High Velocity Vent Valves, Vacuum Valves, P/V Valves

High Velocity Vent Valves, Vacuum Valves, P/V Valves

Simple to check, easy to maintain, don’t let them be a problem again.

A recent marine casualty resulted in significant damage to tank structures, piping and components of

a combination chemical / oil product carrier. Vessel-to-vessel loading operations were taking place at

anchorage and one tank became over-pressurized. Subsequently, three cargo tanks and three

ballast tanks on the ship suffered catastrophic structural failures that allowed hazardous cargo to

migrate throughout those areas. This incident caused the vessel to list and created a very dangerous

explosion hazard requiring costly and time-consuming lightering and repair operations. Additionally, it

presented a hazard to the port and persons involved.

The investigation is nearly complete and investigators have identified

several causal factors, one being the failure of the high velocity vent

valve which did not open and prevent over pressurization of the tank

while it was being filled.

The high velocity vent valve is part of the pressure / vacuum

protection piping of the cargo tank. It prevents damage to cargo tanks

by normally staying closed and only opening at a preset positive

pressure. Large shipboard tanks can be easily damaged by relatively

low pressures. Use of these valves minimizes such occurrences.

Other benefits of the valve being in a closed position is that breathing

of the tank is minimized thus preventing unwanted cargo vapor

releases into the atmosphere, protecting the environment and limiting

the loss of cargo due to vaporization. The valves are also equipped

with flame screens to prevent explosions due to ignition. The vacuum

breaker component of pressure / vacuum protection piping of the

cargo tank similarly protects the tank by preventing excessive

vacuum from being formed.

After this specific casualty, the high velocity vent valve of the tank being filled was found stuck closed

and inoperable using the manual test lever. A valve for another tank was also found stuck. Scientific

testing of the substance between the stuck valve disc and the seating surfaces indicated the

presence of oxidized vegetable oil or fat likely from previous cargos. It appears doubtful that

crew members exercised the valves prior to their cargo operations as per their operating procedure.

As a result of this casualty and others involving similar circumstances, the Coast Guard strongly

recommends to vessel Owner / Operators, Crew members, Classification Society Inspectors, Vetting,

and other inspection personal ensure that tank high velocity vent valves and vacuum valves, or

combination pressure / vacuum valves are maintained in operating conditions at all times and are

routinely checked as procedures require.