 1). These are fitted through a yoke that bridges the opening so the bolts can pull the cover back against the gasket. This bolting limitation, combined with the distance between the bolts and the gasket, results in low and often uneven compressive force.
Since the manway is inside a tank or vessel, the internal pressure acts differently on the assembly than it would on a conventional bolted flange connection. In the latter, the internal pressure acts to separate the flanges and unload the gasket. With a manway, the internal pressure actually pushes the flange into place, loading the gasket and, in many cases, generating more compressive force than the bolts (Figure 2). Notwithstanding, this dynamic, compressive load rarely reaches recommended levels.
Figure 2. The internal pressure on manway covers can exceed the compressive force of the bolts, serving to seat and load the gasket.
Pressure vessels commonly utilize internally seating and sometimes hinged doors for interior access. Such doors must be oriented so the internal system pressure generates sufficient stress to seat the manway gaskets. 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.
For the purposes of this discussion, it is helpful to understand that a sealing surface is normally measured to the inside edge of the gasket. So, an oval shape will be expressed as inner width x inner length x gasket surface width – e.g. a gasket listed as 12 inches x 16 inches x 1 inch x 1/8 inch has inner measurements of 12″x 16″, with a 1″ flange width and ⅛″ thickness.
There are several different types of gaskets suitable for sealing manways and handholes in pressure vessels. Spiral-wound gaskets are available for both lower pressure (less than 999 psi) and higher pressure (greater than or equal to 1,000 psi) applications. Although their compressive loads are usually below recommended levels, they typically seal effectively. Made from long strips of thin metal, wound and filled with a soft material between the windings, or wraps, these gaskets come in two configurations – windings only and windings with inner compression rings.
Lower-pressure systems dictate the use of a thicker grade of filler to make the gasket softer, whereas higher-pressure vessels require gaskets with a thinner gauge of filler. This not only increases the number of metal plies in the gasket, but also makes it denser and more capable of withstanding higher compressive loads. System pressure is exerted on the outside diameter (OD) of the gasket, which is seated at the door-to-vessel interface. This makes sealing failures readily apparent if the gasket is seen protruding toward the ID of the connection. Where this type of failure has been observed and in applications with high-pressure cycling, a solid metal inner ring can be designed for the gasket.
The first choice for manways with internal pressures under 1,000 psig are corrugated metal core gaskets with graphite facing (Figure 3). These gaskets consist of a relatively thick metal core that is corrugated to create concentric or parallel ridges and faced with graphite sheet material. They work best when the flange width of the gasket is one-half inch or greater. For flange widths less than one-half inch, the gasket supplier should be consulted.
Figure 3. Corrugated metal core gaskets with graphite facing are recommended for manways with internal pressures of less than 1,000 psig and flange width one-half inch or greater.
Flexible graphite sheets can be used to effectively seal boiler manholes and handholes as long as the gasket is at least one-half inch wide, and preferably three-quarters of an inch or more. For connections requiring a gasket width of less than one-half inch, spiral-wound gaskets should be used since uneven compressive load can adversely affect graphite sheet gasketing. Most graphite sheet gaskets contain a metal insert, and in some cases multiple inserts, to facilitate handling without damaging the gasket.
Figure 4. Kammprofile gaskets seal less-than-perfect flanges and withstand extreme temperature and pressure excursions.
Kammprofile gaskets (Figure 4) are an excellent alternative for applications where the available seating stress is too low for a spiral-wound gasket, but gasket cross-section, system pressures, surface irregularities and other conditions are not conducive to the use of flexible graphite or non-metallic gasketing materials. These type of gaskets consist of a metal ring with deep grooves and faced with a soft material such as expanded graphite, micro-cellular PTFE or expanded PTFE.
High compressibility, low-creep PTFE gaskets are biaxially oriented with either a micro-cellular structure or filled with micro-balloons. They have been used successfully in sealing manway and handhole flanges for chemical services, but are not usually recommended for steam/boiler applications. The chart below provides a quick reference for matching gaskets to service conditions.
Since manway and handhole gaskets are installed on the inside of a vessel, pressure-sensitive adhesives are sometimes used to affix them gasket to the covers. However, these adhesives soften or melt at steam temperatures, and may break down in chemical service.
Non-metallic gaskets should be installed without adhesive, but a few small spots of spray contact adhesive can be used to hold them in place. Metal gaskets are available with retention tabs to hold them in place during installation.
Sealing internal manways and handholes can be difficult given the pressure differentials and other conditions to which they are subjected. Most assemblies generate low compressive load during bolt-up, and some produce high compressive stresses when internal pressure is applied. Different types of gaskets are available to seal these openings, but their effectiveness will depend on selecting the right type of gasket, correctly dimensioned for the specific application.