Equipment Available Onboard

There are two types of equipment on board that ensure the tanks are not exposed to their designed pressure, these are:

• The Pressure Vacuum Relieve Valve.
• The Secondary Pressure Vacuum Monitoring System including pressure sensors inside the tanks and on the vent line.

Pressure Vacuum Relieve Valves

These valves are designed to provide protection for all the cargo tanks against over-pressurisation and under-pressurisation and should operate prior to the pressure vacuum breaker, where an Inert Gas system is in use. A Pressure Vacuum Relieve Valve is made up of two valves:

• The Pressure Valve.
• The Vacuum Valve.

Usually, Pressure Vacuum Valves are set to activate at a pressure of 2000 mmWG (millimetres of water gauge) and a vacuum of -350 mmWG. These valves are extremely important. 

When loading a closed tank, the cargo quantity increasing inside the tank causes the pressure inside to build up since the air is not allowed to escape. Once the pressure reaches 2000 mmWG, the pressure valve will lift and allow the air or inert gas (if the tank is inerted prior to loading) inside the tank to escape. Once the pressure reduces below the set pressure of the Pressure Vacuum Valve, the pressure valve will close.

On the other hand, when a vessel is discharging its cargo, a vacuum is created. As the negative pressure of the Pressure Vacuum Valve is reached, the vacuum disk lifts and allows the air to flow inside the cargo tank reducing the vacuum as a result. 

With that said, it is extremely important to note that this procedure cannot happen when the cargo inside the tank carries a risk of fire or explosion because then we would be introducing Oxygen which as discussed in the previous lesson is one of the factors of the fire triangle. 

In such a case, it is important for personnel to ensure no entry of air is allowed, and instead, inert gas is introduced constantly during the discharge operation ensuring no vacuum occurs.

Secondary Pressure Vacuum Monitoring System

SOLAS Chapter II-2, Part C, Reg. 11, 6.3.2, states that;

‘’A secondary means of allowing full flow relief of vapours, air or inert gas mixtures shall be provided to prevent overpressure or under pressure in the event of failure of primary means of venting’’

The most common secondary means of venting fitted onboard chemical tankers nowadays consist of pressure sensors. The purpose of these sensors is to alert the operator by means of an alarm in the case that the primary method of venting, therefore the Pressure Vacuum Relieve Valve, fails.

As per SOLAS II-2, Reg 6.3.2, pressure sensors fitted on each tank will be considered as an acceptable alternative to the secondary means of venting.

‘’Alternatively, the pressure sensors may be fitted in each tank protected by primary means of venting, with a monitoring system in the ship’s cargo control room or the position from which cargo operations are normally carried out. Such monitoring equipment shall also provide an alarm facility which is activated by detection of over-pressure or under pressure conditions within a tank.’’

Since this is the secondary means, the alarm is required to go off at a higher pressure than the Pressure Vacuum Relieve Valve. OCIMF (Oil Companies International Marine Forum) recommends a pressure setting 10% higher than that of the primary means. So if the primary means is set at 2000mmWG the secondary means would be set to 2200mmWG.

The following should be kept in mind when it comes to the secondary means of a pressure vacuum monitoring system:

• The alarm settings for the pressure sensors are set to activate at a pressure above the normal setting of the Pressure Vacuum Relieve Valve, as mentioned above.
• The set pressures must be clearly indicated inside the Cargo Control Room.
• During cargo operations, the audible alarm cannot be disabled.

In regards to the low-pressure alarms, this depends on the ship’s inerted state:

• For Inert tanks, the pressure inside should never be allowed to fall below zero and therefore, the pressure sensors within the Inert Gas system, as required by SOLAS, should be set to alarm at 200 mmWg and 100 mmWG.
• Discharging operations must be suspended at once if a low-pressure alarm sounds or the Inert Gas system fails. This is done to ensure that the pressure does not continue to fall.