Refinery turns to thermal mass flow meter for nitrogen tank blanketing

Tuesday, 04 August, 2015 | Supplied by: AMS Instrumentation & Calibration Pty Ltd


An independent refiner in the US Midwest recently approved a project to improve the safety of its plant and reduce its costs for nitrogen gas used for tank blanketing. The company operates refineries in multiple states, with a combined crude oil processing capacity of more than 500,000 barrels per day.

Nitrogen blanketing is a practice commonly used in the chemical and petroleum refining industries to reduce the hazards associated with flammable liquids. It is the process of applying nitrogen gas to the vapour space of a tank or vessel, which minimises the possibility of an explosion or fire by reducing the oxygen content or the concentration of flammable or explosive vapours in a tank or vessel.

Blanketing also helps decrease product evaporation and protects the tank from structural corrosion damage caused by air and moisture. Nitrogen usage varies based on the size of the tanks and vessels used in the production, transfer, transportation and packaging of the product. There are three common methods of blanketing: continuous purge, pressure control and concentration control. The continuous purge method employs a constant flow of nitrogen. This approach is simple, but nitrogen consumption is high. The pressure control and concentration control methods are more costly to implement and rely on the pressure in the tank or the concentration of the oxygen to initiate the flow of nitrogen, but these methods improve overall safety and the efficiency of the process.

The refiner’s plant team needed to measure the flow rate of nitrogen more accurately in its tank blanketing applications. This would maximise safety while improving efficiency; lower the plant’s nitrogen consumption and costs; and eliminate any unexpected supply shortages.

The amount of nitrogen pumped into or released out of the tank’s vapour space is controlled by a predetermined pressure set point. As product is pumped from the tank, the vapours inside expand and the pressure falls below the set point; more nitrogen is then introduced. As the vessel is filled, the vapours begin to compress and the nitrogen is released and usually sent to a vapour recovery system.

Product falling and rising can create static electricity, so the amount of nitrogen in this vapour space is an extremely important safety factor. The refiner needed a flow meter that could provide a mass flow output and measure a low flow rate of 36 to 600 SCFH (1 to 17 NCMH) at 70°C with a pressure maintained at 60 psig (5 bar) in a 1″ schedule 80 pipe with limited straight run. With these process requirements, the plant team had many challenges to overcome.

After consulting with the applications team at Fluid Components International (FCI), the refinery’s process engineers selected the Model ST100L thermal dispersion air/gas in-line mass flow meter and the Vortab flow conditioner.

The Vortab flow conditioner provided a low pressure-loss solution for flow profile irregularities produced by elbows, valves and other disturbances that are commonly present when sufficient straight run is not available to generate the necessary flow profile. Vortab flow conditioners combine swirl removal technology with a mixing process to achieve thorough and efficient flow conditioning. Tabs are located strategically within the conditioner to promote rapid mixing that creates a uniform flow profile for proper meter flow measurement by eliminating swirl and distortion or any other profile irregularities that could be present through the transitional flow range (Reynolds number 1000-4000).

The ST100L meter was factory calibrated in FCI’s NIST certified laboratory for nitrogen service. Flow meters calibrated using the actual gas to be measured provide high accuracy, as opposed to a precalculated air equivalent methodology that often fails to transfer accurately in real-world installations.

Once installed, the flow meter provided an accurate, repeatable and reliable output necessary for the tank blanketing valve to operate as designed and provide the safety and cost savings expected in this application. Some tank blanketing applications have larger lines, and FCI has a solution for this field challenge too. The ST100 insertion style air/gas meter has the same accuracy specifications as the ST100L, but allows the end user the capability to insert the probe directly into the line.

Multiple ST100L flow meters have now been installed at the Midwest refinery for its nitrogen tank blanketing application. The refiner has reduced its nitrogen consumption considerably, which in turn reduced its overall operating costs. According to the plant engineering team, the flow meters are working better than expected. The ST100L was a perfect fit with application conditions because of the installation challenges and process requirements.

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