1、 ASME PTC*L7.7 80 0757b70 0052723 T PART 7 Measurement of Power ANSI /ASME PTC 19.7-1980 lNSTRUMENTS AND APPARATUS THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS United Engineering Center 345 East 47th Street New York, N.Y. 10017 No part of this document may be reproduced in any form, in an electronic
2、 retrieval system or otherwise, without the prior written permission of the publisher Date of Issuance: August 31, 1980 Copyright 1980 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS All Rights Reserved Printed in U.S.A. ASME PTC*L7.7 80 M 0759670 0052725 3 FOREWORD The Performance Test Codes Superviso
3、ry Committee in December 1974 activated a Committee to revise PTC 19.7 (1961) on Measurement of Shaft Power. This Instruments and Apparatus Technical Committee has prepared an lnstrumentsand Apparatus Supplement which incorporates the latest technology on the Measurement of Shaft Power. The Scope of
4、 the work of PTC 19.7 on Measurement of Shaft Power is limited to descriptive material which will enable the user to select an appropriate system or procedure for his applica- tion. It includes criteria for the operating conditions of the equipment whose power is being measured. The Object of this S
5、upplement is to describe the function, characteristics, advan- tages, disadvantages and accuracy of equipment and techniques currently available for the measurement of shaft power in rotating machines. Only the methods of measurement and instruments, including instructions for their use, specified i
6、n the individual test codes are mandatory. Other methods of measurement and instru- ments, that may be treated in the Supplements on Instruments and Apparatus, shall not be used unless agreeable to all the parties to the test. This Supplement was approved by the Performance Test Codes Supervisory Co
7、mmittee on July 2, 1979. It was approved and adopted by the American National Standards Institute as meeting the criteria for an American National Standard on April 28, 1980. ASME PTC*L7*7 BO m 0757670 0052726 5 m PERSONNEL OF ASME PERFORMANCE TEST CODES COMMITTEE NO. 19.7 ON INSTRUMENTS AND APPARAT
8、US MEASUREMENT OF SHAFT POWER Hunt Davis, Chairman andSecretary Robert R. Piepho, Vice Chairman Arthur L. Beaman, Ir., Fellow Design Engineer, Medium M-otor and Gearing Division, Westing- Robert Clelland, Senior Principal Engineer, RotatingEquipment Section, Pullman Kellogg Divi- Hunt Davis, Senior
9、Staff Engineer, Pullman Kellogg Division, Pullman, Inc., Three Greenway Ralph J aeschke, Chief Engineer, Eaton Corp., Industrial Drives Division, 3122 14th Avenue, Donald R. Jenkins, Associate Professor, Mechanical Engineering Department, Lafayette College, Irving 1. Kahn, President, Kahn Industries
10、 Inc., 885 Wells Road, Wethersfield, Connecticut 06109 Douglas C. FoIkner, Senior Test Engineer, Cameron Test Department, lngersoll Rand Co., Phil- Richard A. Mayer, Senior Consultant, Applied Physics Division, Southwest Research Institute, Daniel Nobles, Research Engineer, Worthington Pump Internat
11、ional, Harrison, New Jersey 07029 Robert R. Piepho, Product Manager, Fuel Preparation Systems, Fossil Power Generation Divi- sion, Babcock Advantages and disadvantages. Table 1 provides guidance on the range of application of the various torque and power measurement systems. 1 .O6 Sbme of the proced
12、ures and equipment measure shaft torque, and require concurrent determination of rotational speed to provide a value for shaft power. 1.07 There are three general methods available for meas- urement of rotational speed: (a) Devices which display, indicate, or record a number of revolutions within a
13、known time in terval. (b) Devices which display, indicate, or record time- averaged rotational speed. (c) Devices which continuously record instantane- ous angular velocity. Reference 12 (Appendix D) contains acomplete descrip- tion of types, methods, and classification of rotational speed measuring
14、 devices. 1.08 The scope does not include operating instructions for specific measurement apparatus. 1.09 It is expected that the main equipment Performance Test Codes will contain instructions concerning the fre- ASME PTC*27*7 80 E 0759670 0052731 7 E SECTION 1 ANSI/ASME PTC 19.7-1980 quency of cal
15、ibrations, number of observations, limitations 1.10 Specific instructions about the itatistical treatment on time variations, etc., which are appropriate for the pur- of test data should be found in the main equipment Per- pose of the equipment tests. formance Test Codes, “ ._ TABLE 1 Application Ra
16、nge of Torque and Power Measuring Systems POWER HP 50 O00 20 O00 10 O00 5000 2000 1 O00 O O B E E E E B B B I I I I BC E- EF EF E E B BC BC BC BC I I I I HI E EF E E E B B BC HI BC BC BC BC BC I I I I E EF EF EF EF EF EF BC BC BC BC BC BC I I I G I GHI HI E EF EF EF EF EF B BC BC BC BC BC I I I G I
17、HI HI E EF EF EF EF ;HI I I I G I GHI GHI ;HI EF EF EF EF EF bEF B BC BC rBCABCABCABC I I I G I GHI GHI 1000 2000 5000 10 O00 20 O00 50 O00 ROTATIONAL SPEED r/min POWER MW 37 15 KEY 7.5 A AC or DC motor or generator dynamometer B waterbrake D prony brake E surface strain gage torquemeter 1.5 F angul
18、ar twist -electrical torquemeter G angular twist - optical torquemeter H calibrated motor or generator 3.7 . C eddy current dynamometer .75 I heat balance methods* *Limited only by driver capability and heat exchanger size. ASME PTCmL9-7 BO m 0757670 0052732 O W SECTION 2 DEFINITIONSAND DESCRIPTIONS
19、 OF TERMS TORQUE - SPEED - POWER RELATIONS 2.01 The most common situation for power measurement is one in which the power being transmitted and the angu- lar velocity are both constant with time; that is, there are no transients in either torque or angular velocity within the time interval required
20、to make the measurement. 2.02 The measurement of shaft power of rotating ma- chines in the absence of transients, is accomplished by either direct or indirect methods, The direct methods, utilizing a dynamometer or a torque meter, involve deter- mination of the variables in the following equation: P
21、hysical equation P=w T where P = power W = angular velocity T= torque Po wer expressed in SI units P=oT where P= power, watts (W) W = angular velocity, rad/s T= torque, newton meters (N*m) Power expressed in English units 2nn T P=- 550 where P= power, horsepower (hp) . n = rotational speed, revoluti
22、ons/sec (r/s) T= torque (Ibf-ft) 2.03 There are cases in which transients in angular velocity and torque occur, Some of the apparatus described herein may, under certain circumstances, be capable of making measurements of instantaneous power, when angular velocity and torque vary with time; or measu
23、rements of average power when angular velocity and torque vary r cyclically. 2.04 For these cases of non-steady angular velocity and torque the instantaneous value of power is, in physical terms, P=w T Torque meter systems with appropriate data recording systems may be used to determine the value of
24、 T at any instant. A similar recording of angular velocities, W, then provides a basis for determination of the instantaneous value of P. If the values of T and W vary cyclically the average power may be determined as follows. Let the period of one cycle of torque and speed be the time A, and the ro
25、- tational travel for one cycle be the angle e, in radians. Also let cp be defined as cp= neither the exact value of the quantity being measured, nor the exact error associated with the measurement can be found. The techniques of statistics may be used to pro- vide an estimate of the true value of a
26、 quantity, and an estimate of the standard deviation. References 1 and 2, Appendix D, provide the foundations and specific proce- dures for applying the appropriate statistical methods. 2.06 Each measurement of a single physical quantity X is accompanied by an error e such that X=X+e where x is the
27、true value of the- single quantity being measured. The error has two components: a random error and a systematic error. 2.07 When repeated measurements are taken, random errors produce the scatter about the average of the results. The term precision is used to characterize random errors. Precision i
28、s quantified by an estimate of the standard deviation. Systematic errors are those which produce re- sults consistently too high or too low with respect to the true value. Systematic errors are characterized by bias, or accuracy. Calibration procedures provide quantification of bias. Figures 1 and 2
29、 illustrate the concepts. 2.08 The confidence interval statement indicates a range .centered on the estimated value, within which the true value is believed to lie. This is accqmpanied by a probability statement which indicates the assurance that the stated range contains the true value. For example
30、, a complete ANSIIASME PTC 19.7-1980 NBS REFERENCE FUNCTION = “TRUE“ v THERMOCOUPLE CHARACTERISTIC MEAN VALUE ERROR DISTRIBUTION TEMPERATURE, T FIG. 1 SYSTEMATIC AND RANDOM ERRORS ILLUSTRATED FOR CASE OF A THERMOCOUPLE CALIBRATION (Ref. 1) confidence interval statement is given by power = 386 k 6 kW
31、; (95%) where 386 kW is the estimated value, based on averaging a number of measurements, 6 kW is the confidence interval; and the probability of the true value of power being in the range 380 to 392 kW is 95%. For a given set of observa- tions, as the confidence interval is made larger, the prob- a
32、bility becomes greater. 2.09 Measurements of shaft power involve determination of multiple physical quantities. The complexity of the ap- plication of the foregoing statistical. concepts, and the pro- cedures of References l and 2, Appendix D, varies greatly depending on the method and apparatus to
33、be used. 2.10 The contribution of systematic errors is minimized by careful calibration of individual components. The con- tribution of random errors is minimized by increasing the number of readings of output for fixed values of the con- trolled operating parameters. 2.1 1 The termprobableerrorrefe
34、rs to the confidence inter- val around the estimated value for which the probability is 50%. 2.12 In the succeeding parts of this Supplement, numerical values are assigned to the “errors“ or “overall errors“ of the various systems. Each of these is to be considered as the probable error of asingle d
35、etermination, having a 50% probability that the given range includes the real value. These overall errors are usually dominated by random sources because the procedures require calibrations be 4 ASIE PTCm17-7 80 m 0757670 0052734 4 ANSI/ASME PTC 19.7-1980 BIAS”-, SECTION 2 POOR PRECISION -POOR ACCUR
36、ACY POOR PRECISION -GOOD ACCURACY -1 - xx BIAS cl GOOD PRECISION -POOR ACCURACY GOOD PRECISION -GOOD ACCURACY FIG. 2. ACCURACY AND PRECISION (Ref. 1) performed where appropriate. The nature of the systems which are subject tocalibration is such that the confidence interval (50%) of the calibration d
37、ata is usually consider- ably smaller than the stated overall error. This fact can be understood when one realizes that calibrations are usually conducted repeatedly and a history of consistency in data is obtained; furthermore the calibrated component often provides only one of numerous inputs requ
38、ired fora deter- mination of shaft power. 2.13 Causes of errors given for the various measurement methods are categorized as predominantly random or sys- tematic, where possible. These listings provide guidance about a) the potential for improvement in results attain- able by calibration, and b) by
39、making multiple determina- tions of the shaft power for each value of the independent operating parameters. 2.14 If an individual instrument has from experience and numerous calibrations a known probable error, signified by C/; (50%), the confidence interval for higher probabil- ities may be assumed
40、 to be related as follows: C/; (90%) 1.6450 - 2.44 C/ (50%) 0.6740- C/; (99%) N 2.5760 - 3.82 C/;(50%) - 0.674a- These ratios may be useful in relating historical “prob- able error” descriptions of uncertainty to confidence inter- vals having high probability. The latter are used in Refer- ences 1 a
41、nd 2, Appendix D. 2.15 The application of statistical procedures to deter- mine the confidence interval statements for shaft power measurements is a matter for judgment of the parties con- cerned with the testing of equipment under one of the ASME PTC*L9*7 80 0757670 0052735 b m SECTION 2 ANSI/ASME
42、PTC 19.7-1980 Performance Test Codes. The basis for statistical treatment cedures should be agreed in advance. Table 2 outlines of data, calibration reference standards, number of data typical values of probableerror for the methods described. points, probability levels desired and computational pro
43、- TABLE 2 Summary of Typical Probable Errors for Different Shaft Power Measurement Methods I Confidence Interval (50%) for a Single Measurement Reaction Torque Systems Cradled Dynamometers Uncradled Dynamometers f 0.5% to f 1 .O% for torque* f 0.5% to f 1 .O% for torque* Shaft Torque Measurement Sur
44、face Strain Systems Angular Displacement Systems Mechanical Electrical Optical Calibrated Motors or by attaching a counterweight to the frame so that the center of gravity of the frame is shifted horizontally, but not vertically. The scales can be reset to zero to com- pensate for the preload weight
45、. The reversing linkage con- tains a lever having a 1 to 1 ratio and any error in this ratio will cause the scale calibration in one direction to differ from that in the other direction; hence, the calibration should be checked in both directions. The preload should be reversed whenever the directio
46、n of torque is reversed. 3.12 Friction in the scale, linkage or knife edges, or cradle bearings will show up as a difference of scale read- ings for the same calibrating point dependent upon the direction from which the point is approached. For this reason it is necessary to calibrate the scale by a
47、dding weights carefully so as not to overshoot the readings, and then to remove them in the same manner. The difference between the increasing and decreasing curves is an indica- tion of possible error due to friction. 3.13 If the dynamometer weighing system uses a dash- pot to damp the torque arm f
48、luctuations, its adjustment ASME PTC*17*7 AO W 0757670 0052737 3 m SECTION 3 should be checked periodically to determine that the dashpot effect is equal on the “in“ and “out“ stroke. This check should be performed with the weighing system free of influence from the dynamometer. The natural frequenc
49、y of the dashpot should be at least twice, and preferably five times, the highest rotational speed of the dynamometer. 3.14 The overall error of power measurement by acradled dynamometer results from several contributing factors: (a) Trunnion bearing friction. (b) Force measurement system error. (c) Torque arm and calibrating arm length error. (d) Static unbalance of dynamometer housing. (e) Restriction of free movement caused by water lines, lubrication lines and electrical leads connected to the dynamometer housing. (f) Static reaction from Bourdon t
