Page 254 - Design of Simple and Robust Process Plants

240 Chapter 6 Process Design Based on Reliability
A base case (case 0) was selected for the design with single components with an
exception for those components which also served another process section. This was
to avoid outage of more sections/plants by failure of a component. This approach is
common to be followed in designs where independent operation of process sec-
tions/plants is the selected operational strategy. The results of the base case are
given in Table 6.3.
It is clear from the data that pumps make the major contribution to mechanical
failures, and the number of unplanned stops (6.4 per year) was considered quite
large. The total number of pumps was 15, and the number of installed spare pumps
was five. The average repair time for the pumps in this study was 12 ± 2h.

Table 6.2. Overview of some typical MTBF, MTTR (h), and its distribution function of process
equipment.

Component MTBF Distribution MTTR Distribution
Drum/vessel 920 000 Exp. 60 Exp.
Column 162000 Weibull 120 Log N
Agitator 31 500 Weibull Exp
Compressor 13 000 Exp 32Log N
Pump liquor seals 9 900 Weibull 12Log N
Pump gas seals* 19 800 Weibull 12Log N
Seal-less pumps 12600 Exp 12 Normal
Vacuum pump 13 500 Weibull 12Log N
Exchanger 108 500 Log N 20 Log N
Condenser 107 700 Exp 20 Log N
Reboiler 150 000 Weibull 20 Log N
Air cooler 29 300 Weibull 12 Normal
Centrifuges 13 000 Exp
* Limited data are available, but comparison between liquid and gas
seals for compressors justified doubling of MTBF.
Note: Repair times of pumps are based on replacement of pump by
spare pump at stock.

Table 6.3. Base case 0, overview of down-time and number of stops and unavailability as function
of type of mechanical equipment failures.

Type of equipment Down-time (h/year) No. of unplanned Unavailability
stops/year as % of time

Pumps 61.9 5.1
9
Heat exchangers 24.4
> >
>
Drums 7.7 > 1.3
=
Columns 2.4
> >
>
>
Rest of equipment 1.9 ;
Total 98.3 6.4 1.12