Monday, January 3, 2011

AR-AFFF ON CHEMICAL TANKERS -- CAPT AJIT VADAKAYIL






AR-AFFF  -  Alcohol Resistant  Aqueous Film Forming Foam concentrate ( free flowing ). 

Four stripers who go to shipyards to take delivery of brand new chemical tankers, must be familiar with the following.  I had once joined a 12 year old stainless chemical tanker with animal protein foam inside the foam tanks, and i had to twist my company's arm ( packed with pretenders )  by threatening to hold up the ship ,  before they would spend budget money and change the foam concentrate.  

Remember this is NOT about certification or inspectors or audits -- it is about lives at sea, you have sworn to protect.

The rate of foam supply on a chemical tanker is 3 times more than that of a usual annex 1 tanker.  Portable foam is also required ( in addition to fixed ) for small isolated fires.

Regular protein foam should not be used on chemical tankers. For cargoes for which foam is NOT compatible, dry powder or water spray can be used. Take this information from column “ l “ of the IBC code table for min requirements.

AR-AFFF is a synthetic foam developed for BOTH hydrocarbon and polar solvent materials.

Polar solvents are combustible liquids that destroy conventional fire fighting foam. These solvents extract the water contained in the foam, breaking down the foam blanket. Therefore, these fuels require an alcohol or polar solvent resistant foam.

Alcohol resistant foam must be bounced off of a surface and allowed to flow down and over the liquid to form its membrane, compared to standard AFFF that can be sprayed directly onto the fire.

AR-AFFF is intended for use at a 6% proportioning rate means 6 parts AR-AFFF concentrate to 94 parts water. It can be used on polar solvent/water miscible fuels such as alcohols, ketones, esters, etc.  It is suitable for use with either fresh or salt water

First, an aqueous film is formed in the case of a conventional hydrocarbon fuel, or a polymeric membrane in the case of a polar solvent fuel. This film or membrane forms a barrier to help prevent the release of fuel vapor

Second, regardless of the fuel type, a foam blanket is formed which excludes oxygen and from which drains the liquids that form the film or the polymeric membrane.

Third, the water content of the foam produces a cooling effect. When used on polar solvent liquids (such as alcohols, ketones and esters) a tough, polymeric membrane layer is formed to separate the foam blanket from the polar fuel, and help prevent vapor emissions. ) with the addition of a synthesized polymer. Such polymers allow the foam to form a layer that encases it from the fuel. 

As a result, AR-AFFF compounds can be applied to fires where the fuel is miscible with water (e.g., alcohols). function is to make the foam more stable and heat resistant, resulting in better burnback resistance and sealability compared to conventional AFFF

Alcohol resistant foams should be used in fighting fires of fuels containing oxygenates, eg. MTBE or fires of liquids based on or containing polar solvents .  

Foam concentrates for use on polar solvents differ from standard AFFFs because they contain hydrophilic polymer which enable these products to effectively extinguish polar solvent fires when proportioned at their designed percentages, either at 3% or 6%.

The hydrated hydrophilic polymer coagulates and comes out of solution when the foam contacts the water soluble polar solvent, forming a cohesive polymeric layer on the fuel surface. This polymeric layer prevents the polar solvent from attacking the foam. The presence of the polymer in AR-AFFF makes the concentrate look different than a standard AFFF;  

AR-AFFF is thicker and also appears more opaque than AFFF.  

Despite its increased apparent viscosity, AR-AFFF is sheer thinning, non-newtonian fluid which will proportion effectively through equipment designed to proportion standard AFFFs.

Care must be taken when storing AR-AFFF to ensure that polymer remain fluid and do not gel in the storage tank or piping. The following precaution must be taken to prevent gelling problems :
            
Since the product is designed to gel when come in contact with a polar solvent, polar solvents introduced in storage will cause gelling. This includes ethylene glycol which is sometimes used to reduce the freezing point of products. Ethylene glycol or any other polar solvent must not be added to the foam concentrate in storage.
               
Other foam liquid concentrates should not be mixed with AR-AFFF in storage. This includes AFFFs, other brands of alcohol resistant concentrates, fluoroprotein and protein foams.

Different types of foam concentrates should never be mixed since they may be incompatible and all manufacturer warranties would be voided since long-term performance of a given agent could not be validated.

This is recognized by NFPA 11 - Low Expansion Foam Combined Agent Systems, “Different types and brands of concentrates might be incompatible and shall not be mixed in storage.”

AR-AFFF is sensitive to high concentrations of metal ions in solution. Excess metal ions can come from a carbon steel tank or piping particularly when a large surface area of carbon steel is in contact with a small amount of AR-AFFF. This is usually the case in small diameter pipe. A large carbon steel tank filled with AR-AFFF has a relatively small amount of surface contact area compared to the mass of AR-AFFF, Carbon steel is not the most suitable material of construction for an AR-AFFF storage tank. The most common recommended materials of construction in use today for AR-AFFF are : the original container, carbon steel with an approved coating, stainless steel, approved plastic or fiberglass, or a bladder tank.

Another situation to avoid is the use of dissimilar metals in an AR-AFFF foam storage system. Dissimilar metals such as a stainless steel tank and a brass valve will cause galvanic corrosion and will increase the metal ion content in the AR-AFFF concentrate which can cause gelling. Dissimilar metals should either not be used in the concentrate system or they should be isolated by a non-conductive gasketing material. By avoiding or isolating dissimilar metals, the tank and piping will last longer as galvanic corrosion is prevented.
 
If AR-AFFF concentrate is allowed to evaporate freely, the water and solvent in the concentrate will evaporate causing a skin of polymer to form on the surface of the concentrate. The skin could plug the proportioning orifices. To eliminate this possibility, the concentrate tank must be sealed to prevent evaporation and a vacuum breaker installed to break the seal when the system is in use. 

Evaporation losses will also be minimized by keeping the concentrate tanks full. If it is not possible to seal the tank, a thin layer of mineral oil can be applied to the surface of the concentrate to reduce evaporation.

The concentrate should be stored within its usable temperature range which is 20- 49C) for non-freeze protected concentrates. Freeze protected concentrates should stored at temperature specified/designated by their manufacturer. If the product freezes, it can be thawed and agitated before using.

Container should not be exposed to direct heat and light.

METHODOLOGY OF ADDITION OF OIL OVER THE SURFACE OF FOAM CONCENTRATE (ALCOHOL TYPE) WHEN STORED IN STORAGE TANKS

The quantity of oil required should be calculated as per the size and area of the tank.
The thickness of oil layer over the foam concentrate should be minimum 4 - 6 mm.
By using a funnel and a pipe, oil should be gently poured over the surface of the foam concentrate.
The one end of the pipe will be attached to the funnel and other end of the pipe should be just above the surface of the foam concentrate while pouring the oil.
The oil should be added at different points of the tank, so that the oil is uniformly spread over the surface of the foam concentrate.
If there is a single opening or a small opening, then add half the quantity of oil over the surface of the foam concentrate gently. Wait till the oil spreads over the foam concentrate and then add the balance quantity of oil so that the complete area is covered by oil with a minimum thickness of 4 - 6mm.
The discharge of the foam concentrate when stored in a tank should be from the bottom of the tank.
The foam in the tank should not be completely utilized as there are chances of oil entering into the fire fighting system.

The Specification of the Oil to be used is usually Gear Oil SAE 90

ADDITION OF FRESH FOAM CONCENTRATE TO THE STORAGE TANKS

The foam should not be added over the surface of the oil.
Foam concentrate should be poured by using a funnel and a pipe.
The one end of the pipe will be attached to the funnel and other end of the pipe should be at the bottom of the tank or should be immersed in the foam concentrate while pouringthe foam concentrate.
For any reason, if you cannot use a funnel and a pipe, the foam concentrate can be put directly over the surface of the oil
Even if foam falls over the surface of the oil, it will not affect the quality of the foam because:
As the density of foam is higher than the density of oil, it will settle down and the oil will float over the surface. Oil is immiscible in foam.

PROPORTIONING
Fixed or portable in-line eductors
In-line balanced pressure and pump pressure proportioning skid
Bladder tank balanced pressure proportioning systems
Around the pump proportioners Handline, air-aspirating nozzles with fixed eductor pickup tube

AR-AFFF is biodegradable, low in toxicity and can be treated in sewage treatment plants.

CAPT AJIT VADAKAYIL
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3 comments:

  1. what test we do for afff on board to check for its condition .

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  2. Dear Captain,

    I belong to the fire fighting industry and i have delt with AFFF foam but not for the ships cause of the DNV certification. Better about the article on AFFF however there is a company in US called Ansul much known for innovative fire solutions and they have more variety on the AR AFFF foams. You can contact for more details on the same.

    ReplyDelete
  3. Captain You have posted very nice information.

    ReplyDelete