FLAME ARRESTORS AND SHIELDING
The intention of the requirement to shield the valve discharge was to reduce the possible danger to personnel from flame emission. However explosion testing has shown that whilst a flame arrestor will work satisfactorily when shielding is not fitted, when shielding is fitted, the energy from the discharge is focussed in one direction, and there will be an emission of flames during an explosion. The fitting of shielding also reduces the effective outflow area of a valve.
Since July 2002 it has been a Lloyds Register rule requirement to fit flame arrestors and to test the relief valve with any proposed shielding to be fitted to the valve when installed on the engine.
A test procedure for new crankcase relief valves has been developed over the past four years by MAN B&W, The Physical Test Institute in the
The purpose of the test is to:
· verify the effectiveness of the flame arrester.
· verify that the valve closes after an explosion.
· verify that the valve is gas/air tight after an explosion.
· establish the level of over pressure protection provided by the valve.
Inspection and Testing of Valves in Service.
Explosion relief valves are to be periodically inspected visually. The valve should be inspected for damage, deformation, leakage and loose fittings on a monthly basis. Particular attention should be paid to the condition of the flame arrester to ensure that it has not become choked. It is vitally important that the flame arrester is in good condition. A damaged flame arrestor will render it useless, and will result in the ignition of the oil mist outside the crankcase.
The O-ring used for sealing the valve is subject to hardening and should be renewed within 5 years of delivery. Only manufacturers supplied spare O-rings should be used in the renewal process.
Crankcase relief valves are subject to survey. This usually entails stripping the valve for inspection, and renewing the O ring seal. After reassembly the valve is tested using a spring balance to measure the force needed to open the valve.
Hoerbiger recommend testing their valves using a pressure rig to ensure that the valve opens and seals again after opening: Their instructions state:
Air pressure testing of explosion relief valve is to be carried out using a suitable test rig assembly. The valve seat must be clean before lowering onto the rig. With the valve bolted to the test rig open the air supply ball valve connection to atmosphere and record the gauge pressure. Close the ball valve.
Connect a clean 3 -10 bar air supply to the test rig and open the ball valve. Audible chattering of the valve plate opening/closing will be observed. Close the ball valve and observe the gauge pressure. A pressure indication of between 40 and 60 mbar should be held for one minute to demonstrate the valve tightness.
If the pressure is not held for one minute, the valve should be dismantled and the rubber Oring renewed.