Page 252 - Design of Simple and Robust Process Plants

238 Chapter 6 Process Design Based on Reliability
± Total number of outages.
± Outages per group of similar components or individual components such as
pumps, vessels, compressors, and exchangers.
± Total lost time (unavailability).
± Lost time per group of components or individual components.
± Ranking items in order of failures/outages and time lost.
. Evaluation of results and development of alternatives. Evaluation of perfor-
mance make the components that contribute mainly to failures and unavail-
ability clearly visible. Also, the cost of these outages can be related to the dif-
ferent components. In line with the design philosophies for simple and
robust process plants, we must evaluate the following options to meet the
objectives:
± Can unreliable component be avoided? Examples to be considered include:
replacement of a pump by taking advantage of gravity flow or pressure differ-
ence, or replacement of an agitator by a jet mixer.
± Consider replacement of less reliable components by more reliable ones.
Examples include: replace liquid sealed pumps with seal-less pumps, or pro-
vide gas seals; alternatively, compressors might replace liquid seals by gas
seals An agitator with a long shaft might be replaced with a bottom entry
agitator. The long shaft of an agitator creates large forces on the bearings and
seals, due to any imbalance of the rotor, and may be subject to extreme wear.
± Identify components as less critical.
In some situations, as in the case of pump failure, these failures do not have
to result in a momentary process stop. When a process can withstand this
interruption during the MTTR of the pump, then it is called a ªnon-criticalº
item. This might be the case for some dosing pumps. For example, cooling
water systems but also boiler water systems can stand interruption for several
hours. In those cases, we can compensate the interruption by a temporarily
higher dosing stream after repair.
Another option to be explored is to build some temporary hold-up in the pro-
cess. The bottom hold-up of a distillation tower might be increased to capture
the volume of a small tar stream during a short outage of the bottom pump.
The same applies to the size of a reflux drum in case of a small top product
stream.
Consider installation of redundant components. The redundancy of items
should be the last explored option for a design, although there might be no
other choice ± particularly in the case of damage situation after a failure. The
option of sharing an installed spare needs to be explored, for noncompetitive
process streams, in terms of safety or product quality. One should respect
total quality control (TQC) as objective.
Note: In the above, most attention has been paid to rotating equipment as being one
of the major contributors to component failures. Much progress has been made dur-
ing the past few decades on dynamic seals and bearing development, in order to
make the design more robust.