Check Valves
Special check valves are used in applications where a simple mechanical non-return valve is not sufficient due to variable pressure conditions, high flow velocity, requirements for controlled opening, or the need to limit hydraulic shock. This category includes the Cla-Val 81AF-8 special check valve for aviation fuel systems, configured as a non-surge reverse-flow prevention valve with controlled transition during pump start-up and shutdown.
The 81AF-8 operates using a Cla-Val 100-34 HYTROL hydraulically actuated main valve installed in reverse-flow orientation, with a CV opening speed control, X101 position indicator, and two 81-01 check valves in the control circuit. When downstream pressure exceeds upstream pressure, the auxiliary check valves route the higher pressure to the cover chamber of the main valve, forcing closure and preventing reverse flow. When upstream pressure exceeds downstream pressure, cover chamber pressure is relieved to the outlet side and the main valve opens. This control arrangement provides damped valve movement and reduces surge risk associated with rapid pump transients.
Selection of this valve arrangement is based on fuel flow rate, allowable pressure drop, pump dynamic response, surge potential, required opening time, tight shut-off requirements, and material compatibility with aviation fuels. The control trim and elastomers must be suitable for hydrocarbon service and aligned with the applicable line pressure class and operating temperature range.
During installation, the flow direction must be verified and the cover chamber and pilot control system must be fully vented. The X101 position indicator should remain accessible for inspection and commissioning. Opening speed is adjusted at the CV control; clockwise stem rotation reduces the opening rate of the main valve. Maintenance should include inspection of pilot tubing, check valve function, diaphragm condition, and seat leakage, particularly in systems where reverse-flow isolation is critical to pump protection and line stability.