Thursday, September 27, 2012



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 ):

Pressure test

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.

Vacuum test

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 


---  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.



  1. hello sir
    one more great article; thank you very much.
    already on 1st position on Google image search.

    can you please explain "...However they still give protection against flame velocity of 7.5 metres/ sec."
    flame travels at 3 m/s in air and 20 m/s in a pipe like structure like p/v valve, then how can 7.5 m/s exit velocity stop the flame from entering

  2. hi bhoj,

    when the exit velocity of flammable gas from PV vent ( which has no flame arrestor) is greater than the flame travel velocity-- it can catch fire.

    the flame will burn like a candle.

    however the flame will remain outsise the tank--it will NOT be drawn in.

    this is why, when the PV catches fire during loading-- do NOT stop loading, till you extinguish the flame by IG / nitrogen or by cutting off the flame..

    capt ajit vadkayil

    1. Dear Sir,

      I just got the opportunity to read your update regarding PV importance and testing method. even though i have tested several PV valves in the past by using a water coloumn method i didn't know exactly how does it works and what is the correct method of testing.(oil & gas tank PV's).
      currently i am working on ship's PV valves and wish to understand in details with industry standards.
      could you please advise me the following;
      1. best testing method for PV instead of water coulmn method.
      2. testing standard (ABS/BV/ DNV code.
      3. there is few types of PV and what is the common types and how does it works or perform on the cargo tanks.
      4. i am still no clear about the PV on the toxic application, which is not allowed to inhale the outside air. please advise.

      Your kind advise is highly appreciated.
      Thanks & Regards,
      V. Ashok Kumuar,


  3. Good post sir

    i am a follower of your blog for two years since i first read about heating coils on a chemical tanker i started my carrier as a deck cadet on a chemical tanker i learned everything the hard way i will mention some experience chief officer washing ss tanks with hot water containing residues of caradol and it get polymerized and as a cadet i was manually cleaning the tanks for continuous 36 hrs then same chief officer loading stearic acid and do not know how to control the heating i was running at night as soon as we cross the straights of Gibraltar for adjusting temperature(All Indian officers and crew if i ask something either they will get angry or tell me to find my self and in a chemical tanker sailing in Europe you will know how much time a cadet get to learn )

    some how i manage to get my degree and now in Singapore doing my MBA in ship brokering and vessel chartering part time working in the same field

    the way you giving away your knowledge for free is awesome sir and your initiative to show the real truths is wonderful keep going sir

    I have a request sir can you post a page teaching how to test the BHC of a mooring winch all of my ship all chief officers were copying from the former one and taking beautiful pictures and giving us fundas
    we will be stenciling the date and final photo and it is done for finishing touch pump man will adjust the pointer and re paint it

    I Never got a chance to work as an officer on board but i think that was a good decision future officers like me knowing nothing and not properly trained will be like my chief officers one day (Shikkari Shambhoos) i am happy now learning the trade being in the sea helped me a lot but for thirst for the open sea sailing on board a vessel trough the seven seas still in side my hart

  4. hi vishnu,

    i have put a post already--


    this is the biggest bullshit going on at sea.

    all fools of the first order !

    the sea is NOT a place for intelligent people.

    this is why i did NOT allow my sons enter this field.

    i am telling this after 4 decades at sea.

    capt ajit vadakayil


  5. I love your Posts - infact came across it recently. I'm also an Ex Ch. Engineer from Oil Tankers & had sailed for almost 30 yrs. I've a craving to visit Calicut as that's where my so called intellectual birth took place way back in 60's in REC Calicut (Now NITC). Infact, Sir your posts are proving to be quite helpful to me as now I pass time in delivering lectures in Maritime Traning Institute for Post Sea Tanker Students.
    Thank you very much & Keep up the Good Work - May God Bless you

    A K Kulshrestha

  6. nice to hear from a sea brother

    thank you anil-saab!

    capt ajit vadakayil

  7. Dear Sir,
    Himanshu here , iam doing my mates from delhi.
    i have a question regarding pv valve setting . how do we determine the pressure setting for PV valves.
    Thanks for all your posts , they are really helpful.

    1. hi himanshu,

      PV valves on chemical tankers operate between +2000mm AQ to -350mm AQ and are clearly marked.

      you will find these values in the class endorsed vapor return system manual and the class approved P@A manual.

      in certain strongly built chemical tankers which are approved to carry propylene oxide -- you have a higher setting -- so that the BP of the liquid is NOT breached in high ambient temperatures..


      capt ajit vadakayil

  8. Hello Captain Ajit

    I just wondering about the incident occurred recently in Labuan, Malaysia according to Methanol vessel tank explosion.

    The vessel was suspected using PV Valve to control the overpressure and underpressure how come that the PV Valve still can catch fire?

    and if so..what is the best action to take by the crew?

    Is it possible that the fire on PV Valve can enter into the tank and explode?

    or maybe the fire was started inside the tank and explode due to electrostatic charge or something like that?? do you have any idea???


  9. hi mohd,

    this explosion happened during methanol loading at labuan .

    the PV riser was struck by a bolt of lightning.

    when the PV riser catches fire, loading MUST CONTINUE to prevent the flame from being drawn into the tank -- till it is extinguised by injection of nitrogen into the riser pipe.

    also punch into google search--


    capt ajit vadakayil

    1. dear ajit
      this is interesting , appreciate more technical details on this subject.

  10. dear sir, in the above explanation kindly confirm if the vacuum side as u stated is 0.035 bar or is it "-ve 0.035" bar??
    best regards Pritish

  11. Hello sir,
    on what principle does High Velocity Valve works?

    1. hi sudhanshu,

      it is a floater type relief valve to prevent cargo tank rupture-- during loading and also during sea carriage of heated chemicals and high VP chemicals.


      gravity model and magnetic model is in use.

      capt ajit vadakayil

  12. Good day Sir,

    I have been following your articles for quite sometime now and cant be silent to say they are of great help.
    I had a question for you if you could comment whether the PV breaker requires to have a flame screen to it?if no then under what regulation.

    Best Regards,
    Capt Gagandeep Singh

    1. hi gagan,

      any orifice to a cargo tank which cannot expel the gas greater than the flame travel velocity needs a flame screen.

      the is the reason why the pressure side of PV valve does NOT have a flame screen , while the vacuum ( breather ) side has it.

      IMO had a standard tanker equipment publication for this, which will be in your ship's library on the bridge.

      capt ajit vadakayil

  13. hii
    sir plz advise me on the following
    What will happen on the PV breaker, when level of the liquid is less or more than the required. also how to correct it???

    1. hi vc,

      the PV breaker has to be filled with a liquid.

      the manual will tell you what liquid is to be filled , and what height in the glass tube.

      35 years ago, we used to fill turbine oil. nowadays annex 1 oils are NOT allowed due to marpol rules.

      so now you fill water plus anti freeze.

      a PV breaker must lift at the correct pressure.

      if you fill more tha tbak will rupture.

      on an oil tanker ( say VLCC ) it hardly takes 0.3 kilos to rupture a tank on a new ship.

      you fill 2.1 kg air in your car tyre.

      if you fill too less the PV breaker lifts early . and then you have to stop cargo work to fill the liquid.

      capt ajit vadakayil

  14. Dear Capt, I have just started following your blogs and find them very useful. Could you please help clearing following doubt? The fwd fairleads on my last 2 ships VLCC (for use with SPM and ETA) had 2 SWLs marked on it. (Permanently bead welded) 350 T for ETA and 200 T for SPM (I may be wrong in recollecting It may be the other way around)
    But my doubt is it does not make sense to mark 200 T when the equipment is certified for 350 T. Why is it done? Is there a requirement?? May be by terminals// Oil majors??
    Thank you

    1. hi uj,

      OCIMF requires that tankers of 50,000 dwt and above to provide:

      a chock (fairlead) arrangement, with suitable reinforcement, having a minimum SWL of 200 metric tonnes; and

      a strong point arrangement, with suitable reinforcement, having a minimum SWL of 200 metric tonnes when used with a single eye towing line or grommet.

      (Note: I metric tonne force = 9.81 kilo Newtons).

      it does NOT matter what a crane can lift -- all that matters it that there must be a valid cerificate specifying the SWL.

      capt ajit vadakayil



    1. hi rohit,

      the PV is supposed to life automatically if the cargo is inflammable or toxic.

      it will lift at 2000 MM AQ --

      you are supposed to test the lifting pressure, say every 6 months , and record it with masters and chief officers signatures-- and stencil it too.

      before every loading you are supposed to lift manually and record-- to prevent freezing of movable parts.

      capt ajit vadakayil

  16. Hello sir,
    This is Aditya
    Can you please tell how much back pressure a wet type deck seal can take from tanks and why does the liquid present in the deck seal does not go to the engine room, no matter how much is the pressure and that's why it is known as fail safe device?
    Thank you

    1. hi ap,

      when a tank gets pressurised--

      the tank PV lifts first.

      then the PV breaker ( oil tankers ).

      then the deck seal, which is basically a NON-return valve ( using sea water ).

      the pressures depend upon the equipment fitted and the tank structural strengths--which is available in the P& A manual.

      capt ajit vadakayil

  17. Hello,
    These include Waste Tankers, non-hazardous, hazardous and full A.D.R., Jetting Tankers and Combination Units. Specialised vehicles for Airport maintenance and Road applications and Rail applications are also in our range.

  18. Dear Sir
    can you kindly clarify whether the pv valve has the capacity to escape gas at a rate higher than flame velocity at pressure less than 2000mmAQ(when pv valve is lifted manually)

    1. hi vsk,

      sometimes you lift the PV valve manually, to release gas pressure before doing UTI sampling etc.

      you must know that the vaccum side has a flame arrestor-- and gas can be released from here . .
      the pressure side ( has NO flame arrestor ) can be lifted if the conditions are good.

      capt ajit vadakayil

  19. hello sir,
    i am anish can u put the actual photographs of the ig system used in a tanker
    not the schematic diagram,i need the full system

  20. Hello sir,
    Can you please explain difference between Flame screen and Flame arrester

    1. hi g,

      A spark arrestor works by trapping large carbon particles in the exhaust system. It prevents the emission of flammable debris from combustion sources.

      A flame arrester works by forcing a flame front through channels too narrow to permit the continuance of a flame—like a wire mesh.

      In school you held a flame screen on a Bunsen burner and lit the gas on top of the wire mesh only.

      capt ajit vadakayil

  21. Thank you very much sir for sharing your immense knowledge.......
    I had a doubt regarding PV valve .......... it gets lifted automatically when MARVS is reached and the pressure is released.........
    understand that PV valve is set to operate at that pressure............
    but as soon as it operates the pressure must fall below MARVS then why doesnt the PV valve shuts????


    1. hi as,

      punch into google search-

      capt ajit vadakayil

  22. Are you going to do a follow up article? Would love to know what happens next.

    pneumatic valves

  23. Sir,

    Haven't been blogging much.
    But as your articles are of great help to us, I just wished to say Thankyou.

    I am a deck cadet onboard a chemical tanker.

    You post have been of great help to understand how things work. I have a lot of interest in "How does it happen?"

    And it helps me to complete my DLP after understanding things properly.

    Cadet Prabhjot Singh Maan

  24. thank u in advance
    hv a gud day sir

  25. Good Day, Captain Vadakayil

    I am CHO on the bitumen tanker and doing one risk assessment: chipping cargo tank hatches
    I have a question regarding fire/ exposion hazard, and could not find an answer:

    Will it be risk of explosion if cargo tanks if they have a negative constant pressure abt -33mBar with CH4 gas inside between LEL and UEL?

    Thank You

    1. hi in,

      put you readings on the graph-

      help yourself !

      capt ajit vadakayil

  26. Our pneumatic factory was founded in 2000,CHECK HERE. In decades of serving China pneumatic industry, we have built our reputation on product knowledge, world class suppliers, application support, and value add services. Our commitment to our employees, customers and vendors is the core of our business beliefs. In our factory, there are more than 80 front-line workers, 3000 squares working shop.Our main products are air Pneumatic Cylinder, solenoid valve and also a lot of pneumatic fittings like Air Gun.

  27. Hello Sir

    I m an Engine cadet preparing for my class 4 exams. i have a doubt on IG system.
    From IG main line , it branches to supply to individual tanks through a butterfly valve for each tank. what will be the position of that valve during loading and discharging and PV valves and PV breakers comes into action at what time?
    Waiting for your reply
    Thanks Sir

    1. hi u,

      capt ajit vadakayil

  28. Sir plz guide .....are high velocity vent and PV valve both same?...if pressure inside tha tank falls then outside air is suck into the tank...wont this air increase oxygen content in the tank?....during sounding the tank wont some IG escape through sounding pipe n depressurize the tank?


  29. Dear sir,

    I love your blogs and learned a lot from it. My question sir, suppose sir during discharging in a normal condition, my PV release on -25 mb instead of -35, what are the steps that i should do as the duty officer and what do you think goes wrong with the pv? If there is something to be checked there what do you think could it be? I hope you would spare some time to answer my question. Thank you sir.

  30. Posts from you is a eye opening session . Tq so much sir. Most respected.
    I will suggest my friends too.
    One more question pls post ff arrangement in mast raiser.