Check pressure standard and interpretation of check valve
Check pressure refers to the minimum inlet pressure required for the valve to close automatically, and its value depends on the valve type and design:
1. Swing check valve: 0.035-0.05MPa, relying on the balance between the valve disc gravity and the medium pressure to achieve sealing.
2. Lift check valve: 0.02-0.03MPa (API 594 standard), using the valve core deadweight to close, more sensitive to low pressure conditions.
3. Double disc check valve: 0.01-0.02MPa (EN 12334), spring-assisted design suitable for vertical pipes.
Key factors affecting reverse pressure of working conditions
Reverse pressure refers to the pressure bearing capacity of the valve when the medium flows back, which needs to be comprehensively evaluated in combination with the following parameters:
1. Valve material: cast iron valve body pressure bearing ≤1.6MPa, stainless steel valve body can reach 6.4MPa (ASME B16.34).
2. Medium characteristics:
- Liquid medium: The reverse pressure is usually 1.5 times the rated pressure (ANSI/FCI 70-2 standard).
- Gas medium: A 20% safety margin needs to be added, such as the reverse pressure limit of PN16 valve is 19.2MPa.
3. Installation direction: The reverse pressure is reduced by 30%-50% when installed vertically (due to the weakening of sealing due to gravity).
Project case: A swing check valve was used in the circulating water system of a power plant. When the reverse pressure was measured to be 1.2MPa, leakage occurred. Later, it was replaced with a spring-assisted model (with a pressure of 2.0MPa) to solve the problem.
Selection and maintenance recommendations
1. Selection formula: Reverse pressure ≥1.5×system maximum working pressure (API 6D recommendation).
2. Special working conditions:
- Medium containing particles: Use rubber sealing surface, and the check pressure is increased to above 0.04MPa.
- High temperature steam: chrome-molybdenum steel valve body is required, and the reverse pressure bearing capacity decreases by about 15%.
3. Failure prevention: Regularly check the wear of the sealing surface, and replace the elastic element of the spring-type valve every 2 years.