Single Control Loops

PRESSURE CONTROL LOOPS

Pressure control loops vary in speed—that is, they can respond to
changes in load or to control action slowly or quickly. The speed
required in a pressure control loop may be dictated by the volume of
the process fluid. High-volume systems (e.g., large natural gas storage
facilities) tend to change more slowly than low-volume systems
(Figure 7.21).

FLOW CONTROL LOOPS

Generally, flow control loops are regarded as fast loops that respond
to changes quickly. Therefore, flow control equipment must have fast
sampling and response times. Because flow transmitters tend to be
rather sensitive devices, they can produce rapid fluctuations or noise
in the control signal. To compensate for noise, many flow transmitters
have a damping function that filters out noise. Sometimes, filters are
added between the transmitter and the control system. Because the
temperature of the process fluid affects its density, temperature
measurements are often taken with flow measurements and
compensation for temperature is accounted for in the flow
calculation. Typically, a flow sensor, a transmitter, a controller, and a
valve or pump are used in flow control loops (Figure 7.22).

LEVEL CONTROL LOOPS

The speed of changes in a level control loop largely depends on the
size and shape of the process vessel (e.g., larger vessels take longer to
fill than smaller ones) and the flow rate of the input and outflow
pipes. Manufacturers may use one of many different measurement
technologies to determine level, including radar, ultrasonic, float
gauge, and pressure measurement. The final control element in a level
control loop is usually a valve on the input and/or outflow
connections to the tank (Figure 7.23). Because it is often critical to
avoid tank overflow, redundant level control systems are sometimes
employed

TEMPERATURE CONTROL LOOPS

Because of the time required to change the temperature of a process
fluid, temperature loops tend to be relatively slow. Feedforward
control strategies are often used to increase the speed of the
temperature loop response. RTDs or thermocouples are typical
temperature sensors. Temperature transmitters and controllers are
used, although it is not uncommon to see temperature sensors wired
directly to the input interface of a controller. The final control element
for a temperature loop is usually the fuel valve to a burner or a valve
to some kind of heat exchanger. Sometimes, cool process fluid is
added to the mix to maintain temperature (Figure 7.24).