1、Application and Installationof Continuous-BeltWeighbridge ScalesApproved 30 March 1984ISARP74.011984RECOMMENDED PRACTICEISA The Instrumentation,Systems, andAutomation Society TMCopyright 1984by the Instrument Society of America. All rights reserved. Printed in the UnitedStates of America. No part of
2、 this publication may be reproduced, stored in a retrieval system, ortransmitted in any 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
3、 27709ISARP74.011984, Application and Installation of Continuous-Belt Weighbridge ScalesISBN 0-87664-814-6ISA-RP74.01-1984 3PrefaceThis preface is included for information purposes and is not a part of ISA-RP74.01-1984.This recommended practice has been prepared as a part of the service of ISA towar
4、d 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 welcomes all comments and criticisms, and asks that they be addressed to the Secretary, Standards and Practices Boar
5、d, ISA, 67 Alexander Drive, P.O. Box 12277, Research Triangle Park, NC 27709, Telephone (919) 549-8411, 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 (SI)
6、in particular, in the preparation of instrumentation standards. The Department is further aware of the benefits to U.S.A. users of ISA standards of incorporating suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this end, t
7、his Department will endeavor to introduce SI-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 Electronic Engineers as ANSI/IEEE Std. 268-1982, and future revisions will be
8、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. Participation in the ISA standards-making process by an individual in
9、 no way constitutes endorsement by the employer of that individual, of ISA, or of any of the standards that ISA develops.The information contained in the preface, footnotes, and appendices is included for information only and is not a part of the recommended practice.Prior to issuance of this recomm
10、ended practice, NBS Handbook 44 (Specification Tolerances and Other Technical Requirements for Commercial Weighing and Measuring Devices) and OIML IR50 (Laws of the Member States Journal of European Communities Relating to Continuous Totalizing Weighing Machines) were available to guide specifiers a
11、nd buyers of weighbridge-type, continuous-belt weigh scales. These two documents are intended for use on the regulation of weighing and apply to sales approved by weights and measures personnel.The purpose of this recommended practice is to furnish design criteria to simplify the specification of we
12、ighbridge-type, continuous-belt weigh scales and to provide recommendations for their installation, calibration, and maintenance. It is recognized that nuclear-radiation-type belt weigh scales and “bolt-on“ sensors perform a valuable service in the industry and can provide Class III accuracy (1-perc
13、ent error). This recommended practice, however, addresses only the weighbridge-type scale, leaving the application and installation of nuclear scales and other weighing devices to subsequent consideration.The persons listed below served as members of ISA Committee SP74:NAME COMPANYN. L. Kautsky, Cha
14、irman Stearns Catalytic CorporationR. W. Brockmeyer AMAX CorporationP. Chase Ramsey Engineering Company4 ISA-RP74.01-1984S. Ciesiewski Peabody Coal CompanyW. J. Coulam Kennecott Research Center E. Duncan K belt idlers; or slider beds for handling bulk materials, packages, or objects placed directly
15、upon the belt.Bell-conveyor scale: A device installed on a belt-conveyor structure that continuously weighs the material being conveyed.Belt-speed sensor: A device that generates a signal as a function of belt speed.Belt-speed transmitter: A device that transmits a belt speed signal to a receiver.Be
16、nd pulley: Any pulley used to change the direction of travel of a belt.Calibration test: A test using known weights and forces to load the scale in order to determine the performance of a belt-conveyor scale.Concave curve: A change in the angle of inclination of a belt conveyor where the center of t
17、he curve is above the conveyor.Convex curve: A change in the angle of inclination of a belt conveyor where the center of the curve is below the conveyor.Conveyor stringers: Support members for the conveyor on which the idlers are mounted.Design capacity: The maximum weight load that the scale is des
18、igned to weigh in one hour within the designated class accuracy. It is customarily 125 percent of normal capacity and is also known as “scale capacity.“Head pulley: The pulley at the discharge end of the belt conveyor. The power drive for the belt is generally applied to the end pulley.Idler spacing
19、: The center-to-center distance between consecutive idler rollers, measured parallel to the belt.Idlers (idler rollers): Freely-turning cylinders mounted on a frame to support the conveyor belt. For a flat belt, the idlers may consist of one or more horizontal cylinders transverse to the direction o
20、f belt travel. For a troughed belt, the idlers may consist of one or more horizontal 8 ISA-RP74.01-1984cylinders with one or more additional cylinders at an angle that lifts the sides of the belt to form a trough.Impact idler: A belt idler incorporating resilient roll coverings to absorb large amoun
21、ts of shock at the loading point.Load reactor: A device that generates a signal proportional to the force imposed upon it by the load sensor.Load sensor: See “weigh carriage.“ Also called “load receiving element.“Loading point: The location at which material to be conveyed is applied to the conveyor
22、.Normal capacity: Normal capacity is 80 percent of design capacity.Retreat idler: The first idler reached after the material on the belt leaves the weigh carriage. Also called “departure idler.“Service idlers: Those idlers in the weighing area, including scale-borne idlers and several idlers on eith
23、er side of the scale-borne idlers. They must be of the same type and grade and receive maintenance as weigh idlers.Simulation tests: Tests on the weighing unit in which either the movement of the belt, the effect of the material thereon, or both are simulated by using known weights and forces.Skirti
24、ng: Stationary sideboards or sections of the belt conveyor attached to the conveyor support frame or other stationary support to prevent the bulk material from falling off the side of the belt.Snub pulley: Any pulley used to increase the arc of contact between the belt and the drive pulley.String li
25、nes: Wires, piano wire, or monofilament line of suitable tensile strength and visibility, strung over each of the three rolls of the weigh idlers to confirm idler alignment and elevation (three-wire line alignment).Tail pulley: The pulley at the opposite end of the conveyor from the head pulley.Take
26、up (gravity): A device plus a calculated quantity of dead weight to provide sufficient tension in a conveyor belt to ensure that the belt will be positively driven by the drive pulley. A counter-weighted takeup consists of a horizontal pulley free to move in either the vertical or horizontal directi
27、on, with dead weights applied to the pulley shaft to provide the tension required.Test chain: A calibrating device consisting of a series of rollers or wheels linked together to ensure that their weight is uniform and they move freely (so that chain weight loss due to wear is minimized).Totalizer: A
28、 device used with a belt-conveyor scale to indicate the total weight of the material that has been conveyed over the scale. The master weight totalizer is the primary indicating element of the belt-conveyor scale. An auxiliary vernier counter used for scale calibration should not be part of the mast
29、er weight totalizer. Totalizers can be remote auxiliary totalizers as well as local masters. The totalizer shows the accumulated weight; a totalizer may be nonresettable or resettable to zero to measure a definite amount of conveyed material.Training idlers: Idlers of special design or mounting that
30、 are intended to counteract any tendency of the belt to shift sideways.Tripper: A device for unloading a belt conveyor at a point between the loading point and the head pulley.Weigh carriage: A structure supporting the weigh idlers, which in turn transmits weight to the load reactor.Weigh idlers: Id
31、lers positioned in the weigh carriage assembly so that they sense the weight of the material on the conveyor belt and transmit the weight through the carriage to the load reactor.ISA-RP74.01-1984 9Weighbridge-type belt scale: A scale mounted above or below a belt conveyor that supports a section of
32、the conveyor belt via a structural suspension system (weigh carriage) and weigh idlers.Figure 1: Belt conveyor and weighbridge assembly10 ISA-RP74.01-19843 Conveyor design recommendationsThe design and installation of the conveyor leading to and from the weigh scale are crucial to enable the scale t
33、o perform within accuracy specifications.3.1 Conveyor lengthThe maximum conveyor length may be dictated by the desire to make one belt revolution within a given time period or a given tonnage. The minimum conveyor length is 40 ft (12 m), unless the conveyor is specifically designed as a weighing con
34、veyor and certified by the scale manufacturer.3.2 Maximum angle of inclination The maximum angle of inclination is a function of material properties and belt speed. The primary objective is that the material must not slide back on the belt.3.3 Belt speedBelt speed is a function of conveyor length an
35、d application. In general, accuracy can be maintained in the range of 100 to 1,000 ft/min (30 to 305 m/min).3.4 Troughing angle carrying idlersThe maximum troughing angle for carrying idlers is 35.3.5 Idler spacingIdler spacing is a function of conveyor load, troughing, and belt construction, e.g.,
36、stiffness, and belt width. The scale can function as long as the conveyor does not spill the load between idlers.3.6 TakeupWhere space permits, the takeup is vertical gravity type, preferably near the drive or head pulley.3.7 Concave/convex curvesNo vertical curve in the belt should be located betwe
37、en the loading point and the scale. The scale should be located not less than 40 ft (12 m) from the convex curve and not less than 70 ft (21.3 m) from the concave curve. The belt should be in contact with at least eight idlers on either side of the scale.3.8 Conveyor stringersConveyor stringers shou
38、ld be continuous to accommodate scale weigh idlers, with at least four idlers on either side of the scale, to minimize deflection under load. If the stringers are not continuous, welding them together can be helpful. (See Section 3.9.)3.9 Conveyor vertical supportsConveyor vertical supports are loca
39、ted at weigh-section, load-bearing points, with additional supports spaced at 10 ft (3.1 m) to span an area equal to at least four idlers on each side of the weigh idlers. Relative deflection between the idlers (eight idlers plus weigh idlers) should not exceed 0.010 in. (0.0254 cm) under load.ISA-R
40、P74.01-1984 113.10 Support footingPreferably, support footings are located on concrete foundations, but they may be located on a suitably reinforced concrete floor.3.11 Training idlersTraining idlers shall not be allowed within 40 ft (12 m) on either side of the scale or within 10 idler spacings, wh
41、ichever is greater.3.12 Weigh idlersWeigh idlers shall be evenly spaced and normal (square) to the conveyor. At least four idlers on either side of the weighbridge shall be similarly aligned.3.13 Conveyor load pointConveyor loading shall be preferred at one point only. If multiple load points are re
42、quired, they should be grouped in close proximity to one another. Where a load point is within eight approach idlers of the weighbridge, multiple impact idlers are recommended. The conveyor loading mechanism should be designed to provide uniform belt loading.3.14 Wind loadingThe conveyor in the scal
43、e area should be provided with wind shields on the sides, ends, and top and bottom of each end of the scale for a distance of 30 ft (9 m) measured along the belt, where exposed to outdoor environment. The wind screen should protect against wind, at least 6 in (15 cm) below the return belt.3.15 Mater
44、ial transitionThe distance from the loading point to the scale shall allow a minimum settling time of 2 s for the material on the belt before it is weighed. Appropriate apron feeders, profilers, etc., should be used to produce as uniform a loading as possible.3.16 Idler alignmentThe manufacturers in
45、stallation instructions should be followed for the stringline alignment of weighing, approach, and retreat idlers.3.17 Belt trackingThe belt should have sufficient flexibility to ensure contact with all weighing idler rolls and be centered when it is running empty. The belt should track within 1/2 i
46、n (1.25 cm) from empty to fully loaded. This can be aided by loading material onto the belt in the direction of travel rather than transverse to it. Lagging on the head pulley is advisable; it will grip the belt and carry it into a well-centered position. It will also permit belt tension to be reduc
47、ed to a minimum, which will allow better weighing accuracy.3.18 Calibration chain storageA suitable protective support should be provided to allow storage of the test chain, where used, to facilitate handling of the chain onto the conveyor belt.12 ISA-RP74.01-19844 Calibration4.1 MethodsOperate the
48、conveyor under load for a sufficient time, depending upon weather conditions, to ensure normal operating conditions and instrument warmup. In temperatures below freezing, it is advisable to operate with a load at least 1 hour prior to starting calibration.Assure that the belt is clear, that all idle
49、rs are free, and that the belt is in alignment. To account for variations in belt thickness, zero-balance the weigh signal transmitter with an integrated average belt weight. To determine the average belt weight, run the belt for at least five integral revolutions or 3 min, whichever is greater (to reduce the error caused by inaccurate observations). Begin and end zero balance while the conveyor is running. On long belts (e.g., 1,000 ft (305 m), three revolutions should be sufficient for Class II accuracy. (See Section 5.)4.1.1 Material run preferred methodMaterial is passed ov
