PRESSURE- VACUUM VALVE OR PV VALVE ARE A SAFETY EQUIPMENT USED IN TANKER SHIP TO PROTECT THE CARGO HOLDS FROM OVERPRESSURE OR VACUUM CONDITION. BUT BEFORE WE CONTINUE WE SHOULD KNOW AS TO HOW OVER PRESSURE AND VACUUM CONDITION IS CREATED INSIDE THE TANK.
REASONS FOR OVER PRESSURE:-
- DUE TO FAST RATE AT WHICH THE TANK IS FILLED.
- DUE TO HIGH TEMPERATURE IN DAY TIME THE FORMATION OF VAPOR OF THE CARGO FLUIDS AND WATER VAPOR INCREASES THE PRESSURE INSIDE THE TANK.
REASON FOR VACUUM:-
- DUE TO FAST RATE OF DISCHARGE.
- DUE TO LOW TEMPERATURE AT NIGHT THE VAPORS CONDENSES WHICH LEAD TO DECREASE IN PRESSURE.
PV VALVE IS USED ONLY FOR OVERCOMING MODERATE CHANGES IN PRESSURE INSIDE THE TANK AND NOT THE RAPID CHANGES WHICH MAY BE CAUSED DUE TO EXPLOSION INSIDE THE TANK IF THAT MAY HAPPEN.
PV VALVES ARE NOT USED AS VENTS. THAT WORK IS DONE BY THE HELP OF MAST RISER.
AS WE CAN SEE IN THE PIC ABOVE THAT THERE ARE 2 VALVES, THE VALVE AT LEFT IS FOR OVER PRESSURE AND THE ONE IN LEFT IS FOR VACUUM. THE PRESSURE IS SET BY THE HELP OF THE VALVE STEM.
INSPECTION CARRIED OUT IN PV VALVES:-
- INSPECT THE DIAPHRAGM, GUIDE AND SEATING SURFACE TO ENSURE THAT VALVE OPEN FREELY.
- ALL GUIDES (18) SHOULD BE INSPECTED FOR BUILD UP , CORROSION AND BUILD UP.
- INSPECT GUIDE HOLE IN THE VACUUM COVER (19).
- CHECK METAL SEATING SURFACE FOR PITTING, CORROSION OR PRODUCT BUILD UP.
- FLAME ARRESTER TO BE CHECKED FOR DAMAGE AND FOR CLOGGING.
MAINTENANCE CARRIED OUT IN PV VALVE :-
- DIAPHRAGMS (13,14,24,25) TO BE REPLACED.
- ORINGS (27) AND COVER GASKET (22) TO BE REPLACED.
- IF METAL SEATING IS DAMAGED THEN LAPPING CAN BE CARRIED OUT BY USING FLAT GROUND METAL DISC AND A FINE GRIT EMERY CLOTH ATTACHED TO IT.
- IF STEM IS RENEWED THEN CORRECT SIZE STEM TO BE USED.