Abschnitt 4.3 - 4.3 Annex: Structural requirements set out in DGUV Vorschrift 59 concerning the heat treatment of aluminium and wrought aluminium alloys in nitrate salt baths (formerly BGV D14)
Knowledge of the requirements to which plants were subject at the time of their placing on the market may be useful for their assessment when in operation. The requirements of BGV D14 concerning the heat treatment of aluminium and wrought aluminium alloys in nitrate salt baths (in the version of 30 March 2007) were for a very long time binding for plants for this purpose.
Excerpt from BGV D14 concerning the heat treatment of aluminium and wrought aluminium alloys in nitrate salt baths (in the version of 30 March 2007) regarding structural requirements:
I. Scope
Section 1 Scope
This accident prevention regulation governs the heat treatment of aluminium or wrought aluminium alloys with a magnesium content of up to 10 % in salt baths of potassium nitrate, sodium nitrate or their mixtures (nitrate salt baths).
Implementation instruction:
Salt baths of this kind generally employ mixtures of potassium nitrate and sodium nitrate salts in ratios of 1: 4 to 1: 2. Commercial salt mixtures may also contain nitrite and colour additives for the purposes of identification. For certain materials that must not be treated in salt baths of this kind, refer to Section 11.
II. Definitions
Section 2 Definitions
(1) Aluminium in the sense of this accident prevention regulation is ultra-pure aluminium or pure aluminium.
Implementation instruction:
For the composition of ultra-pure aluminium and pure aluminium, refer to DIN 1712-1, Aluminium; Ingots and DIN 1712-3, Aluminium; Half-finished products.
(2) Wrought aluminium alloys in the sense of this accident prevention regulation refers to aluminium alloys which are processed into semi-finished products in the plastic state (e.g. rolling, extrusion, drawing, forging).
Implementation instruction:
For the composition of wrought aluminium alloys, see DIN 1725-1, Aluminium alloys; Wrought alloys. The maximum magnesium content is governed by Section 1 of this accident prevention regulation.
...
(4) Heat treatment installations in the sense of this accident prevention regulation are the nitrate salt bath (the container and the molten salt within it) and the heating, temperature monitoring and other safety equipment.
(5) Containers in the sense of this accident prevention regulation are baths, crucibles or other vessels for holding the molten salt.
(6) Heating equipment in the sense of this accident prevention regulation is equipment located inside or outside the container with the function of attaining and maintaining the required temperature of the molten salt.
(7) Temperature monitoring devices in the sense of this accident prevention regulation are equipment for measuring and controlling (temperature control equipment) and limiting (temperature limiting equipment) the relevant permissible temperature of the molten salt.
(8) Sludge in the sense of this accident prevention regulation comprises impurities in the molten salt which settle on the bottom of the container, such as scale from the container or residues of lubricants on the treatment material.
III. Construction and equipment
Section 3 Characteristic data
(1) The following information must be marked clearly and permanently on each heat treatment plant:
Manufacturer or supplier
Type or model number
Year of manufacture
(2) In addition to the data stated in Paragraph 1, the following must be applied:
- a)
On electric heating equipment:
Rated voltage
Rated current
Total electrical output (connected load)
Type of current
Frequency
- b)
On gas heating equipment:
Gas type
Maximum permissible gas pressure
Total fuel output (connected load)
Implementation instruction:
For the designations of heating equipment, refer to DIN 24 201, Industrial furnaces; heating and heat-treating furnaces; concepts
(3) Each container must be marked permanently with the following information:
Manufacturer or supplier
Type or model number
Year of manufacture
Maximum permissible container temperature
Section 4 Container material
Containers must be made of material resistant to scaling and corrosion.
Implementation instruction:
Examples of materials resistant to scaling and corrosion are Armco or calorized iron.
Section 5 Container covers
Containers must be equipped with covers that prevent persons or objects from falling into them and melt from spattering out.
Section 6 Heating equipment
(1) Heating equipment for nitrate salt baths may employ only electric current or gas for heating purposes.
Implementation instruction:
The most important technical rules for the heating of nitrate salt baths are:
DIN 57116/VDE 0116, Electrical equipment of furnaces
VDE 0721 Part 2b, Industrial electroheat installations
DIN 4788-2, Induced and forced draught gas burners
DIN 4788-3, Gas Burners; Flame Monitoring Devices, Flame Monitors, Control Devices and Automatic Firing Units
DVGW-Arbeitsblatt G 610 concerning gas burners on industrial furnaces
(2) Where the interior of the container is heated electrically, it must be ensured that neither charging equipment nor workpieces are able to rest on the heating pipes.
(3) Heating pipes must be situated such that sludge can settle only below the pipes.
(4) Gas heating systems must be equipped with automatic ignition and low-pressure cut-off devices. Observation of the flame must be possible.
(5) Gas heating equipment must be designed such that the container is not in direct contact with the flames.
(6) Gas heating equipment must be designed such that soot does not form.
Implementation instruction:
The avoidance of soot formation requires not only a suitable design, but also optimum adjustment of the gas burners.
(7) Heating equipment must be situated such as to exclude the possibility of local overheating.
Implementation instruction:
Local overheating may also cause chemical reactions within the bath, leading to explosions.
Local overheating can occur owing to:
Uneven heating
Insufficient clearance between heating equipment and container
Sludge deposits on the bottom of the tank
(8) It must be possible for the heating equipment to be switched off manually from a safe location. The switching position of the switching device for electric heating and the shut-off device for gas heating must be clearly identifiable.
(9) When solidified bath contents are melted, the type of heating equipment and its location must be such that during heating, contents of the bath that are still solid are prevented from being ejected from the container by the pressure of gasses trapped in the salt.
Implementation instruction:
This pressure can be prevented from building up when for example the gas is able to escape through salt that has already melted. On deep baths with internal electric heating, it may be necessary for channels first to be melted into the solidified bath contents by means of a vertical heater arrangement, such has heating bars, before heating proper is initiated.
Should a bottom heating system be required in addition to the side heating system on baths with external heating, the requirement can be met for example when
Side and bottom electric heating can be activated separately.
Or
A gas-heated furnace system is designed such that the side walls are heated first.
Section 7 Temperature monitoring devices
(1) Nitrate salt baths must be equipped with temperature control devices which is designed to prevent the maximum permissible temperature of the molten salt from being exceeded. The temperature of the molten salt detected by the sensors must be displayed visibly and recorded graphically..
(2) Nitrate salt baths must be equipped with temperature limiters which function separately from the temperature control equipment. The temperature limiters must switch off the heating equipment automatically and at the same time issue a reliably perceptible warning signal when the maximum permissible temperature of the molten salt for the hardening material concerned is exceeded.
Implementation instruction:
The maximum permissible temperature of the molten salt depends on the magnesium content of the hardening material (see also Section 10).
Owing to the requirement for temperature control and limiting devices to act independently of each other, these devices must also have separate temperature sensors.
The warning can be acoustic or visual, depending on the operating conditions.
(3) The temperature monitoring devices must be situated such that it is protected against melt spatter ejected from the container.
(4) The measuring sensors must be situated such that they are not damaged during charging of the bath.
Section 8 Additional safety devices
(1) Nitrate salt baths must be equipped with an additional safety device which switches off the heating equipment safely and triggers a reliably perceptible warning device when the molten salt exceeds a temperature of 560 °C. This safety device must act independently of the temperature limiting devices described in Section 7 (2).
Implementation instruction:
The molten salt temperature of 560°C is that which must not be exceeded in nitrate baths even when no hardening material is present. This maximum temperature should not be confused with the temperatures stated in Section 10 for heat treatment of the hardening material according to the magnesium content.
(2) Once the supplementary safety device has tripped, it must not be possible for the heating equipment to be restarted until the safety element which is no longer effective has been replaced by a functional element.
Implementation instruction:
Examples of effective safety elements are suitable fuses.
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