Page 40 - Design of Simple and Robust Process Plants

24 Chapter 2 Simple and Robust Plant Design

A B

Fig. 2.1. How to protect back flow.

In that case we normally have two pressure measurements converted in a pres-
sure difference, and a control device with an actuated shut-off valve. This is indepen-
dent from any pressure protection (low and high) on the vessels, and from any flow
device to control the flow. The tank shown in Figure 2.2 has 10 possible connec-
tions, and in case of any change in level it will be difficult to analyze the cause. To
illustrate this, consider a set of manifolds with a number of flow interconnections
(Figure 2.3). One single line has only two flow interconnections from vessel A to
vessel B and vice versa. In case of a manifold of seven connections, there are already
42 flow interconnections. The number of flow interconnections in relation to the
connections of a manifold is described as:

iˆn 1
I=2” P i 1†, (1)
iˆ1
where I is number of flow interconnections, and i is the number of connections.
We can list this for the number of flow interconnections as a function of the num-
ber of connections in a manifold (Table 2.1).
It will be clear that when the number of connections increases, the number of
potential errors in lining up and eventual error detection points increase steeply. Of
course, not all manifolds are as complicated as this, and when one considers a
steam header system, the situation is often designed in such a way that there is one
high-pressure inlet and all other connections have a low pressure. This prevents
back flow and also considerably reduces the number of potential flow interconnec-
tions.
When we compare a TV set with a piping manifold, it might be concluded that a
TV set is a simple device because everybody understands its function, and even

Fig. 2.2. Vessel with ten connections, what causes the increase in level?