Page 44 - Design of Simple and Robust Process Plants

28 Chapter 2 Simple and Robust Plant Design
where C is called the complexity factor of the unit. By summation of individual fac-
tors and adding a weight factor for each term, we have the following formulae (the
impact of interaction or external disturbance may be larger than the effect of an
additional valve):
Complexity becomes: C unit = mM + nN + oO + pP + qQ + rR,
where m, n, o, p, q, r, are the weight factors per item.
n
In generic terms: C unit = P a n A n , (3)
1
where a are the weight factors and A the items.
A specific term can be further split up if we want to give different weight factors
within a term. Let us look at the manipulators N: these consist of manual valve W,
actuated valves in general connected to control system X, switches DOFs Y, set
points Z, the respectively weight factors are in small letters.
n
N = wW + xX + yY + zZ in generic terms N = P b n B n ,
1
where b are the weight factors and B the manipulators.
Equation (3) can be simplified by using 1 as the weight factor for all terms. Thus,
Complexity is now simplified to: C unit = M + N + O + P + Q + R
n
In generic terms: C unit = P A n (4)
1
Now, we can calculate a complexity number for a unit. Some examples for quanti-
fication of complexity are as follows:

. For a household refrigerator M = 1, N = 2 on/off switch and temperature set
point, O = 0, P = 1 electricity, Q = 0, R = 0. Now the complexity is C = 4.
. For a simple TV set M = 1, N = 3 on/off switch, program selector and volume,
O = 0, P = 2 electricity and signal cable, R = 0. Now the complexity is C = 5
. For a manifold with seven branches with its connecting equipment and the
limitation of only one single line up at the time, M = 7, N = 7 number of
valves, P = 14, Q = 0, R = 0. Now the complexity is C = 28
For a normal distillation column a P&ID was analyzed. With regard to the manipu-
lated items, the following were excluded: instrument valves, blinds, safety devices.
Included were vents and drains.
The values of the different terms are: M = 9 (tower, reflux drum, reboiler, conden-
ser, vent condenser, two reflux pumps, two bottom pumps); N = 89 (control valves 7,
manual valves 67, DI/DOs 9, control loops 6); O = 28; P = 12 (feed, bottom, distillate,
vent stream, steam, condensate, cooling water in and out twice, electricity twice); Q
= 2; and R = 3 (feed flow, feed composition, cooling water temperature). So, the
complexity for the selected distillation column is: C = M + N + O + P + Q + R =143
For a process plant, which is a summation of individual units, we can add up the
individual complexity factors:
n
Complexity of process plant: C plant = P c n C n units, (5)
1
where c n are the weight factors per unit.