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.
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.
The following points are critical:
Fig:Chemical tanker pressure vacuum valves
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:
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.
Fig: Failure of venting system leading to Cargo tanks explosion
PV valves maintenance
The correct maintenance of these valves is essential to the safe operation of the vessel.
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.
Related Info
Pumps operation parameters - risk of cargo pump overload or underload
Risk & hazards of chemical contamination onboard
Cargo hoses handling ,connection and use
Control & operation of centrifugal pumps
How to test a tank environment prior entry ?
How to determine the level of a liquid in a chemical tank
Chemicaltankerguide.com is merely an informational site about various aspects of chemical tankers and safety tips that may be particular value to those working in: Chemical Handling, Chemical Storage, Liquefied Chemical Suppliers, Chemical Shipping, Chemical Transportation, Chemical Terminals, Bulk Chemical Services and Chemical Processing. If you are interested in finding out more about chemical tanker guideline please visit IMO official website. For any comment please Contact us
Copyright © 2011 Chemical Tanker Guide.com All rights reserved.
- 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 disabled.
Fig:Chemical tanker pressure vacuum valves
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:-
- 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.
A notice 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 rate.
Fig: Failure of venting system leading to Cargo tanks explosion
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
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.
Related Info
Pumps operation parameters - risk of cargo pump overload or underload
Risk & hazards of chemical contamination onboard
Cargo hoses handling ,connection and use
Control & operation of centrifugal pumps
How to test a tank environment prior entry ?
How to determine the level of a liquid in a chemical tank
Chemicaltankerguide.com is merely an informational site about various aspects of chemical tankers and safety tips that may be particular value to those working in: Chemical Handling, Chemical Storage, Liquefied Chemical Suppliers, Chemical Shipping, Chemical Transportation, Chemical Terminals, Bulk Chemical Services and Chemical Processing. If you are interested in finding out more about chemical tanker guideline please visit IMO official website. For any comment please Contact us
Copyright © 2011 Chemical Tanker Guide.com All rights reserved.