A valve class such as 300 or 2500 is a designation that selects a row in a rating table, not a pressure in psi. The pressure that row allows depends on the body material group and on the design temperature, and a lower rated seat, seal or bellows can cap the assembly below the class. This note explains how the rating is actually determined, what an unlisted grade needs before it can be rated, and which of those facts belong on a forging enquiry.
A class is a designation, not a pressure
Class 300 does not mean 300 psi, and Class 2500 does not mean 2,500 psi. The class fixes which row of a pressure-temperature table applies. What that row allows depends on the body material and on the working temperature. ASME B16.34 carries those tables for steel and nickel alloy valves, and its published scope is ratings, dimensions, tolerances, materials, examination, testing and marking, with no single pressure tied to a class. [S1]
Ratings are tabulated by material group
The tables are sorted by material group, not by grade name. B16.34 places carbon, carbon alloy and low alloy steels in Group 1, ferrous stainless steels including martensitic, ferritic, austenitic and duplex grades in Group 2, nickel alloys in Group 3 and bolting alloys in Group 4. A carbon steel forging and a stainless forging of the same class do not share a curve, because their allowable stress at temperature differs. That is why the material and class columns on a drawing are read together, and why a class alone tells a forge shop very little. [S2]
Temperature moves the number
Temperature moves the allowable pressure more than most buyers expect. One published table for a 316 stainless needle valve rated Class 2200 in material group 2.2 gives 5,280 psig from -20 to 100 F, 4,541 psig at 200 F, 3,767 psig at 400 F and 3,256 psig at 650 F. The class never changed; the allowable stress did. A valve selected using only its ambient rating can exceed its rating in a hot line if the operating pressure is above the value permitted at that temperature, even though the nameplate still reads Class 2200. [S3]
An unlisted grade has to be rated, not copied
A grade not in the table does not inherit a rating from a similar grade. ASME B16 Case 2 requires the material specification to be listed in Section II of the Boiler and Pressure Vessel Code, allowable stress and tensile and yield strength at temperature to be available, size dependent data to be taken at the lesser value, and then applies the Standard or Special Class rules with their ceiling pressures. A rating is a calculation from stress data at temperature, not a label moved because the chemistry looks close. [S2]
The same document treats Standard, Special and Limited Class as separate routes, and bars flanged end valves from Special Class. Two valves of one size and class can therefore differ in wall and examination because different rules rated them. That decision sits with the valve designer. [S2]
The body may not be the lowest rated part
A rating belongs to the assembly. Seats, seals, bonnets and bellows can cap the valve below its class. In the same catalogue, a bellows sealed valve is listed at Class 1500 on the pressure boundary components, with a note that it must not run above the 2,500 psi maximum allowable working pressure of the bellows. The forged body may be good for more. The line is limited by the lowest rated component, so process conditions belong with whoever selects trim and seals. [S3]
What belongs on a forging enquiry
For a forging order, give the class, the grade and its standard, the design temperature and the service, and say which standard the finished valve will be rated to. We forge the body or cover to that grade, heat treat to the cycle the grade and order call for, and test to the scope agreed on the order. We do not issue the valve rating. A material certificate proves the steel is F316, not that a valve is Class 600 at 400 F.
Name the grade standard precisely. ASTM A105 covers carbon steel piping components for ambient and higher temperature service; A182 covers alloy and stainless components for high temperature service and says grade selection depends on design and service requirements. Temperature is part of the grade choice before the class table is opened. Correspondence is in English, and a complete enquiry usually gets a quotation within 24 to 48 hours. [S4] [S5]
Related Reading
Frequently Asked Questions
Does Class 300 mean the valve is good for 300 psi?
No. The class selects the table row, not the pressure. Read the rating for the actual material group at the design temperature. At moderate temperature a Class 300 carbon steel valve may be allowed well above 300 psi, while a stainless valve of the same class may be allowed less.
Can the forging supplier certify the valve rating?
We certify the grade, heat, heat treatment cycle, mechanical results and NDT within the agreed scope. The rating belongs to the valve design and is issued against the valve standard the designer works to. Our certificate is material evidence underneath that rating, not a substitute for it.
Why do two valves of the same size and class have different wall thickness?
The class is not the only input. Material group changes the allowable stress, design temperature changes it again, B16.34 offers more than one rating route, and corrosion allowance adds to that. The drawing in front of the forge is the result of those decisions, and we work to it.
Published by Zhejiang Longrui Forging Co., Ltd.
Technical reference information. Applicable specifications and inspection requirements must be agreed for each order. Questions about this note: sales@longruiforging.com