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Corrosion resistance testing

Metal chimneys are subject to corrosion caused by flue gas condensates, and this needs testing says Arnold Teekaram
Corrosion resistance testing
For safety prducts such as flues and chimneys the requirements within the European test standards are becoming more stringent. As a consequence, there is increasing pressure on manufacturers to ensure that their products are fit for purpose and safe to use before they are placed on the market.

Furthermore, the drive by the Government for energy-efficient products and use of condensing boilers has created further pressure to deal with condensate in the flue, which, if not tackled, causes corrosion and leaks. Chimneys are subject to significant corrosion caused by flue gas condensates which occurs from the inside of the chimney liner and is difficult to detect early.

Corrosion in metal chimneys is due to acidic condensate forming when the temperature of the inner surface of the liner is at or below the dew point temperature of the flue gases. Under certain conditions, particularly combustion of fuels containing high concentrations of sulphur, very localised and severe corrosion can occur due to the formation of sulphurous and sulphuric acid leading to perforation of the chimney liner.

Performance requirements
European Standards under the Construction Products Directive (CPD) for metal chimney systems is BSEN 1856-1:2009-'Chimneys-Requirements for metal chimneys Part 1: System Chimney products' specifies the performance requirements for rigid, single and multi-wall system chimney products with metallic liners required for CE marking, and states that all metal chimneys and their fittings must be classified and labelled in accordance with the scheme from the results of laboratory tests.

The test standards define a set of performance classes for chimney systems which must be complied with. These include: temperature class, pressure class, condense resistance, corrosion resistance, liner material specification and soot fire resistance class - in accordance to EN1856-1 test standard:
System Chimney -EN1856-1 T400- P1- W -Vx -L40045- G or O (xx)
Where: EN1856-1 represents the test standard.
T450 temperature class is specified by the manufacturer and dedicates the temperature of the flue gases at which the heat stress tests are to be performed at P1 pressure level at which the gas tightness tests are performed are carried out before and after the thermal performance tests. Other pressure levels (N & H) are also given in the standard.

W condensate resistance of the chimney - when the chimney is designated to operate normally with the temperature of the inner surface of the liner at or below the water dew point temperature, the chimney is designated (W)wet; for example in the case of condensing boiler applications. Where the chimney is designated to operate normally with the temperature of the flue liner above the water dew point temperature, the chimney is designated (D)-dry
Vx represents the corrosion resistance class of the chimney
L40045 indicates a chimney liner made of 1.4401(316 stainless steel) with a thickness of 0.45mm
G signifies that the chimney is soot fire resistant, and O not soot fire resistant. XX is the distance of the chimney to any combustible material in mm and is declared by the manufacturer prior to testing.

The durability of the chimney liner with respect to corrosion is given the designation Vm. In the UK, the minimum chimney liner material specification acceptable for various applications in terms of corrosion resistance for solid fuel, gas and oil is given in the UK National Annex to BS EN 856-1.

The durability can be declared on the basis of material type and thickness of the chimney liner or determined by one of three tests: Vm, V1, V2 or V3.

V2 accelerated corrosion tests are carried out in accordance with the requirements of the same BSEN 1856-1 standard. Tests are conducted within a specially constructed test rig where the test chimney is exposed to 30 cycles, each cycle comprising of a long duration of a condensing phase and a drying phase. Sulphur and chlorine compounds are pre-mixed with the fuel oil to achieve the desired concentration of acidic species in the flue gases.

Assessment of the pitting corrosion and mean pitting corrosion for stainless steel is carried out at end of the test using standard metallographic examination procedures to verify compliance with the standard.

The table below gives the possible fuel types and sulphur content which determine the corrosion test undertaken.

Further information on V2 corrosion testing can be obtained by contacting BSRIA Test at www.bsria.co.uk

// The author is head of chimney testing and combustion at BSRIA //
18 June 2012

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