Valve working pressure at high temperature: reading GOST 356 and datasheets
A PN 25 gate valve does not always hold 25 bar. At high temperature metal loses strength and the allowable pressure drops. Standards build this in, and PN selection must account for it.
TAZ Engineering Team · · 6 min
Nominal, working and test pressure
GOST 356 sets the series of nominal pressures and links nominal, test and working pressure for valves and pipe components by material and temperature. Nominal pressure equals the maximum working pressure at the reference temperature; above it, working pressure reduces according to the tables.
Why material matters
Materials lose strength differently. Grey iron is limited to moderate temperatures; carbon steel keeps much of its strength to about 400 °C; alloy steels go higher. The same PN therefore means different allowable pressures for iron and steel valves at 200–300 °C.
The ASME approach
ASME B16.34 gives allowable pressures in tables by material group and class. The logic is the same. When comparing GOST and ASME offers, compare allowable pressure at your operating temperature, not the nominal numbers.
Worked example
A steam line at about 13 bar and 250 °C may be at or beyond the limit for a PN 16 steel valve depending on material, so PN 25 is often chosen. Cold water at 13 bar is well within PN 16.
What to do
State working pressure and temperature as well as PN in the RFQ. TAZ checks the PN against the standard for the body material and proposes the next class where needed.
Checklist
- working pressure
- working temperature
- body material
- pressure-temperature table
- selected PN
- pressure margin