1、 AMERICAN NATIONAL STANDARD ANSI/ISA77.42.011999 (R2011) Fossil Fuel Power Plant Feedwater Control System Drum Type Reaffirmed 4 August 2011ANSI/ISA77.42.011999 (R2011) Fossil Fuel Power Plant Feedwater Control System Drum Type ISBN: 978-1-937560-05-8 Copyright 2011 by ISA. All rights reserved. Not
2、for resale. Printed in the United States of America. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic mechanical, photocopying, recording, or otherwise), without the prior written permission of the Publisher. ISA 67 A
3、lexander Drive P.O. Box 12277 Research Triangle Park, North Carolina 27709 USACopyright 2011 ISA. All rights reserved. ANSI/ISA-77.42.01-1999 (R2011) - 3 -Preface This preface, as well as all footnotes and annexes, is included for information purposes and is not part of ANSI/ISA-77.42.01-1999 (R2011
4、). This document has been prepared as part of the service of ISA, the international society of automation, toward a goal of uniformity 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 welco
5、mes all comments and criticisms and asks that they be addressed to the Secretary, Standards and Practices Board; ISA; 67 Alexander 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 Departm
6、ent is aware of the growing need for attention to the metric system of units in general, and the International System of Units (SI) in particular, in the preparation of instrumentation standards. The Department is further aware of the benefits to USA users of ISA standards of incorporating suitable
7、references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this end, this Department will endeavor to introduce SI-acceptable metric units in all new and revised standards, recommended practices, and technical reports to the greatest extent
8、possible. Standard for Use of the International System of Units (SI): The Modern Metric System, published by the American Society for Testing the firebox area, including burners and dampers; and the convection area, consisting of any superheater, reheater, and/or economizer sections, as well as drum
9、s and headers. 3.2 cascade control: A control action in which the output of one controller is the setpoint for another controller. 3.3 controller: Any manual or automatic device or system of devices for the regulation of boiler systems to keep the boiler at normal operation. If automatic, the device
10、 or system is motivated by variations in temperature, pressure, water level, time, flow, or other influences. 3.4 drum (steam): A closed vessel designed to withstand internal pressure. A device for collecting and separating the steam/water mixture circulated through the boiler. 3.5 feedwater flow co
11、ntrol system: A control system that uses input signals derived from the process for the purpose of regulating feedwater flow to the boiler to maintain adequate drum level according to the manufacturers recommendations. 3.6 mass feedwater flow rate: The mass flow rate of all water delivered to the bo
12、iler; it is derived either from direct process measurements and/or calculations from other parameters. When volumetric feedwater flow rate measurement techniques are employed, and the feedwater temperature at the flow-measuring element varies 100F (37.8C), the measured (indicated) flow shall be comp
13、ensated for flowing feedwater density to determine the true mass feedwater flow rate. 3.7 mass steam flow rate: The mass flow rate of steam from the boiler, derived either from direct process measurements and/or calculations from other parameters. If volumetric steam flow-rate measuring techniques a
14、re employed, the measured (indicated) flow shall be compensated for flowing steam density to determine the true mass steam flow rate. 3.8 primary/secondary control loop controller: The controller that adjusts the setpoint for the secondary control loop controller in the cascade control-action scheme
15、. Copyright 2011 ISA. All rights reserved. ANSI/ISA77.42.011999 (R2011) 10 3.9 protective logic circuits: Logic circuits designed to prevent damage to equipment by related system equipment malfunctions, failure, or operator errors. 3.10 pump drive control: A control component of the final device tha
16、t translates a control system demand signal into an electronic, hydraulic, pneumatic, or mechanical signal that affects pump speed. 3.11 redundant (redundancy): Duplication or repetition of elements in electronic or mechanical equipment to provide alternative functional channels in case of failure o
17、f the primary device. 3.12 runback: An action by the boiler control system initiated by the loss of any auxiliary equipment that limits the capabilities of the unit to sustain the existing load. Upon runback initiation, the boiler demand signal is reduced at a preset rate to the capability of the re
18、maining auxiliaries. 3.13 rundown: An action by the boiler control system initiated by an unsafe operating condition; i.e., fuel air limit (cross-limiting), temperature limits, etc. Upon rundown initiation, the boiler demand signal is reduced in a controlled manner to the load point where the unsafe
19、 operating condition is eliminated. 3.14 shall, should, and may: The word “SHALL“ is to be understood as a REQUIREMENT; the word “SHOULD“ as a RECOMMENDATION; and the word “MAY“ as a PERMISSIVE, neither mandatory nor recommended. 3.15 shrinkage: A decrease (shrinkage) in drum level due to a decrease
20、 in steam-bubble volume. This condition is due to a decrease in load (steam flow), with a resulting increase in drum pressure and a decrease in heat input. 3.16 single-element feedwater control: A control system whereby one process-variable drum level is used as the input to the control loop that re
21、gulates feedwater flow to the drum to maintain the drum level at setpoint. 3.17 steady-state: A characteristic of a condition, such as value, rate, periodicity, or amplitude, exhibiting only negligible change over a long (arbitrarily chosen) period of time. NOTE It may describe a condition in which
22、some characteristics are static, others dynamic. 3.18 swell: An increase (swell) in drum level due to an increase in steam-bubble volume. This condition is due to an increase in load (steam flow), with a resulting decrease in drum pressure and an increase in heat input. Swelling also occurs during a
23、 cold start-up as the specific volume of the water increases. 3.19 three-element feedwater control: A control system whereby three process variables (steam flow, feedwater flow, and drum level) are used as inputs to the control loop that regulates feedwater flow to the drum to maintain the drum leve
24、l at setpoint. This is a cascaded feedforward loop with drum level as the primary variable, steam flow as the feedforward input, and feedwater flow (feedback) as the secondary variable. 3.20 transient: The behavior variable during the transition between two steady states. Copyright 2011 ISA. All rig
25、hts reserved. 11 ANSI/ISA77.42.011999 (R2011) 3.21 two-element feedwater control: A control system whereby two process variables (steam flow and drum level) are used as inputs to the control loop that regulates feedwater flow to the drum to maintain the drum level at setpoint. The feedforward input
26、is steam flow, with the output of the drum level controller as the primary control signal. 3.22 two-out-of-three logic circuit (2/3 logic circuit): A logic circuit that employs three independent inputs. The output of the logic circuit is the same state as any two matching input states. 4 Minimum des
27、ign requirements for a feedwater control system The control system shall meet operational requirements and correctly interface with the process. To accomplish this objective, the following requirements are defined for minimum system design: a) Process measurement requirements b) Control and logic re
28、quirements c) Final control device requirements d) System reliability and availability requirements e) Alarm requirements f) Operator interface requirements 4.1 Process measurement requirements 4.1.1 Instrument installations for feedwater control Instruments should be installed as close as practical
29、 to the source of the measurement, with consideration given to excessive vibration, temperature, and accessibility for periodic maintenance. Recommendations for the location of instrument and control equipment connections can be found in the joint publication, Recommendations for Location of Instrum
30、ent and Control Connections for the Operation and Control of Watertube Boilers, by SAMA (Scientific Apparatus Makers Association) and ABMA (American Boiler Manufacturers Association). Specific requirements for the location of drum water-level measuring devices are contained in Section 1 of the Ameri
31、can Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. Separate isolation valves, head chambers (when used), and impulse lines shall be provided for each instrument (see figure 1). 4.1.2 Drum-level measurement A drum-level signal is required for single-element, two-element, and
32、three-element feedwater control systems (see figures 1, 2, 3, and 4). If the instruments used to measure drum level are sensitive to density variation, then density compensation techniques shall be employed. 4.1.3 Steam-flow measurement A mass steam-flow signal is required for two-element and three-
33、element feedwater control systems. If volumetric steam flow-rate measuring techniques are employed, the measured (indicated) flow shall be compensated for flowing steam density to determine the true mass steam flow rate. Copyright 2011 ISA. All rights reserved. ANSI/ISA77.42.011999 (R2011) 12 4.1.3
34、Feedwater-flow measurement A mass feedwater flow signal is required for three-element feedwater control systems. When volumetric feedwater flow rate measurement techniques are employed and the feedwater temperature at the flow-measuring element varies 100F (37.8C), the measured (indicated) flow shal
35、l be compensated for flowing feedwater density to determine the true mass feedwater flow rate. Figure 1 Typical drum-level differential pressure transmitter connections 4.2 Control and logic requirements The function of the feedwater control system is to maintain drum water level within the boiler m
36、anufacturers specified limits. The flow of feedwater to the drum is controlled by the variation of boiler feedpump speed and/or by the action of a control valve(s). Feedwater control can be accomplished by using the following control strategies: a) Single-element control b) Two-element control c) Th
37、ree-element control Copyright 2011 ISA. All rights reserved. 13 ANSI/ISA77.42.011999 (R2011) 4.2.1 Single-element feedwater flow control Single-element control (figure 2) is the minimum feedwater flow control system and shall be used for the following applications: a) During the start-up or at low-l
38、oad operation, when flow measurements are generally not accurate b) When steam flow rate of change is minimal and feedwater supply pressure is essentially constant 4.2.2 Two-element feedwater flow control Two-element control (figure 3) is the minimum feedwater control for a variable steam-flow appli
39、cation and is not recommended for new applications. 4.2.3 Three-element feedwater flow control Three-element control (figure 4) shall be used for applications experiencing variable steam flow and/or variable feedwater supply pressure at the flow control valve inlet. 4.2.4 Feedwater protective logic
40、Requirements for protective logic signals shall be those determined by the specific equipment manufacturers. Protective logic signals that trip the fuel to the boiler may include high drum level and low drum level. The following conditions shall produce a signal usable by other control systems, as c
41、overed under other standards within the ISA77 series: a) Loss of a boiler feedpump b) Exceeding the maximum capability limit of the feedwater system c) High drum level d) Low drum level Copyright 2011 ISA. All rights reserved. ANSI/ISA77.42.011999 (R2011) 14 KTf(x)AAXLT PTf(x)FINAL CONTROL DEVICEM/A
42、 CONTROL STATIONSET POINTPRESSURECOMPENSATIONDRUM PRESSUREDRUM LEVELTypical single-element, single-drive control system. For simplicity, redundant transmitters have not been shown on this typical control drawing. Figure 2 Single-element feedwater control functional control diagram using the Function
43、al Diagramming of Instrument and Control Systems (previously SAMA PMC 22.1-1981) format 4.2.5 Feedwater-flow control Feedwater flow shall be controlled by varying the speed of the boiler feedpump(s) and/or by varying the position of the feedwater control valve(s). 4.2.5.1 Variable-speed feedwater pu
44、mp(s) control When feedwater flow is controlled by using a hydraulic coupling, a variable-speed motor, or a turbine-driven pump with varying speed, the following features shall be provided: a) In the automatic mode, the output of the flow controller shall have an adjustable low limit. This low limit
45、 should be set to maintain minimum pump flow, per the pump manufacturers specifications. The flow controller output is speed demand (position demand of the hydraulic coupling), and the low limit will be a pump-speed, demand-low limit. b) When using a flow controller with a speed controller, the flow
46、 controller sets pump-flow demand. The speed controller shall serve in a cascade configuration to linearize the flow response of the pump. Copyright 2011 ISA. All rights reserved. 15 ANSI/ISA77.42.011999 (R2011) 4.2.5.2 Control valve positioning control When feedwater flow is controlled by positioni
47、ng a flow control valve(s), a single- or multiple-feedwater control valve(s) may be used. Choice of valve configuration shall be based on consideration of the pressure drop across the valve, rangeability, cavitation, and excessive valve wear with the valve nearly closed during low-load operation. If
48、 multiple valves are used, the control system shall provide for a smooth transition during the crossover to each additional value, both on flow increases and decreases. To minimize wear across the main control valves and to improve controllability, the control system should be configured to close of
49、f the start-up valve(s) as the main control valve(s) assumes the load. KTf(x)AAXXKLTPT PT FT TEf(x) f(x) f(x)FINAL CONTROL DEVICEM/A CONTROL STATIONSETPOINTPRESSURECOMPENSATIONTEMPERATUREAND PRESSURECOMPENSATIONLEVELPRESSURESTEAMTEMPERATUREFLOWDRUMPRESSURETypical two-element, single-drive control system. For simplicity, redundant transmitters have not been shown on this typical control drawing. Figure 3 Two-element feedwater control (functional-control diagram) functional control diagram using the Functional Diagr
