Page 225 - Design of Simple and Robust Process Plants

210 Chapter 5 Process Simplification and Intensification Techniques
. ªprevent versus cureº; and
. ªfirst pass primeº.
These are basic approaches to keeping piping designs simple, and when discussions
begin about the design of P&IDs, any questions which include the term ªwhat if ?º
should precipitate one of the following responses:
What is the likelihood of this situation?

. What would happen if we did not have the proposed item at that time?
. Could we prevent such a situation, and what would we then need to do?
. If we were to design for first pass prime, would it still be likely to occur?
The operation that should lead to first pass prime operation will be discussed in
Chapter 8.

5.7.8
Instruments

Instruments are recognized as being the ªeyesº, ªearsº, and ªhandsº of the process,
and it was during the eighties and nineties that a number of somewhat striking
statements came to light:

. A major proportion of the instruments lie or die.
. Instruments are often unreliable during the start up and restart of processes.
. Most process outages are caused by instrument problems.
It is clear that hands-off operation of a simple and robust designed processes, cannot
be obtained under these conditions. With respect to instruments, we will here only
discuss those issues that are essential for simplification, as instrument design is de-
scribed in greater detail in Section 8.2 in Chapter 8.
From a perspective of simplification, the following design statement should be
borne in mind: ªBetter a good reliable instrument then lots of unreliable onesº.
This statement was underlined implicitly by a company which had a large num-
ber of remotely operated air separation plants. The company improved operation by
eliminating and upgrading instruments on their remotely operated processes,
mainly because too many of the instruments did not give the correct information
and this led to a great deal of confusion.
In order to simplify instrumentation, it is obvious that by eliminating equipment
and combining process functions will result in a need for fewer instruments, in
much the same way as was discussed for piping.
The specific instrument elements to achieve simplified instrumentation include:

. Selection of the essential process variables for operation, and their location.
Avoid measurements which are only in place to measure equipment perfor-
mance for the design engineer. Operation is the primary purpose of instru-
mentation ± design information can also be measured by clamp-on measure-
ments during test runs.