-
SAMA DIAGRAMS FOR
BOILER CONTROLS
We're Siemens. We can do that.TM
APPLICATION DATA
PURPOSE
Functional control diagrams for the power industry are of-ten
referred to as SAMA diagrams. They are based onsymbols and
diagramming conventions developed by theScientific Apparatus Makers
Association (SAMA). Theyare used to describe and document control
strategies andsystems designed for both industrial and utility
boiler ap-plications. Although similar in concept to ISA
diagrams,there are significant differences between the two meth-ods
of diagramming control systems.
SAMA Standard PMC 22.1-1981 Functional Diagrammingof Instrument
and Control Systems is no longer sup-ported by SAMA or any other
standardization committee.It is anticipated, however, that the
symbols and conven-tions contained in this standard will continue
to be usedfor the foreseeable future. The purpose of this
documentis to describe the basic symbols and a few of the
manyvariations of these symbols that have evolved over
theyears.
These SAMA diagrams are used in the Siemens Mooreseries of
boiler control application notes.Therefore, a gen-eral
understanding of the diagrams is helpful to effectivelyuse the
applications notes.
HIGHLIGHTS
SAMA diagrams represent the language of choicethroughout the
power and pulp & paper industries for in-strumentation and
control systems. This documentshows and explains the following
types of symbols, in-cluding variations and examples of their
application. Enclosure symbols Signal continuation symbols Signal
processing symbols
FIGURE 1 ISA vs. SAMA Functional Diagrams
ISA Diagram
FT
FIC
FCV
FT
P I
A T A
FCV
SAMA Diagram
SYMBOLS
Figure 1 shows a simple flow control loop using both ISAand SAMA
diagrams. Only the symbol for the flow trans-mitter (FT) is
identical in both cases. The ISA diagramshows a very symbolic
representation of the flow indicat-ing controller (FIC). The SAMA
diagram provides a moredetailed block diagram of the proportional
plus integral (PI)controller with setpoint and manual adjustments
and auto/manual transfer switch. The SAMA and ISA versions alsouse
different symbols to represent the flow control valve(FCV).
Siemens Moore is a member of theAmerican Boiler Manufactuers
Association
-
A SAMA diagram uses various types of enclosure sym-bols as shown
in Table 1 to represent the various ele-ments or functions of the
control system. The enclosuresymbols are tied together by the
continuation symbolsshown in Table 2. Table 3 shows the many signal
process-ing symbols used to describe the functions within
eachenclosure.
The use of most of these symbols will be fairly obviousafter
reading a few SAMA diagrams. It is important to re-alize, however,
that very few designers adhere strictly tothe SAMA standard, and it
is not uncommon to see slightvariations in the symbols shown
here.
TABLE 1 Enclosure Symbols
AND
OR
_ _n
NOT
S
R
SO
R
S
RO
FUNCTION ENCLOSURESYMBOL FUNCTIONENCLOSURE
SYMBOL
t_ _
Measuring or Readout
Manual Signal Processing
Automatic Signal Processing
Final Controlling
Final Controllingwith Positioner
Time Delay orPulse Duration
Logical AND
Logical OR
Qualified Logical OR
Logical NOT
Maintained Memory
Optional Reset
page 2
-
TABLE 2 Processed Signal Continuation Symbols
* The on-off signal symbol may be a solid line if on a
separatedigital logic diagram or if on an inset detail on a
funtionaldiagram.
SIGNAL SYMBOL
Continuously Variable Signal
Incremental Change Signalor Rate of Change of a Continuously
Variable Signal
On-Off Signal *
TABLE 3 Signal Processing Symbols
FUNCTIONSIGNAL
PROCESSINGSYMBOL
Summing
Averaging
Difference
Proportional
Integral
Derivative
Multiplying
Dividing
Root Extraction
Exponential
Non-Linear FunctionTri-State Signal
Integrate or Totalize
High Selecting
Low Selecting
High Limiting
Low Limiting
Reverse Proportional
Velocity Limiting
Bias
Time Function
Variable Signal Generator
Transfer
Signal Monitor
(Raise, Hold, Lower)
or +
/ nor
K or P
or |d/dt or D
x
x
f(x)
Q>
f(t)A
T
H/, H/L, /L
FUNCTIONSIGNAL
PROCESSINGSYMBOL
Logical Signal
Logical AND
Logical OR
Qualified
n = an integer
Logical NOT
Set Memory
Reset Memory
Pulse DurationPulse Duration
Analog
Digital
Voltage
Frequency
Hydraulic
Current
Electromagnetic
Pneumatic
Resistance
of the Lesser Time
Generator
Logical OR
Time Delayon Initiation
SignalConverter
or Sonic
B
ORAND
GTn
LTn
EQnNOT
> n
< n
= n
S, SO
PD
R, RO
LT
DT
DI or GT
A
ED
F
H
IO
Input/ Output
Examples: D/A I/P
R
P
Time Delayon Termination
n
n
page 3
-
VARIATIONS
PID ControllerThe fundamental function of most control loops is
the PIDcontroller. PID stands for the Proportional plus
Integralplus Derivative control algorithm. Figure 2 shows
fourvariations of the symbol used to describe this algorithmin SAMA
diagrams.
The PID controller generally has two inputs representingthe
process variable (PV) to be controlled and the setpoint(SP) value
at which it is desired to maintain the PV. Thecontroller calculates
the difference ( ), or control error,between these two signals and
generates an output todrive the PV to SP. Depending on the number
of controlmodes specified, the controller output is proportional
(P)to the magnitude of the error, the integral (I) of the error,the
derivative (D) of the error, or various combinations ofthese three
functions.
Figures 2A shows the classic SAMA symbol for a PIDcontroller
using the standard mathematical symbols forthese functions. The
rectangular enclosures indicate thatthese signals and functions are
processed automatically.Figure 2B simply substitutes P, I, and D
for the standardmathematical symbols. Figure 2C simplifies the
drawingof the symbol by combining the P, I, and D functionswithin a
single rectangle.
Figure 2D shows the actual structure of the standardSiemens
Moore implementation of the PID algorithm. Inthis form, the
derivative mode is a function of a change inprocess variable
instead of a change in control error. Thisavoids a derivative
"kick" on changes in setpoint. In mostcases, however, it is not
necessary to make thisdistinction when drawing a SAMA diagram.
FIGURE 2 PID Controller Symbol Variations
2A
K ddt P I D
2B
P I
D
PID
2C 2D
page 4
-
Single Loop ControlThe fundamental single loop control diagram
includes aprocess measurement, a PID controller with
adjustablesetpoint and auto/manual transfer, and a final control
ele-ment such as a control valve or drive mechanism. Figure3 shows
three variations of the collection of symbolsused to describe
single loop control on SAMA diagrams.
Figure 3A shows the classic SAMA diagram for singleloop control
using a PI controller. The three diamondsganged together represent
the adjustable setpoint (left A),the adjustable manual output
(right A), and the auto/manual transfer switch (T). The diamond
shaped enclo-sures indicate that these are all manual functions
per-formed by an operator. The location of these functions onthe
diagram is probably symbolic of the equipment de-signs in use when
the standard was originally developed.These adjustments were
typically provided by separatecomponents mounted within a control
station.
Figure 3B simply relocates the setpoint adjustment di-rectly on
the symbol for the PI controller. It also showsthe FCV equipped
with a valve positioner. It should benoted, however, that many
diagrams omit the positionersymbol for simplicity, and the drawing
should not be con-strued as the final authority regarding the
presence or ab-sence of a valve positioner.
Figure 3C shows another variation of the classic arrange-ment of
the three diamonds. Also note the non-linearfunction symbol [f(x)]
instead of the FCV. This may beused to represent an inherently
non-linear valve character-istic (e.g. equal percentage), or it may
represent the useof a positioner that includes a characterizer
function (eventhough the positioner symbol is not shown). Also
beaware that some SAMA diagrams routinely show the f(x)
symbol on all final control elements without regard for
theactual characteristics of the valve or positioner.
Equipment DetailSAMA diagrams are used to describe the
functional ele-ments of a control strategy. The symbols are generic
andare not specific to the control hardware manufactured byany
particular vendor. There are instances, however, whenit may be
necessary to show equipment details to fullydocument the control
strategy. Figure 4 shows two varia-tions of the single loop control
diagram showing equip-ment details. In general, this level of
detail should beavoided since it obscures the basic control
strategy andmakes the diagram less generic.
Figure 4A shows one of the single loop variations withthe
addition of three separate indicators for monitoring
andmanipulating the loop. The indicators are shown as
circularenclosure symbols with the ISA indicator symbol (I).
Theydisplay the three key variables of the control loop (pro-cess,
setpoint, and valve). The operator needs to seethese three
variables to determine the state of the controlloop. In addition,
the operator must be able to see thevalue of the setpoint and valve
loading to adjust thesevariables in auto and manual modes,
respectively. Since itis standard practice to provide these
readouts on everycontrol loop, it is generally not necessary to
show themexplicitly on the SAMA diagram.
Figure 4B shows additional automatic signal processingfunctions
to describe equipment details. The transferblocks on the setpoint
signal and controller output areused to describe setpoint and
controller tracking. The rect-angular transfer symbols switch
automatically based onthe state of the discrete input signal
represented by thedotted line. This discrete signal indicates the
position of
FIGURE 3 Single Loop Control Variations
FT
P I
AT
A
f(x)
3B 3C3A
FT
P I
A T A
FCV
FT
P IA
T A
FCV
page 5
-
FIGURE 4Equipment Detail
Variations
value of the PV. Another equipment detail shows resetfeedback
from the valve signal to drive the integral actionof the PI
controller. These are control hardware implemen-tation details that
are not usually necessary to convey theoverall control strategy.
They should be shown only asnecessary in the judgment of the
designer or end user.
EXAMPLES
Three-Element Drum Level ControlA common application in boiler
control is three-elementdrum level control. Boiler drum level is a
critical variablein the safe operation of a boiler. Low drum level
risksuncovering the boiler tubes and exposing them to heatstress
and damage. High drum level risks water carryoverinto the steam
header and exposing steam turbines tocorrosion and damage. The
level control problem iscomplicated by inverse response transients
known asshrink and swell.
Figure 5 is a SAMA diagram of the cascade plusfeedforward
control strategy that is normally used to solvethese control
problems. The three transmitters, or vari-ables, are the three
elements referred to in the name ofthe control strategy. The
feedwater flow setpoint is setautomatically by the steam flow
signal to keep thefeedwater supply in balance with the steam
demand; thisis the feedforward component of the control strategy.
Thedrum level controller trims the feedwater flow setpoint
tocompensate for errors in the flow measurements or anyother
unmeasured load disturbances (e.g. blowdown) thatmay effect the
drum level; this is the cascade componentof the control strategy.
The summing function is used tocombine these two components.
The square root functions on the flow transmitters linear-ize
the relationship between flow and differential pressurein head-type
flowmeters.
FT
P I
AT
A
FCV
I I I
4A
4BFT
P I
T A
FCV
T
TA
FIGURE 53-Element Drum Level
Control
DrumLevel
SteamFlow
FeedwaterFlow
Feedwater Valve
LT
P I
FCV
A
FT
P I
FT
T A
page 6
the manually operated auto/manual transfer switch. In themanual
mode, the setpoint tracks the process variable,and the controller
tracks the manually adjusted valve load-ing signal. This prepares
the controller for bumpless trans-fer back to auto, and aligns the
setpoint with the present
-
Logic DiagramTo avoid damage to equipment and injury to
personnel,boiler control systems include safety systems for
com-bustion control and burner management. These systemsprovide
permissives and interlocks to ensure safe operat-ing conditions and
to shutdown or "trip" the unit if safeoperating conditions are not
maintained.
Figure 6 shows a typical logic diagram for a small portionof the
safety system. This diagram shows the derivationof a boiler trip
command signal and alarm indicators,based on high or low drum
level, and power failures inthe combustion control or burner
management systems.For fail-safe operation, the drum level and
power switches(LSH, LSL, JS) are ON for safe conditions and OFF for
tripconditions. The NOT functions reverse the logic to accom-modate
this input convention.
The time delay functions on the drum level switch signalsprevent
nuisance trips due to brief violations of the levellimits that may
be caused by noise or transient behavior.As indicated in Table 1,
these symbols require a time set-ting "t" and a two-letter
designator for the function type(e.g. PD, DI, DT, etc.). DI stands
for "Delay On Initiation";in this example, the delay time is set
for 20 seconds. Theinput signal must stay ON for at least 20
seconds beforethe output signal will turn ON.
One OR function provides a common alarm and trip signalfor high
or low drum level conditions. The other OR func-tion provides a
common boiler trip signal for a level trip, aCCS power failure, or
a BMS power failure.This example uses dotted lines to designate
discrete logicsignals. However, it is also acceptable to use solid
lineswhen the logic diagram is separate from the continuouscontrol
diagram.
APPLICATIONS
SAMA diagrams are generally used to describe boiler con-trol
systems for the power industry. Although there is noreason they
cannot be used to describe control systemsin other industries,
convention dictates that ISA diagramsare used in those industries.
Therefore, the control engi-neer should be conversant in either
method of diagram-ming control systems.
Siemens Moore assumes no liability for errors or omissions in
this document or for the appli-cation and use of information
included in this document. The information herein is subject
tochange without notice.
Siemens Moore is not responsible for changes to product
functionality after the publication ofthis document. Customers are
urged to consult with a Siemens Moore sales representative
toconfirm the applicability of the information in this document to
the product they purchased.
FIGURE 6 Typical Logic Diagram
NOT 20 SECLSH DIDrum LevelBelow + 7 in.
Drum LevelAbove - 7 in.
CCS PwrOK
BMS PwrOK
OR LA
NOT 20 SECLSL DI
NOTJS
OR
NOTJS
JA
JA
DrumLevelTrip
BoilerTrip
CCS PwrFailure
BMS PwrFailure
page 7
-
ADi-201 Rev 1 9/00
500 SH
www.smpa.siemens.com
For prompt, personal attention to your instrumentation and
control needs, contact the
Siemens Moore location nearest you. Information on other Siemens
Moore representa-
tives in your area is available from these regional
locations.
Siemens Moore Process Automation, Inc.1201 Sumneytown PikeSpring
House, PA 19477-0900
We're Siemens. We can do that.TM
2000 Siemens Moore Process Automation, Inc.
Procidia is a trademark of Siemens Moore Process Automation,
Inc. We'reSiemens. we can do that is a trademark of Siemens Energy
and Automation.All other trademarks are the property of the
respective owners.
The information in this document is subject to change without
notice.Customers are urged to consult with a Moore sales
representative to confirmavailability and specifications.
INDIA
34/35, MIDC Estate
H Block
Pimpri, Pune - 411 018
Tel: +91 20 7470171, +91 20 7470121
Fax: +91 20 7477049
ITALY
Via S. Aleramo 2
20063 Cernusco sul Naviglio (Milano)
Tel: +39 02 929054 1
Fax: +39 02 92905454
MEXICO
Tintoreto No. 32 - 302
Col. Nonoalco, Mixcoac
Mexico, D.F.
C.P. 03910
Tel: +52 5 611 9858; +52 5 615 1948;
+52 5 615 0262; or +52 5 615 0238
Fax: +52 5 611 4541
SINGAPORE
2 Kallang SectorSingapore 349277Tel: +65 740 7842
Fax: +65 740 7839
THE BENELUX
Wagenmakerstraat 3
2984 BD RIDDERKERK
Tel: +31 180 461111
Fax: +31 180 461132
CANADA
P.O. Box 370
Brampton, Ontario L6V 2L3
Tel: +1 905 457 9638
Fax: +1 905 457 4182
CHINA
Automation & Drives Group
10/F China Marine Tower
No. 1 Pudong Da Dao
Pudong New Area
Shanghai 100120
China
Tel: +86 21 5888 2000
Fax: +86 21 5879 0144
FRANCE
Logistique A&DTour Centre 5eme etage39-47 Boulevard
Ornano
93527 Saint Denis
Cedex 2, FranceTel: +33 1 49 22 93 18Fax: +33 1 49 22 93 20
SOUTH AFRICA
Moore House
39 Monza Place
Kyalami Business Park
Midrand
Tel: +27 11 466 1673/9
Fax: +27 11 466 1618
UNITED KINGDOM
Copse Road
Lufton Industrial Estate
Yeovil, Somerset BA22 8RN
Tel: +44 1935 706262
Fax: +44 1935 706969
UNITED STATES
1201 Sumneytown Pike
Spring House, PA 19477-0900
Tel: +1 215 646 7400
Fax: +1 215 283 6358
