Safe flow meter installation and flow monitoring in hazardous environments
11 April 2023
Flow meters are used in a wide variety of industries, processes and applications and installed in a broad range of environments. Because of this spectrum of variables, flowmeter manufacturers supply detailed technical datasheets and installation instructions for each specific flow measuring device. These documents should be fully reviewed by the design engineer to ensure the chosen meter is fully applicable for the process conditions.
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Suppliers of flow measuring devices will be familiar with most process scenarios that their products are applicable for, as well as common specification and installation errors encountered by their customers. So it is always worthwhile to communicate early with the supplier to ensure the correct meter is chosen and trouble free installation occurs.
General safety considerations for safe flow measurement
Safety covers a wide array of scenarios with liquid flow measurement. Ranging from the requirement for safety critical measurement to installing within an explosive atmosphere.
Key areas of focus for the design engineer will be the process liquid chemistry and physical attributes within the required operation parameters; the required safety operation and accuracy of the measurement; and the environmental conditions the instrument will have to endure. Coupled with all these they should also ensure the practicalities of installation and maintenance are taken into consideration to help the installation engineer who has to fit the meter and ensure its correct operation.
As the specialist for an application process, the design or process engineer will have more specific knowledge of the chemical and physical operation concerns and should therefore liaise with the flow meter manufacturer where possible, highlighting these. Often customers send a basic operation specification and ask for a guarantee from the flow meter manufacturer that the meter is appropriate. This is not always possible however, as the chemicals and process conditions may be outside the capability of the devices in question. For example, chemical compatibility of materials is key to a safe installation, but data relating to the most appropriate material for the conditions in question may not be available. The specialist engineer should always check that the specification from the supplier is correct and most suited to the specific system - choosing the wrong material can be catastrophic to a system install.
Physical conditions must also be designed in. Typically, a customer will know the general operation maximum and minimums, such as temperature and pressure; and will share that with the manufacturer. What should also be shared is the level of cycling of these parameters the equipment is likely to see. Most processes operate at relatively stable conditions, which all instruments are designed to cope with. But if large cycling of pressure and/or temperature is expected, the manufacturer should be consulted as this will reduce the life span of the device and potentially incur catastrophic failures from fatigue or moisture ingress. As a rule, flow meters do not perform well in large cycling conditions, so manufacturers will always request this is minimised where possible.
Once the general process conditions are satisfied, we then come to the environment that the meter is installed in. As already mentioned, temperature and pressure conditions need to be accounted for and environmental conditions will contribute to these. Additional environmental conditions will include UV, water and dust, pressure (e.g. submerged), vibration, electromagnetic and Hazardous or Explosive atmospheres.
Flow monitoring in hazardous environments
Some installations will require ATEX (or similar) compliant equipment. Solutions for accurate flow measurement in hazardous environmental conditions include flowmeters that offer a variety of compliance conditions, such as Explosion Proof, Intrinsically Safe, Encapsulated and Non-Sparking.
In an area where an explosive mixture of vapour or powder can be present either continuously or intermittently, steps must be taken to minimise the risk of an ignition source within this space. There can be no high temperatures, voltages or the capability of generating a static or frictional spark. Operators must comply by wearing suitable anti-static clothing and carry only ATEX approved electrical devices.
If the flow measuring device is to be installed into a system that measures the flow of kerosene for example, it may be deemed to be sited within a hazardous atmosphere and must therefore be proven to be intrinsically safe. Because of the wide variety of scenarios that may produce an environment with an explosive atmosphere there is no one method of installing a flow meter in such an area. Operators working within the area must ensure that correct standards are maintained for the full electrical installation of the ATEX compliant flow meter.
There are additional considerations that can be useful to note when selecting and installing flow measuring equipment for use in a potentially hazardous location.
1. Use a totally mechanical flowmeter
Purely mechanical flowmeters, such as Variable Area flow meters, Oval Gear and Turbine flow measuring devices, remove the requirement for any electrical input into the device, making them suitable for use within an explosive atmosphere. These measurement sensors must still be tested and compliant with safety standards such as ATEX and there may still be temperature and pressure limitations that must be taken into account.
The main restriction is that there is no method to remotely read the flow into an automatic control system. These meters are purely for local display and require manual reading.
2. Use a passive device
Simple passive devices such as switches cannot in themselves induce or generate a spark. These can be used with flowmeters with a pulse output, such as those seen in Turbine and Oval Gear sensors, which use a rotating element with internal magnets. Passive switch devices have carefully designed circuits to ensure there is insufficient energy to discharge a spark. It must be noted that any instrumentation connected to these components must be intrinsically safe and ensure any component failure would not cause another component to overheat.
A simple option for ATEX compliant flow meters for use in an explosive atmosphere is a pulse output flow element based upon using a reed switch operating via the Operator’s intrinsically safe circuit. By employing this design there is no potential for storage or generation of electricity, allowing the flow sensor to safely pass “Simple Apparatus” safety requirements. These units can therefore be used without the need for full ATEX certification.
Where standard mechanical flowmeters use electrical pulse outputs, options of housing materials and active sensors (see point 4) are available in the market to ensure they meet the requirements for safe use in hazardous atmospheres. For example, Stainless Steel, Aluminium, PEEK and Hastelloy housing options can provide a comprehensive range of chemical compatibility to suit almost any application.
3. Use a device with remote electronics installed in a ‘safe’ area
Flowmeters that incorporate remote electronics have the power transmitted through them limited by a barrier at the safe/hazardous environment interface. This restricts the amount of energy that can be transferred into the hazardous location. The benefit of this approach is that standard instrumentation can be used as it can be installed in the safe area.
4. Use a flow meter fitted with an approved active sensor
Neil Hannay, Titan Enterprises Ltd.
An active sensor is a detection device that requires input energy from a source other than that which is being measured. Examples of active sensors include Namur, Hall Effect and Coil switches. In this instance the sensor itself is approved for use in the hazardous area but the power to it must also be controlled.
A fully-certified Namur Proximity Switch is an intrinsically safe sensor specially designed for use in hazardous locations that have explosive gas, dust or fibres present. For example, a Namur switch integral to the flow meter and supplied with an appropriate amplifier can comply with ATEX requirements, enabling the flowmeter to deliver highly accurate flow measurement in such an environment. In addition, the electronics of the flow device would ideally be isolated in safe housing or safe zone.
5. Install your flowmeter in an explosion proof box
In this option the flow meter is placed in a controlled environment container. This ruggedized box is able to contain any explosive event spreading to the ‘outside’ world. This is far from ideal and a relatively expensive method but where no other flow measurement solution is possible, it may be the only answer.
Processes that operate within harsh environmental conditions, such as explosive or corrosive atmospheres, require flow meters to be able to withstand these tough environments whilst maintaining accurate and reliable measurement. It is critical to the safe installation and monitoring, that the specialist engineer provides the detailed parameters and conditions that the flow equipment is required to operate within, to ensure all safety considerations are taken into account. Closely following the installation instructions for the flowmeter will help to mitigate any failures that can lead to costly repairs and process downtime.
About the author:
Neil Hannay is Senior R&D Engineer at Titan Enterprises. He joined the company in 2018 to manage the development of new flow measuring systems. His previous experience over the last 25 years includes project managing industrial research and development, focusing on process devices and processes for gas and liquid systems for the oil and gas, chemical, food, water and pharmaceutical sectors. Neil began his career in water treatment process and equipment design, focusing on flocculation, ozonation and biological treatment processes and devices and then moving to biological and chemical process development in the food and chemical industries. Over the years, Neil has accumulated expertise in gas and liquid flow control and measurement and customer process integration.
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