Class 12 dead time, p & i diagram basics

ICE401: PROCESS INSTRUMENTATION
AND CONTROL
Class 12
Dead Time, P & I Diagram Basics
Dr. S. Meenatchisundaram
Email: meenasundar@gmail.com
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015

Comparison of Variables:
• A comparison of variables in the three different physical
systems we have discussed is given in the table below.
• It is important to have a general idea of the physical meaning
of process time constants so you can observe a process with
some understanding of its capacity to store material or
energy.
• In this way, you can gain some insight into the process
dynamics of a system.
• Such an understanding is very important in process control
design.

Comparison of Variables:
Variable
System Type
Electrical Liquid Thermal
Quantity Coulomb (c) Cubic meter (m3) cal
Potential Volt (v) meter (m) °C
Flow Ampere (A) m3/s cal/s
Resistance Ohms (Ω) m/m3 /s °C/cal/s
Capacitance Farad (F) m /m3 cal/°C
Time Second (s) Second (s) Second (s)

Dead Time Lag:
• In general, not all processes can be neatly characterized by
first-order lags. In some cases, a process will produce a
response curve like that shown in Figure 9.1.
• In this process, the maximum rate of change for the output
does not occur at time zero (to) but at some later time (t1).
• This is called dead time in process control: the period of time
(td) that elapses between the moment a change is introduced
into a process and the moment the output of the process
begins to change.
• Dead time is shown in Figure 9.1 as the time between t1 and
to , or td = t1 - to.

Dead Time Lag:
Figure 9.1
• Dead time is the most difficult condition to overcome in
process control.
• During dead time, there is no process response and therefore
no information available to initiate corrective action.
• To illustrate the concept of dead time, consider the temperature
feedback control system shown in Figure 9.2.

Dead Time Lag:
Figure 9.2

Dead Time Lag:
• In this process, steam is used to heat process fluid that is used
in other parts of a plant.
• A temperature detector in the heat exchanger discharge line
measures the temperature of the process fluid.
• The control system increases or decreases the steam into the
heat exchanger to maintain the outlet fluid at a desired
temperature. In the design of this control loop, the location of
the temperature detector is critical.
• It is tempting to say that the detector should be installed
farther down the outlet pipe and closer to the point at which
the process fluid is used.

Dead Time Lag:
• This seems correct because the temperature of the process
fluid in the heat exchanger or at the discharge point of the
exchanger is of little importance to the user of the heated
process fluid.
• However, this line of reasoning can be disastrous because as
the detector is moved farther and farther down the line, a
larger and larger dead time is introduced.
• In the example in Figure 9.2, the process dead time (td) is
equal to the distance (D) between the heat exchanger and the
temperature sensor, divided by the velocity (υ) of the water
flowing through the discharge pipe, or td = D/υ.

Dead Time Lag:
• If excessive dead time is introduced into this temperature
control loop, the loop performance will deteriorate and may
reach a point where it is impossible to achieve stable control.
Problem: Determine the dead time for the process shown in
Figure 9.2 if the temperature detector is located 50 meters from
the heat exchanger and the velocity of the process fluid in the
discharge pipe is 10 m/s.
• Solution: The dead time is given by td = D/υ. Since D = 50 m
and υ = 10 m/s.
→ 50 m / 10 m/s = 5s

Process & Instrumentation Diagram:
• Piping and instrumentation diagrams belong to a family of
flow sheets that includes block flow diagrams and process
flow diagrams. Technology advances have transformed these
resources into intelligent documents, capable of storing layers
of digital information.
Flowsheets:
• The terms flowsheet and flow diagram are often used in the
context of engineering and design applications.
• Although this terminology is not the most accurate way to
describe P&IDs, it is sufficient to describe the overall family
of process-based diagrams to which P&IDs belong.

• The block ﬂow diagram (BFD) (See Figure) is a very simple
diagram that can condense an entire process onto as little as a
single sheet.
• More detailed information can be found in the process ﬂow
diagram (PFD), which is considered the precursor to the P&ID.
• Typically, the PFD is used by plant designers to conduct initial
layout studies of a plant’s process systems and major pipework.
• Since PFDs use many of the same symbols as P&IDs, they
allow viewers to more easily identify items and processes by
sight.

• This is in contrast to the BFD’s standard block and line
diagram, which emphasizes the descriptions contained in
those blocks.

Block flow diagrams:
• The advantage of a BFD is its ability to out-line the complete
process on little more than a single sheet.
• These diagrams usually resemble an organizational chart,
containing mainly text enclosed by boxes, inter connecting
lines and the process commodities they transport, and flow
arrows to indicate process flow directions.
• A good BFD typically contains the following:
large individual pieces of equipment, or equipment as part
of a combined process, that are denoted by a single
symbol, typically a rectangle

Block flow diagrams:
clear labels illustrating function (since no equipment or
package numbers appear on this document)
the order of process flow arranged from left to right and, if
possible, with a gravity bias, i.e., if hydrocarbons are shown
entering a separation process, then gas leaving the process
should be shown exiting from the top of the block and
condensate from the bottom
lines linking equipment or processes to show flow direction
wherever more than one line leaves a process, then the
processed commodity in each line should be clearly marked

Process flow diagrams:
• Process flow diagrams (Figure 9.4) carry more information
than the block flow diagrams from which they are derived.
• They show more detail about major equipment and
subsystems and the flow of product between them.
• PFDs include information on the pressures and temperatures
of feed and product lines to and from all major pieces of
equipment, such as vessels, tanks, heat exchangers, pumps,
etc. Also indicated are main headers and points of pressure,
temperature and flow control, plus the main shutdown points
in the system.

Process ﬂow diagrams:
• For rotating equipment, PFDs carry important information,
such as pump capacities and pressure heads, and pump and
compressor horsepower.
• For tanks, vessels, columns, exchangers, etc., design
pressures and temperatures are often shown for clarity.

A typical PFD shows the following items:
• process piping
• process flow direction
• major equipment represented by simplified symbols
• major bypass and recirculation lines
• control and process-critical valves
• processes identified by system name
• system ratings and operational values
• compositions of fluids
• connections between systems.

References:
• Measurement and Control Basics, 3rd Edition by Thomas A.
Hughes, ISA Press.
• CEP (Chemical Engineering Progress), May 2009

Class 12 dead time, p & i diagram basics

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