With demand for sugar products growing worldwide, production is rising at many mills including one in Latin America, which relies on steam heat as part of the process that converts raw sugar cane into molasses, sugar and related products.
The mill’s production team was tasked recently with transferring excess steam from its production system to a nearby bio-ethanol plant operated by the same parent company.
A typical sugar mill first extracts raw sugar from the cane, and then processes it further for a variety of consumer and commercial uses. The mill cleans, cuts, chops and shreds the cane and then adds water before heavy rollers crush out the sucrose juice. The liquid is then mixed with other substances prior to further refinement in several complex steps that eventually create granulated sugar or other products.
The left-over pulverised sugar cane material is burned to fuel the mill’s process steam boilers, which send steam to large turbines that create co-generation electric power. The co-gen electric power is then used in the plant and/or exported to the power grid for use by others. Surplus plant material also can be incorporated in feed for livestock and paper product production as well.
The challenge
The process engineers at the sugar mill needed a new flowmeter for steam custody transfer purposes. For cost accounting purposes, they were required to measure the steam transferred via a 400 mm line from the mill to a sister company’s bio-ethanol plant. The mill’s process engineers were looking for a reliable and accurate steam flow measurement solution without routine maintenance requirements for operation in a high heat, high humidity and dirty plant environment.
Choosing a new flowmeter can be a complex and time-consuming process. There are numerous flowmeter measurement technologies and not all of them are equally suitable for measuring all media: steam, gas or liquids. A review of the plant’s process requirements, however, will generally narrow the field of candidates if the following criteria are included:
• Fluid to be measured: steam at the sugar mill.
• Accuracy requirement: custody transfer for plant-to-plant cogeneration.
• Reliability or repeatability: potentially hazardous environment.
• Environment: high pressure, high temperature, high humidity
• Installation: straightrun requirements for accurate measurement.
• Maintenance: no cleaning or recalibration.
• Long life: 25+ years.
• Price: low life-cycle cost.
In this particular application, the sugar mill’s high pressure operating environment was a cause for concern with some flow sensing technologies. Those technologies that rely on moving turbines or plates can be problematic in high pressure (steam) applications. They can even pose a serious safety hazard if a piece should break off during operation and pass through the line into other equipment.
Variable line pressures occur when the steam flow is irregular due to seasonal high/low climate temperatures or changes in steam production relating to a drop or increase in feedstock. The result can be depressurisation or a pressure spike that causes stress to the mechanical parts. These types of conditions eventually require costly maintenance and recalibration with some flow sensing technologies.
The solution
After the process engineers at the sugar mill reviewed a number of flow sensor technologies, they contacted the flow measurement applications team at McCrometer. The company has been supplying flowmeters that rely on differential pressure, electromagnetic and propeller technologies for over 55 years in the process and other industries.
The team at McCrometer quickly recognised the harsh operating environment and low maintenance requirements, which led them to suggest the V-Cone flowmeter for this application.
The V-Cone meter not only provides precision flow measurement in steam, liquid or gas, but requires virtually no maintenance in demanding processes where high pressure conditions exist. It is ideal for use in a wide range of industries from food and beverage, pharmaceutical, pulp and paper, oil and gas, and water and wastewater treatment.
The instrument is based on advanced differential pressure technology requiring no moving parts and has no spaces that may clog during use. Built-in flow conditioning allows it to achieve accuracy of 0,5% and repeatability of 0,1% (high reliability) with straight pipe runs of only 0-3 pipe diameters upstream and 0-1 pipe diameters downstream.
The smaller footprint requires up to 70 percent less straight pipe without being affected by flow disturbing equipment up or down stream. Its self-conditioning design allows the sugar mill process team to place the flowmeter exactly where it’s needed without the costly addition of extra pipe, external flow conditioners or complicated space-consuming layouts.
The V-Cone flowmeter measures fluid flow by utilising the conservation of energy theory, which basically states that in a closed system, energy can be neither gained nor lost. According to the PV=nRT equation, pressure multiplied by volume equals temperature while ‘n’ and ‘R’ are constants. Imposing a volume change within the pipeline, therefore, results in a differential pressure drop that can be measured directly.
The V-Cone places a ‘V-shaped’ conical intrusion centrally in the line, redirecting the fluid to the outside of the pipe and around the cone. One pressure sensing tap located upstream from the cone measures static pressure while another pressure sensing tap measures the low pressure created by the cone on the downstream face of the cone itself.
This pressure difference is incorporated into a derivation of the Bernoulli equation to determine fluid flow. As the fluid moves past the cone, very short vortices are formed that result in a low amplitude, high frequency signal optimal for excellent signal stability. The meter maintains 0,5 percent accuracy and 0,1 percent repeatability over a 10 to 1 turndown, and the cone conditions the fluid such that there is relatively low permanent head loss.
The low permanent head-loss results from the shape of the cone itself, which minimises the energy losses commonly caused by areas of low flow, cavitation and erratic flows. Each V-Cone flowmeter is sized to meet desired application requirements and may be specifically designed to have high or low head loss. Regardless, the overall energy consumed by the V-Cone flowmeter is minimised because of its inherent characteristics.
The rugged device measures clean, abrasive, dirty, and particle-laden fluids over a wide range of Reynolds numbers without wear or clogging concerns, resulting in a standard 25-year operating life with generally no need for maintenance. Reynolds numbers are a measure of whether flow is laminar or turbulent.
The turbulent vortices condition the fluid flow to be homogeneously distributed and extremely stable. It is this turbulent flow that actually protects the cone as well as the surrounding pipe. It forms a boundary layer against the pipe wall and cone protecting it from particle impingement, which can cause deterioration or build-up on the surfaces.
Conclusion
The first V-Cone was installed in a 400 mm line and has been operating successfully since 2005. The owners of the sugar mill are expanding the distillery plant where they will have the same application for a larger 750 mm line. Since the plant found the original V-Cone flowmeter reliable, repeatable, and accurate with no need for recalibration or adjustments, purchasing a new V-Cone for the larger was an easy choice.
For more information contact Stuart Brown, UIC Instrumentation, +27 (0)31 468 2561, [email protected], www.uic.co.za
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