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LOAD CELL INSTALLATION

The correct installation of load cells is vitally important to ensure optimum performance and weighing system longevity. Load cells are designed to measure loads through their primary axis and any off-axis loads can cause serious problems. Although careful consideration may have been given to the overall design of the weighing system, it should not be taken for granted that every thing on site will be 'hunky dory'. Manufacturing tolerances, together with discrepancies in civil engineering work can cause problems. It is therefore vitally important to carry out a thorough check of the main components, load bearing structure and load receiving element before proceeding with the installation. Check also that adequate drainage has been provided if load cells are fitted on the ground in order to avoid them sitting in water or process chemicals. Load cells may have been stored on site prior to installation and these should be carefully checked to ensure that the original packaging is in good condition and that they are the correct type and capacity.

Any threads in steel plates or girders should be checked for integrity and depth. Fixing bolts may appear to tighten correctly but in reality, reach a dead-end before actually securely fixing what they are supposed to. This can often be the case where concrete or grout has migrated into the thread.

USING DUMMY LOAD CELLS

Most process weighing system installations involve the 'craning' or otherwise 'lifting-in' of tanks or silos. If this is the case, it is always prudent to use dummy mounting assemblies during this procedure to avoid damaging the load cells. Well designed, modern mounting hardware assemblies usually now allow the fitting of these in the jacked up position without the load cell. This permits adjustments to be made, final welding to be carried out and pipework to be aligned and connected before the load cells are installed. The levels of dummy assemblies should be adjusted to align with both the support structure and the load receiving element. This is usually achieved by adjusting the level of the base plates before they are finally grouted in position. If the load cells are mounted on top of steel structures, then leveling can be carried out using steel spacers or shims. On no account should mating surfaces be pulled together using the connecting bolts! This can cause permanent damage to the load cells or leave one or more load points in a negative load situation.


Once the basic leveling and adjustments have been made, the live load cells can be carefully fitted and the load receiving element lowered. Remember that the threads of any jacking bolts are almost certainly not strong enough for this operation and suitable jacks must be used.

RETROFIT SYSTEMS

It is assumed that in new weighing installations that the structures involved are designed to be more than adequate for their purpose. However, if load cells are retrofitted to existing structures, then care must be taken to ensure that system integrity is maintained. Although most loading assemblies are designed to prevent excessive vessel movement, they must never be used as part of a structure's overall strength. Fitting load cells to existing structures typically involves cutting into the support legs or unbolting the structure from the ground or similar, in order to fit the load cells. In this situation, additional cross braces or gussets will almost certainly be required to provide sufficient strength. Unbolted legs can easily splay and buckle under load if not suitably braced.

TEMPERATURE CONSIDERATIONS

The effects of temperature on load cells and weighing systems can be a major source of error, resulting not only in poor weighing but also causing major damage. Huge forces can be generated by expansion and contraction if adequate clearances are not allowed for.

One topic often ignored when fitting load cells relates to the possible relative expansion and contraction of the load receiving element and the support structure or ground works. When fitted, the loading assemblies must allow for movement which occurs over the whole of the anticipated operating temperature, taking into account the maximum possible deviations which may occur between extreme winter and summer conditions. If the system is operated in doors, then large temperature extremes may occur especially in areas such as steel smelting plants. Remember also, that during installation, the ambient temperature may be well below that at which the system will operate in practice.

PIPEWORK

Pipework attached to weighing systems can be a major source of error and non-repeatability. Hopefully, careful design criteria will have been followed during the design stage to minimise the effects in order to provide a well balanced system. However, turning theory into reality on site, may not be as straightforward as anticipated. Good pipework alignment with vessel flanges is vital. Although it may be tempting to pull flanges together using the connecting bolts, this will inevitably lead to problems. If pipework does not align, then it is prudent to carry out remedial work before final connections are made.

It is quite common for pipes to carry hot liquid which therefore requires them to be lagged or insulated. Lack of diligence in the fitting of this can introduce problems. If possible, all insulation should be carried out prior to commissioning.

LOAD DISTRIBUTION ADJUSTMENT

Once the weighing system has been installed it is important to carry out a final leveling procedure prior to calibration. This 'cornering' process will ensure equal load distribution amongst the load cells- especially important in a four load cell system. Three load cell systems will naturally provide a stable loading situation ( as borne out by the three legged stool ) but those with four or more load points will tend to rock about one point. Note that from an overall stability point of view, a four point support system is around 30% more stable.

In the cornering process, the electrical output from each individual load cell is measured for comparison and, where practical, these readings should agree within 1mV or better. Note, that ideally, the weighing system should be loaded to full capacity and emptied before carrying out any measurements in order to remove any irregular mechanical stiffness and to 'bed' the loading assemblies in. If the readings are outside the acceptable tolerance, then shims can be fitted between the top of one or more loading assemblies and the load receiving element in order to redistribute the load sharing.

MECHANICAL PROTECTION

It is important to ensure that the load cells and their cables are provided with suitable mechanical protection. Remember that maintenance engineers and other work personnel may not be as diligent as you and may use load cells as suitable step ups during work on the system. Damage to cables can be very costly and they should be routed through conduit where possible. Remember also not to allow cables to become somewhere for water to track down towards the load cell.


DO's

1 Use dummy load cells during system installation and final alignments/adjustments.

2 Store and handle load cells carefully prior to and during installation. Ensure that their certificates of conformity are kept in a safe place.

3 Check load cells before fitting for correct model, capacity and thread combination.

4 Check that any threaded fittings screw smoothly into the load cell before final assembly.

5 Always use high quality bolts and fittings

6 Use care when tightening mounting bolts and restraints such as tie bars.

7 Use the recommended fixing torque.

8 Use lock nuts as appropriate on threaded fittings especially if vibration is present.

9 Check out cable colour code for load cell prior to connection. Use good quality connecting terminals and junction boxes- solder joints if possible.

10 Provide adequate and effective earthing between weigh system and ground. Ensure that every load cell is at the same earth potential. Fit earth straps between top and bottom of mounting assembly.

 

DON'Ts

1 Carry out electric welding near load cells.

2 Forget to check specific storage and operating temperatures.

3 Ever carry load cells by their cables.

4 Force bolts or threaded assemblies.

5 Use mounting bolts to pull uneven mounting surfaces together.

6 Use excessive force when fitting or tightening mounting bolts or hardware- especially on low capacity cells.

7 Twist 'S' cells when tightening threaded fittings.

8 Cut load cell cables unless absolutely necessary. This may affect performance and invalidate warranties.

9 Allow moisture to get into any interconnections.

10 Leave load cell cables unprotected.

11 Allow load cell to be the electrical link between ground and metal weighing structure


COMMISSIONING AND CALIBRATION

Once a weigh system has been designed, built and installed, the final working performance will depend on the initial calibration procedure. Depending on the type and mode of operation of any weigh system, there are a number of calibration methods that can be used, each with their own merits and uncertainty of measurement.

The main methods are:

1. Standard weights

2. Reference weights

3. Substitute Material

4. Metered flow

5. Force transfer method

Essentially calibration is the procedure of applying known and traceable loads to the system in a controlled manner. To ensure optimum performance, the system must be loaded to full rated capacity. Often this is not carried out, either due to physical restraint or as a result of short cuts being taken and system performance is, at best, unreliable. Applying loads to high capacity vessels can be very difficult and consideration should be given to this during the design stage so that provision can be made to facilitate calibration. Remember that the accuracy of the calibration method used must be at least three times better than the required overall accuracy of the weighing system.

PREVENTATIVE MAINTENANCE

In order to ensure a long and healthy life for your weighing system it is important to carry out regular inspections. This is especially important for systems that are in harsh environments. Waiting until the system fails before carrying out remedial work is definitely false economy.

Simple and regular checks can highlight problems and preventative checks should be made on load cells, mounting assemblies and cabling. Simple remedial work can usually prevent further problems.

If load cells show sign of initial corrosion then often a coat of paint can prevent further problems. However, the cause of such problems may be due to a change in the local environment such as a leaking pipe, allowing chemicals or water to drip onto the load cell or blocked drainage holes allowing load cells to become submerged during heavy rain. If this is the situation, then remedial action must be taken.

Another cause or problems can be build up of debris around to load cell. This can impair weighing operations and also mask on-going corrosion underneath. Debris around a load cell can also act as a path for unwanted electrical current during electrical storms, resulting in load cell failure.

If a load cell does fail through ingress of chemicals or moisture, it may be tempting to electronically disconnect the rogue cell and re-calibrate the weighing system without it. From a metrological point of view this may provide adequate performance. However, the load cell may then subsequently continue to corrode inside, to a point where catastrophic mechanical failure takes place which may cause the system to collapse .


CONCLUSION

Care in the original design and choice of critical components for a weighing system should ensure optimum performance and reliability. At the start of a project, it is vitally important that users provide weighing companies with all relevant information on what they expect from a weighing system and how and where it will operate. Failure to do this in the initial stages will almost inevitably lead to misunderstandings and poor system performance once the system is installed.

Load cells should not be chosen on cost considerations alone. Long term reliability and ongoing support are vital considerations when choosing a supplier.

Once a weighing system is operational it should not be a case of `fit and forget'. Preventative maintenance can save considerable time and money, highlighting and remedying problems before they cause expensive system failure.

Download this useful WELMEC document on load cells