ANSI ISA TR67.04.08-1996 Setpoints for Sequenced Actions.pdf

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1、Setpoints for Sequenced ActionsApproved 21 March 1996ANSI/ISATR67.04.081996TECHNICAL REPORTCopyright 1995 by the Instrument Society of America. All rights reserved. Printed in the UnitedStates of America. No part of this publication may be reproduced, stored in a retrieval system, ortransmitted in a

2、ny form or by any means (electronic, mechanical, photocopying, recording, orotherwise), without the prior written permission of the publisher.ISA67 Alexander DriveP.O. Box 12277Research Triangle Park, North Carolina 27709ANSI/ISA-TR67.04.08 Setpoints for Sequenced ActionsISBN: 1-55617-591-4ANSI/ISA-

3、TR67.04.08-1996 3PrefaceThe information contained in the preface, footnotes, and annexes is included for information only and is not part of ISA-TR67.04.08.This Technical Report has been prepared as part of the service of ISA, the international society for measurement and control, toward a goal of u

4、niformity in the field of instrumentation. To be of real value, this document should not be static but should be subject to periodic review. Toward this end, the Society welcomes all comments and criticisms and asks that they be addressed to the Secretary, Standards and Practices Board; ISA; 67 Alex

5、ander Drive; P. O. Box 12277; Research Triangle Park, NC 27709; Telephone (919) 549-8411; Fax (919) 549-8288; E-mail: standardsisa.org.The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and the International System of Units

6、 (SI) in particular, in the preparation of instrumentation standards, recommended practices, and technical reports. The Department is further aware of the benefits to USA users of ISA standards of incorporating suitable references to the SI (and the metric system) in their business and professional

7、dealings with other countries. Toward this end, this Department will endeavor to introduce SI and acceptable metric units in all new and revised standards to the greatest extent possible. The Metric Practice Guide, which has been published by the Institute of Electrical and Electronics Engineers as

8、ANSI/IEEE Std. 268-1992, and future revisions, will be the reference guide for definitions, symbols, abbreviations, and conversion factors.It is the policy of ISA to encourage and welcome the participation of all concerned individuals and interests in the development of ISA standards, recommended pr

9、actices, and technical reports. Participation in the ISA standards-making process by an individual in no way constitutes endorsement by the employer of that individual, of ISA, or of any of the standards, recommended practices, and technical reports that ISA develops.The following people served as m

10、embers of ISA Subcommittee SP67.04:NAME COMPANY*T. Hurst, Chairman Hurst Consulting Inc.C. Sossman, Vice Chairman Westinghouse Savannah River Company*R. Webb, Managing Director Pacific Gas then several typical types of systems are analyzed, with specific numerical examples provided to show how the m

11、inimum separations for some actual systems are calculated. The effects of system configuration on the calculated values are also discussed.4.1 Minimum separations between setpointsFor the generalized instrument system shown in Figure 1, calculations of the minimum separations between setpoints for s

12、equenced actions must consider terms such as those illustrated in Figure 2. ANSI/ISA-TR67.04.08-1996 15Figure 1 Instrument channel layout for the generalized systemThe various uncertainty terms in Figure 2 have superscripted “ and “+“ signs to indicate whether the uncertainties are for process varia

13、ble values above or below their associated setpoints. The time response and reset deadband terms are shown overlapping the applicable uncertainty terms to illustrate the effects of the SRSS combinations of the random independent components.In order to keep Figure 2 from becoming too complex, the ran

14、dom (R) and bias (B) components of the appropriate uncertainty terms have not been shown. The following equations define the breakup of these terms into their components:TRANSMITTER ATRANSMITTER BTRANSMITTER CSIGNALCONDITIONER ASIGNALCONDITIONER BSIGNALCONDITIONER CBISTABLE 2FOR HIGH-HIGHBISTABLE 1F

15、OR HIGHBISTABLE 3FOR LOWAU1_AU1R_AU1B_+=AU2+AU2R+AU2B+=AU1+AU1R+AU1B+=AU3_AU3R_AU3B_+=16 ANSI/ISA-TR67.04.08-1996Figure 2 Illustration of relationships that may apply in determining minimumseparations of setpoints for sequenced actionsANSI/ISA-TR67.04.08-1996 17Referring to Figure 2 and the four def

16、ining equations provided, the minimum separations between the setpoints in the generalized system of Figure 1 are given by the following two equations:SR12= ( AU1R2+ AU2R+2)1/2+ AU1B+ AU2B+ TR12(Eq. 1)whereSR12= minimum separation between setpoint 1, the high setpoint, and setpoint 2, the high-high

17、setpoint;AU1R= random component of AU1, the applicable uncertainty for setpoint 1, in the direction of setpoint 2;AU2R+= random component of AU2, the applicable uncertainty for setpoint 2, in the direction of setpoint 1;AU1B= bias component of AU1in the direction of setpoint 2;AU2B+= bias component

18、of AU2in the direction of setpoint 1; andTR12= additional bias separation between setpoint 1 and setpoint 2 due to time response(s), if applicable.SR13= ( AU1R+2+ AU3R 2)1/2+ AU1B+ AU3B+ max(DB1, DB3) + OB (Eq. 2)whereSR13= minimum separation between setpoint 1, the high setpoint, and setpoint 3, th

19、e low setpoint;AU1R+= random component of AU1, the applicable uncertainty for setpoint 1, in the direction of setpoint 3;AU3R= random component of AU3, the applicable uncertainty for setpoint 3, in the direction of setpoint 1;AU1B+= bias component of AU1in the direction of setpoint 3;AU3B= bias comp

20、onent of AU3in the direction of setpoint 1;DB1= reset deadband for setpoint 1;DB3= reset deadband for setpoint 3; andOB = normal operating band of the process variable.The reason for including the maximum of DB1or DB3in the equation for SR13is to prevent an overlap between the reset of setpoint 1 an

21、d the actuation of setpoint 3, or between the reset of setpoint 3 and the actuation of setpoint 1. Only one of the deadbands is effective for a given process variable excursion up or down, and the one that is larger should be used for conservatism.The inclusion of an OB term in Equation 2, while not

22、 theoretically required for the determination of the minimum separation SR13, is necessary in practice because some allowance must be made for the operating band of the process variable. There are several factors that may affect the size of the normal operating band; a discussion of these is beyond

23、the scope of this Technical Report.18 ANSI/ISA-TR67.04.08-1996In Figure 1, separate transmitters and signal conditioners are shown for each bistable, but this is often not the case when a single process variable is involved. If bistables 1 and 2 of Figure 1 should both be driven by the same transmit

24、ter and signal conditioner, the applicable uncertainties AU1and AU2+shown in Figure 2 will become the bistable uncertainties, as discussed earlier. Also, if bistables 1 and 3 of Figure 1 should both be driven by the same transmitter and signal conditioner, the applicable uncertainties AU1+and AU3sho

25、wn in Figure 2 usually will become lower in value because of shared errors.Figure 2 also shows the maximum and minimum normal operating points of the process variables, the earliest and latest actuation points for the setpoints, and the analytical/process limits, together with the associated channel

26、 uncertainties for an instrument system of the type shown in Figure 1. Channel uncertainties must always be applied between the analytical/process limits and the setpoints closest to them.4.2 Typical types of systems4.2.1 Interrelating high pre-trip and high trip setpoints derived from the same tran

27、smitterThe instrument channel layout for this type of system is shown in Figure 3, and the setpoint relationships are illustrated in Figure 4. The high pre-trip setpoint may be used to generate an alarm indicating that the high trip setpoint is being approached. The minimum separation between setpoi

28、nts may be expressed in an equation similar to Equation 1, with the applicable uncertainty AU1becoming the bistable uncertainty BU1for the high pre-trip setpoint, and the applicable uncertainty AU2+becoming the bistable uncertainty BU2+for the high trip setpoint.The following equations define the br

29、eakup of the appropriate uncertainty terms in Figure 4 into their components:BU1=BU1R+BU1BBU2+=BU2R+BU2B+The equation for minimum separation between setpoints in this type of system therefore becomesSR12=( BU1R2+BU2R+2)1/2+BU1B+BU2B+TR12(Eq. 3)whereSR12= minimum separation between setpoint 1, the hi

30、gh pre-trip setpoint, and setpoint 2, the high trip setpoint;BU1R= random component of BU1, the bistable uncertainty for the high pre-trip setpoint, in the direction of setpoint 2;BU2R+= random component of BU2, the bistable uncertainty for the high trip setpoint, in the direction of setpoint 1;BU1B

31、= bias component of BU1in the direction of setpoint 2;BU2B+= bias component of BU2in the direction of setpoint 1; andTR12= bias time response term to account for plant operator actions and equipment response times to avoid a high trip after receiving a high pre-trip alarm.ANSI/ISA-TR67.04.08-1996 19

32、Figure 3 Instrument channel layout for system having high pre-trip and high tripbistables driven by the same transmitterFigure 4 Setpoint relationships for interrelating high pre-trip and high trip bistables driven by the same transmitter, with example values fora typical PWR pressurizer pressure sy

33、stemTRANSMITTERSIGNALCONDITIONERBISTABLE 2 FORHIGH TRIPBISTABLE 1 FORHIGH PRE-TRIPPROCESS VARIABLEANALYTICAL / PROCESS LIMIT 2475 PSIACU 57 PSI2-SETPOINT 2 (HIGH TRIP) 2418 PSIAEARLIEST ACTN POINT FOR SETPOINT 2LATEST ACTN POINT FOR SETPOINT 1SETPOINT 1 (HIGH PRE-TRIP) 2363 PSIAEARLIEST ACTN POINT F

34、OR SETPOINT 1 2339 PSIABU 3 PSI2+BU 3 PSI1-AU 24 PSI1+TR 50 PSI1220 ANSI/ISA-TR67.04.08-1996Figure 4 also shows the earliest and latest actuation points for the high pre-trip setpoint, the earliest actuation point for the high trip setpoint, and the high analytical/process limit, together with the a

35、ssociated channel uncertainty for this type of system. (The setpoint margins are assumed to be zero in Figure 4.)As a specific numerical example, the determination of the high trip and the high pre-trip setpoints and the minimum separation between them for a high pressurizer pressure trip channel in

36、 a PWR plant will be discussed in relation to Figures 3 and 4, and to Equation 3. The analytical limit is specified as 2475 psia, and if a restricted range transmitter with a calibrated span of 1500 to 2500 psia is used, the value for CU2calculated by the methodology of ISA-RP67.04, Part II, is 57 p

37、si. The maximum high trip setpoint is therefore (2475 - 57) = 2418 psia. If the values of BU1and BU2+are 3 psi each, with BU1R= BU2R+= 2 psi and BU1B= BU2B+= 1 psi, the minimum separation between the high pre-trip and high trip setpoints (with the SRSS values rounded to the closest whole number) wil

38、l be ( 22+ 221/2+ 1 + 1 + TR12) = (5 + TR12) psi. On the basis of engineering and operations judgment, a value of 50 psi is assigned for TR12, giving a minimum separation between setpoints of (5 + 50) = 55 psi. The high pre-trip setpoint thus becomes (2418 - 55) = 2363 psia. If the value of AU1+is 2

39、4 psi, the earliest actuation point for this setpoint will be (2363 - 24) = 2339 psia. These example values are shown in Figure 4.4.2.2 Interrelating high and low alarm setpoints derived from the same transmitterThe instrument channel layout for this type of system is shown in Figure 5, and the setp

40、oint relationships are illustrated in Figure 6. Since both bistables are driven by the same transmitter and signal conditioner, but the operating band is traversed when the process variable goes from one setpoint to the other, AU1+and AU2usually will have values that are intermediate between those f

41、or the full channel uncertainties and those for the bistable uncertainties.The following equations define the breakup of the appropriate uncertainty terms in Figure 6 into their components:AU1+=AU1R+AU1B+AU2=AU2R+AU2BFigure 5 Instrument channel layout for system having high and low alarm bistables d

42、riven by the same transmitterTRANSMITTERSIGNALCONDITIONERBISTABLE 1 FORHIGH ALARMBISTABLE 2 FORLOW ALARMANSI/ISA-TR67.04.08-1996 21Figure 6 Setpoint relationships for interrelating high and low alarm bistablesdriven by the same transmitter, with example values for a typical spraypond level system22

43、ANSI/ISA-TR67.04.08-1996The minimum separation between the setpoints may be expressed in an equation similar to Equation 2, as follows:SR12= ( AU1R+2+ AU2R2)1/2+ AU1B+ AU2B+ max(DB1, DB2) + OB (Eq. 4)whereSR12= minimum separation between setpoint 1, the high alarm setpoint, and setpoint 2, the low a

44、larm setpoint;AU1R+= random component of AU1, the applicable uncertainty for the high alarm setpoint, in the direction of setpoint 2;AU2R= random component of AU2, the applicable uncertainty for the low alarm setpoint, in the direction of setpoint 1;AU1B+= bias component of AU1in the direction of se

45、tpoint 2;AU2B= bias component of AU2in the direction of setpoint 1;DB1= reset deadband for setpoint 1;DB2= reset deadband for setpoint 2; andOB = normal operating band of the process variable.Figure 6 also shows the maximum and minimum normal operating points of the process variable, the earliest ac

46、tuation points for the two setpoints, and the two analytical/process limits, together with the associated channel uncertainties for this type of system. (The setpoint margins are assumed to be zero in Figure 6.)As a specific numerical example, the determination of the high alarm and the low alarm se

47、tpoints and the minimum separation between them for a spray pond level alarm system will be discussed in relation to Figures 5 and 6, and to Equation 4. The high process limit is specified as 180.0 inches, and the values of CU1and CU2+calculated by the methodology of ISA-RP67.04, Part II, are each 4

48、.5 inches. The maximum high alarm setpoint is therefore (180.0 - 4.5) = 175.5 inches. If (1) the values of AU1+and AU2are each 2.5 inches, of which 1.5 inches are random components, and 1.0 are bias components; (2) it is desired that the low alarm setpoint be as high as possible without causing conf

49、usion between high and low alarms; (3) the minimum reset deadbands for the two bistables are each 2.0 inches; and (4) the desired normal operating band is 3.0 inches, then the minimum separation between the setpoints using Equation 4 would be ( 1.52+ 1.521/2+ 1.0 + 1.0 + 2.0 + 3.0) = 9.1 inches. The maximum low alarm setpoint would then be (175.5 - 9.1) = 166.4 inches, which would be satisfactory if the process low level limit is no higher than (166.4 - 4.5) = 161.9 inches. These