Page 302 - Design of Simple and Robust Process Plants

288 Chapter 8 Instrumentation, Automation of Operation and Control

Business

Operation
supervision
Operation
optimization

Model based
control system
Emergency
system Basic control
system

Actions to valves,
switches, output
Measurements,
input

Fig. 8.2. Control structure.

8.2.3
Instrumentation Design

Instrument design was influenced by the introduction of micro-electronics using
integrated circuits (ICs) also called chips. The application of these techniques at the
instrument level led to the introduction of so-called ªsmart transmitters and instru-
mentsº. These provided new means of increasing accuracy, applying fault detection
and auto-check and robust instrument design resulting in improved instrument
reliability. Accuracy was improved by the application of corrections for ambient tem-
perature variations for the different type of measurements.
In combination with digital communication, signal analysis over a broad spec-
trum may be applied for the early detection of approaching undesirable process con-
ditions. An example is the detection of pump or valve cavitation by signal analysis of
pressure measurements located close to the device; alternatively, the effects of bad
functioning control valves can be identified by flow or pressure analysis The speci-
fics for the most common instruments are discussed in the following sections.
Flow measurements were ± and still are ± typically performed with a cheap orifice
measurement provided with a dP cell. The element is reliable and robust, but it
2
measures rV (r = density; V = velocity); hence, the measurement of flow is in units
of neither mass nor volume. This can be seen as strange, since although the mass
2
flow was required for an operation, we were satisfied with obtaining a value for rV .
Normally, mass flow is derived by calibration at a fixed density. The signal is wrongly