1、 ISATR77.42.022014 Fossil Fuel Power Plant Compensated Differential Pressure Based Drum Level Measurement Approved 5 June 2014 Copyright 2014 ISA. All rights reserved. ISATR77.42.022014 Fossil Fuel Power Plant Compensated Differential Pressure Based Drum Level Measurement ISBN: 978-0-876640-87-6 Cop
2、yright 2014 by the International Society of Automation (ISA). All rights reserved. Not 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,
3、recording, or otherwise), without the prior written permission of the Publisher. ISA 67 Alexander Drive P. O. Box 12277 Research Triangle Park, North Carolina 27709 USA 3 ISATR77.42.022014 Copyright 2014 ISA. All rights reserved. Preface This preface, as well as all footnotes and annexes, is include
4、d for information purposes and is not part of ISATR77.42.022014. This document has been prepared as part of the service of The International Society of Automation (ISA), toward a goal of uniformity in the field of instrumentation. To be of real value, this document should not be static but should be
5、 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 Alexander Drive; P. O. Box 12277; Research Triangle Park, NC 27709; Telephone (919) 549-8411; Fax (919) 549-8288; E-
6、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 (SI) in particular, in the preparation of instrumentation standards. The Department is further aware of the be
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18、T ADDRESSED THE POTENTIAL ISSUES IN THIS VERSION. The following people served as voting members of ISA77.42 and reaffirmed the document: NAME COMPANY J. Gilman, Chair JFG Technology Transfer LLC P. Toigo, Chair Burns the firebox area, including burners and dampers; the convection area, consisting of
19、 any superheater, reheater, economizer sections or any combination thereof, as well as drums and headers. 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. 3.11 redundant (redundancy
20、) Duplication or repetition of elements in electronic or mechanical equipment to provide alternative functional channels in case of failure of the primary device. 3.15 shrinkage A decrease (shrinkage) in drum level due to a decrease in steam-bubble volume. This condition is due to a decrease in load
21、 (steam flow), with a resulting increase in drum pressure and a decrease in heat input. 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 hea
22、t input. Swelling also occurs during a cold start-up as the specific volume of the water increases. 3.22 two-out-of-three logic circuit (2/3 logic circuit) A logic circuit that employs three independent inputs and whose output of the logic circuit is the same state as any two matching input states.
23、ISATR77.42.022014 12 Copyright 2014 ISA. All rights reserved. 4 Introduction One of the more critical measurements made is the water level in the boiler steam drum. Failure to maintain this water level within reasonable boundaries can and has resulted in damage to the boiler itself (on an extreme lo
24、w level) or damage to downstream equipment such as the superheater piping system, or the steam turbine to which steam is provided (on extreme high level). Because of the critical nature of this measurement, a variety of devices are usually applied to monitor the steam drum water level. A single stea
25、m drum may have several level measuring instruments including sight glasses, electrode columns, guided wave radar and differential pressure transmitter based indicators and transmitters. The differential pressure based level devices (and even most sight glasses and electrode columns) experience inac
26、curacies in measurement when boiler steam drum pressure is not at its design value. These changes in pressure can be overlooked for boilers operating at lower pressures; but for most utility boilers, and many industrial boilers, compensation is necessary to reduce these inaccuracies. 5 Background A
27、schematic of a typical differential pressure-based drum level transmitter installation is shown in Figure 1. The differential pressure instrument is connected to the steam drum by two sensing lines. The reference leg connection is in the drum steam space. One sensing line is connected to the upper s
28、ection of the drum and the other line is connected to the lower section. As depicted in Figure 1, a reservoir may be used on the sensing line connected to the upper section of the drum. The upper sensing line is called the reference leg and the lower sensing line is called the sensing leg. The top o
29、f the upper sensing line defines the top of the measurement range. In operation, the reference leg will fill up with steam, the steam will condense to water, and the water will fill the entire reference leg. The reservoir if present provides a larger volume of water in the reference leg. Reservoirs
30、were required by the older, mechanical type drum level transmitters. The horizontal lines connecting the drum to the sensing and reference legs must be of sufficient length (typically 30 inches) to subcool the water entering the reference leg to meet the assumed temperature being used for compensati
31、on to prevent inaccurate readings during pressure transients. The sensing leg connection is in the drum water space. The sensing leg fills up with water rather than steam since it is connected directly into the water space. A boiler steam drum is a closed, pressurized vessel. The boiler drum pressur
32、e is effectively cancelled out in this differential pressure measurement, since the drum pressure is applied to both the sensing leg and the reference leg of the transmitter. The differential pressure measured by the transmitter is inversely related to the level of water in the steam drum. As the wa
33、ter level goes up, the sensing leg water column approaches the height of the reference leg water column. When the water level reaches the highest drum connection, the two legs will be the same height, and there would be a differential pressure of zero between the two legs of the transmitter if the d
34、ensity of the fluid in both legs were the same. 13 ISATR77.42.022014 Copyright 2014 ISA. All rights reserved. LTH LNorm al W at e r Le vel (NWL )Drum Lev elTransm it te rSensi ngLegRe fer e nceLegRe ser vo i rBoil erStea m D ru mFigure 1 - Schematic of a typical differential pressure-based drum leve
35、l transmitter installation 6 Methods of compensation There are several approaches that can be taken to calibrate the differential pressure transmitter (LT) being used for drum level measurement. These approaches are as follows: No compensation - calibrate for atmospheric pressure in the drum No comp
36、ensation - calibrate for drum operating pressure Calibrate for normal drum pressure and compensate for deviations from that pressure Calibrate for atmospheric pressure and compensate for deviations from that pressure No compensation - calibration for atmospheric pressure is acceptable for industrial
37、 boilers operating at fairly low drum pressures. Below about 200 PSIG on small drums, the error in measurement may not be very significant. No compensation and calibration for drum operating pressure was used on many older drum boilers. The drum level measurement was only correct when operating at n
38、ormal pressure. Sight glasses or other level measurements are necessary when starting up a boiler in order to maintain proper water level. Calibration for atmospheric pressure and compensation for deviations from that pressure was a common technique with analog electronic measurement systems because
39、 it was easier to implement than the method with calibration at atmosphere pressure. With digital systems the implementation difficulty is the same for both compensation methods. 6.1 Calibration at atmospheric pressure with compensation for operating pressure The differential pressure transmitter is
40、 calibrated in the same way it would be if drum pressure was not a problem. Compensation curves can be generated without knowing the exact normal operating pressure. Once these curves are generated they can be used even if the normal operating pressure changes. Figure 2 has been provided to demonstr
41、ate the potential measurement errors if pressure compensation is not performed. The Figure 2 example is based on measurement taps that are 30 inches apart for a resulting reference leg of 30 inches. ISATR77.42.022014 14 Copyright 2014 ISA. All rights reserved. U n c o m p e n s a t e d D r u m L e v
42、 e l M e a s u r e m e n t E r r o r E x a m p l eA ct u a l L e v e l = 0 . 0 “ ( m i d - sp a n ) , S p a n = - 1 5 “ t o + 1 5 “D r u m P r e s s u r e ( p s i g )W a t e r D e n s i t y ( l b / f t3)S t e a m D e n s i t y ( l b / f t3)R e f e r e n c e L e g D e n s i t y 1 0 0 F ( l b / f t3)M
43、 e a s u r e d D i f f e r e n t i a l P r e s s u r e ( i n w c )I n d i c a t e d W a t e r L e v e l ( i n c h e s )E r r o r w i t h o u t C o m p e n s a t i o n ( % o f s p a n )15 5 8 . 8 1 0 0 . 0 7 2 1 6 2 . 0 0 2 1 5 . 6 7 4 - 0 . 6 7 4 2 . 2100 5 6 . 0 2 0 0 . 2 5 7 0 6 2 . 0 1 8 1 6 . 3
44、0 8 - 1 . 3 0 8 4 . 4200 5 4 . 1 4 9 0 . 4 6 8 2 6 2 . 0 3 7 1 6 . 7 1 7 - 1 . 7 1 7 5 . 7600 4 9 . 5 3 7 1 . 3 3 2 0 6 2 . 1 1 2 1 7 . 6 5 5 - 2 . 6 5 5 8 . 91200 4 4 . 6 9 5 2 . 7 9 8 4 6 2 . 2 2 4 1 8 . 5 2 2 - 3 . 5 2 2 1 1 . 72000 3 8 . 8 7 9 5 . 3 6 8 1 6 2 . 3 7 2 1 9 . 3 7 4 - 4 . 3 7 4 1 4
45、. 62600 3 3 . 8 9 1 8 . 3 5 2 3 6 2 . 4 8 1 1 9 . 9 0 9 - 4 . 9 0 9 1 6 . 4Figure 2 Potential error without compensation Uncompensated Drum Level Measurement Error 0.0% 2.0% 4.0% 6.0% 8.0% 10.0% 12.0% 14.0% 16.0% 18.0% 0 500 1000 1500 2000 2500 3000 Drum Pressure (PSIG) Error (% of span) 15 ISATR77.
46、42.022014 Copyright 2014 ISA. All rights reserved. 6.2 Differential pressure calculation A detailed schematic representation of the basic reservoir column installation can be seen in Figure 3. In this installation the portion of the reference leg between the measurement taps is assumed to be at 100
47、degrees Fahrenheit, which is a reasonable assumption for a boiler whose drum is located indoors. 7 Practical considerations Sensing piping is very important. The sensing lines to the transmitter must never run horizontally. Running these lines horizontally would allow air to collect in various spots
48、 and may change the effective height of the column of water bearing on the transmitter. The pressure at the transmitter is directly related to the height of the column of water so large errors can be introduced by air in the sensing lines. Install the sensing lines as close to vertical as possible w
49、ith the transmitter always below the vessel. A minimum slope of one inch per foot of run is acceptable but the closer to vertical the better. Shutoff valves should be provided at the vessel (root valves) and at the differential pressure transmitter. Blow down capability should be provided and the differential pressure transmitter should be located with easy access to the blow down, usually located in the back of the transmitter. A leak in the blowdown valve could lead to a huge error on the differential pressure transmitter rea
