The importance of flowmeter selection to improve process safety
12 June 2019
This article will illustrate the importance of selecting the correct flowmeter to improve process safety. Additional benefits will be outlined.
To some extent this simply involves picking the right meter for the job. The first 6 paragraphs do that. It is the last 5 that can really influence process safety and aren’t necessarily considered.
Flow rate range
The first variable to consider is the flow rate, or rather the range of flow rates over which the device must operate. This doesn’t just include the ‘normal’ flow rate but also start-up and shut-down and maybe some emergency conditions as well. These flow rates should also encompass night and day variations, summer and winter variations and predicted plant and efficiency changes in the years after first installation. The meter should retain the desired accuracy and repeatability properties where required through the range.
Often the accuracy takes a place behind repeatability and reliability. It must be considered where process quality is affected. Does it vary with pressure or temperature or viscosity or another variable?
Often the process output quality will be improved if the process is more repeatable. Sometimes the flowmeter will be the least repeatable of the process variables. Matching the process requirements will go beyond just flow rate considerations.
This may be a simple subject – i.e. pick a rating that’s greater than the pump capacity. However, the piping design might lend itself to water hammer where a travelling ‘slug’ of fluid can magnify the pressure by a factor of 10. Check your pipework – there won’t be many meters that will contain that increase without a fracture or seal weep. Make sure the connection type is appropriate to fluid and site standards. Some fluids are just not suitable for thread connections. The sealant used for sealing threads may be inadequate, as may be flange gaskets. Some installations will have specifications for pipework, valves, instrumentation etc depending on service and will specify in some detail acceptable materials, techniques, seals and piping systems.
Hazardous Area level
Perhaps this is the first subject that springs to mind when ‘process safety’ is invoked. Normally, the instrument supplier relies on data from the customer or the customer’s H&S representative with regards to the zone level in the installation area. An experienced supplier will be able to interpret this with a suitable offering to meet this or exceed this. More importantly, they can realise that the level discussed doesn’t seem appropriate and probe a little further.
Lastly, we’ll discuss materials of construction of the meter. Usually, these are well known with selection appropriate to chemical compatibility at process temperatures. Due diligence should include the range of Design Temperatures and include cleaning regimes. Seals should be chosen based on compatibility first and then be selected on re-usability and resistance to the range of chemicals expected during the process life. Consider that corrosion might be controlled but to a fraction of a millimetre per year but is that accelerated by temperature or concentration change or affects the most delicate part of the device first. Some plastics and seals have changed properties in contact with some fluids. The material manufacturer may consider that compatibility is ‘good’ but what exactly does that mean?
To get the best out of your system some output types will be better than others. For example, if the process calls for a batch of liquid then a pulse output will be simpler to process than integrating a flow rate signal or, arguably, some digital communications protocols. In another example, where the flow rate is monitored for a continuous process then monitoring the flowmeters analogue output maybe better than observing a pulse train with slow update time. If there is a safety issue when the flow rate or total exceeds a value, then the tolerance or selection of the alarm value will be critical.
Is maintenance required to keep the meter operating correctly – does the meter have to be returned to the factory for an overhaul, or can it be fixed locally by a trained engineer? Is there a method for self-diagnosis giving confidence to the user that all is well? These questions are best asked early in the specification process to improve process safety.
Ease of installation
To achieve accurate results then, undoubtedly, the manufacturers manual should be read, and instructions adhered to. From a safety perspective, some meter types will misread if poorly installed. Some will require set-up or calibration in-situ and regular
This could be based on a given Mean Time Between Failure figure. More likely, it depends on the cost of the process going wrong. Where a hazardous output may occur if the flowmeter fails then perhaps two meters should be installed, or two different types of meter, or two different types of variable measurement. In the author’s experience, one pharmaceutical client had a £250,000 batch of drugs go bad all for the sake of installing some additional measurement apart from a £500 flowmeter. It’s common in subsea design to have two seals of different types to protect a critical area. Not only will one hold the pressure on its own but, if an unforeseen problem occurs that takes out one of the seal types, there’s always a backup. An expert will be able to select a meter that fails safe according to the factory requirements and has a Safety Integrity Level (SIL) as needed.
This is a double-edged sword. It boils down to finding a confidence level. Additionally, if removing a meter or device from a line creates further handling issues or downtime then is the recalibration worth it. Sometimes this information can only be built up over time. A flowmeter expert should be able to recommend the right meter for each application and provide further advice on suitable calibration periods and techniques. Some ‘solid state’ meter types have in-built diagnostics and provide a calibration commentary.
Get advice from experts. They will be aware of the many forms a flowmeter can take and their suitability for the many different process types.
About the author
Charles Wemyss is CEO of UK-based Litre Meter Ltd. His main expertise is in oil & gas flow measurement and chemical injection measurement, positive displacement meters and coriolis mass meters. Litre Meter manufactures rotary piston positive displacement flow meters and pelton wheel turbine flowmeters. In the UK, Litre Meter also distributes a wide range of flowmeters from other international manufacturers.
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