Weighing requirements for the chemical industry

The critical elements for any static weighing system being used in the chemical industry remain basically unchanged, explains Rod Morgan, sales director, Applied Weighing Ltd:

Reading, UK - The weighing industry has seen significant change over the past decade, driven not least by developments in information technology, the growing importance of manufacturing traceability, health & safety legislation and the adoption of much stricter working methods.

The critical elements for any static weighing system being used in the chemical industry, however, remain basically unchanged. The four key determining factors for specifying such a system, but in no particular order, are still - accuracy, reliability, durability and safety.

Accuracy is often the first consideration but sometimes it’s confused with similar terminology such as 'displayed weight' and 'display readability'. These terms are sometimes used when referring to the accuracy of a weighing system, but are not necessarily the same thing.

Stand-alone weighing systems, even small platform scales are now accurate to 0.1g, or even better. However, the factory environment and other practical considerations mean that for process applications, a platform scale with a capacity of, for example 30kg, will usually have weight increments of 2g, or a scale with a capacity 3000kg by 0.5kg.

A modern weight controller or terminal can be simultaneously linked to a number of platform scales. The controller will guide the operator to confirm that the correct product is being weighed and that the most accurate scale has been selected.

Manufacturing schedules can be downloaded from other higher-level systems and weighing data and other management information can be uploaded. Systems such as this can provide detailed reports and are an invaluable aid to product traceability.

Modern controllers are capable of simultaneously taking inputs from up to 28 separate weighing systems or from other measurement devices such as flow meters, temperature sensors and pressure transducers. Each system can work independently or as part of a sequence for process control. Typical outputs available include, Profibus, Devicenet, Ethernet, Modbus RTU and 4-20mA.

The accuracy of vessel and tank weighing systems in the chemical industry is largely determined by mechanical considerations, such as pipe-work, motors and other less obvious considerations such as the ratio of the vessels’ dead weight to its contents weight.

A good installation, using flexible connections on pipe-work and by using the appropriate model of weighing assemblies and load cells, should achieve at least 0.1% system accuracy, for example, 5000kg x 5kg.

Many applications are able to achieve much better than this, sometimes achieving 0.01% or more. However, very often, the main consideration of a manufacturing process is the way that the system is constructed to give good repeatable results. So, although achieving the best accuracy possible is important, it’s only a part of the equation.

Reliability is a key factor in the chemical industry and equipment used should be fit for the purpose intended. Clearly, as with other capital equipment and weighing equipment is no different, you usually pay for what you get.

Using unsuitable components and poorly constructed platform scales for applications that call for robust designs will usually mean that reliability will be compromised. Sometimes though there has to be a trade-off between robustness and accuracy. For example, very high accuracy equipment, designed for laboratory conditions, may not have the robustness required for the manufacturing areas.

The critical component for most modern chemical weighing systems are the load cells and it is the one electronic device which is the most exposed to its environment; whether incorporated within a platform scale or under the leg of a vessel. Fortunately, they are so robustly constructed and reliable these days that the leading companies are confidently able to offer a five-year warranty.

A chemical engineer’s worst nightmare is for a failure to occur in his weighing equipment, resulting in incorrect quantities being used and spoiling a process that can then result in the loss of revenue and even a client.

For vessel weighing applications this can be prevented by using equipment such as an ‘intelligent’ load cell junction box – a clever device that can detect an imbalance or damage to a load cell and warns the engineer via an integral display or output to an alarm before any harm is done.

Durability plays a big part when it comes to the chemical industry. Sometimes, there are strict hygiene regulations. To comply with these regulations, systems need to be designed to prevent chemicals from gathering in screw heads, between crevices, etc. This applies to platform scales and components used for vessel weighing systems.

Critical components such as load cells and weight instrumentation are usually manufactured in stainless steel and constructed in such a way as to minimise the ingress of material. These components should also be hermetically sealed to IP68 to withstand frequent wash down, often at high pressure, which is mandatory in some chemical industries.

Pit-installed platform scales are unobtrusive and allow trolleys to be easily pushed on but in recent years they have been a less popular choice because pits can be difficult to clean and be kept clean. However, new platform scale designs such as those using gas-lift assisted lids make it much easier for the pit and the scale to be accessed for washed down.

For those applications where pit installations are not desirable very low profile platform scales are available instead. Some designs are only 50mm off the ground so that even tall trolleys can be pushed on or off without to much effort.

Safety, in the chemical industry is a paramount consideration and weighing equipment, must be suitable for the environment it is being used in. Installations in hazardous areas must meet ATEX directive requirements. Platform scales or vessel weighing systems can either be either installed completely in the hazardous area; or partly in the hazardous area by utilising a local safe area and using appropriate safety barriers or isolators.

Although process weighing for the chemical industry has come along way, the fundamental elements remain the same. Our rapidly growing need for more factories and manufacturing capability introduces further competition into the market place.

Consequently, the need to keep costs to a minimum is more important than ever. As technology changes and the way in which items are manufactured and stored continue to develop we will see further advancements in weighing systems at competitive prices. Increasingly, for example, digital load cells and communication bus systems are being used for applications where there is no need for local weight instrumentation or operator intervention.