Boilers & vitreous enamel bosom friends
Drinking water
According to the Italian Decree No 31, dated 2 February 2001, “waters intended for human consumption”, are any treated or untreated waters intended to be drunk or to be used for preparing food and drink or for other domestic uses.
The Decree N° 31/2001, amended by Decree N° 27/2002, stipulates that water must be wholesome and clean, not contaminated by micro-organisms and parasites, nor by substances in quantities or concentrations that could constitute a danger for human health.
Water must therefore comply with a set of minimum requirements from the microbiological and chemical point of view (absence of various toxic substances).
The boiler
The aim of boilers powered by electricity, gas or any other means is to heat water and to keep it warm, without modifying its organoleptic properties
The CEI EN 60335 standard defines the different types of storage boilers, for domestic use:
:: Storage boiler
:: Closed boiler
:: Cistern-fed boiler
:: Cistern-type boiler
:: Open-oultet boiler
:: Open boiler
All these types of boiler, regardless of their operation, feature a tank designed to store hot water for a long period or also only temporarily.
The requirements of the tank
To store water intended for human consumption, a tank must be manufactured using materials which do not modify the quality of the water in any way.
The tank shall guarantee:
:: Hygiene
:: Resistance to corrosion produced by water and steam
:: A good mechanical resistance
The materials
The materials used to manufacture tanks are:
:: Steel and
:: Stainless steel AISI 316 L.
Stainless steel
Stainless steel has an advantage:it does not require any further treatment and two disadvantages:
:: The risk of rust formation on welds
:: When in contact with water it yelds up nickel, a high allergenic material
Steel
Steel rusts when in contact with water,the tank must therefore be coated with a water-resistant material.
The most common finishes used for coating tanks are:
:: Vitreous or porcekain enamels
:: Epoxy resin paints
:: zinc coating
:: zinc coating + organic coatings
Boiler during firing process
After pre-treatment (sanding) and the application of the enamel, boilers are fired at about 30° C.
During this process, the enamel vitrifies.
During firing, chemical – physical reations take place, which give the material all the chemical and physical properties of glass and an optimal adherence to the base metal.
Hygiene
What is better than glass for storing hot water?
Glass – and vitreous enamel is a glassy material – does not transmit any odour to water and does not affect its flavour. It is absolutely hygienic, as its closed surface does not allow bacteria to accumulate and proliferate. Vitreous enamel does not give off any toxic or allergenic substances.
Comparative Bacteriological Study
The comparative bacteriological study carried out by the Modena Hospital Institute has shown that vitreous enamel is the hygienically safest material. In fact…. vitreous enamel achieved the best results in the tests for
:: Spontaneous pollution
:: Caused pollution
:: Ease of decontamination
Results of test: Spontaneous pollution
| Table 1 | |||||
| SURFACE | Bacterial load | ||||
| 0 h | 24 h | 48 h | 72 h | 144 h | |
| Stainless steel | 5.030 | 3.050 | 3.090 | 3.030 | 3.020 |
| Vitreous enamelled steel | 5.070 | 2.015 | 2.000 | 2.020 | 2.020 |
| Painted steel | 5.070 | 3.100 | 3.060 | 3.060 | 3.060 |
| ABS polymer | 5.070 | 3.025 | 3.070 | 3.090 | 3.025 |
| Polythetrafluorethylene (P.T.F.E.) | 5.080 | 3.000 | 3.000 | 3.000 | 3.000 |
| N.B. Figures are in thousands. |
Results of test: Caused pollution
| Table 2 | |||||
| SURFACE | STAPHYLOCOCCOS PIOGENE – Number of bacteria at | ||||
| 0 h | 24 h | 48 h | 72 h | 144 h | |
| Stainless steel | 200.000 | 50.000 | 40.000 | 30.000 | 20.000 |
| Vitreous enamelled steel | 200.000 | 15.000 | 5.000 | 1.000 | 650 |
| Painted steel | 200.000 | 20.000 | 20.000 | 10.000 | 10.000 |
| ABS polymer | 200.000 | 30.000 | 28.000 | 20.000 | 10.000 |
| Polythetrafluorethylene (P.T.F.E.) | 200.000 | 60.000 | 60.000 | 60.000 | 20.000 |
| N.B. Figures are in thousands. |
| Table 3 | |||||
| SURFACE | ESCHERICHIA Coli – Number of bacteria at: | ||||
| 0 h | 24 h | 48 h | 72 h | 144 h | |
| Stainless steel | 300.000 | 75.000 | 450.000 | 30.000 | 25.000 |
| Vitreous enamelled steel | 300.000 | 18.000 | 10.000 | 5.000 | 5.000 |
| Painted steel | 300.000 | 40.000 | 40.000 | 10.000 | 10.000 |
| ABS polymer | 300.000 | 30.000 | 30.000 | 30.000 | 18.000 |
| Polythetrafluorethylene (P.T.F.E.) | 300.000 | 75.000 | 50.000 | 50.000 | 50.000 |
| N.B. Figures are in thousands. |
| Table 4 | |||||
| SURFACE | PROTEUS MIRABILIS – Number of bacteria at: | ||||
| 0 h | 24 h | 48 h | 72 h | 144 h | |
| Stainless steel | 400.000 | 100.000 | 90.000 | 60.000 | 50.000 |
| Vitreous enamelled steel | 400.000 | 28.000 | 14.000 | 10.000 | 850 |
| Painted steel | 400.000 | 60.000 | 40.000 | 30.000 | 10.000 |
| ABS polymer | 400.000 | 70.000 | 60.000 | 60.000 | 60.000 |
| Polythetrafluorethylene (P.T.F.E.) | 400.000 | 90.000 | 90.000 | 70.000 | 50.000 |
| N.B. Figures are in thousands. |
Decontamination
| Table 5 | |||||
| SURFACE | Decontamination – Number of bacteria at: | ||||
| 0 h | 24 h | 48 h | 72 h | 144 h | |
| Stainless steel | 900.000 | 5.000 | 5.000 | 5.000 | 4.000 |
| Vitreous enamelled steel | 900.000 | 1.000 | 700 | 600 | 600 |
| Painted steel | 900.000 | 7.000 | 5.000 | 5.000 | 5.000 |
| ABS polymer | 900.000 | 5.000 | 5.000 | 4.000 | 3.000 |
| Polythetrafluorethylene (P.T.F.E.) | 900.000 | 7.000 | 7.000 | 3.000 | 3.000 |
| N.B. Figures are in thousands. |
Resistance to corrosion
CISP’s WG 7 has carried out a comparative study to evaluate the resistance to corrosion using two different test methods (A and B).
The tests – which lasted several months – covered steel boilers coated with
:: vitreous enamel and
:: paint
Method A
An equal number of enamelled and painted samples was used, each measuring 100 mm square50% of specimens were dipped in demineralised water at 90°C and 50% were in contact with steam.
This method requires the water to be kept stirred.
Checking the specimens at periodical intervals, the following points were noted:
Results of the tests conducted using Method A:
Enamelled samples were only slightly matt and the enamel had lost a little weight. By extrapolating the weight loss data, which became constant after a few hundred hours, it was established that the enamel would be worn away after several years.
Painted samples were already suffering blistering after 100-200 hours.
Test Method B:
An equal number of enamelled and painted samples was used, each measuring 100 mm square.
This series of samples was tested with still water, using the AKZO procedure, which stipulates:
:: Demineralised water, kept still and at a constant temperature of 90°C
:: pH between 6.9 and 7.4
:: Resistance between 12 and 20 Mohms
:: Water change every 96 hours
Results of the tests conducted using Method B
The enamelled samples were found to be slightly matt and to have lost a little weight.
By extrapolating the weight loss data, which became constant after a few hundred hours, it was established that the enamel would be worn away after several years.
After a few hundred hours of testing, the painted samples blistered visibly and oxide had formed under the blisters.
In conclusion…
Vitreous enamel can be said to be the safest coating for resistance to corrosion.
Even manufacturers offer different guarantees for their products, according to the coating used.
Resistance to corrosion of common finishes used to coat boilers
| Type of coating | Guarantee/years |
| Vitreous enamel | 5 -10 |
| Epoxy resin paints | 2 – 3 |
| Zinc plating | 1- 2 |
| Zinc plating + organic coating | max 5 years |
Mechanical Resistance
Stress analysis has enabled the influence extered by a boilrt’s dimensions and geometry on its mechanical resistance to be evaluated as a function of the stress it undergoes during work or enamel firing so that both tensions and stresses can be minimized.
The next step was to determine the precise maximum stress tolerable by the composite steel-vitreous enamel material.
For this purpose, tensile tests were conducted on vitreous enamelled test pieces to determine the steel extension at which vitreous enamel starts crazing.
Results of tensile tests
The tensile tests carried out by WG 7 show that vitreous enamel does not craze as long as the steel does not sag.
This means that the vitreous enamel does not break as long as the steel remains within its elastic limits.
Boilers and vitreous enamel
| Functional characteristic of enamel coatings for boilers | ||
| Characteristics | Properties | Reference Standards |
| Hygiene | The closed surface of vitreous enamel does not allow bacteria to accumulate and proliferate | Comparative bacteriological study carried out by the Istituti Ospedalieri di Modena |
| Physiological innocuity | Vitreous enamel for boilers must not give off any toxic or allergenic substances. | UNI 9905 |
| Chemical and physical resistance | ||
| Resistance to hot water and its vapour | Boiling water and vappour do not corrode vitreous enamel | EN 14483 -2 ISO 2744 |
| Acid Resistance | Vitreous enamel for boilers must be at least class A | EN 14483 -1 |
| Thermal shock resistance | Thermal shock from 150 to 15 °C does not damage the vitreous enamel surface | UNI 9905 |
| Mechanical Resistance | ||
| Adherence | Vitreous enamel must adhere to the base metal | UNI 8883 |
| Resistance to Abrasion | Because it is a vitreous cladding, vitreous enamel does not abrade easily, even after a long time | UNI 7233 |
| Resistance to shock | Only quite significant shocks may chip enamel. As a general rule, a good enamel should have no surface damage whose diameter is greater than 2 mm 24 hours after a shock of 20 N | UNI 9613 |
Vitreous enamel in the law
Vitreous or porcelain enamel has been recognised as a material suitable for contact with water intended for humsn consumption.
:: in the Italian Legislative Decree N°. 174 , dated 6 April 2004
:: in the French Decree dated 29 May 1997
:: in the EC Mandate M 136
Enamelled boilers and technical standards
:: UNI 9905 – Vitreous Enamel – Enamelled water heaters – Requirements and tests
:: UNI 10821-2 – Vitreous Enamel – Guideline for the design of steel boilers intended
:: DIN 4753 – Part 3 – Wassererwaermer und Wassererwaermeranlagen fuer Trink-und Betriebswasser – Wasserseitiger Korrosionsschutz durch Emaillierung – Anforderung und Pruefungto be enamelled
Acknowledgements
ing. Claudio Contato – Project leader of WG 7 and the companies:
:: Arcelor
:: A.T.I. s.n.c.
:: Baxi S.p.A.
:: C.L.N. S.p.A.
:: Colorobbia S.p.A.
:: Ferro Italia S.r.l.
:: Fismar S.p.A.
:: Gruppo Lorenzi S.p.A.
:: Idropi S.p.A.
:: MTS S.p.A.
:: Pangborn Europe S.r.l.
:: Pemco Emails S.r.l.
:: Wendel Email Italia S.r.l.
:: Zuk S.r.l.
Whose experts,with their experience, contributed significantly to the development of the studies conducted by C.I.S.P. -Working Group 7
IEI 