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Pressure/Vacuum (PV) valves for chemical tankers

Pressure/Vacuum valves are designed to provide protection of all cargo tanks against over/under pressure and provide for the flow of small volumes of tank atmosphere resulting from temperature variations in the cargo tank(s) and should operate in advance of the pressure/vacuum breaker, where IG system is in use.




Secondary PV System

To meet the requirements of SOLAS II-2, Part C, Reg. 11, 6.3.2, a secondary means of full flow venting for all tanks is to be provided – this is typically the combination of IG line P/V breaker / vent rise plus individual tank P/V valves. An alternative means of compliance is for a pressure monitoring and alarm system to be fitted. As this is the secondary means, the alarm is required to go off at a higher pressure than the P/V valve.

Chemical tanker pressure vacuum valves

Fig:Chemical tanker pressure vacuum valves


The following points are critical:
  1. 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.
  2. These pressures are to be clearly indicated in the Cargo Control Room.
  3. During cargo operations, loading or discharging, the audible alarm must not be disabled.

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 state:-
A notice is to be posted in the CCR stating:
  1. Alarm set-point values,
  2. Procedures to be followed in the event of alarms wounding.
  3. 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 rate.


PV valves maintenance

The correct maintenance of these valves is essential to the safe operation of the vessel. 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 necessary.

Flame screens on ballast tank and bunker tank vents must be inspected as per PMS and replaced, as necessary.


P/V Breaker

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 instructions.

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.


Recommendations:
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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