In This Article
- 1.ASME B16.5 Pressure Class System
- 2.ASME B16.5 Material Groups
- 3.Pressure-Temperature Rating Table (Class 150 & 300, Group 1.1 WCB)
- 4.When to Use RTJ Instead of RF Facing
- 5.Class Selection Breakpoints — Practical Decision Guide
- 6.Body Material Selection by Pressure Class
- 7.Piping Specification Reference (Typical Industry Practice)
When a process engineer writes 'Class 600 RF flanged gate valve, A217 WC9', the pressure class designation carries more information than a simple pressure number — it encodes the maximum allowable pressure at every temperature up to the material's design limit, the flange facing type, and the body material group. Understanding ASME pressure class is foundational to industrial valve specification.
ASME B16.5 Pressure Class System
ASME B16.5 defines seven pressure classes: 150, 300, 400, 600, 900, 1500, and 2500. The class number is not a pressure in psi — it is a designation tied to a pressure-temperature (P-T) rating table. The actual maximum allowable pressure (MAP) at a given temperature depends on the material group. For example, a Class 150 valve in Group 1.1 (WCB carbon steel) has a MAP of 285 psi at ambient temperature (-29°C to 38°C) and only 170 psi at 260°C. A Class 300 valve in the same material group has 740 psi at ambient and 455 psi at 260°C. The P-T tables in ASME B16.5 Appendix E must be consulted for every combination of pressure class and material group.
ASME B16.5 Material Groups
ASME B16.5 organises flange and valve body materials into Material Groups based on their pressure-temperature allowables. Group 1.1: WCB, A105, LCB (carbon steel) — the most common; Group 1.4: CF8M (SS 316 cast) and F316 (SS 316 forged); Group 2.1: WC6 (1.25Cr-0.5Mo) and F11; Group 2.2: WC9 (2.25Cr-1Mo) and F22; Group 3.3: C12A (9Cr-1Mo-V, Grade 91); Group 6.8: CF3M (SS 316L); Group 7: Alloy 20 (CN7M). The P-T ratings differ between groups — for example, Group 1.4 (SS 316) has lower pressure ratings than Group 1.1 (WCB) at ambient temperature due to the lower yield strength of austenitic stainless steel.
Pressure-Temperature Rating Table (Class 150 & 300, Group 1.1 WCB)
| Temperature (°C) | Class 150 MAP (bar) | Class 300 MAP (bar) | Class 600 MAP (bar) | Class 900 MAP (bar) |
|---|---|---|---|---|
| -29 to 38°C | 19.6 | 51.1 | 102.1 | 153.2 |
| 100°C | 17.7 | 46.6 | 93.2 | 139.8 |
| 150°C | 15.8 | 45.1 | 90.2 | 135.3 |
| 200°C | 13.8 | 43.8 | 87.6 | 131.4 |
| 260°C | 12.1 | 43.8 | 87.6 | 131.4 |
| 316°C | 10.8 | 43.1 | 86.2 | 129.3 |
| 371°C | 9.5 | 38.6 | 77.2 | 115.8 |
| 427°C | 8.1 | 33.6 | 67.2 | 100.7 |
When to Use RTJ Instead of RF Facing
Raised Face (RF) flanged joints with spiral-wound gaskets are appropriate for Class 150 through Class 300 service in most process industries. For Class 600 and above, most refinery and petrochemical piping specifications require Ring-Type Joint (RTJ) facing for hydrocarbon, H₂S, or high-temperature service. The rationale: at Class 600 and above, the bolt loads required to compress a spiral-wound gasket are very high, creating significant bending stress in the flange — a metal-to-metal RTJ joint is more reliable at these pressures. ASME B31.3 Process Piping does not mandate RTJ at Class 600, but most company piping specifications (Aramco, Shell DEP, ExxonMobil GP, Chevron) require RTJ for Class 600 hydrocarbon service as standard practice. Always check the applicable company piping specification.
Class Selection Breakpoints — Practical Decision Guide
- Class 150: up to ~19.6 bar (285 psi) at ambient in WCB — general utilities, water, low-pressure steam, cooling water
- Class 300: up to ~51 bar (740 psi) at ambient in WCB — medium-pressure steam, process gas, general refinery utility
- Class 600: up to ~102 bar (1480 psi) at ambient in WCB — main process pipework in refineries, high-pressure steam, gas compression
- Class 900: up to ~153 bar (2220 psi) at ambient in WCB — high-pressure steam (power plants, IGCC), HP gas compression
- Class 1500: up to ~256 bar (3705 psi) at ambient in WCB — supercritical steam, very high-pressure gas injection
- Class 2500: up to ~427 bar (6170 psi) at ambient in WCB — ultra-high pressure, wellhead Christmas trees, specialty services
Body Material Selection by Pressure Class
Pressure class alone does not determine the body material — temperature and fluid corrosivity also govern material selection. For general process service up to 400°C, WCB (ASTM A216) is appropriate for all classes 150–900. Above 400°C (power plant steam service), WC6 or WC9 is required to avoid graphitisation of carbon steel. For cryogenic service (-46°C to -196°C), low-temperature carbon steel (LCC, ASTM A352) or SS 316L is required — WCB loses impact toughness below -29°C. For sour service (H₂S), body material must comply with NACE MR0175 hardness limits regardless of pressure class. For Class 1500 and 2500 in high-stress service, forged body (ASTM A182 F22, F91, F316) is preferred over cast for better metallurgical consistency and defect-free pressure boundary.
Piping Specification Reference (Typical Industry Practice)
Industrial piping systems are governed by company-specific Piping Material Specifications (PMS), also called Piping Classes. A piping class (e.g., 'A1A' or 'C6B') defines the pressure class, material, facing, gasket type, and bolt material for a specific service. All valves on that line must comply with the piping class. Cross-referencing the piping class specification with ASME B16.5 P-T tables and the applicable material group is mandatory before finalising valve procurement. When the piping specification is unavailable, request it from the project engineering team — specifying valves without a confirmed piping class is a procurement risk that can lead to rejection and re-order.
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