Fig:Chemical tanker pressure vacuum valves
The following points are critical:
- The alarm settings for the pressure sensors must be set to activate when the tank
pressure or vacuum reaches a reasonable margin of safety above the normal
actuation settings of the Pressure/Vacuum valves themselves.
- These pressures are to be clearly indicated in the Cargo Control Room.
- During cargo operations, loading or discharging, the audible alarm must not be
Vetting inspectors will frequently ask what the alarms are set at and for a demonstration
of the system working. As a guidance as to what level to set the pressure alarms, Masters
are referred to the most recent OCIMF Vessel Inspection Questionnaire (VIQ). Petroleum
tankers should set the high pressure alarms at 10% above the design opening settings
of the pressure valve.
With regards to the low pressure alarms, this will vary depending on the vessel’s inerted
- For inert tanks the pressure in the tank should never be permitted to fall below zero
and so the pressure sensors within the IG system required by the Inert Gas Code and
SOLAS should be set to alarm at positive 200 mmWg (LOW) and 100 mmWG (LOW -LOW).
Should the low pressure alarms sound or there be a failure of the IG system,
discharging operations must be suspended immediately.
With regards to the individual tank pressure sensors, these should be set to positive
50 mmWg in order to give warning in the event that the main IG alarms have failed
to suspend cargo discharging operations. This equally applies where nitrogen has
been taken from shore to provide padding .
- For non-inert tanks, the sensor should be set at a vacuum 10% greater than the
normal actuation settings of the vacuum valves.
Chemical Tankers and smaller tankers not fitted with Inert Gas systems, may, by design,
be capable of discharging to a higher vacuum which could be up to 20% of the P/V valve
opening setting. In such cases Masters should refer to the ship’s specific operating
manuals or contact the office for advice.
is to be posted in the CCR stating:
- Alarm set-point values,
- Procedures to be followed in the event of alarms wounding.
- A notice stating that audible alarms are not to be disabled.
Note that this only refers to alarms fitted on tankers where the alarm system is provided
to meet the requirement for a secondary means of venting. If a vapour recovery system
(VRS) is fitted, a pressure alarm will be fitted in the vapour return line. This must be set
to actuate before the P/V valve design pressures. The USCG requires this alarm to be set
to 90% of the designed P/V valve actuation settings.
Full details of the VEC system will be in the ship specific Vapour Emission Control Manual.
Hi-Jet type high velocity pressure/vacuum valves are designed to provide
protection to individual tanks and are capable of allowing high volumes of tank
atmosphere to pass, as would be the case during loading/discharging. They are also
designed to throw the vented gases clear of the deck area. They are not designed to be
operated in the “jacked-open” position. The maximum PV valve flow capacity is to be
readily available in the cargo control room. This flow capacity is 125% maximum loading
PV valves maintenance
The correct maintenance of these valves is essential to the safe operation of the vessel.
- At each dry-dock all P/V valves are to be overhauled and tested according to maker’s
guidelines and a certificate is to be issued by a competent authority.
- A spare P/V valve for each type fitted is to be carried onboard.
- Each P/ V valve is to be dismantled over a 12 month cycle interchanging with the
spare valve. This is to be done on the ballast voyage with tank open to atmosphere
and all supply valves to the tank shut. Maintenance to be as required by maker’s
instructions. On re-assembly, valve tightness is to be tested using soapy water.
- Each P/V valve is to be numbered and a record to be kept of all maintenance for each
valve maintained in PMS.
Flame arrestor gauzes fitted to cargo related systems are to be checked prior to cargo
operations to ensure that they are free from damages and polymerised substances which
may prevent freedom of vapour flow.
Flame arrestor gauzes
Flame gauzes/screens on P/V valves, Hi-Jet type valves, vapour lines, mast risers, purge
pipes, p/v breakers and on ullage ports are to be inspected as per PMS and replaced as
Flame screens on ballast tank and bunker tank vents must be inspected as per
PMS and replaced, as necessary.
Every inert gas system is required to be fitted with one or more pressure/vacuum
breakers or other approved devices. These are designed to protect the cargo
tanks against excessive pressure or vacuum and must therefore be kept in perfect
working order by regular maintenance in accordance with the manufacturer’s
When these are liquid filled it is important to ensure that the correct fluid is used and
the correct level maintained for the density of the liquid used. The level can normally
only be checked when there is no pressure in the inert gas deck main. Evaporation,
condensation and possible ingress or sea water must be taken into consideration when
checking the liquid condition, density and level.
In heavy weather, the pressure surge
caused by the motion of the liquid in the cargo tanks may cause the liquid in the
pressure/vacuum breaker to be blown out. When cold weather conditions are
expected, liquid filled breakers must be checked to ensure that the liquid is
suitable for low temperature use, and if necessary anti-freeze is to be added.
The P/V breaker(s) are to be clearly marked with their high pressure and vacuum
opening pressures and also with the type and volumetric concentration of antifreeze (if
water filled type), and minimum operating temperature.
During a recent loading operation at Odfjell terminal, it was observed that PV Valves
of the passive tanks (tanks which were not being loaded or discharged) were found to be covered with canvas.
Investigation revealed that intention to cover the PV valves was to prevent ingress of water from the passing squalls.
It is important to note that PV valves are not required to be covered at any time due to simple reason as it has direct impact on the safe working of the PV valve and would defeat the very purpose of the equipment.
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