Sealing Challenges and Gasket Solutions for Vessel Manways & Handholes

Flanged openings—manways and handholes—are common access points in pressure vessels, often using standard ASME‑type flanges bolted like pipe connections. Internal covers sit inside the vessel and are drawn back against a gasket and an external flange, creating unique sealing dynamics that differ from conventional flanged joints.

Typical flange surfaces are rarely perfect, and the cover must be larger than the opening to provide clearance for insertion. Consequently, oval or obround gaskets are employed so the cover can be rotated sideways during installation. Oval gaskets offer a continuous curvature for strong hoop support, while obround gaskets contain a straight segment that reduces radial strength. These gaskets are manufactured to OEM specifications in a wide range of sizes, always dimensioned from the inside diameter (ID). Both major and minor axes are specified, along with flange width and, if present, the width of any inner ring.

Figure 1. The single‑stud bolt on this manway cover applies low, uneven compressive force, illustrating the sealing challenge.

Internal covers typically feature one or two stud bolts welded to the cover’s center (see Figure 1). These bolts pass through a yoke that bridges the opening, pulling the cover against the gasket. Because the bolt spacing is limited and the bolts are positioned away from the gasket, the resulting compressive force is often low and uneven.

In a vessel, the internal pressure acts opposite to a conventional bolted flange: instead of pulling the flanges apart, it presses the flange into the gasket, creating additional compressive load that can exceed the bolts’ contribution (Figure 2). However, this dynamic load rarely reaches the level recommended for reliable sealing.

Figure 2. Internal pressure can surpass bolt force, aiding gasket seating.

Gasket Selection for Pressure Levels

Choosing the right gasket is critical because the internal pressure directly influences the required sealing strength. Lower‑pressure systems (≤999 psi) benefit from softer, more deformable gaskets that can accommodate minor surface irregularities. Higher‑pressure vessels (>1,000 psi) demand rigid, high‑density seals such as metal‑reinforced gaskets, which, while more challenging to seal, provide the necessary strength and resistance to extrusion.

Gaskets are typically expressed as inner width × inner length × gasket surface width, e.g., a gasket listed as 12" × 16" × 1" × 1/8" has inner dimensions of 12" × 16", a 1" flange width, and a 1/8" thickness.

Recommended Gasket Types

Spiral‑Wound Gaskets
Suitable for both <1,000 psi and ≥1,000 psi applications. Made from thin metal strips wound and filled with a compliant material, they come in two forms: winding only and winding with an inner compression ring. For <1,000 psi systems, a thicker filler grade is used to increase softness; for higher pressures, a thinner filler provides higher density and load capacity. In high‑pressure cycling, a solid metal inner ring may be incorporated.

Corrugated Metal Core Gaskets with Graphite Facing
Ideal for internal pressures below 1,000 psi and flange widths ≥0.5" (Figure 3). These gaskets feature a corrugated metal core and a graphite sheet surface, delivering excellent sealing against imperfect flanges. For flange widths <0.5", consult the supplier for appropriate specifications.

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 effectively seal boiler manholes and handholes when the gasket width is at least 0.5" and preferably 0.75" or more. For <0.5" width, spiral‑wound gaskets are preferred due to uneven compressive loading.

Kammprofile Gaskets
These are metal rings with deep grooves faced with expanded graphite, micro‑cellular PTFE, or expanded PTFE. They excel when seating stress is low and the application cannot accommodate flexible graphite or non‑metallic materials. They are also highly tolerant of surface irregularities and extreme temperature/pressure excursions (Figure 4).

Figure 4. Kammprofile gaskets seal less‑than‑perfect flanges and withstand extreme temperature and pressure excursions.

High‑compressibility, low‑creep PTFE gaskets—biaxially oriented with micro‑cellular or micro‑balloon structures—are well suited for chemical service but generally not recommended for steam or boiler applications. A quick reference chart (Figure 5) can aid in matching gasket type to service conditions.

Installation Practices

Because the gasket sits inside the vessel, pressure‑sensitive adhesives are occasionally used. However, these adhesives can soften or melt at high steam temperatures or degrade in chemical environments, so they are generally avoided. Non‑metallic gaskets are installed without adhesive, though a few spots of spray contact adhesive may be applied to keep the gasket in place during bolting. Metal gaskets often feature retention tabs that help secure them during installation.

Successful sealing hinges on selecting the appropriate gasket type, correctly dimensioning it for the specific pressure and flange conditions, and ensuring proper installation technique. Properly matched and installed gaskets provide reliable, long‑term seals for vessel manways and handholes.