The design, installation, calibration and ongoing maintenance of process weighing systems
12/02/2013 Weightron Bilanciai Ltd
Process weighing plays a vital role across a broad spectrum of industries and, given the current difficult economic climate, there is an increased focus on reliability, performance and accuracy. Weightron Bilanciai's Sales Director Emlyn Roberts reviews key issues relating to the design, installation, calibration and ongoing maintenance of process weighing systems. An overview of legislative requirements is also discussed.Process weighing can be broadly subdivided into the following categories, each with their own specific requirements:
-€¢ Basic level control
-€¢ Stock/inventory control
-€¢ Batching and blending
-€¢ Recipe formulation
-€¢ Loss in weight
-€¢ Bulk discharge weighing
-€¢ Filling and decanting
Design parameters have to take into account a wide range of operational, environmental and legislative issues, all of which can have an impact on overall cost and performance. Installations can be divided into new and retrofit. Although the same fundamental design rules apply to both, retrofit weighing systems may present different challenges, especially relating to structural integrity.
Measuring chain
Process weighing systems typically consist of primary measuring sensors (usually strain gauge load cells), appropriate mounting hardware and instrumentation.
Instrumentation, software and data management
Although many of the fundamental mechanical principles of process weighing have not seen quantum changes over the past decade, advances in electronics, software and, of course, the web have had a major impact on the versatility of process weighing systems. Weighing systems used to operate in 'splendid isolation'. The collection and transfer of critical weight data from the factory floor was carried out manually and integration with management software systems was virtually non-existent. Effective weight data management now plays a growing role in many applications, both locally and remotely. To achieve this there is now a wide range of dedicated instrumentation, software systems and interfaces. These bring real time bi-directional communication between the factory floor and essential management systems (ERP, Sage, Navision etc). Versatile colour touch screen weighing terminals that can be readily user-defined have brought significant advantages and removed the need for local PCs in many applications.
Weighing system performance
Accuracy is an important factor in process weighing systems. Unfortunately the term is often misunderstood, mixed up or confused with other measurement parameters, especially repeatability and resolution. Although theoretical accuracy levels can be calculated for weighing systems, these are modified by a number of external factors and it is therefore more important to understand what can be realistically achieved in practice within the everyday working environment. In parallel, accuracy levels are often mixed up with error (or more correctly 'uncertainty of measurement'). In common parlance it is not uncommon for levels of accuracy to be quoted, for example, as 1 part in 1000 or 0.1%. What is really meant is that the system has an 'uncertainty of measurement' or error of 0.1% and in fact the accuracy is 99.9%.
Accuracy versus repeatability and resolution
The definitions (courtesy of Institute of Measurement and Control document WGC0496) are as follows:
Repeatability: the measure of agreement between the results of successive measurements of a weighing system output for repeated applications of a given load
Resolution: the smallest change in weighing system output that can be meaningfully distinguished
The time frame over which the weighing process takes place has a major impact on overall weighing performance. Three key factors need to be taken into account:
-€¢ The overall long term zero stability of the weighing system
-€¢ The linearity of the weighing system between zero and full load
-€¢ Differences between increasing and decreasing load outputs (hysteresis)
Different weighing applications can have different accuracy requirements and priorities. The importance and relevance of these are best illustrated by the following examples:
Silo weighing system
Such systems are typically used to manage stock control for essential raw materials to ensure processes do not run out of product and to calculate inventory values for fiscal purposes. Required accuracy levels may be relatively low, with errors of ±0.5 -1% usually being acceptable. However achieving these levels long term is not as straightforward as it may first appear and it is important that key weighing elements such as load cells provide good long term stability.
Unlike many weighing installations, once the silo weighing system has been installed and calibrated the zero stability of system may not be checked for months or years, especially if the vessel does not become completely empty at any stage. Over time if the zero point of the weighing system changes, then the displayed weight will not represent a true contents value even though any differential weight readings may appear to be consistent.
Batching and recipe formulation
By contrast, batching and recipe formulation systems have different fundamental requirements. High quality load cells must be used but the weighing process usually takes place over a defined, relatively short period. This means that the system can be reset to zero before each batch thereby improving short term accuracy capabilities, typically by up to a factor of 10. This allows small amounts of minor ingredients to be added with acceptable precision. The short weighing period also minimises the effects of any temperature changes. Here good repeatability can be more important than overall accuracy.
Environmental considerations
The ability for key weighing system elements to provide long term reliability is essential, especially in harsh environmental conditions. Material of construction, surface finish and sealing levels for critical equipment such as load cells play a key role. In the absence of clearly defined sealing nomenclature for load cells, manufacturers adopt the IP rating system (NEMA in the US). Unfortunately the IP system only refers to water ingress and does not give useful information with regard to chemical compatibility. This emphasises the importance of regular preventative maintenance discussed below.
Temperature Effects
Temperature changes can have important adverse effects on weighing system performance especially for outdoor systems. Key issues are the rate of temperature change and differential heating/cooling. Rapid or excessive temperature changes affect load cell compensation performance and also impacts on load introduction (cosine error) through expansions and contractions.
ATEX weighing systems*
Certain weighing applications take place in designated hazardous areas (ATEX), where the presence of dust and gases together with oxygen (air), presents the finite chance of an explosion occurring through sparks or heat sources above critical temperatures. It is of paramount importance that any ATEX weighing system must be designed and installed by fully qualified, competent engineers. The weighing system must also be made up of fully ATEX certified components.
Hazardous areas are divided into different Zone Classifications - Zones 0, 1 and 2 for gases, 20, 21 and 22 for dusts. The classification of each zone and the methods of ensuring electrical equipment cannot cause explosions are defined through European Directives under the auspices of CENELEC. Directive 94/9/EC covers equipment, whilst a complementary Directive, 1999/92/EC, is specifically concerned with worker safety and places requirements on employers whose staff may work in an explosive atmosphere.
*ATEX legislation is mandatory in Europe and is also accepted in many areas of the world. However the US system is controlled by Factory Mutual (FM). (There is close cooperation between FM and CENELEC)
For individual applications, the plant is divided up into so-called hazardous and safe areas. There are a number of ways to provide protection for weighing equipment, depending on the zone classification and working environment. The most common way is based on intrinsic safety which essentially limits the maximum energy that can be applied to the load cell under clearly defined 'normal' and 'fault' conditions. (Note: load cells are classified as non-energy storing devices) This is normally achieved through the use of zener barriers between the instrumentation in the safe area and the load cells in the hazardous area. Alternatively specialist instrumentation is available that can be installed directly in certain hazardous areas, removing the need for separate barriers. Weightron's D440IS hazardous area indicator can operate in Zones 1, 2, 21 and 22 without the need for additional zener barrier protection. This minimises installation time and cost.
System and structural integrity
The installation of a large percentage of process weighing systems involves mechanical changes to the structure of the tank, hopper or vessel being weighed. It is therefore important that this does not adversely affect the structural integrity of the system. This is particularly important for large vessels, especially those installed out doors. Key factors can include wind forces and seismic activity. (The UK, for instance, has a surprising number of earthquakes which can test the integrity of weighing systems even though they register relatively low readings on the Richter Scale). Any restraints, such as check rods, used to maintain integrity can have adverse effects and must be carefully designed and installed. It is therefore essential that expert structural advice is taken both during the design phase and the installation.
Pipe work and other force shunts
Very few process weighing systems stand in isolation without external attachments such as pipe work, gantries, ladders and other force shunts. Anything connected to the weighing systems can, to a greater or lesser degree, have an adverse effect on performance. Particular care should be taken with silo weighing systems where several vessels may share a common support structure. So called 'vessel crosstalk' can occur whereby weight changes in one vessel are erroneously detected and measured in adjacent ones. However careful design can minimise the problems and ensure any force shunt effects are repeatable.
Selecting the correct type and capacity of load cell
Load cells form the basis of over 99% of process weighing systems. Selecting the correct type and capacity is a critical issue that needs careful consideration from the outset. The key factors in the decision process must take into account:
-€¢ The overall capacity of the weighing system
-€¢ The dead weight of the weighing system itself when empty
-€¢ The working range of the weighing system
-€¢ Any overload requirements (e.g. wind forces, shock loading, vibration, seismic forces)
-€¢ Environmental factors (e.g. wash-down, chemicals, temperature changes)
-€¢ Accuracy requirements
-€¢ Legislative requirements (e.g. Legal for Trade, ATEX)
It should be remembered that in many applications, the actual live working range of the load cell (and hence the available electrical output) will be reduced depending on the dead weight of the vessel and any overload requirements. This can have an important affect on performance and further endorses the need to ensure load cells are of the highest quality.
Operationally weighing systems essentially fall into two main categories - compression and tension, depending on whether the vessel or similar 'sits on' or is 'suspended from' the load cells. These two fundamental modus operandi can then be sub-divided into the following main types of load cells:
-€¢ Single ended bending beam
-€¢ Shear beam (single and double ended)
-€¢ 'S' tension beams (bending and shear)
-€¢ Compression canister
-€¢ Ring torsion
Material of construction includes aluminium, plated tool steel, painted tool steel and stainless steel Any final decision in the selection process will almost certainly involve a level of compromise to optimise performance versus system integrity and environmental compatibility.
Mounting hardware
In the majority of applications, mounting load cells without the use of proprietary mounting hardware can be difficult and very unsatisfactory. Well designed hardware not only plays an important role in terms of load introduction, vessel restraint, lift-off protection and overload protection, but also facilitates load cell fitting and removal.
Calibration procedures
Calibration is a critical process in ensuring initial verification and ongoing weighing system performance. Essentially calibration involves the checking of weighing system performance by applying known loads (these may be physical or electrical) over as wide a part of the weighing range as practical. There are a number of recognised methods of applying load, each with there own merits and precision. Clearly the type of weighing system plays a major part in deciding on the method of calibration. For instance, it is quite simple to add relatively small amounts of calibrated weights to a platform scale or small hopper, yet this becomes very difficult with large vessels. The main calibration methods are;
-€¢ Use of dead loads (calibrated weights added to system)
-€¢ Substitution (previously weighed load (via weighbridge or similar) added to vessel)
-€¢ Flow meter (vessel filled with known amount of water measured via flow meter)
-€¢ Force calibration (known loads applied via calibrated load cells and hydraulic jacks)
-€¢ Load cell output calculations
Where practical the design of weighing systems should take into consideration how calibration will be achieved.
Maintenance
It is prudent to utilise expertise of an external service provider to ensure ongoing reliability, safety and performance for process weighing systems. Regular checks on calibration and system integrity will prevent expensive and time consuming breakdowns, whilst protecting revenue. In parallel it is also advisable that the user carries out simple regular checks on weighing systems as part of a structured in-house preventative maintenance regime. This can highlight problems including product build up, leakages that can cause component failure, unwarranted structural changes and mechanical damage. Note: it is essential that electric welding is not carried out on weighing systems without seeking professional advice.
Conclusion
For optimum performance and reliability, modern process weighing systems should be based on a blend of sound mechanical design principles, versatile instrumentation. Flexible user-friendly software with inbuilt future proof capabilities will ensure comprehensive weight data management.
Key issues in the design and installation process mandate the use of qualified engineering expertise, especially for hazardous area applications and where structural integrity is an issue. Overall performance will always be affected by a range of environmental and operational factors. Cutting corners through the use of lower quality weighing components and instrumentation is false economy and will result in costly and time consuming ongoing poor performance and reliability.
For further information, please visit: www.weightron.com or contact: Tel: 01246 260062
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