What are the pressure-temperature ratings for gasket, stud bolt & nut combinations?

Ask ten buyers what a Class 150 flange joint is rated for and most will answer "150 psi." In practice the number on the class tag is only a label — the pressure-temperature rating of a bolted joint, and the rating of the gasket, stud bolt & nut set inside it, is a curve that drops as temperature climbs. Get the combination wrong and the flange will hold; the gasket or the bolting will quietly decide when to let go. This article walks through how pressure-temperature ratings actually work for gasket, stud bolt & nut combinations, and how EZ Steel Industrial applies them when packing flange packages for oil and gas, petrochemical, and power projects.

The rating is a curve, not a single number

A pressure-temperature rating states the maximum working pressure a component can safely carry at a specific temperature. The two values move against each other: steel loses strength as it heats up, so the allowable pressure falls as the service temperature rises. That is why every ASME component ships with a rating table rather than a headline number, and why a flange that carries 740 psi at 100 °F only carries a fraction of that once steam service pushes the joint beyond 700 °F.

This matters for the gasket, stud bolt & nut combination in a particular way. A bolted flange joint is a system of three parts — flange, gasket, bolting — and its effective rating is set by the weakest of the three. You can fit a Class 1500 flange with a soft PTFE gasket and you gain nothing beyond the gasket’s own ceiling. Likewise a correct gasket on an undersized stud will still let the joint creep open during a thermal cycle. Understanding the rating means reading all three envelopes and taking the lowest.

Start from the ASME class framework

Flanges and their bolting in North American and most international projects follow the ASME B16.5 class system for pipe sizes up to NPS 24, and ASME B16.47 for larger diameters. The classes are 150, 300, 400, 600, 900, 1500 and 2500. As a rough ambient-temperature guide for carbon steel, a Class 150 flange is rated near 285 psi, Class 300 near 740 psi, Class 600 near 1480 psi, and Class 1500 near 3705 psi. These figures are a memory aid, not a substitute for the standard’s tables — the exact value depends on flange material and face type, and it falls steadily with temperature.

The class system assumes a matched assembly. ASME B16.5 specifies not only the flange dimensions but the bolt count, bolt size, and minimum bolting material for each class and size. When a gasket, stud bolt & nut order is drawn from that table — rather than from a habit-driven guess — the joint inherits a rating that is actually achievable.

The gasket sets the practical ceiling

Gaskets fall into three families, and their pressure-temperature envelopes are very different from one another. Matching the family to the service is the highest-leverage decision in the whole joint.

Non-metallic (soft) gaskets — compressed non-asbestos fibre, PTFE, and flexible graphite — handle the low-pressure utility and chemical duties. Compressed non-asbestos is a Class 150 workhorse up to roughly 200 °C. PTFE gives outstanding chemical resistance but cold-flows, so it stays under about 150 °C and 2.5 MPa, and well away from serrated faces. Flexible graphite reaches the widest temperature band of the group (roughly −200 to +650 °C) but lacks mechanical strength, so it needs a metal eyelet or ｉｎｓｅｒｔ as class rises.

Semi-metallic gaskets — led by the spiral-wound gasket — are the refinery and petrochemical workhorse. A stainless winding strip (304 or 316L) is wound with a graphite or PTFE filler and finished with inner, outer, or both centering rings. The structure absorbs thermal and pressure cycling that would tear a soft gasket, which is why the spiral-wound gasket carries ratings from Class 150 all the way to 2500, and with a mica or Inconel filler can follow the flange to roughly 870 °C. EZ Steel Industrial supplies spiral-wound gaskets in all four style combinations (basic, with inner ring, with outer ring, and with both rings), along with kammprofile and metal-jacketed types for heat exchangers and large diameters.

Metallic ring-joint gaskets — octagonal R and oval RX / BX profiles — are the only sensible choice at Class 600 and above, and non-negotiable for hydrocarbons above roughly 600 °C or anywhere a leak is unacceptable. The solid-metal ring deforms into the flange groove under bolt load, which gives it a far higher pressure envelope than any fibre or spiral-wound gasket of the same size. For these joints, the practical pressure ceiling usually moves from the gasket to the flange geometry and the bolting.

Stud bolt and nut temperature limits

Bolting is the component people actually temperature-derate last, yet it is the one most exposed to long-term heat. ASTM A193 covers the stud material and ASTM A194 the nut, and the two are designed as a matched pair. In high-temperature hydrocarbon service the default is A193 B7 (chrome-molybdenum, quenched and tempered), rated to roughly 538 °C and matched with an A194 grade 2H nut. Where sustained elevated temperature and creep resistance matter more, A193 B16 advances the ceiling to about 595 °C with an A194 grade 7 nut. Stainless grades B8 (304) and B8M (316) carry an oxidation-applicable window up to roughly 800 °C in clean services, but they relax more under heat and need care on chloride duty, where B7M or a 321/347 grade may be the wiser pick.

Two heat-related behaviours deserve attention. First, creep relaxation: at elevated temperature, the stud slowly sheds its initial preload, so the joint must be re-tightened or designed with a higher initial stress. Second, differential thermal expansion between a stud and its nut — which is exactly why the nut is specified harder than the stud and why the pairing is not left to the site stores. A gasket, stud bolt & nut set ordered from a single source arrives as a matched, heat-traceable unit instead of three unrelated parts that may not share an envelope.

Putting the three envelopes together

A worked example makes the logic concrete. Imagine a saturated-steam line designed to Class 300, running near 400 °F. The Class 300 welded-neck flange is comfortably within its envelope. But a buyer who slips a compressed non-asbestos gasket into that joint caps the whole assembly at roughly the gasket’s own ceiling around 200 °C — squarely on the steam temperature — so the joint leaks or blows through before the flange is ever stressed. The correct call is a spiral-wound gasket with a graphite filler and outer ring, which sits comfortably above 400 °F, paired with A193 B7 studs and A194 2H nuts, which are rated well above the same temperature. Now the flange, the gasket, and the bolting all share one envelope, and the joint can actually be rated at its class value instead of a phantom number.

This is why reading a single component’s table is not enough. The rating that reaches the datasheet is the minimum of the three, usually driven by the gasket in mild service and by the bolting in hot service. Flag the operating pressure, operating temperature, fluid, and flange class in your inquiry, and the correct gasket, stud bolt & nut combination can be matched against your tables in one pass.

Ask for the table, not just the class

When you receive a quotation, insist on the pressure-temperature envelope for the exact material combination, not a bare class tag. For the flange, that means the ASME B16.5 or B16.47 table for your material group. For the gasket, it means the manufacturer’s P-T curve for the specific filler and ring style. For the bolting, it means the A193 grade and its temperature ceiling together with the A194 nut grade. EZ Steel Industrial’s gasket, stud bolt & nut line and pipe flange range are supplied with matched sets, delivered with mill test certificates and positive material identification, so the rating you specify is the rating you actually bolt up.

If your project still treats a gasket as a consumable and a stud as a generic rod, that gap is exactly where flange leaks, unplanned shutdowns, and rework costs come from. Pressure-temperature rating is not paperwork — it is the single most reliable check that a flange joint will hold what the design says it will hold.