LPG VESSEL CORROSION & STRUCTURAL PROTECTION

Last Updated 2 months ago by Kenya Engineer

LPG vessel protection requires serious evaluation considering the potential consequences if failure occurs through corrosion or inadequate fire protection. The overriding concern is maintaining structural integrity, safety of employees, containment of corrosion and protection against a possible fire event.  Evaluating authorities discover there are a variety of options available, the most suitable are those that can be applied most economically while efficiently providing fire and corrosion resistance under their environmental and operating conditions.

Recognised methods of fire protection :

• Separation from a potential heat source
• Burying or mounding
• Insulation applied to the tank (passive materials)
• Radiation barriers
• Water deluge systems
• The use of Pressure Relef Valves.(PRV)

Water deluge systems are the most common in use, these are ‘cooling systems’ as distinct from permanent physical protection. Deluge systems alone cannot ensure total fire safety no matter how elaborate Whilst they protect against radiation heat; they cannot be trusted to cope with a concentrated flame impingement.

Studies has shown the weakest link begins with gas leaks from valves or ruptured corroded feed pipes resulting in release of a vapour gas cloud heavier than air. Perhaps the most dangerous property of LPG is its weight. Released vapour remains at ground level undiluted spreading beyond the confines of the vessel perimeter where water systems become ineffective in protecting other structures in the vicinity.

Permanent passive insulating materials combined with water cooling is preferable to the palliative approach of only using water cooling systems. Water systems are vulnerable to interruption, require either manual operational procedures or automatic devices to bring them into action. Passive protection is proving the safest and most viable option especially in areas where water pressure may be variable, insecure, or subject to drought.

Testing of water systems dates to the 1930’s, studies in the UK established that a minimum of 10 litres/M2 of steel surface / per minute is needed for protection against the effects of a pool and radiant heat from any adjacent fire.

The quantity needed to protect a typical vessel, 3.5 metres diameter x 25 metres  would require approximately 180,000 litres/hour, excluding any additional water to protect adjacent vessel or structures. Failure of one spray nozzle or uncooled area greater than 300mm diameter is sufficient to cause a local “Hot Spot”, resulting in structural collapse exposing the contents to the surrounding fire. The consequences of which is the possibility of a catastrophic Bleve (boiling liquid expanding vapour explosion).Ref You Tube.

Special attention needs  be given to :

• The potential time delay between the outbreak of the fire and the actuation of the system
• Vessel size and quantity of water required.
• Fixed water systems require regular inspection and maintenance to ensure that piping is not corroded, and nozzles free of blockages. Maintenance is required on a yearly basis which comes at a cost.
• There is evidence to suggest water systems are not effective against jet flame impingement or torching type fires.
• Positioning of associated pipe work and valves are often on the underside of the vessel away from the water deluge system.

Passive Material Development:

Passive protection materials perform either by:

1. A thermal barrier ( Lightweight cementitious mixtures)

OR

2. Ablative/Intumescent ( Epoxy coatings)

Thermal barrier, lightweight cement mixers are applied by concrete pump/spray equipment incorporating reinforcing galvanized mesh to thicknesses of between 35-40mm (depending on vessel wall thickness) to provide the customary two (2) hour protection.

Ablative intumescent  epoxy paint materials are applied by airless spray-painting equipment. When exposed to fire they undergo sublimation utilising a massive amount of heat energy which transforms solid material directly into vaporising gases to block the heat input of the fire.  Coating thickness is in the order of 20 mm plus. They are one off systems and always need to be replaced following any fire event, unlike cementitious materials they produce toxic fumes and smoke.

Design Philosophy

The design philosophy for these systems was to limit  heat absorption and maintain a temperature within defined limits for specific emergency measures to be put into effect.

Materials are required to meet the following  parameters:

• Under stated fire conditions, limit the temperature of the vessel below the specified fire rating requirement.
• The system must not fail suddenly at the end of the specified period but continue to offer a predictable measure of protection beyond that point.
• Material to remain in place during a fire and should be able to withstand thermal shock and impingement from fire hoses. There is evidence epoxy materials are not entirely effective against jet flame impingement.( Fig 3 )
• The products be non-corrosive to the substrate and not affected by environmental conditions, whether natural or from local leaks, spillages, or pollution.
• Materials must not become a hazard in a fire whether by spalling, spreading flame or producing toxic fumes.
• Products should be easy to apply in various environmental conditions without causing hazard.
• The material should be durable and easily repaired.
• For existing tank installations – be compatible with a wide range of existing paint systems (including lead-based paint materials) where removal may not be appropriate due to environmental or safety considerations.

Fire Types

Testing programs are required to ensure the system could combat heat input from three types of fires-

1. An intense torching type of impinging flame, e.g. high-pressure gas or vapour leak which could produce a heat flux of up to 250 KW / M2 with flame temperatures of up to 1600°C.
2. An engulfing pool fire from a flammable liquids, with heat flux of approximately 129 KW/M² with flame temperatures of l000 – 1200°C.
3. Radiant heat from adjacent fire.

Test Data Evaluation:

Materials are required to undergo testing appraisal by an “Independent Authority” as close as possible to the conditions of fire exposure to satisfy the fire endurance rating required.

Consideration should be given to specific obligations regarding health, safety responsibilities, and insurances which may be applicable. Without question the implications which arise from the use of materials without “total compliance”, provide insurers the opportunity to deny any liability. Furthermore, should there be any  injury or death would expose all parties concerned with  selection. Proven performance with independent test data is the only safe and secure basis for selection.

Standard fire test requirements

Standard fire test (stickability, insulation, & integrity) was designed to act as a benchmark to determine in a consistent manner, the performance of products to a particular fire condition. What these tests “cannot do” is  determine the performance of (1) Wind turbulence, and (2) Thermal shock caused by high pressure water from fire hoses.

Whilst all considerations are important, product selected  should have been subjected to a water impingement thermal shock test, this simulates actual fire conditions and complements the standard fire test.

Another important consideration, fires can last considerably longer than for the designed protection period. Products should not fail  but be capable of continuing to offer protection beyond that point, therefore it is prudent to build in a safety factor over and above the requirement.

Cementitious materials have been exposed to fire situations where the performance has well exceeded the design parameters (e.g. 1.5-hour rating extending to over 5 hours of protection).

Epoxy materials by their very nature are sacrificial products, when exposed to fire produce a loose porous char layer which can be  removed when exposed to thermal shock created by either the normal fire hose or jet fire flame impingement. ( Fig 3&4 ). For the most part epoxy materials require thicknesses of approximately 20 mm plus to pass the thermal shock test, which comprises of the following cycles:

• 0· 30 mins Thermal torch
• 31 – 45 mins Fire water hose
• 46 – 75 mins Thermal torch
• 76 – 90 mins Fire water hose
• 91 -120 mins Thermal hose action

There have been test procedures where a steel plate has been subjected to a flame impingement to one side, where the transmitted radiate heat goes to atmosphere. This is not how vessels are exposed in hydrocarbon fire situations. The heat transmitted to the vessel and its contents cannot be re radiated to atmosphere from what is a closed system. Testing of this nature are fundamentally unsound and should be rejected. However, in practice as an added safety measure pressure relief valves ( PRV ) are fitted to relieve pressure build up if it happens to occur.

Cement based materials incorporate galvanized reinforcement mesh as an extra built in safety factor. Installed in a monolithic nature, locked within itself in such a manner that it cannot be the cause of delamination. There is some evidence epoxy systems are not totally effective under jet fire flame exposure. Independent hydrocarbon jet flame impingement tests are highlighted in ( fig 3 & 4.)

Epoxy materials, incorporate over lapping Fibreglass or Kevlar matting to reinforce the “ Cohesive “ strength of the coating film. This is necessary as the underlining anti-corrosive paint systems are smooth and don’t have anchor pattens to enhance or promote mechanical adhesion This gives no guarantee for firm and complete adherence to the substrate, particularly after the anti-corrosive coating system has failed. Whilst these materials have passed the standard fire test, there is always an element of risk under actual fire conditions. They are not monolithic nor locked in the same manner as  galvanized mesh used in light weight Cementous materials. If epoxy materials are to be used, Galvanized or Stainless-steel mesh would be a far better choice.

CORROSION IMPLICATIONS

Regardless of any fire considerations, structural integrity needs to be maintained throughout the operating life of any vessel hence the need for an underlining anti- corrosive paint system.

Fire protection products, whilst they contribute to protection are barrier materials they were not designed for anti-corrosive purposes.

Steel corrosion is a complex electro-chemical reaction, this in its simplest form occurs when bare steel is in the presence of an electrolyte (i.e. water, oxygen, chlorides containing airborne pollutants).The industry test method for determining this characteristic within the fire protection industry for anti-corrosion paint systems is ASTM-96-53-T.

Corrosion can be prevented successfully by numerous methods, the most common, the use of an envelope barrier system (i.e. protective coatings to prevent the electrolyte:- water, oxygen, and soluble chlorides from reaching the steel surface). The addition of a fireproofing materials provides a further barrier to the electrolyte.

The use of cement-based materials has an extra anti-corrosive benefit. Products which exhibit alkaline reading of PH 11 plus, inhibit the corrosion process of steel whether it be completely bare or coated for as long as the cement remains alkaline. Corrosion will only commence at the interface when the anti-corrosive paint system is depleted due to the penetration of airborne chloride contaminants or when the cement has lost its alkalinity due to carbonisation.

• Carbonation – involves the reaction of carbon dioxide with calcium hydroxide naturally found in cement mixers, to produce calcium carbonate. Build-up of calcium carbonate with time reduces the alkalinity to the point where it loses its pacifying effect against corrosion. This process in mild environments can take many years although experience has shown that even ‘un top coated’ materials have retained their alkalinity in some cases more than 20 years.
• Where airborne chloride contaminates penetrate unprotected fire insulation and form acidic solutions which then provides the basis for corrosion of the steel.
• To prevent and slow carbonisation, it is necessary to waterproof all cement-based materials with a flexible high build Acrylic or Chlorinated Rubber-based waterproofing coatings.

Ablative intumescent materials are formulated with epoxy resins and reactive chemicals to provide the subliming or intumescent action, typical thickness for 2-hour rating is 20mm plus depending on steel wall thickness. Top coating is always necessary due to their poor U.V. resistance. It is often claimed they provide good corrosion resistance. However, they exhibit poor Water and UV resistance compared to their anticorrosive counterparts, hence the reason for over coating with epoxy or polyurethane topcoat systems.

The use of fibre contributes to corrosion through the phenomenon known as wicking whereby water is transferred through the material to the metal surface by way of capillary action.

Epoxy anti corrosive primers are often used however they are renowned for under film corrosion creep. When their protective properties have been depleted corrosion commence, the process is continuous and spreads rapidly at the steel interface. To prevent this, Zinc-based primers are now and should always be recommended followed by epoxy topcoats.  Zinc primers have the tendency to contain corrosion creep,  if it happens to occur the corrosion spread is relatively slow. This has large implications for future operational activities and maintenance costs.

When LPG is converted to a liquid state for storage, volume is reduced 270 times to release  back into its gaseous form for transferring to mobile tankers heat is required. To accomplish this heat vaporizers are needed to reduce tank vapour pressure to a consistent pressure that can be run through the piping system.

When vessels are shut down, they cool and are invariably steam cleaned, the result is rapid steel and epoxy expansion. Epoxy materials have high cohesive strength, meaning they have very little flexibility, on cooling they remain detached from the surface and crack (Fig 5  ) leaving a void at the interface for water and chlorides to accumulate. Cracking only applies to vessels, when applied to structural members detachment is highly unlikely.

Cementous materials don’t suffer this problem, they have greater flexibility to accommodate structural movement.

HEALTH AND SAFETY

Materials employed should not contain products or elements which pose a threat to the health of the applicators or any adjacent operational personnel, nor the possibility for the development of risk at a future date. These questions need to be addressed prior to final selection, materials should  non-toxic, during application or under fire conditions, be environmentally friendly and non injurious to the facility operators.

COST COMPARISON

Current estimates show that lightweight cement systems are quicker and easier to install than all other forms of fire protection with large cost savings. Case histories in Europe have been installed since the late 1960s have shown no apparent known problems.  Typical application costs  listed  are based on a vessel 2.3m diameter x 14m long, no allowance has been made for the anti-corrosive system, mobilization, or scaffolding. Costs are expressed in Australian currency although similar figures would apply to other international jurisdictions.

Typical Cost Examples:

Cementitious   (thickness 35mm)       A$750/m²

Epoxy (thickness 20mm)                   A$2300/m².

This article is only an overview and not all inclusive, the intent is to provide a general Insight into the engineering and corrosion principles associated with LPG vessel protection.

LPG is not the only material that requires protection, other products include Methyl and Ethyl Chloride,Vinal Chloride Monomer,Isoplopylaimine,and Oderant .

References:

1. AS1596-1989 (LPG storage and handling)
2. UL1709 (rapid rise hydrocarbon test)
3. AS/NZS2312 (Guide to the Protection of Iron and Steel Against Exterior Atmospheric Corrosion)
4. British Standard BS476 Part 20, 1987, appendix D
5. ASTM E93 53-T
6. BLEVE illustrations available on You tube
7. Petrochemical Corrosion and Fire Protection. N. Karakasch
8. Corrosion under Fireproofing Compounds.N.Karakasch
9. DGL Contracting P/L( Fire cost estimates)