Since the previous articles solely represented an account of the main aspects of project design work, the question of process protection has not been dealt with explicitly. It therefore has to be said that this question can be a particularly important additional factor of project design work in relation to process technology. For example, the requirements for the automation of an atomic power station will be considerably higher than those for the automation system of a brewery. Consequently, the project design engineer must be aware of the respective essential requirements for process protection and incorporate these into the project design. Picture 1 sets out the basic aspects of process protection.
Picture 1: Overview of basic aspects of process protection
According to this, the project design engineer will be confronted with problems such as explosion protection (explosion-proof design of automation equipment and systems). There is generally sufficient company documentation available for this, whereby the requirements arising from process technology can be implemented quite successfully.
The problem of EMC (electro-magnetic compatibility), which is also shown on Picture 1 is even more complex compared to an explosion proof design of automation equipment. However, depending on process requirements it also needs to be taken into consideration in the design of an automation system. Here, the project engineer’s experience is of some importance, since the appropriate know-how is at best only partially available in technical literature. It is therefore, as previously mentioned, up to the experience of individual companies to realise a well configured automation, system as far as EMC is concerned.
Similarly, the problem of lightning protection must be accorded its proper place in this context, since it also plays an important role in advanced system automation. Again, only fragmented specifications can only be found in the technical literature, including DIN documentation, hence the experience of the project design engineers is again crucial as far as this aspect of process protection is concerned. Apart from the above mentioned topics, which doubtless require further details, there remains the important matter of VDI/VDE2180. This guideline is of the greatest practical relevance and is to be incorporated unequivocally and practically in the project design.
Picture 2: Mode of operation of monitoring and safety equipment
VDI/VDE guideline 2180 is based on the following basic precept. (Picture 2):
>> Case 1: In the case of a process technological system, differentiation is made between the so-called acceptable range, the permissible error range and the unacceptable error range. Here, it is assumed that a process technological system in the course of normal operation (steady-state operation) moves within the so-called acceptable range, i.e. in the event of a drifting of process parameters from the working point, the system operates in the permissible error range. The protection of this operating regime is realised by means of the actual automation system, whereby a corresponding monitoring device (limit value encoded) ensures that if the acceptable range is exceeded (steady-state operating status), the system is automatically returned to the acceptable range or, if governed by the process technology, by manual intervention.
>> Case 2: If this monitoring device fails, then a so-called safety device (safety device 1) is indicated, which is also realised by means of a corresponding limit value encoder and at the very least returns the process parameters into the permissible error range.
>> Case 3: Irrespective of these measures described, an additional safety device (safety device 2) must be provided, which comes into effect in the event of failure of the previously described safety technology. This safety device requires a hardware basis which is completely independent of the previous automation equipment, which encompasses both sensors and actuators and also processors.
This ensures that in the event of failure of monitoring or safety equipment 1, an additional automation structure becomes effective in the process technology system, thereby preventing any drifting of the process parameters into the unacceptable error range (system damage or breakdown). Depending on the process class, the project design engineer must therefore decide to what extent the specifications of VDI/VDE 2180 must be incorporated in the project design work.
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