Tuesday, December 28, 2010

STATIC ELECTRICITY ON CHEMICAL TANKERS -- CAPT AJIT VADAKAYIL

A few years ago , a chemical tanker exploded with loss of most hands. 


There the crew had attempted to "fast forward dry"  the sump of a cargo tank containing MTBE by blowing air via the Framo suction , rather than doing a bottom flush with water. 


MTBE is a static accumulator.


This real reason is buried--as AFTER AN ACCIDENT AT SEA , TRUTH IS ALWAYS THE FIRST CASUALITY. 



People involved have to protect their backsides, company has to collect insurance and laugh all the way to the bank , ignorant souls in the safety dept have to write beautiful post mortems and personally shine, Vetting houses have to be fooled .  Hardly anybody cares for the poor dead sailors or their families.



Static electricity is generated by friction which happens during relative motion. Electrostatic charges can accumulate in poor conductors or insulated good conductors. If two such bodies with accumulated static electricity are brought close together, a spark jumps between the bodies. Almost everybody has experienced such crackling discharge when removing a nylon shirt from a warm body in a  cold dry cabin. If this spark is incendive it could ignite a flammble atmosphere. Such discharge must be prevented, unless the atmosphere is inert. 

Static electricity can happen due to-  Charge separation (turbulent flow of a static accumulator liquid) ,  Charge accumulation (introduction of a non bonded object like a sampler) , Electrostatic discharge (the non bonded object with accumulated charge comes close to an earth and an spark will occur. such spark is incendive ),  Electrostatic discharges with sufficient energy can ignite flammable hydrocarbon gas/air mixtures.
All three of these stages are necessary for an electrostatic ignition. 

Whenever two dissimilar materials come into contact charge separation occurs at the interface. The interface may be between a solid and a liquid or between two immiscible liquids.  

Examples: 
The flow of liquids (e.g. through pipes or fine filters,  the settling of an immiscible liquid through a liquid (e.g.  water through kerosene),  the ejection of steam from a nozzle , the splashing or agitation of a liquid against a solid surface (e.g. water washing operations or the initial turbulence involved in filling a tank with oil), the vigorous rubbing together of certain synthetic polymers (e.g. the sliding of a polypropylene rope through PVC gloved hands).

When the charges are separated, a large potential difference develops between them. Also a voltage distribution is set up throughout the neighbouring space and this is known as an  electrostatic field .If an uncharged conductor is present in an electrostatic field it picks up the same voltage as the field it occupies. Charge one end of the conductor gives an equal and opposite charge at the other end. This is capable of contributing to an electrostatic charge.

Charges which have been separated attempt to recombine and to neutralise each other. This process is known as charge relaxation. If one, or both, of the separated materials carrying charge is an insulator recombination is impeded and the material  accumulates the charge upon it. The period of time for which the charge is retained is characterised by the relaxation time of the material, which is related to its conductivity; the lower the conductivity the greater is the relaxation time. 

If a material has a comparatively high conductivity, the recombination of charges is very rapid and no static electricity accumulates on the material. Such a highly conducting material can only retain or accumulate charge if it is insulated and the rate of loss of charge is then dependent upon the relaxation time of this insulator. 

Electrostatic discharge, is dependent upon the strength of the electrostatic field in the space between the points. This field strength, or voltage gradient, is given approximately by dividing the difference in voltage between the points by their distance apart. A field strength of about 3,000 kilovolts per metre is sufficient to cause breakdown of air or petroleum gases.  


The field strength near protrusions is greater than the overall field strength in the vicinity and discharges therefore generally occur at protrusions. A discharge may occur between a protrusion and the space in its vicinity without reaching another object. These single electrode discharges are not incendive in the context of normal tanker operations. The alternative is a discharge between two electrodes adjacent to each other.


Examples are: 
Between sampling apparatus lowered into a tank and the surface of a charged petroleum liquid.  Between an unearthed object floating on the surface of a charged liquid and the adjacent tank  structure.   Between unearthed equipment suspended in a tank and the adjacent tank structure  

All  liquids the whole range of aqueous solutions including sea water are conductors. The human body, consisting of about 60% water, is effectively a liquid conductor. The important property of conductors is that they incapable of holding a charge unless insulated. But if they are insulated and an opportunity for an electrical discharge occurs all the charge available is  instantaneously released . 

Sparks between two conductors are more potent than the ones between objects, one of which is not a conductor. In the latter case, discharges often take a more diffuse and much less dangerous form, known as corona or brush discharge, rather than a spark.  Corona or st Elmo’s fire does not have sufficient energy to start an explosion ( ion emission of blue colour ). 


Spark energy may be reduced by physical factors such as electrode resistance, spark gap distance, and large gap areas. Discharges are sometimes in the form of a "corona", an ionization of gas which is not incendive but may precede an incendive spark.

The second group is the insulators who retain a received charge for a very long period.  Charged non-conductors cause concern because they can transfer charge to, or induce charge on, neighbouring insulated conductors which may then give rise to sparks. Very highly charged non-conductors may themselves contribute directly to incendive sparks. 

Gases are good insulators. Charged mists are formed during the ejection of wet steam. Though water, has high conductivity, the relaxation of the charge on the droplets is retarded by the insulating properties of the surrounding gas.

Electrostatic discharges can occur when a static accumulator oil (such as toluene) pumped turbulently into a tank.,electrically isolated liquid or solid conductors, for example mists in air, or a metal rod hanging on the end of a synthetic fibre rope. 


The most important countermeasure that must be taken to prevent an electrostatic hazard is to bond all metal objects together. Bonding eliminates the risk of discharges between metal objects, which can be incendive. To avoid discharges from conductors to earth, it is normal practice to earth ( grounding). On ships, bonding to earth is effectively accomplished by connecting metallic objects to the metal structure of the ship, which is naturally earthed through the sea.

Some examples of objects which might be electrically insulated in hazardous situations and which must therefore be bonded are: 

Ship/shore hose flanges if more than one length of non-conducting hose is used . Portable tank cleaning machines.  Conducting manual ullaging and sampling equipment. The probe of an UTI tape .  Floating cans must not be inadvertently be left inside cargo tanks.

Charge generation is enhanced ----if water droplets are suspended in the oil as it flows through the pipes, or flow through a  filter of the kind used for aircraft jet fuels. These filters have the ability  to charge fuels to a very high level, probably because all the fuel is brought into intimate contact with the filter surface, where charge separation occurs or by turbulence and splashing in the early stages of pumping say toluene into an empty tank or the settling of water droplets stirred up by a static accumulator oil in the tank.

Fine particles during discharge of pressurised liquid carbon dioxide are  charged .During the discharge of pressurised liquid carbon dioxide, the rapid cooling which takes place can result in the formation of particles of solid carbon dioxide which become charged on impact  with the nozzle and can thereby lead to incendive sparks. Carbon dioxide should therefore not be injected into cargo tanks or pump rooms which may contain unignited flammable gas mixtures.

Only conductors are able to release sufficient energy to ignite an explosive vapor.

When the field strength around a charged body reaches a certain value, different kinds of discharge may occur. To start a fire or explosion such a discharge must possess sufficient energy. It has been shown that a spark between 2 electrodes has to possess an amount of energy of about 0.2mj to ignite an explosive propane mixture. For ammonia the necessary amount is > by 600 times.

Liquids are considered to be static accumulators if have conductivities less than 50 picoSiemens/metre (pS/m) giving relaxation times greater than 0.35 seconds. A static accumulator may carry sufficient charge to constitute an incendive ingnition hazard during loading into the tank, and for upto 30 min after completion of loading. Examples: Clean oils (distillates). An antistatic additive is a substance which is deliberately added to a petroleum distillate to raise its conductivity.

Antistatic additives raises the electrical conductivity >100 pico siemens/ metre.  Antistatic precautions are not necessary in an inerted tank. The effectiveness of the antistatic additives  is dependent upon the length of time since the additive was introduced into the product, the ambient temperature , and satisfactory product mixing.  It can never be certain that a products conductivity is above 100 pS/m unless it is continuously measured. 

The generation of static electricity cannot be prevented absolutely, because its intrinsic origins are present at every interface. The conductivity of a liquid determines whether or not it retains the generated static charge. Loading rate precautions are not necessary for gasolines and some aviation fuels because of their low viscosity and low friction. "Avjet JP 4" and middle distillates require loading precautions to prevent static accumulation

Known static accumulators are—
Cumene
Cyclohexane
Diethyl ether
Heptane
Jet fuels
Kerosene
MTBE
Nonene
Octane
Styrene
Toluene
Xylene
Gasolene 

Electrostatic generation is restricted in the initial stages of loading by reducing the loading rate until all turbulence in the tank has ceased. 

At the commencement of loading an empty tank the linear velocity in the branch line to each individual cargo tank should not exceed 1 metre/second.  Mixtures of oil and water constitute a most potent source of static electricity. When an accumulator cargo moves through a pipeline it leaves a charge on the metal and picks an opposite charge. It also picks up a charge due to turbulence when it first enters a tank. 

By reducing the initial and max loading rates—the accumulated static charge is kept too low to cause an incendiary spark. Inerted tanks do not need the 30 minute moratorium.

Fixed washing machines or high level alarms, may act as isolated probes. A metal probe remote from any other tank structure but near a highly charged liquid surface will have a high voltage gradient at the probe tip. During the loading of static accumulator oils, this high voltage gradient may cause electrostatic discharges to the approaching liquid surface. 

A person who is highly insulated from earth by his footwear or the surface on which he is standing can become electrostatically charged. Electrostatic discharges caused by clothing and footwear do not present a significant hazard at high humidities. However boiler suits should not be made of synthetic materials.
Metallic components of any equipment to be lowered into a tank should be securely bonded together and earthed to the ship before introduction and should remain earthed until after removal.


A significant electrostatic charge can be generated when a polypropylene rope runs rapidly through a PVC-gloved hand. For this reason only natural fibre ropes should be used for dipping, ullaging and sampling.
Lightning may strike ship if an ionized path is created by gas flow to the atmosphere. So it is better to stop loading/gas freeing in still air conditions, to prevent gas accumulation on deck, when lightning threatens. A distant thunderstorm may produce heavy charges on board. Wind speed of > 5m/ sec is required to disperse gases.
  
Static accumulator cargoes should not be handled at flow rates > 7ms/ s when loading kerosene, toluene start at slow speed <1 m/ sec in pipes. Until the turbulence at the tank bottom has ceased.

Insulating flange at shore connection is better than bonding wire via a flame proof switch ( to prevent ship and jetty to form a voltaic cell as the hoses have bonding wire.

Ship to shore bonding is not effective as a safety device and may even be dangerous. If local regulations require the use of a bonding cable, then the Chief Officer must ensure that it is connected before the first hose is put aboard and not disconnected until after the last hose has been removed. The connection must be at the shipside, well clear of the manifold area.

When a ship-shore bonding cable is used, it must be used in the following manner:

When connecting:
-open switch
-connect bonding cable to ship away from any possible source of ignition such as drip trays
-close switch
-connect hose.

When disconnecting:
-disconnect hose
-open switch
-remove bonding cable.

Only if the jetty is without an insulating flange and is devoid of impressed current protection, should consideration be given to switching off the ship's system.

The ship's system should only be switched off as provided for by the above, or if required by terminal regulations. In view of the possibility of an incendiary spark above the water line, on the ship's initial contact with a jetty or other ship, it is necessary to curtail all activities, which would provide a source of hydrocarbon vapour during the berthing process.

If in any doubt as to whether a particular cargo is a static accumulator or not it should be treated as such a product and the precautions described followed.


It must be ensured that the MF/HF SSB (GMDSS) is earthed at the berth during cargo operations..
Before cleaning a tank containing a static accumulator like toluene the bottom should be flushed with water and stripped.

If portable cleaning machines are used all hoses and hose connections should be tested for electrical continuity. Connections should not be removed unless the machine has been removed from the tank..

The tank should be kept drained during washing. Ropes of synthetic fibres should not be used to support the tank cleaning machines. The UTI MMC tapes used must be earthed. UTI tube is slotted so as not to allow any significant electrical charge to accumulate on the surface of liquid within the sounding pipe. UTI tapes which have quick couplings to connect to vapour lock do not need bonding wires—the internal bonding of such unit should be checked every 6 months in accordance to man instrns.


Avoid recirculation in a non gasfree tank. Steam  must never be injected, if tank is not gas free. No machine may have greater throughput than 60m3/h and no nozzle may have greater throughput than 17.5 m3/h. The total water throughput per cargo tank must not exceed 110 m3/hr.

Water washing, causes electrostatically charged mist. The electrostatic levels vary widely from tank to tank, both in magnitude and in sign. 

When washing is started in a dirty tank the charge in the mist is initially negative, reaches a maximum negative value, then goes back through zero and finally rises towards a positive equilibrium value. The level and polarity of charge depends on the wash water and the degree of cleanliness of the tank. Recirculation of tank cleaning chemicals,  may cause very high electrostatic potentials in the mist. Potentials are higher in large tanks than in small ones. The size and number of washing machines in a tank affect the rate of change of charge.

Steaming can produce electrostatically charged clouds. Steam can cause very much higher levels of charging at a lesser time than those produced by water washing. Steam should not be injected into cargo tanks where there is a flammable atmosphere.

Experiments have shown that high capacity, single nozzle fixed washing machines can produce water slugs which, owing to their size, trajectory and duration before breaking up, may satisfy the criteria for producing incendive discharges. On the other hand, there is no evidence of such water slugs being produced by portable types of washing machine.

Unearthed conducting objects must not be introduced in a tank when there is charged mist.

Tankcleaning hoses must be tested for electrical continuity. Max 6 ohms/ meter length.

The plastic or polyethylene sleeves used for gas freeing can accumulate charge.

If air or inert gas is blown into the bottom of a tank containing a static accumulator oil a strong electrostatic field can be generated, especially in the presence of water or particulate matter. So it may not be a wise idea to blow through the sump to dry it out . It is better to bottom flush and strip. Inert gas too blown into the bottom of a tank can generate a strong electrostatic charge by bubbling action and agitation of the fluid.

An isolated probe configuration can be avoided by installing the device adjacent to a wall or other tank structure to reduce the voltage gradient at the probe tip. Alternatively, a support can be added running from the lower end to the tank structure so that the rising liquid meets an edge rather than the isolated tip of a probe. Or the protrusion could be earthed by a wire as is usually done on chemical tankers. The best solution is to construct the probe-like device entirely of a non-conductive material. These measures are not necessary if the vessel is limited to crude, black oil service where the tanks are inerted.

It is not a right practice to allow  violent wave motion in static accumulator tanks due to free surface..
A person who is highly insulated from earth by his footwear or the surface on which he is standing can become electrostatically charged. Electrostatic discharges caused by clothing and footwear do not present a significant hazard at high humidities. However boiler suits should not be made of synthetic materials.


Metallic components of any equipment to be lowered into a tank should be securely bonded together and earthed to the ship before introduction and should remain earthed until after removal.

A significant electrostatic charge can be generated when a polypropylene rope runs rapidly through a PVC-gloved hand. For this reason only natural fibre ropes should be used for dipping, ullaging and sampling.


Lightning may strike ship if an ionized path is created by gas flow to the atmosphere.so better to stop loading and gas freeing especially in still air conditions, to prevent gas accumulation on deck. A distant thunderstorm may produce heavy charges on board. Streamers from lightning rods can ignite—relates in tanks as sparks coming from protruding objects towards a charged mist.




IBC code requires that all gasketed cargo pipes, joints and hose connections  to be electrically bonded.

Air driven lamps must have water traps for the air supply. The supply hose should be of low electrical resistance.

All bonding arrangements on board must be regularly inspected with records of the inspections maintained. When maintenance work is carried out on the cargo system, checks must be made to ensure that the bonding   arrangements have been reinstated correctly.

It is our policy NOT to allow Toluene spraying due to danger of static explosion. 



 CAPT AJIT VADAKAYIL ( 28 YEARS IN COMMAND )

14 comments:

  1. so what additional precautions should be done to avoid generating sparks from static electricity while loading or discharge ?

    i mean is there any special equipment should be used in this case ?

    thanks in advance Capt.

    can i have your e-mail ?
    i have many questions and need your help with it

    ReplyDelete
  2. hi mahmoud,

    Electrostatic charges can accumulate in static accumulator cargoes such as toluene, kerosene , jet fuel, heptanes, heptenes—l. All whose conductivity is <50 ps/m.

    Before cleaning a tank containing static accumulators flush the bottom with water and strip.

    Then gas free to< 10% LEL. LEAN ATMOSPHERE. During washing if LEL rises to 50% ( 35% is water temp is >60C ) then gasfree again to< 20% LEL.

    Steam must not be injected into a non gas free tank.

    UTI tapes must be earthed.

    If the tank is not gas free no tankcleaning nozzle can have greater through put than 17.5 cum/hr. the total water throughput per cargo tank must not exceed 110 cum/hr.

    Antistatic additives are used to raise the conductivity >100 pico siemens/ metre.

    Electrostatic discharges can occur when a static accumulator is pumped turbulently into tank. So reduce loading rate till all turbulence has ceased.

    Bonding eliminates the risk of discharges between metal objects.

    Static accumulator cargoes should not be handled at flow rates >7 ms/sec. Start at low speed in pipelines of 1 m/sec

    IBC code requires that all gasketed cargo pipes , joints and hose connections be electrically bonded.

    Insulating flange at shore connection is better than bonding wire via a flame proof switch—to prevent ship and jetty to form a voltaic cell as hoses have bonding wire.

    Tank cleaning hoses must be tested for electrical continuity—max 6 ohms/ m length.

    At port ensure MF/HF GMDSS aerials are earthed.

    When an electrical storm is present transfer of static accumulator cargoes, ballasting of non gas free tanks, tankcleaning and gas freeing must be stopped and all openings shut.

    If air or inert gas is blown into the bottom of a tank containing a static accumulator oil a strong electrostatic field can be generated.

    Violent wave motion must be avoided in a slack tank containing static accumulator cargo, due to free surface.

    Only natural fibre ropes must be used for dipping, ullaging and sampling.

    Sliding of polypropelene rope through PVC gloves creates static electricity.

    No non bonded metallic equipment can be introduced into the tank for 30 minutes after loading has been completed for the tank—unless tank is inerted.

    Recirc of tank cleaning chemicals must be done in an inert atmosphere.

    Boiler suits should not be made of synthetic materials.

    I DO NOT GIVE OUT MY EMAIL ID TO MY BLOG READERS

    capt ajit vadakayil
    ..

    ReplyDelete
  3. Respected Sir Capt. Ajit,
    I sincerely respect and admire your work pertaining to various aspects concerning vessels as well as your other topics(although I still have to go through each of them in detail). My name is Rayan Webster Pinto.I am a 2nd mate navigating officer trainee DPO currently preparing myself for phase 1. For certain topics, I keep checking on google so that I can prepare notes for reference. During the course of scanning through google, I happened to come across your blog pertaining chemical tankers (phosphoric acid) and I did take them down on writing for future reference. I do have a blog as well, Sir. Besides the marin aspect, I am an ordinary rock singer, songwriter,music composer, a creative visionary computer artist and a perceptionary blog writer. This is the address to my blog : ryanpinto777.blogspot.com
    I thank you for your efforts through your blog work and hope to seek further guidance in due course of Infinity of time. Thank you Sir.
    Yours Sincerely,
    Rayan Webster Pinto

    ReplyDelete
  4. Good day capt,
    i am still unable to understand the insulated flanges used in manifold connexins while every where else a bonding wire is used.
    I am not so bright with chemistry. So i may need more effort to get some concepts.
    Would you have the time to explain?
    Thank you

    Udayakumar J

    ReplyDelete
    Replies
    1. hi uj,

      i usually do NOT take academic questions like this. i will be inundated if i take it.

      when two metal surfaces are a vastly different potential , as you bring together a static electricity arc jumps . electrons jump fomhigher potential to lower potential.

      it can be avoided two ways.

      -- by insulation

      -- by bonding one surface to the other surface at a far away safe place. ( potential made equal )

      capt ajit vadakayil
      ..

      Delete
  5. There is some confusion in the trade literature concerning the electrical continuity of flexible hoses.

    Here is ISGOTT 18.6.2.4:
    "Electrically discontinuous hose should have a resistance of not less than 25,000 ohms measured between nipples (end flange to end flange). The testing of electrically discontinuous hoses should be carried out using a 500 V tester.
    Electrically continuous hoses should not have a resistance higher than 0.75 ohms/metre measured between nipples (end flange to end flange)."

    However, the USCG is comfortable with ohmic values far higher:
    "The recommended use of conductive materials for fixed piping also addresses this problem. The Coast
    Guard has specified that plastic pipes (such as post-chlorinated polyvinyl chloride (CPVC) ) in tanks
    containing combustible and flammable fluids have a maximum resistance to ground of 1 megohm (10E6
    ohms). 200 kilohms is specified in tanks which are adjacent to pump rooms and which contain static
    accumulating cargoes (conductivity > 100 pS/m) [20].
    The International Maritime Organization (IMO) is considering a similar standard: the resistance of
    plastic piping would not exceed 100 kilohms/meter, and nowhere should exceed 10E6 ohms [21]. "

    20. United States Coast Guard, Navigation and Vessel Inspection Circular #11-86, "Guidelines
    Governing the Use of Fiberglass Pipe on Coast Guard Inspected Vessels", September 1986.
    21. International Maritime Organization Subcommittee on Ship Design and Equipment, 35th Session,
    Report to the Maritime Safety Committee, DE 35/35, 15 April 1992.

    Then there is simple physics and common sense. You state in your treatise above that discharge spark energies capable of ignition are attained only between good conductors.
    Absolutely true.
    Then what is the problem with using hoses that are electrically DISSIPATIVE, made of elastomers with resistances in the kilohm to megohm per meter range?
    This is standard practice when conveying chemicals in other industries.

    ReplyDelete
    Replies
    1. hi cm,

      rules are to be read , after forgetting that you are a sailor.

      when i read rules i tell my wife to expalin it to me-- because the people who make rules have zilch experience or brains.

      BE VERY CLEAR WHICH ARE MEANT TO BE "SHIP'S HOSES" AND WHICH ARE MEANT TO BE "SHORE HOSES".

      no point in sweating over SHORE hoses on the ship.

      capt ajit vadakayil
      ..

      Delete
  6. Sir, I'm about to load Mtbe without purging the tanks with n2 before loading. How can I top off the tanks in this case with uti or I have to do by fixed gauging system like kongsberg

    ReplyDelete
    Replies
    1. hi id,

      topping up is better with UTI, with HL / OF alarms running.

      kongsberg often malfunctions-- use it as secondary.

      once it gives same ullages as UTI after corrections you can rely on it --again only as secondary means.

      capt ajit vadakayil
      ..

      Delete
  7. Tx Sir for your input, I was enquiring that if we can use Uti properly grounded for topping off electrostatic accumulator cargoes like mtbe when the tanks are not inerted. Isgott says we need to wait 30 mins after completion of cargo for gauging. In that case, shouldn't the kongsberg be used for topping off if its reliable.

    ReplyDelete
    Replies
    1. hi id,

      only old models UTI need to be grounded.

      for static accumulator cargoes follow the rules.

      if someone does NOT allow you the delay as required by the rules -- he is in BIG trouble.

      like I said - kongsberg can be trusted ONLY if it tallies with UTI ( after index corrections ).

      capt ajit vadakayil
      ..

      Delete
  8. Tx Sir, btw I'm a big fan of your blog. Learnt a lot from it.

    ReplyDelete