Page 256 - Design of Simple and Robust Process Plants

242 Chapter 6 Process Design Based on Reliability
The cost of theses outages may be high. Site vulnerability is discussed in the next chap-
ter. Here, the results of a study by Shor and Griffin (1991) are summarized, regarding
the design of co-generation plants to provide a reliable, continuous steam supply.
The objective for the study was to evaluate the reliability of steam supply of mini-
mum 380 Mton/h for different co-generation design cases. Four design cases were
considered, while case 2had a variant A and B (see Figures 6.9 and 6.10):

. Case 1 consisted of three gas turbines (GT), frame 6, each provided with a
waste heat boiler (WHB) and two existing fossil boilers (FB).
. Case 2 consisted of four gas turbines LM 2500 with WHB and two existing
fossil boilers.
. Case 2 (A and B) differentiate themselves in the capacity of the WHB, 75
versus 100 Mton/h.
. Case 3 consisted of three boilers with steam turbines provided with back-up
de-superheaters and the two existing FBs.
. Case 4 emphasized four large-capacity boilers with three steam turbines pro-
vided with de-superheaters.
The maximum steam and power capacities for these cases are summarized in
Table 6.5. The steam production of these cases is also given if a boiler or a turbine is
eventually combined with a WHB is out of service. In all these cases the minimum
amount of steam (380 Mton/h) is still available. This is a design requirement, as
utility plants have to provide their service all-year round, and so scheduled mainte-
nance needs to be performed during operation. At a small site, the maintenance of
utility systems might be performed during planned process plant stops. For the dif-

Case 1 Case 2A Case 2B
100 t/hr 100 t/hr 100 t/hr
Boil 1 Boil 1 Boil 1
100 t/hr 100 t/hr 100 t/hr
Boil 2 Boil 2 Boil 2

75 t/hr 100 t/hr
160 t/hr LM LM
2500 WHB 2500 WHB
Fr 6 WHB
75 t/hr 100 t/hr
LM LM
160 t/hr WHB WHB
2500 2500
Fr 6 WHB 75 t/hr 100 t/hr
LM LM
2500 WHB 2500 WHB
160 t/hr
Fr 6 WHB 75 t/hr 100 t/hr
LM LM
2500 WHB 2500 WHB

Fig. 6.9. Alternative designs for a co-generation plant.