SAMA DIAGRAMS FOR BOILER CONTROLS APPLICATION …

1 sama DIAGRAMS FORBOILER CONTROLSWe're Siemens. We can do DATAPURPOSEF unctional 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 Standard PMC Functional Diagrammingof Instrument and control Systems is no longer sup-ported by sama or any other standardization 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 sama DIAGRAMS are used in the Siemens Mooreseries of BOILER control APPLICATION , a gen-eral understanding of the DIAGRAMS is helpful to effectivelyuse the applications DIAGRAMS represent the language of choicethroughout the power and pulp & paper industries for in-strumentation and control systems.

2 This documentshows and explains the following types of symbols, in-cluding variations and examples of their APPLICATION . Enclosure symbols Signal continuation symbols Signal processing symbolsFIGURE 1 ISA vs. sama Functional DiagramsISA DiagramFTFICFCVFTPIATAFCVSAMA DiagramSYMBOLSF igure 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 AssociationA 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.

3 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 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 1 Enclosure SymbolsANDOR_ _nNOTSRSORSRO FUNCTIONENCLOSURESYMBOLFUNCTIONENCLOSURE SYMBOLt_ _Measuring or ReadoutManual Signal ProcessingAutomatic Signal ProcessingFinal ControllingFinal Controllingwith PositionerTime Delay orPulse DurationLogical ANDL ogical ORQualified Logical ORLogical NOTM aintained Memory Optional Resetpage 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 Variable SignalIncremental Change Signalor Rate of Change of a Continuously Variable SignalOn-Off Signal *TABLE 3 Signal Processing SymbolsFUNCTIONSIGNALPROCESSINGSYMBOLS

4 UmmingAveragingDifferenceProportionalInt egralDerivativeMultiplyingDividingRoot ExtractionExponentialNon-Linear FunctionTri-State SignalIntegrate or TotalizeHigh SelectingLow SelectingHigh LimitingLow LimitingReverse ProportionalVelocity LimitingBiasTime FunctionVariable Signal GeneratorTransferSignal Monitor(Raise, Hold, Lower) or+ / nor K or Por|d/dt or Dx xf(x)Q> <> <-K or -Pv > f(t)ATH/, H/L, /L FUNCTIONSIGNALPROCESSINGSYMBOLL ogical SignalLogical ANDL ogical ORQualifiedn = an integerLogical NOTSet MemoryReset MemoryPulse DurationPulse DurationAnalogDigitalVoltageFrequencyHyd raulicCurrentElectromagneticPneumaticRes istanceof the Lesser TimeGeneratorLogical ORTime Delayon InitiationSignalConverteror SonicBORANDGTnLTnEQnNOT> n< n= nS, SOPDR, ROLTDTDI or GTAEDFHIOI nput/ OutputExamples: D/A I/PRPTime Delayon Termination nnpage 3 VARIATIONSPID ControllerThe fundamental function of most control loops is the PIDcontroller.

5 PID stands for the Proportional plus Integralplus Derivative control algorithm. Figure 2 shows fourvariations of the symbol used to describe this algorithmin sama 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 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 2B simply substitutes P, I, and D for the standardmathematical symbols.

6 Figure 2C simplifies the drawingof the symbol by combining the P, I, and D functionswithin a single 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 2 PID Controller Symbol Variations2 AKddt PID2 BPIDPID2C2 Dpage 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 3A shows the classic sama diagram for singleloop control using a PI controller.

7 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 adjustments were typically provided by separatecomponents mounted within a control 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 3C shows another variation of the classic arrange-ment of the three diamonds.

8 Also note the non-linearfunction symbol [f(x)] instead of the FCV. This may beused to represent an inherently non-linear valve character-istic ( 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 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.

9 In general, this level of detail should beavoided since it obscures the basic control strategy andmakes the diagram less 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 4B shows additional automatic signal processingfunctions to describe equipment details.

10 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 ofFIGURE 3 Single Loop control VariationsFTPIATAf(x)3B3C3 AFTPIAT AFCVFTPIATAFCV page 5 FIGURE 4 Equipment DetailVariationsvalue 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 Drum Level ControlA common APPLICATION in BOILER control is three-elementdrum level control .