Page 45 - Design of Simple and Robust Process Plants

2.2 The Level of Complexity 29
In the above formula a weight factor is introduced per unit. This might be used if
we have to deal with identical or similar units operations. For an identical unit, 0.1
might be used as weight factor, and for a similar unit with the same physical separa-
tion 0.2, while for a similar unit with another physical system to be separated the
weight factor would be 0.5.
It should be realized that particular interactions and disturbances in a process
plant can be a severe handicap, and that this should properly be reflected in the
weight factor. In these situations, weight factors of 10 might be applicable.
Weight factors have been chosen arbitrarily, and listed in Table 2.2. Those compo-
nents/units that have identical or similar functions have received a lower weight
factor.
Table 2.2. Weight factors for different items.

Item Weight factor
Identical unit 0.1
Similar unit for the same application 0.2
Similar unit for another application 0.5
Identical measurement in radial position of vessel 0.1
Interaction with direct single relation 5
Interaction with multiple relation 10
Disturbance with simple clear correction 5
Disturbance with multiple corrections 10

We have now introduced a complexity factor that can be used to compare different
designs.
When we analyse the formula it is clear how we can reduce the complexity of a
unit. The answer is to reduce the individual terms such as equipment, manipula-
tors, input and output streams, and measurements, and thus achieve a higher level
of automation and control. Under these circumstances the DOFs, interaction and
disturbances are reduced, and in principle a unit can offer single-button operation.
An example of this might be a burner management system, which is operated
hands-off while the operators do not have access to the parameters or the manipula-
tors, as all the software is protected. Another example might be a refrigeration unit
which is operated totally hands-off, with only a start/stop button being available.
Any other level of action is taken by nonoperational people.
Future investigations are required to address the relationship between complexity
and weight factors more accurately. In addition, the span of control of operators as a
function of complexity needs to be addressed (Wei Zhi-Gang et al., 1998; Stassen et
al., 1993). This might be a basis for deciding how far we should go with simplifica-
tion in order to make one operator responsible for the operation/supervision of
more units. Another side to the problem is that the minimum activity level of opera-
tors required to ensure correct, attended operation needs to be defined.