1. Standard Tank Manway Sizes
Storage tank manways generally range from 16″ to 36″ nominal diameters depending on access needs and vessel geometry:
| Diameter | Typical Vessel Application | Matching Components |
|---|---|---|
| 16″ | Small vertical tanks, chemical dosing vessels, inspection access, and pump suction chambers. | Manway, Cover, Gasket |
| 18″ | Light industrial tanks, ag nurse tanks, and roof inspection access points. | Manway, Cover, Gasket |
| 20″ | Roof manways; smallest size listed in API 650 for shell and roof tank manholes. | Manway, Cover, Gasket |
| 24″ | The industry standard for shell manways; provides comfortable personnel entry for maintenance and cleaning. | Manway, Cover, Gasket |
| 30″ | Harnessed confined-space entry, hydro-blasting equipment, internal scaffolding erection, and rapid solids washout. | Manway, Cover, Gasket |
| 36″ | Large municipal water reservoirs, heavy crude storage, mixer installation, and emergency worker rescue. | Manway, Cover, Gasket |
| 24″ x 36″ Obround | Flush-shell bottom cleanouts for sediment removal, mud scraping, and tank drainage. | Cleanout Frame, Cover, Gasket Roll |
2. Anatomy of a Welded Tank Manway Assembly
A complete tank manway consists of five coordinated structural elements:
- Neck (Collar): The short rolled-steel cylinder welded directly into the tank wall. Its inside diameter defines the nominal manway size (16″ to 36″). The projection length (4″, 7″, or 12″) determines how far the bolting flange stands off the tank shell to accommodate insulation and bolt clearance.
- Bolting Flange: The heavy ASTM A36 steel ring welded to the end of the neck, pre-punched with 9/16″ holes for structural bolting. Available in single-punched or double-punched patterns.
- Cover (Blind): A solid steel disc precision-cut to match the flange outside diameter and bolt circle. Thicker covers (up to 3/4″) are specified on larger diameters to resist hydrostatic deflection.
- Sealing Gasket: An engineered full-face or ring gasket compressed between the flange and cover to create a leak-tight seal against liquid head pressure.
- Reinforcing Pad (Repad): An annular doubling plate welded around the shell opening on large diameter or thin-wall tanks to restore hoop stress strength lost by cutting the vessel shell.
3. SteelFront Manway Dimensions, Weights & Pricing
SteelFront fabricates every round manway assembly to order at our Hutchinson, Kansas plant from ASTM A36 steel with 100% domestic components:
| Nominal Size | Flange & Cover OD | Bolt Circle (BCD) | Assembly Price Range | Cover Price | Fibreflex Gasket |
|---|---|---|---|---|---|
| 16″ | 20-1/2″ | 19″ | $624.28 – $1,080.98 | from $244.87 | $30.25 |
| 18″ | 22-1/2″ | 21″ | $507.58 – $1,077.06 | from $277.13 | $33.40 |
| 20″ | 24-1/2″ | 23″ | $569.90 – $1,319.22 | from $329.66 | $44.28 |
| 24″ | 28-1/2″ | 27″ | $739.12 – $1,688.58 | from $405.52 | $56.82 |
| 30″ | 35″ | 33″ | $1,131.78 – $2,319.86 | from $508.06 | $86.93 |
| 36″ | 41″ | 39″ | $1,371.62 – $2,881.64 | from $609.52 | $98.21 |
4. How to Measure a Storage Tank Manway Gasket
When replacing a degraded, swollen, or leaking manway gasket in the field, accurate measurements ensure the new gasket seats perfectly without interfering with bolt torque. Follow these four measurement steps:
- Step 1: Measure Inside Diameter (ID): Using a steel tape or caliper, measure across the clear internal opening of the neck cylinder from inside wall to inside wall. Common standard sizes are 16″, 18″, 20″, 24″, 30″, or 36″. The gasket ID should equal or slightly exceed the neck ID so it does not protrude into the fluid stream.
- Step 2: Measure Outside Diameter (OD): Measure across the outermost edge of the bolting flange face. For a full-face gasket, the gasket OD matches the flange OD (e.g., 28-1/2″ for a 24″ manway). For a ring gasket (raised face), measure only out to the inner edge of the bolt holes.
- Step 3: Measure Bolt Circle Diameter (BCD): Measure from the exact center of one bolt hole, straight across the center of the opening, to the center of the opposite bolt hole. On a SteelFront 24″ manway, the BCD is exactly 27″.
- Step 4: Count Bolt Holes & Measure Hole Size: Count the total number of bolt holes around the circumference and measure the hole diameter (typically 9/16″ for 1/2″ bolts on SteelFront assemblies).
Universal Double-Punched Gaskets
To eliminate bolt-hole alignment errors, SteelFront’s Fibreflex replacement gaskets are precision die-cut and double punched. A single gasket fits both standard single-pattern and double-pattern bolt circles. You only need to know your manway diameter (16″ through 36″) to order a guaranteed fit.
Selecting the Right Gasket Material
- Fibreflex (1/8″ Vegetable Fiber Sheet): Saturated with glue-glycerine to deliver superior resistance to petroleum fuels, crude oil, lubricating oils, solvents, and gasolines. Retains dimensional stability up to 250°F (121°C) without softening or extruding under bolt pressure.
- Neoprene Rubber (Continuous Roll): Engineered for municipal water storage, wastewater, rainwater harvesting, and weatherproofing exterior cleanout frames. Provides outstanding resistance to UV degradation, ozone, and atmospheric oxidation. Available in 100 ft rolls for custom field cutting.
5. API 650 Manhole Reference Dimensions
API Standard 650 (welded steel tanks for oil storage) designates access openings as “manholes.” While SteelFront manways are engineered for general atmospheric and commercial storage vessels and are not API-monogrammed, these reference dimensions assist engineers comparing specifications:
| API 650 Opening Type | Standard Nominal Sizes | Bolting Configuration | Standard Gasket Dimensions |
|---|---|---|---|
| Shell Manhole | 20″, 24″, 30″, 36″ | 20″ & 24″: 28 bolts (3/4″ in 7/8″ holes); 30″ & 36″: 42 bolts (3/4″ in 7/8″ holes). | 1/8″ Ring Gasket; OD 25-3/8″ (20″), 29-3/8″ (24″), 35-3/8″ (30″), 41-3/8″ (36″). |
| Roof Manhole | 20″, 24″ | 20″: 16 bolts on 23-1/2″ BCD (26″ cover); 24″: 20 bolts on 27-1/2″ BCD (30″ cover). | Full-Face Gasket; ID 20″ / 24″, OD 26″ / 30″. |
6. How to Order a Matching Manway Set
- Pick the diameter required for your vessel access needs (24″ is standard for personnel entry).
- Select the neck length (4″, 7″, or 12″) to provide adequate stand-off clearance for insulation, jackets, and bolt torque wrenches.
- Select neck plate (1/4″ to 1/2″) and flange thickness (1/4″ to 3/4″) to match your design pressure requirements.
- Choose single or double punched bolting patterns, and order the matching cover.
- Add a Fibreflex gasket in the same nominal diameter, along with spare gaskets for scheduled turnaround maintenance.
Order Custom Manways & Gaskets Online
SteelFront builds all 16″ to 36″ storage tank manways, replacement covers, and Fibreflex gaskets to order with posted pricing and instant nationwide shipping from Hutchinson, Kansas.
Frequently Asked Questions
What is the standard size of a tank manway?
24″ is the most common size for a shell manway designed for worker entry. API 650 specifies shell manholes in 20″, 24″, 30″, and 36″ sizes and roof manholes in 20″ and 24″. Smaller vessels frequently use 16″ or 18″ for inspection access.
What size manway is required for confined space entry?
OSHA’s permit-required confined space standard (29 CFR 1910.146) does not mandate a single rigid diameter, but requires rescue retrieval systems that make non-entry extraction feasible. Openings of 24″ and larger are strongly recommended across the industrial sector to accommodate personnel wearing full-body harnesses, escape respirators, and retrieval lanyards.
What are the dimensions of a 24 inch manway?
The 24″ designates the clear inside diameter (ID) of the neck collar. A SteelFront 24″ manway has a 28-1/2″ flange outside diameter (OD), a 27″ bolt circle diameter (BCD), and 9/16″ bolt holes for standard 1/2″ bolts.
How do I measure a manway gasket?
Measure the inside diameter of the neck opening, the outside diameter of the flange face, the bolt circle diameter (center-to-center across opposite holes), and the hole count and size. For SteelFront manways, you only need the nominal diameter: our Fibreflex gaskets are precision cut and double-punched to fit either bolt pattern.
Does a bigger manway need a thicker cover?
Yes. The cover acts as an unsupported circular flat plate subjected to uniform hydrostatic fluid pressure. Larger diameters experience significantly higher bending moments at the bolt circle, requiring 1/2″, 5/8″, or 3/4″ plate thickness on 30″ and 36″ manways.
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