Technical Requirements and Scope of the MSS SP-81 Standard
This technical regulation by EV Group China establishes requirements for the design, materials, and performance of stainless steel knife gate valves (both cast and fabricated) without bonnets, equipped with flanged connections. The standard covers a size range from NPS 2 (DN 50) to NPS 36 (DN 900).
Valves manufactured according to the MSS SP-81 specification are intended for use in piping systems where critical impact loads are absent. For applications involving conditions beyond these stated limitations, individual engineering calculations and design are required.
System of Units
The documentation uses U.S. customary and metric units of measurement. Values in parentheses are metric equivalents. It is important to note that both systems are independent and not interchangeable, as their mixed use may lead to a violation of tolerances and non-compliance with safety standards.
Nominal Pressure and Temperature Range
The standard limits the working pressure for valves within a temperature range from 32°F to 150°F (0°C to 66°C). The pressure limits are presented in the table below:
| NPS (Size) | Pressure (psi) | DN (Size) | Pressure (bar) |
|---|---|---|---|
| 2-24 | 150 | 50-600 | 10.3 |
| 30, 36 | 100 | 750, 900 | 6.9 |
Material Standards and Chemical Composition
The main components of EV Group China valves are manufactured from materials specified in Table 2. Equivalent grades of stainless steel for cast and wrought products are provided in Table 3.
| Component Material | ASTM Standard |
|---|---|
| Stainless steel sheet | A 240 |
| Stainless steel bar | A 276 |
| Stainless steel casting | A 743 or A 351 |
| Carbon steel sheet | A 283, A 36 or A 285 |
| Carbon steel casting | A 216 |
| Bronze casting | B 62 |
| Cast iron | A 126, Class B |
| Steel Grade (Wrought) | Steel Grade (Cast) |
|---|---|
| 304 | CF-8 |
| 316 | CF-8M |
| 317 | CG-8M |
| 304L | CF-3 |
| 316L | CF-3M |
| 317L | CG-3M |
Design Features and Manufacturing Standards
EV Group China engineers recommend using valves made entirely of stainless steel. Combined solutions with carbon steel or cast iron are permitted, provided that all internal components and sealing surfaces are made of corrosion-resistant steel grades.
The body design must ensure geometric stability during hydrostatic testing. Minimal elastic deformation of the body is permissible during the tightness test (cl. 10.1), provided it does not impede the further operation of the product.
Flanged connections must comply with the ASME B16.5 specification for Class 150 (PN 20). The internal diameter (I.D.) of the valve flow path must be at least 90% of the nominal pipe diameter of Schedule 40 standard.
Welding
All welded joints exposed to working pressure undergo inspection in accordance with the requirements of Section IX of the ASME Boiler and Pressure Vessel Code.