Schwi­hag

Risks of fire and explosion

Tempering – increasing resistance of the metal

Quenching and tempering improves the service properties of steel in terms of strength, toughness and dynamic fatigue strength. The steel is heated to its hardening temperature under inert gas and then cooled in a hardening bath. The hardened components are given the desired service properties by the subsequent annealing process. The products of Schwihag AG also go through this process.

As part of the necessary capacity expansion, a renewal of the exhaust air system was also commissioned. This was designed and carried out by Acritudo Systems GmbH, a company that specialises in exhaust air and filter systems. In this process, the exhaust air from the hardening furnaces is discharged from the production hall via two separate pipelines.

Unknown hazard in the extraction pipe

Heat treatment is an energy intensive process. Both, the exhaust mixture of flue gas and oil vapours that escapes from the oil quenching chamber during hardening and the dry combustion gases from the gas burners have to be extracted. The flammable process gases and oil vapours escaping from the quenching chamber due to the overpressure are specifically ignited at the overpressure flaps of the quenching chamber so that no more flammable gases are present in the exhaust air. In principle, this general flame formation does not pose any risk. The risk is formed by deposits of non-combusted residues (resins, greases or oil components), which settle in the exhaust system at cooler points on the pipe wall and thus form a high fire load.

The following table shows the calorific values and densities as well as the resulting calorific values of various substances. For comparison: Petrol has a calorific value of 43.9 kj/g.

Our solution for Schwihag

In order to avoid dangerous fires, it is important to detect fire events as soon as possible and to initiate suitable countermeasures. In order to prevent a fire in the extraction pipe, ignition initials such as sparks and smouldering nests must be detected as early as possible. To this end, Schwihag has installed highly sensitive spark detectors in the piping. The downstream automatic extinguishing system can extinguish sources of danger in a matter of milliseconds.

As a supplementary protective measure, another automatic extinguishing system was installed at the beginning of the collection pipe. This can be activated via alarm thresholds. If more sparks follow 30 seconds after the first alarm and extinguishing, the pipe fire extinguishing system is triggered. Thus, the system is able to extinguish fires in the pipeline fully automatically. "The GreCon water mist pipe extinguishing system assures us that we will not have any fires in the exhaust air pipes in the future," concludes Frank Laubert, commercial manager.

In an impulse extinguishing process, only short extinguishing blasts in the form of finely atomised water (droplet size approx. 80-100 micrometres) are introduced into the piping. This small amount of water has no effect on the ongoing production. The water evaporates and is largely conveyed out of the hall with the rest of the exhaust air.

The installation was carried out as a complete assembly. In addition to the installation and commissioning of the system, GreCon also planned and carried out accompanying work such as water piping. "The installation progressed to our great satisfaction. A short maintenance break was sufficient to put the system fully into operation," describes Laubert.

The project planning of the new exhaust air system by Acritudo Systems GmbH made it possible, in connection with a GreCon water mist pipe extinguishing system, to replace the existing exhaust air wet scrubber with an electrostatic exhaust air oil separator. This brought about considerable cost reductions thanks to savings in maintenance and servicing costs for the water circuit of the wet scrubber. Also, with the use of a new frequency-controlled and performance-adapted exhaust air fan, a considerable energy saving of 75% could be achieved as compared to the old system, and Schwihag's water consumption was reduced by 50%.