Page 51 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Consider the benzene product line leaving the right-hand side of the P&ID in Figure 1.7. The flowrate of
this stream is controlled by a control valve that receives a signal from a level measuring element placed
on V-104. The sequence of instrumentation is as follows:

A level sensing element (LE) is located on the reflux drum V-104. A level transmitter (LT) also located
on V-104 sends an electrical signal (designated by a dashed line) to a level indicator and controller
(LIC). This LIC is located in the control room on the control panel or console (as indicated by the
horizontal line under LIC) and can be observed by the operators. From the LIC, an electrical signal is sent
to an instrument (LY) that computes the correct valve position and in turn sends a pneumatic signal
(designated by a solid line with cross hatching) to activate the control valve (LCV). In order to warn
operators of potential problems, two alarms are placed in the control room. These are a high-level alarm
(LAH) and a low-level alarm (LAL), and they receive the same signal from the level transmitter as does
the controller.

This control loop is also indicated on the PFD of Figure 1.5. However, the details of all the
instrumentation are condensed into a single symbol (LIC), which adequately describes the essential
process control function being performed. The control action that takes place is not described explicitly in

either drawing. However, it is a simple matter to infer that if there is an increase in the level of liquid in
V-104, the control valve will open slightly and the flow of benzene product will increase, tending to
lower the level in V-104. For a decrease in the level of liquid, the valve will close slightly.

The details of the other control loops in Figures 1.5 and 1.7 are left to problems at the end of this chapter.
It is worth mentioning that in virtually all cases of process control in chemical processes, the final control
element is a valve. Thus, all control logic is based on the effect that a change in a given flowrate has on a
given variable. The key to understanding the control logic is to identify which flowrate is being
manipulated to control which variable. Once this has been done, it is a relatively simple matter to see in
which direction the valve should change in order to make the desired change in the control variable. The
response time of the system and type of control action used—for example, proportional, integral, or
differential—are left to the instrument engineers and are not covered in this text.

The final control element in nearly all chemical process control loops is a valve.

The P&ID is the last stage of process design and serves as a guide for those who will be responsible for
the final design and construction. Based on this diagram,
1. Mechanical engineers and civil engineers will design and install pieces of equipment.

2. Instrument engineers will specify, install, and check control systems.
3. Piping engineers will develop plant layout and elevation drawings.
4. Project engineers will develop plant and construction schedules.

Before final acceptance, the P&IDs serve as a checklist against which each item in the plant is checked.

The P&ID is also used to train operators. Once the plant is built and is operational, there are limits to
what operators can do. About all that can be done to correct or alter performance of the plant is to open,

close, or change the position of a valve. Part of the training would pose situations and require the
operators to be able to describe what specific valve should be changed, how it should be changed, and