Why the Bolting Kit Deserves Its Own Specification

On most RFQs we review, the gaskets and bolting are added as a footnote under the flange line item, with a single line that says "gasket and bolts to suit." That single line turns into a 30 percent cost overrun or a six-week site delay more often than any other flange-related decision. The reason is mechanical: a Class 150 raised-face flange and a Class 600 raised-face flange with the same nominal bore need very different seating stresses, gasket thicknesses, and bolt cross-sections. A kit that works for the lower class will either crush the gasket (and leak) or simply not seat (and leak) when dropped onto the higher class.

The second reason is logistics. Bolting kits for refinery, offshore, and power projects usually run into the thousands of pieces. If a single stud bolt grade is wrong on a 24-inch Class 600 joint, the whole flange assembly has to be re-engineered, and the gaskets have to be re-cut to a different pitch circle. Treating the gasket, stud bolt and nut package as a coordinated kit, sourced against the flange standard, is the cheapest insurance you can buy.

Step 1 — Read the Flange Standard Before You Buy the Gasket

Every flange drawing carries the information you need: standard (ASME B16.5, B16.47, EN 1092-1, GOST, JIS), pressure class, facing type (RF, FF, RTJ, tongue-and-groove), size, and material. From that one block on the drawing, you can derive the gasket dimensions, the bolt circle, the number of bolt holes, and the required bolt cross-section. Skipping this step is how procurement teams end up ordering a non-reinforced graphite gasket for a ring-type joint flange, or a soft PTFE gasket for a steam service that runs at 300 °C.

Quick reference: facing type vs. gasket family

Step 2 — Match the Gasket to the Fluid and the Temperature

A gasket is a controlled leak path. Its job is to relax enough under bolt load to fill flange surface imperfections, then hold that seal for the design life. Each material family has a working envelope, and stepping outside it is what causes the chronic "weeps" operators blame on the flange. Below is the working envelope that drives our most common selections on EZ STEEL INDUSTRIAL projects:

Spiral wound gaskets

The default for ASME B16.5 Class 300 and above in hydrocarbon service. A corrugated metal spiral with a non-metallic filler (graphite, PTFE, mica) gives both resilience and strength. Operating envelope: roughly −196 °C to 850 °C depending on filler and metal, and pressures up to Class 2500 when paired with a properly specified steel flange. Specify the inner and outer ring separately — inner rings protect against turbulent flow erosion, outer rings center the gasket and prevent blow-out.

Ring-type joint gaskets

Solid metal, oval or octagonal, machined to the flange groove. Used on RTJ flanges in high-pressure service — wellheads, high-pressure separators, and the high-pressure side of a steam turbine. Softness and hardness numbers are critical; specify the brinell hardness gap between the ring and the groove to keep the seal from galling.

Compressed non-asbestos sheet

Cost-effective for low-pressure water, air, and weak chemicals on flat-face or raised-face flanges. Useful temperature ceiling sits around 300 °C for aramid-fiber binders; graphite-loaded sheets push that closer to 450 °C. Not a substitute for spiral wound on Class 300 and above hydrocarbon service.

PTFE and graphite envelopes

PTFE covers aggressive chemicals where metallic gaskets would be attacked. Graphite (flexible, foil-reinforced) handles high-temperature steam and hot oil where PTFE would creep. For marine and offshore copper nickel flanges, graphite sheet gaskets are often the correct pick because they tolerate the thermal cycling of seawater cooling circuits without losing recovery.

Step 3 — Pick the Stud Bolt and Nut Grade Against the Pressure Class

Stud bolt grade is what holds the gasket under load. If the grade is too soft, the joint relaxes and the gasket blows out. If the grade is too hard (or the wrong alloy for the temperature), the bolts creep, yield, or fail by stress corrosion cracking. The standard reference for most industrial piping is ASTM A193 for the stud bolt and ASTM A194 for the nut. The two are designed to be used as a matched pair.

Carbon and low-alloy bolting

ASTM A193 Grade B7 is the workhorse — chromium-molybdenum alloy steel, quenched and tempered, used for Class 150 through Class 2500 in hydrocarbon service up to about 450 °C. B7 is paired with ASTM A194 Grade 2H nuts. B7M is the machined-over version used in low-temperature or hydrogen service where stress-corrosion cracking is a concern. B16 is the high-temperature step-up from B7, rated for service up to about 600 °C, paired with A194 Grade 7 nuts.

Austenitic stainless bolting

ASTM A193 Grade B8 (304 stainless) and B8M (316 stainless) are used where corrosion resistance is more important than strength — typically in chemical, food, and marine systems. They are paired with A194 Grade 8 or 8M nuts. B8 Class 2 is the cold-worked variant that brings the strength closer to B7 while keeping the corrosion resistance.

High-nickel and exotic bolting

For sour service (NACE MR0175), cryogenic service, or aggressive chemicals, the bolting grade steps up to L7, L7M, B8MLCuN, or specialty alloys. These are project-specific and almost always specified by name on the piping class sheet.

Thread engagement, length, and point style

Stud bolt length is calculated from the flange thickness, the nut thickness (typically the same as the stud diameter), the washer thickness (if used), and a thread engagement of at least one stud diameter past the nut. Double-end stud (the most common) versus tap-end stud is a project preference. Most Class 150 to Class 600 RF flanges use double-end studs with heavy-hex nuts; higher classes and RTJ joints often use continuous thread studs with one heavy hex nut and one jam nut.

Step 4 — Cross-Check Against the Piping Class Sheet

The piping class sheet is the single document that should reconcile the pipe fittings, pipe flanges, and bolting kit into one design. On a clean project, every line on that sheet points to a single flange standard, a single gasket family, a single stud bolt grade, and a single nut grade. Three signals that the kit has been specified correctly:

1. The flange facing on the drawing matches the gasket family on the BOM. RTJ flanges with spiral wound gaskets, or RF flanges with octagonal metal rings, are a sign that someone copied the wrong line.
2. The bolt grade and nut grade are a matched pair. B7 studs with 2H nuts, B8 studs with 8 nuts, B16 studs with 7 nuts. Mixing pairs (for example, B7 studs with 8 nuts) is a common error that quietly works in testing and quietly fails in service.
3. The bolting quantity matches the flange bolt count at the rated class. A Class 150, 12-inch RF flange needs more bolts than a Class 150, 6-inch RF flange, and that bolt count is fixed by the ASME B16.5 table. The kit BOM should match it exactly.

Step 5 — Don't Forget Storage, Lubrication, and Reuse

A gasket left in a humid yard for three months will swell, pick up surface contamination, and lose recovery. Stud bolts that go into service without anti-seize compound on the threads will gall against the nut and refuse to come out at the next turnaround. A few non-spec items that routinely cause field failures on otherwise correct kits:

Anti-seize compound: a nickel- or copper-based anti-seize on B7 stud threads is standard practice. It reduces friction variation during torque-up, which directly controls the seating stress on the gasket. Skip it and the same torque wrench will deliver very different bolt loads across the joint.

Gasket storage: keep spiral wound and flexible graphite gaskets in their original packaging, indoors, off the floor. Sheet gaskets (compressed fiber, PTFE) are especially sensitive to moisture and oil contamination.

Reuse policy: do not reuse soft gaskets (sheet, PTFE) — they take a permanent compression set. Spiral wound gaskets can sometimes be reused if the visual inspection, thickness, and recovery test pass. Metal RTJ rings are not normally reused; the plating and seating surfaces take damage on the first compression cycle. Stud bolts are usually reusable if the threads and the body are undamaged, but most owner specifications require a new kit after a defined number of thermal cycles.

How to Bundle the Gasket and Bolting Kit With the Flange Order

The cheapest way to keep all of this aligned is to buy the gaskets, stud bolts, and nuts as part of the same flange package, on the same purchase order, against the same flange schedule. That is how EZ STEEL INDUSTRIAL ships its bundled gasket, stud bolt and nut kits: each kit is cross-referenced to the flange line item, with a mill test certificate that traces the stud bolt heat, the nut heat, and the gasket batch back to the original material certificates. For the buyer, that means a single point of accountability if anything on the joint fails the MTR review or the incoming inspection.

For a typical refinery or desalination project, ordering the pipe fittings, pipe flanges, and gasket, stud bolt and nut kits from a single manufacturer also keeps the documentation consistent across the isometric spool drawings, which is what the inspector asks for first on a hydrostatic test.