Recommendations for the construction of a miniature boiler

Prior to starting the construction of a steam boiler the constructor is urgently advised to:

1. Consider thoroughly the various aspects that play a part in the construction of a miniature boiler by studying all available literature on the subject.
2. Consult with skilled constructors if no own experience is available.
3. Thoroughly familiarize himself with current directions and regulations for the construction of miniature steam boliers, particularly those which apply to self-designed boilers. Such directions can be found in the "Handbook Steamboilers" (Handboek Stoomketels), published by the "Dutch Steam Association" (Stoomgroep Holland).
4. Obtain expert advice from an inspector associated with the "SVS" (Cooperation of Live Steam Groups in the Netherlands and Flanders, Belgium) If the design of the boiler is such that the application for a pressure certificate is mandatory. In such cases it is also advisable to enable the inspector to check the drawings and calculations. The safety coordinator of the "SVS" will be able to provide you with the address of an inspector in your area. Binding arrangements will be made between the inspector and the constructor with regard to construction and materials to be used, and in addition to this dates will be set for intermediate inspections at various stages in the course of the project.
   Even if no pressure certificate is required it is advisable to keep in touch with an experienced constructor in order to be able to intensively discuss design and construction and, if necessary, to have access to assistance at any time during the construction period.

It will, under all circumstances, remain the constructor's exclusive responsibility to build a good, dependable and safe steam boiler, and there will be no right of recourse against any other party such as, for instance, the inspector who has supplied advice and/or direction.

IMPORTANT: The pressure certificate issued by the "SVS" is merely a statement confirming a boiler had been tested. It is not an official document under the rules and conditions of the "Stoomwet" or PED (Pressure Equipment Directive). A pressure certificate by itself doesnot permit commercial use of any boiler apparatus.

1. It is necessary to submit clear drawings, showing all visual aspects of the boiler as well as longitudinal and cross sections, specifications of the materials to be used and details re welding and/or soldering. It is also required to state maximum operational pressure and calculated volume. Measurements of such aspects, cross sections and individual parts, including the safety valves, must be supplied. The wall thickness of all parts that come under pressure must be clearly stated. This also applies to drawings not made by the constructor himself but commercially or otherwise obtained. Construction- and testing standards can be found in the "Handbook Steamboilers" published by the Dutch Steam Association.
2. The drawings must be surrendered to the inspector before the construction of the boiler is actually started.
3. A pressure test may be refused if the boiler shows obvious deficiencies or if its construction deviates to an unaccepatble extent from the drawings that have previously been submitted to and accepted by the inspector.
4. For the first test the boiler must be presented without any coatings, fittings, mountings and accessories, All apertures must be plugged off. A connection nipple G ¼" female must be fitted. During this first test, and also at all further tests, the miniature boiler's own pressure gauge will be checked against a calibrated pressure gauge.
5. The pressure test of steel and stainless steel miniature boilers will preferably be carried out with warm (45°C) tap water at a pressure of 1,5 x PS (PS = maximum operational pressure). In the case of copper boilers a pressure of 2 x PS is to be applied. The pressure shall be maintained for at least 15 minutes.
6. Upon succesful completion of the test a serial number and a testing stamp will be affixed to the boiler under the supervision of the inspector. A pressure certificate will thereupon be issued but this document will get its ultimate validity only after the steam test has been successfully carried out.
7. The steam test: with a hot fire, a powerful working fan and open safety valves maximum steam pressure is not to exceed maximum working pressure by more than 10%.
8. The boiler must be equipped with:

• a pressure gauge on which the maximum working pressure position is indicated in red.
• at least one protected level gauge
• two independently acting feeding appliances

At the introduction of the Pressure Equipment Directive, in May, 2001, it was formulated that all equipment with an overpressure of more than 0,5 bar must comply with this directive. Steam boilers come under article 3 sub 1.2 of the PED. Annex II, Table 5 of the PED states the module which is applicable to the boiler in question. Boilers with a volume of less than 2 litres need not to be tested, irrespective of the pressure. At PS (working pressure in barg*) x V (volume in litres) = under 50 Category 1 applies; this means that the boiler must comply with the conformity assessment procedure as described in module A, implying an "internal" production control.

The PED has been formulated for industrial manufacture and for both industrial and household use (e.g. kitchen pressure cookers). Within the framework of such production processes a manufacturer can easily do his own checks (if such is permitted). In the case of miniature steam boilers the situation is quite different. Many a constructor lacks experience. Quite some miniature boilers will be designed for a capacity of less than 2 litres and will therefore not be subject to the rules of the PED.
The larger types of locomotives (scale of 1 : 16 and up) and some stationary boilers will exceed 2 litres. The PED (as well as the "Pressure Equipment Decree" (Besluit Drukapparatuur) of July 5th , 1999 art. 25, Gazette 311of the Dutch Government) also contains a rule stating that pressure equipment not complying with the "Decree" may only be exposed and/or demonstrated at fairs and expositions if their non-compliance with the rules is clearly announced. Such apparatus may not be sold.

Testing by a third party (a so-called Notified Body or NoBo) is financially speaking hardly attainable for the average model constructor. The Board of the Dutch Steam Association and the inspectors of the SVS have therefore resolved to maintain the current criterions for the testing procedure, which implies that a miniature boiler with an operating pressure of more than 3 barg* must be presented for testing, regardless of its volume. The rule that PS x V may not exceed 200 will also remain in force. For copper boilers the maximum operating pressure will always be 6 barg*.

At fairs and expositions constructors and owners will be expected to carry a completed "Certificaat van Deugdelijkheid" (Certificate of Sound Construction, which may be downloaded from this site) if they wish to demonstrate their boiler. The operating pressure is limited to 3 barg*. If the operating pressure is higher than that a valid document certifying a successfully concluded steam test is required. The re-testing period of 2 years is to be observed.

You are in any case advised to thoroughly familiarize yourself prior to starting construction with all requirements regarding choice of materials, procedures such as welding and soldering and construction particulars, and to establish contact with an inspector of the SVS in your region.

We wish you a lot of constructive pleasure and "safe steaming"!

Note: In this article pressure is given in barg, i.e. bar gauge or overpressure in relation to atmospheric pressure.

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For general advice or check of own designs, please apply to:

For carbon steel/stainless steel:
J.Berendsen
Bijenkorf 57
6961 AP Eeerbeek
T 0313 651257

For copper:
J.Q.M.Verouden
Westeinde 11
6904 AA Zevenaar
T 0316 525524