Hazardous Area Weighing

Excerpt from the METTLER TOLEDO Hazardous Area Guide

Weighing is one of the most basic and important process variables in a clear majority of manufacturing pro­cesses. However, it can be one of the most challenging parameters to control.

Accurate and consistent filling, dosing, and batching reduce variability in the end product, which ensures con­sistently high quality. Achieving reliable quality and reproducible results requires efficient capture and com­munication of weighing data to the existing plant network, which can be challenging due to hazardous-area requirements.

To prevent any ignition and provide safe operation of electrical weighing systems in hazardous areas, one possibility is limiting energy to safe levels. To achieve low energy and prevent ignition, the main components of weighing systems, such as load cells, junction boxes and weighing terminals are designed for intrinsic safety. Intrinsically safe technology prevents explosions by ensuring that the energy in intrinsically safe circuits is well below the energy required to initiate an explosion.

Intrinsically safe electrical equipment and wiring are designed and certified mostly for use in Zone 1/Division 1 hazardous areas as long as they are approved for the location. Intrinsically safe circuits often combine ele­ments with the various safety levels. Depending on functionality and the classification of the safety level, the circuit elements can be applied either in hazardous or non-hazardous areas.

Basic system

In hazardous production areas, there are many processes that require simple standalone weighing applica­tions. Filling tanks, drums or bags with hazardous powders or liquids is one example.

A simple weighing system usually consists of strain gauge (analog) or digital weighing platforms or load cells controlled and monitored directly through a PC installed in a safe area. The weighing signal is interpreted by the hazardous-area terminal and transferred to the safe area computer or printer. As all components of the weighing system are intrinsically safe, the weighing system is powered by an intrinsically safe power supply. Communicating the weighing signal from Zone 1/21, Division 1 to the safe area requires energy-limiting devices referred to as intrinsically safe barriers. These are barriers installed in the safe area that interface with the communication device to prevent excess energy from a fault occurring on the safe side from crossing over to the hazardous area.

Under normal operating conditions, intrinsically safe barriers have no arcing or heat-producing contacts, and if specially marked, they can be installed in Zone 2/22, Division 2. In fault conditions, the barriers limit voltage and current to levels that are not sufficient to ignite the hazardous atmosphere.

These barriers consist of three components:

• The zener diode, which limits the voltage to a value referred to as an open circuit voltage (VOC)
• A resistor, which limits the current to a specific value known as a short circuit current (ISC)
• The fuse limits the maximum current that can flow through the diodes. When the current flows through a diode, the fuse will blow. This interrupts the circuit and prevents the diode from failing. As a result, the excess voltage does not reach the hazardous area.

Communication to the safe area requires a safety analysis based on a comparison of intrinsically safe device entity parameters with the hazardous-area approved safety barrier entity parameters.

This determines if the peripheral device is safe for connection to the intrinsically safe equipment. Entity param­eters are usually found on the control drawing of the intrinsically safe device supplied by the manufacturer or on the Examination Certificate (see Table 18). There are some differences in the abbreviations of US Class/ Division and Europe Zone classification. The US Class / Division system uses the abbreviation of entity pa­rameters like VOC, ISC, and CA. In Europe, the safety parameters are referred to as VO, IO, CO, etc.

Figure 26 shows an example of a basic system with direct communication to a PC in a safe area. The intrinsi­cally safe RS232 communication line is simply passing through the grounded intrinsically safe barrier before connecting to the intrinsically safe weighing terminal installed in the hazardous area Zone 1/21, Division 1.

 Benefits:

• Simple application through intrinsically
• safe RS232 interface
• Cost efficiency
• Precise signal response
• Small barrier footprint

 Limitations:

• Short signal distance (15 – 20 m maximum)
• Safety barrier requires a securely implemented earthing system

To ensure safe system set-up and function, terminal and safety-barrier entity parameters must be compared. Tables 19 and 20 show this comparison, reviewing both active and passive signals.

Many different safety barriers are available. Take time to know the technical details and entity parameters of any chosen safety barrier solution to ensure safety.

This article is from Chapter 7 of the METTLER TOLEDO Hazardous Area. Download the guide to learn more.