Page 224 - Design of Simple and Robust Process Plants

5.7 Simplification and Ranking per Unit Operation 209
Although all of the above-mentioned actions will reduce piping, it should be realized
that in case of the removal of spare installed equipment the removal of piping is
more than double the amount of a single unit. Also, the combination of functions
and equipment leads to a need for considerably less piping. Recall the situation of
when a mixer-settler combination was replaced with an extraction column (see
Figures 5.14 and 16), and the comparison of a series of CSTRs versus an adiabatic
plugflow reactor (Figure 4.8 in Chapter 4).
Specific piping elements which result in the simplification/minimization of pip-
ing include:
. Replacement of piping which has many interconnections by a more dedi-
cated line(s). For example, a large manifold might be split into several small-
er ones. When a manifold from six connections is split into two manifolds
each of three connections, this would reduce the number of flow interconnec-
tions from 30, to 2 ” 6 (see Table 2.1 in Chapter 2). Minimization of the num-
ber of connections is not applicable to utility systems, as in these cases the
pressure in the utility system is kept higher than the process operating pres-
sure of the individual users in order to avoid any back flow. If that is not the
case, then special precautions must be taken.
. Minimization of piping items. This might seem difficult to instigate, but
includes:
± Bellows; these can be avoided by changing the pipe design.
± Manually operated bypasses around control or block valves are not preferred
when a decision has been made to operate fully automatically. A totally auto-
mated process should not be operated on partial manual control, as this will
lead to problems.
± Precision piping bends reduces the need for gasketed and sealed joints,
which are the weak points in piping. In high-pressure systems, welding of
piping is preferred.
± ASA flanges are preferred, as these are stronger and provide less chance for
leakage at gaskets.
± Screwed connections should be avoided for chemicals, as they have a greater
tendency to leak.
± Avoid check valves at places where performance on reverse flow is essential,
and not simply convenient. In these situations, install positive shut-off valves
with back-flow detection. Check valves should be avoided as they are neither
reliable nor robust because they are sensitive to fouling and the performance
is difficult to monitor.
± Isolation valves should be carefully selected; too many of these does not add
value as such, as they add gaskets to the system, and losses to the environ-
ment.
The ªwhat if ?º situation (scenario) often results in add-on provisions, and should be
addressed in another way during P&ID (process and instrument diagram) reviews.
The design philosophies which may be beneficial in such situations are: