Stainless Steel High Pressure Flanges

Class 300 is not a pressure. It is a rating that only means something once you say how hot the line runs. A Class 300 stainless flange holds a very different pressure at 400 degrees C than it does at ambient, because the pressure-temperature rating tables derate as temperature climbs, and a flange chosen on class alone can be undersized for the actual duty. Solitaire Overseas supplies stainless steel high pressure flanges from India, Class 150 through 2500, to ASME B16.5 and B16.47. Below: how the class system works, where RTJ facing takes over, and the large-bore standards.

Part of our stainless steel flanges range. For the temperature side of the question, see high temperature flanges.

Technical Data

Class Is a Rating, Not a Pressure ASME B16.5 and B16.47: Where the Handover Happens Raised Face or Ring Type Joint Materials

Class Is a Rating, Not a Pressure

The class number is an index into a pressure-temperature rating table, not a pressure in itself. The same Class 300 flange is rated for one pressure cold and a lower one hot, and the derating is set by the material group as well as the class.

Class	Where It Is Used
Class 150	General low pressure service. The most common class
Class 300	The workhorse for moderate pressure process piping
Class 600	Higher pressure process and steam service
Class 900	High pressure. RTJ facing becomes usual around here
Class 1500	High pressure hydrocarbon and steam service
Class 2500	The highest standard class. Heavy, expensive, and specified only where the duty demands it

The practical rule. Select the class from the pressure-temperature rating table for the specific material group at the design temperature, not from the design pressure alone. The temperature side of that calculation is covered on our high temperature flanges (https://www.solitaire-overseas.com/stainless-steel-high-temperature-flanges.html) page.

ASME B16.5 and B16.47: Where the Handover Happens

Standard	Size Range	Notes
ASME B16.5	NPS 1/2 inch to 24 inch	The standard most flanges fall under. Classes 150 to 2500
ASME B16.47 Series A	NPS 26 inch to 60 inch	Large diameter. Heavier and larger than Series B for the same size and class
ASME B16.47 Series B	NPS 26 inch to 60 inch	Large diameter. More compact, smaller bolts on a tighter circle

SERIES A AND SERIES B ARE NOT INTERCHANGEABLE: They have different outside diameters, bolt circles and bolt counts for the same nominal size and class. A Series A flange will not mate with a Series B flange. On any order above 24 inch, state which series the specification requires, since this is a genuine and expensive mismatch to discover on site.

Raised Face or Ring Type Joint

Facing type changes as class rises, because the sealing mechanism has to change:

Facing	Typical Classes	How It Seals
Flat Face (FF)	150, and low pressure	Full face gasket. Used where a raised face would overstress a mating cast component
Raised Face (RF)	150 through 1500	Gasket compressed on a raised annular area. The general default
Ring Type Joint (RTJ)	900, 1500, 2500 and hot high pressure	Metal ring seated in a machined groove. Metal-to-metal seal, higher integrity

Why RTJ takes over at the top. A compressed gasket relies on bolt load holding it in place, and as pressure rises the load required grows faster than the joint can comfortably deliver. An RTJ seals metal to metal, with the ring deforming into the groove, which holds better at high pressure and high temperature. The trade is cost, machining precision and a joint that needs more care on assembly.

Materials

High pressure duty does not by itself dictate the grade, the service does. Common choices:

Grade	Why It Is Chosen at High Pressure
304 / 304L	General high pressure service without severe corrosion
316 / 316L	High pressure with chloride or chemical exposure
Duplex and super duplex	High strength allows thinner sections, and resists chlorides. Common offshore
904L and 6-moly	Where the medium is aggressive as well as the pressure high

Item	Detail
Material standard	ASTM A182 forged flanges
Dimensional standards	ASME B16.5 (1/2 to 24 inch), ASME B16.47 Series A and B (26 to 60 inch)
Classes	150, 300, 600, 900, 1500, 2500
Facings	FF, RF, RTJ
Types	Weld neck predominantly at high class. Blind, socket weld and others as required
Certification	EN 10204 3.1 or 3.2. PMI, hardness and NDE on request

Flange types are covered separately: weld neck, slip-on, blind, socket weld, threaded and lap joint pages, all linked from the stainless steel flanges hub (https://www.solitaire-overseas.com/stainless-steel-flanges.html).

Frequently Asked Questions

What does Class 300 mean on a stainless flange?

It is a pressure-temperature rating designation, not a pressure. The allowable pressure for a Class 300 flange depends on the material group and the design temperature, and it falls as temperature rises. Always select the class from the rating table at the design temperature rather than from the design pressure alone.

What is the difference between ASME B16.5 and B16.47?

Size range. B16.5 covers flanges from NPS 1/2 inch to 24 inch. B16.47 covers large diameter flanges from NPS 26 inch to 60 inch, in two series. Series A and Series B have different outside diameters, bolt circles and bolt counts for the same nominal size and class, so they are not interchangeable and the series must be stated on any large bore order.

When should I use an RTJ flange instead of raised face?

Generally at Class 900 and above, and in high temperature high pressure service. A raised face joint relies on a compressed gasket held by bolt load, which becomes harder to maintain as pressure rises. An RTJ seals metal to metal with a ring deforming into a machined groove, giving higher joint integrity at the cost of machining precision and more careful assembly.

Which stainless grade is best for high pressure flanges?

Pressure alone does not decide it, the service does. 304 and 316 cover most general and chloride-exposed duty. Duplex and super duplex are chosen where high strength allows thinner sections and chloride resistance is needed, which is why they are common offshore. 904L and 6-moly are for aggressive media rather than for pressure as such.