Field Inspection Manual — Automatic Weighing Devices

Part 2, Section 2: Type 6-11: Automatic Continuous Totalizing Weighing Systems (Conveyor Belt Scale) [CTWS]

Table of contents


Reference

For more information on the inspection of conveyor belt scales consult the Weights and Measures National Technical Training Program Automatic Continuous Totalizing Weighing System training module.

Purpose

This device is designed for the continuous totalizing of bulk commodities across a continuously integrating device commonly known as a conveyor belt scale. Only mechanical, electro-mechanical and full electronic strain gauge load cell scales are covered by this procedure. Devices which use other sensing technologies (nuclear, LVDT, etc.) may have specific requirements not addressed in these procedures.

General

The inspection of an Automatic Continuous Totalizing Weighing System (CTWS) is of a complex nature. Not only because of the inspection procedure itself, but also because it involves a great deal of planning, organization and communication with the parties involved.

This type of inspection requires a large number of pieces of testing equipment, and requires the involvement of many people. A CTWS inspection is also time consuming. On occasion, the test may restrict or stop the operations of the facility where the inspection is performed. Therefore, the cost of a CTWS inspection may be relatively high.

The inspection must be very well planned and organized. Before going to the site to perform the tests, the inspector must ensure the following:

The inspector must, in advance, become familiar with the instrumentation used. The characteristics of the scale, its operation and installation as well as the intended use are some of the elements that must be known by the inspector prior to testing the scale. It is recommended that the inspector follow the product delivery path from loading to discharge to identify any possible areas of concern (product diversion, spillage or other loss).

This information is needed to effectively implement the inspection procedure and to know which limits of error will be applied and how to best perform the inspection of the DUT.

Classification of Automatic Continuous Totalizing Devices

Automatic Continuous Totalizing Devices (CTWS) may be used to weigh product for assessing transportation charges or for buying or selling the product. The intended use of the in-motion scale determines which limits of error apply. Limits of error (LOE) for a CTWS are found in the Weights and Measures Regulations:

Table 1
Intended Use Regular Commodity Inexpensive Commodity Footnote 1
Assess Freight Charges regulation 193 regulation 193
Custody Transfer Buy or Sell regulation 174/175 as appropriate regulation 193

Testing philosophy

The system shall be tested in a manner which will simulate its intended use. This means that although the device is the primary concern, the interaction between the device and the rest of the system must be taken into account in assessing the overall performance of the system. The other system components that may cause issues include the load hopper, feed conveyor, transport conveyor, gates and loading arms.

Typically, a belt scale is used for continuous duty with consistent belt loading and the testing should reflect this. However, in some cases the owner of the device may intend to stop and start the belt during use, or have intermittent loading on the belt. In these cases, the testing procedures should take into account this potential usage.

In developing the test procedure for a particular site, the inspector must give consideration to the type of load, the weather and its impact upon the material to be weighed, the loading characteristics of the belt as well as the speed of operation of the device.

Test Load

Although a scale can be setup and configured using calibrated weights (blocks or chain), a product test load is required to certify a CTWS. Only by using a product test load can the inspector be satisfied that the entire system is working properly.

The test load can either be pre-determined or can be unknown material which has passed through the system, been caught and then weighed. In either case, the amount of material to conduct a suitable test can be very large and appropriate arrangements must be made to move this material around the site.

In the case of a pre-weighed test load, it is important that the load is stored in such a manner so as to ensure all of the material, and no extra material, is ultimately passed over the system. In both cases, utmost care must be exercised to ensure that no material is lost during the test as this will jeopardize the results.

Reference Scale

The weight of the test load will be obtained statically on a scale that has been demonstrated to perform accurately to within the required limits of error. The scale must be tested using NAWDS and suitable standards.

The test load may be weighed on any suitable scale. Typically a bulk hopper scale or truck or rail scale is used. The location and installation of the device and reference scale will be the determining factors in making this decision.

Any inherent error in the reference scale must be identified and documented. Uncertainty in the Test Load due to the Reference Scale must be determined and accounted for.

Development of the Product Test Load shall be done as per the appropriate procedure. Please refer to the procedure for Product Test Load Development for more information.

Visual Examination

Notice of Approval (NoA)

The inspector will ensure that the scale and instrumentation are approved models. The inspector will ensure that the scale complies with all conditions, restrictions or parameters that may be stated in the Notice of Approval or on the certificate from the last inspection. Restrictions may include: belt speed, belt inclination, minimum loads, material types, location, etc.

Manufacturer's Installation Instructions

All components of the system must be installed as per the manufacturer's instructions and recommendations.

Marking

Ensure that the weighbridge is marked as required by section 18 of the Weights and Measures Regulations (model number, approval number, serial number, etc.). The integrating instrumentation must also be appropriately marked.

Sealing

The speed transducer, junction boxes and integrator are typically sealed to ensure that modifications which may affect the accuracy of the device are not made without breaking a seal.

Weighbridge

Belt scales generally contain one or more live rollers. The number of rollers which are live is dependant upon the design of the device. Ensure that the number, size and location of the rollers is as per the approval.

Installation of the weighbridge shall follow the manufacturer's recommendations and installation requirements as appropriate.

Inclination of the belt scale is extremely important and is directly related to the calibration of the scale. As the angle of inclination is increased, the apparent load sensed by the scale decreases. The relationship is related to the cosine of the angle of inclination.

Equation 1

apparent load = cos(theta) × actual load

The result of this is that the angle of the scale must not be changed after calibration unless an approved angle compensator is used and has been tested. Scales which are designed to operate at several different angles shall be equipped with an angle sensor and shall be inspected at the high and low limits. The angle(s) of the belt shall be noted on the inspection certificate.

In addition, extreme angles will result in product slippage on the belt resulting in measurement errors. The angle at which this occurs is dependent upon the product being weighed.

Once the device has been inspected, the angle of the belt scale shall not be changed without verification that the device continues to operate within acceptable limits.

Weigh Belt

Changing the weigh belt will affect the calibration of the device. New weigh belts will stretch over time and should be conditioned by running for several hours, or as per the manufacturer's recommendations, before the initial inspection of the device.

Bad belt splices will result in zero stability issues. The effect of the splice on the integrators indication should be noted. Excessive effect will results in zero stability issues.

Material build up on the weigh belt will result in repeatability issues during testing. The belt should be checked both on the loaded surface and the unloaded surface. Scrapers may need to be provided to reduce product build up.

The trough angle of the weigh belt will affect the calibration of the device and shall not be changed without ensuring that the device remains within limits of error.

Weigh and belt rollers

Weigh rollers must run straight and true. The alignment of the rollers on the scale should be verified with a straightedge. Individual rollers should be examined to ensure that they are true and move freely inline with the weighbelt. All rollers must be free running - a seized roller will cause performance issues and must be replaced.

Lead in and out rollers must be inline with the weigh rollers. Training rollers should not be located immediately adjacent to the weighing element.

Load cells & Levers

Ensure that the load cell(s) are installed in accordance with the approved design. If the scale uses levers, ensure that they are properly aligned and fully supported. Belt scales with mechanical integration and indication will have a lever system mounted beneath the belt and integration is performed with a mechanical disk assembly.

Check and Tension System

Ensure that the check system is in place, and adjusted properly. Any tension take up devices shall be free moving and functioning correctly. Build up of material beneath a take up roller or weight will cause tension problems and must be removed.

Cables and ground

Ensure that the grounding system is in place and that the cables are suitably protected and shielded as per the manufacturer's instructions. Cables should not be rubbing on moving components.

Speed Sensors

Speed sensors or transducers shall not be mounted on a drive pulley. These sensors must be suitably located to ensure that they measure actual belt speed accounting for slippage if any occurs. Speed sensors must be securely mounted and sealed to the device.

Instrumentation

Ensure that the systems instrumentation is suitable and approved for the intended use.

Electronic instrumentation must be approved for Automatic Continuous Totalizing and contain suitable integration circuitry. Instruments approved only as Non-Automatic Weighing devices shall not be used for this purpose.

Manual integration instrumentation, although rare, does exist. Refer to the Notice of Approval (NoA) for details of the configuration.

Regulation 172(3) stipulates that the value of the minimum increment of registration may not exceed 100 kg (200 lb).

Test Procedure

1. Develop Test Load

Product Test loads shall be developed according to the procedure for Product Test Load Development.

Product Test Load size

Each test run must be of sufficient quantity to ensure a proper evaluation of the device. The minimum totalized test load shall equal or exceed 800 intervals for a device subject to LOE from regulation 193 and 1000 intervals for a device subject to LOE from regulation 174/175 or at least one full revolution of the belt, whichever is greater.

Load Established Before Passing Over DUT

Once a test load has been established, it must be protected. Product which forms the test load must be fully accounted for to ensure that it all passes over the DUT. In addition, it is imperative that no additional product be introduced during the test.

Test Load Established After Passing Over DUT

In those cases where the weight of the test load is established after it passes over the DUT, it is important to ensure that all product is captured and accounted for. This can sometimes be difficult as the amount of test product may exceed the capacity of a single truck or rail car. If product is lost, the test run must be rejected.

Note: Ensure that a sufficient quantity of test product is available to conduct all of the required tests. Belt loading throughout the test should remain reasonably constant.

2. Conduct Product Test

This test is designed to assess the systems ability to measure a known quantity of product that it is designed for. It is analogous to a test of a volumetric device, where the product is passed through (in this case, over) the measuring element and the registered quantity is then compared to a standard.

Procedure

  1. Prepare the materials necessary for the test
    1. appropriate quantity of known product to run the tests.
    2. suitable means to transport the product to and from the reference scale.
    3. sufficient supplementary product for pre-run conditioning of the belt prior to the actual test.
  2. Run supplementary product over the scale for at least three complete belt revolutions or ten minutes at the rate of flow which will be used for the test, whichever is greater.
  3. Zero the scale. Ensure that the zero indication is stable - see procedure for zero below.
  4. Run the first test quantity over the scale at maximum expected flow rate. Ensure that no product is lost (or gained) during the transfer of product between the reference scale, the transportation means and the belt scale.
  5. Note the integrator totalizer reading. Allow for 0.5d for digital indication (R184).
  6. Compare this with the know quantity of material. Determine the error.

    Equation 2

    error percentage = (indicated load) − (actual load) ÷ actual load × 100
  7. Continue to run test loads until accuracy and repeatability requirements have been suitably demonstrated. At least three runs shall be completed at the maximum flow rate to demonstrate repeatability.
  8. Conduct at least one run at approximately 35% maximum flow rate.

Note: The weight of the known quantity of product may be determined before or after it is run over the belt scale. This decision will be dependant upon the installation of the belt scale and ease of product access. In either case, it is imperative that no product is lost (or gained) between the time of weighing on the reference scale and the time of passing over the belt scale. Keep in mind that the required test-load size at rated capacity could be a great deal of product.

To certify or reject a CTWS, a product test must be completed. Performance of the DUT must be verified using known test loads at the limits of desired operational speed; both the fastest and slowest operating speeds must be tested.

CTWS are seldom used as stand-alone devices. More commonly, they are installed as part of a loading facility. In these installations, there may be many opportunities for product loss or diversion, both before and after the belt scale. The inspector must make themselves familiar with the installation as well as details regarding product ownership and transfer points. Once this information is obtained, the system should be examined to ensure that any potential product loss or diversion points have been addressed.

3. Supplementary Requirements

  1. Initial Zero Setting

    The integrator or totalizer must only advance when the belt is running and loaded. The nature of operation of a CTWS means that any error in the zero setting will translate into an error in the final weight totalization. Therefore, it is important that the device is capable of maintaining a steady zero while running in an unloaded state. When first started, the system must be allowed to warm-up and exercise the belt. During this time, the zero setting may be adjusted as required. Zero testing should be completed with a whole number of belt revolutions. This allows errors within the belt length to self correct.

    New devices or existing devices with new weigh belts should be run for several hours or as per the manufacturer's recommendations for belt break in. New belts will stretch significantly and the result could be changes to the calibration of the device.

  2. Zero Stability

    Once warmed up, the device shall be tested for zero stability. The belt shall be run unloaded for no less than 3 complete revolutions or 10 minutes operation, whichever is greater. The indicated totalization (zero error) shall not exceed ± 0.05% of the totalized load at full scale capacity for the duration of the test. This test shall be repeated until 3 consecutive tests meet this requirement without adjustments being made to the zero settings.

  3. Minimum Load

    The minimum totalized load shall equal or exceed 800 intervals for a device subject to LOE from regulation 193 and 1000 intervals for a device subject to LOE from regulation 174/175. In no case shall the value of the minimum increment of registration exceed 100 kilograms R172(3). Minimum totalized load shall be calculated and included on the Inspection Certificate as a usage restriction.

  4. Installation

    Installation of the scale must be as per the manufacturers recommendations, design and installation drawings and any requirements contained within the NoA. Location of belt direction changes, loading points, weighing element and non standard rollers shall comply with all manufacturers' recommendations. Distance from the loading point to the scale can affect the accuracy of the measurements.

  5. Certification

    The Inspection Certificate must describe the system and identify the product to be measured. The Certificate must also indicate the manner(s) the scale may be used (restrictions); for instance, the speed of the belt (min & max), the angle of the belt, etc. Section 70 of the Weights and Measures Regulations requires that the restriction(s) be posted.

  6. Sealing and Stamping

    The DUT and all applicable ancillary equipment must be sealed and stamped as required by sections 29, 31 and 32 of the Weights and Measures Regulations. Any additional requirements from the NoA must also be addressed as appropriate (e.g. sealing of speed sensors).

Interpretation of Results

The DUT is deemed to comply if all results are within the acceptable LOE (R174/175) for the quantity of product test load passed over the device.

Revision

Rev. 2

Rev. 1

Footnotes

Footnote 1

Inexpensive Commodity is deemed to mean any item for which a NAWDS Class IIII device would be suitable as per NAWDS Table 62.

Return to footnote 1 referrer

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