SAMA DIAGRAMS FOR

BOILER CONTROLS

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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 FUNCTION ENCLOSURE

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 Function

Tri-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)

Σ or +

Σ / n

or −

K or P

or |

d/dt or D

x

÷

x

f(x)

Q

>

<

>

<

-K or -P

v >

±

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 Duration

Pulse Duration

Analog

Digital

Voltage

Frequency

Hydraulic

Current

Electromagnetic

Pneumatic

Resistance

of the Lesser Time

Generator

Logical OR

Time Delayon Initiation

SignalConverter

or Sonic

B

OR

AND

GTn

LTn

EQn

NOT

> n

< n

= n

S, SO

PD

R, RO

LT

DT

DI or GT

A

E

D

F

H

I

O

Input/ Output

Examples: D/A I/P

R

P

Time Delayon Termination

ƒ

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 ddtƒ

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 I

A

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

Σ 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 SECLSHDI

Drum LevelBelow + 7 in.

Drum LevelAbove - 7 in.

CCS PwrOK

BMS PwrOK

OR LA

NOT 20 SECLSLDI

NOTJS

OR

NOTJS

JA

JA

DrumLevelTrip

BoilerTrip

CCS PwrFailure

BMS PwrFailure

page 7

ADi-201 Rev 1 9/00

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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.

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