PV VALVES, BREAKER AND DECK SEAL ON CHEMICAL TANKERS- CAPT AJIT VADAKAYIL
Failure of PV valves due to freezing, or the sticking effect of high MP chemical vapours , can cause tank over pressurisation and rupture . It hardly takes less than 0.6 kg pressure to flatten like a balloon , the strong corrugated SS bulkheads and rupture it. Over pressurisation can also happen by filling the tanks too quickly, above the max filling rate, which is posted in CCR.
A choked flame arrestor screen , with high MP vapours, or ice while discharging can cause under pressurisation and vaccum rupture.
Company policy requires PV valves to be tested for correct operation by a approved test kit, every 6 months, as a essential part of Planned Maintenance . Pictures must be taken to record that this important task is indeed done. The tank pressure sensors must be tested and calibrated. The relevant company form must be filled and sent to office.
Nothing is served by being dishonest about this. In addition to safety of crew with respect to fire, cancer, bioaccumulation etc , the ship can get hit with huge cargo evaporation claims. Most chemicals (eg- methanol ) are highly volatile . If salt laden air ingresses from vacuum side into the tank , the whole consignment may go off spec. Nitrogen inside tank which is lighter than air will escape, and cause a dangerous situation for crew.
PV valves on chemical tankers are designed to vent gases out which are displaced by chemical while loading. And of course they provide for thermal variations inside the cargo tank. PV valves on chemical tankers operate between +2000mm AQ to -350mm AQ and are clearly marked. The unit may be marked in other notations instead of AQ.
1 Atmos= 760mm HG= 29.92 inch HG= 33.899 feet FW= 1.033 kg/cm2= 14.69 psi= 1.0133 bars= 101.3 kPa (KN m2)= 1013.3 HPa (mb)
1 kpa= 102 mm aq, 1 kg= 100 kpa, 1 mpa= 10 kg.
Lifting PV valves manually prior cargo loading is an essential part of the loading checklist. Prior sailing from berth, it is the chief officers responsibility to ensure the breather valve ( vacuum side ) if used is reset. Regular inspection ( max 3 months ) of flame arrestor of vaccum side must be done , especially after dischg of high MP cargoes.
Most of the chemical tanker vapours have high VD and will sink to the bottom rather that rise up in the air . Common gases like HC , CO2 , H2S are heavier that air. In still air conditions at berth if the eyes water or breathing gets difficult , the reaction should NOT be to wear goggles or face mask , but to ensure adequate precautions if the cargo is flammable, bioaccumultive, carcinogenic or toxic. It may be necessary to stop cargo work completely .
Vapour pockets may travel inside accommodation. During electrical storms , with flammable and static cargo it will be necessary to stop all emission of gases and all keep openings shut .
Most modern chemical tankers are fitted with Pres-Vac magnetic cones which float up and do not shut till most of the pressure in tank has been released, unlike the non-magnetised weighted cones . Once the magnetic cone is forced up by gas pressure >2000mm aq , the magnetic field reduces at the square of the distance introduced.
However they still give protection against flame velocity of 7.5 metres/ sec.
Non-magnetised cones shoot the vapours high above main deck at 2000mm Aq, at >30 metres/ sec upwards and sit down immediately . This is to ensure that the dilution from rich to flammable range happens away from the main deck, for the exit volume and wind speeds.
This also ensures that in case of electrical storms the plume does not cause the PV stack to be ignited and lit up like a candle, which can cause fire to flash back into the cargo tank. Such a scenario can be combated by cutting off the flame with a good quality (28/28 gauge mesh is too small for flame to pass through) flame arrestor sheet on a snorkel, or more easily on ships equipped with nitrogen plant by use of inert gas purge.
With non-magnetised weighted PV cones if the AC intake is on the monkey island, there is danger of gases coming inside accommodation. It is no longer mandatory to have 100% AC recirc inside accommodation at terminal, as a part of the safety checklist.
During loading the PV pressure side cone gets lifted due to pressure buildup inside the tank. Most modern PV valves cannot be jacked and locked open, as there is no flame arrestor on the pressure side. During dischg , the reverse is not true. The vacuum side is not to be lifted due to vacuum build up inside the tank. Air must flow freely into the tank via flame arrestor mesh , to replace the liquid cargo .
In case the ship has a Nitrogen plant, the nitrogen gas will enter the tank , to maintain good positive pressure at full dischg rate.
During cold weather the functioning of PV relief valves should be checked regularly . It is possible that humid air vented from a cargo tank may condense and freeze thus inhibiting ventilation. This is also possible for cargoes with high melting point, such as Phenol, where cargo vapours could solidify in vent line .
If you have high VP cargoes forward whose PV vents get rimed, there will be structural damage. To prevent PV stack from freezing with rime, loose light canvas covers can be used.
The cleaning of PV vent lines is best done with live steam.
Currently, the normal setting on most chemical tankers 2,000 mm WG. To reach a re-seating pressure of 2,000 mm WG, you'd need, say, 300 mm WG for vent line pressure drop and about the same for pressure accumulation in order to avoid any blow-down. A growing number of chemical carriers have 2,500 mm AQ setting. Some European river tankers are now at 5,200 mm WG minimum setting.
Open venting is allowed only when the FP is > 60C and there is no inhalation hazard to crew. However this must be done via a flame screen. Pipe flow resistance includes resistance of flame screens.. Check IBC code chapter 17 for references to open , closed or controlled tank venting.
Vents heights can be reduced from 6 metres if fitted withing 4 metres from catwalk. This height can be reduced to to 3 metres ( above weather deck ) if the exit velocity of the high velocity vent exceeds 30 metres/ sec. It must be 15 metres from the accommodation openings/ vents or machinery spaces if the cargoes are toxic, otherwise 10 metres.
No isolating valves can be fitted to individual PV stacks of chemical tankers.
Flame screens also serve to cool the passage of any flame. Flame screen area should be >150% of the vent pipe area.
Flame arrestors work on the principle of quenching.
Pure parcel chemical tankers WITHOUT Nitrogen plant do not have a PV breaker . Water reacts with some chemicals , cargo vapours may be incompatible and in any case every cargo tank has a dedicated PV stack.
Annex 1 ships with IG and compatible cargoes do have a PV breaker, designed to protect the tanks against excessive pressure or vacuum.. Ensure the correct nonfreezing liquid ( water: glycol 5)% each ) is used and the liquid column head for the relevant density is to the correct mark.
When reading this level, you must ensure that the deck main pressure is zero. After a bout of rough seas, the level must be checked to see if there has been a pressure surge blow out lift. Condensation or evaporation may cause changes in level. The PV breaker must be clearly marked with the pressure vacuum lifts.
Pure parcel chemical tankers do not have a deck seal as some chemicals like Sulphuric acid are water reactive. Annex 1 tankers with IG plant do have it, to maintain a NR water seal between cargo tank and IG plant. Ensure there is never a water carry over into the IG delivery line with venturi type seals, due to wear and tear of orifice .
Regular checks must be done in freezing weather to see if is operational and filters are clear. Alarm systems must be regularly checked by Chief engineer. Deck seal and the manual primary NR swing valve must be opened up as a company policy at least every 12 months.
Pressure test of cargo tanks and vent lines with p/v valves are to be carried out as follows ( WITHOUT TEST KIT ):
Overhaul p/v valve
Put the valve manually in closed position
Connect air hose with calibrated pressure gauge to the vent line
Batten down the tank being tested , close all cargo valves, open the air and keep an eye on the pressure gauge. When pressure achieves 0.21 bar ( set pressure ) the p/v valve should open. If valve remains closed with pressure exceeding 0.21 bar, the valve requires repair.
Don’t pressurise tank more than 0.3 bar.
Close air and thoroughly inspect all opening, deck above the tank, vent line. If you don’t hear air leaks/hissing and pressure 0.21 bar remains stable during 30 min the tightness of the tank/lines is OK.
Usually p/v valve vacuum setting is 0.035 bar. The aim of testing is to check if the valve opens at set vacuum.To test p/v valve for vacuum setting the following procedure is to be carried out:
Plug the vent line from the tank.
Connect Wilden pump or Graco pump with pressure/vacuum gauge to vent line and start pumping with slowest speed to create vacuum in the line
Monitor the gauge and when vacuum (negative pressure ) reach 0.035 bar p/v valve should open. If valve remains closed at vacuum exceeding 0/035 bar, it needs repair.
On chemical tankers the requirement for PV to be able to handle 125% of loading rate –must also include VAPOR GROWTH due to turbulence.
Punch into Google search
THE NAIVE SAILOR, MAGNETISED PV VALVES- VADAKAYIL
--- to check out what nonsense nay-- bullshit goes on at sea, just because the manufacturers are HOLY COWS!
Nowadays all chemical tankers are equipped with a test kit for pressure and vaccum tests-- see below.
Below- Testing without removing the PV valve.
Below-- See the ring magnets are all eaten up and have gone back to mother element earth.
CAPT AJIT VADAKAYIL