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The measurement and control of fluids in small lines and batch processes can be just as critical as those found in larger systems. Accurate measurement of air, process gas, steam or liquid is essential in achieving the desired quality whilst controlling material cost.
Flow Sensor Selection
When determining which flow sensor technology is best for your small line or batch process, one of the first considerations is always the process media: air, gas, steam or liquid. Some technologies are better suited to specific fluids than others, and attempting to save costs on instrumentation can be painful if a product is misapplied. The major technologies within the process industry are:
• Coriolis (Mass)
• Differential Pressure & Multivariable (Mass)
• Electro Magnetic
• Positive Displacement
• Thermal (Mass)
• Turbine
• Ultrasonic
• Variable Area
• Vortex Shedding
Depending upon the media and your application requirements, each of these technologies will present advantages and disadvantages that need to be considered. Measurement range, process temperatures and pressures, chemical compatibility, line sizes, accuracy requirements, etc. are all common factors. Additional constraints may come from your process layout (adequate straight-run conditions), environment, maintenance schedules and ROI, which can limit you to one or two options that meet your requirements.
Typical Small Line or Batch Process Applications
Analyzer Sampling.
Taking samples of a product to ensure product quality and consistency is a common practice. Typically, these sampling lines are smaller in size and can be prone to fouling depending upon the media. In order to ensure the analyzer is receiving a sufficient sample, selection of a switch or meter to validate flow is important. The sensitivity of some thermal flow switches allows the monitoring of low flow rates that are not achievable with technologies such as differential pressure or mechanical vanes switches.
Monitoring injected additives. Injecting additives into a batch process is another area of concern. Too much or too little can result in product that does not meet specification, resulting in material waste. Thermal devices provide a high level of accuracy and repeatability, have negligible pressure drop, are suitable for high process pressures and can monitor extremely low flow conditions.
Improving burner efficiency. When the addition of heat to a process is required, inefficiencies related to burners can add to energy costs. An inadequate mixture of air and fuel can result in poor combustion. Selection of an instrument that can accurately measure these variables over a wide flow range can improve the efficiency from batch to batch under varying demands. Maintaining reactor feed stock. Many chemical processes require hydrogen and nitrogen feedstock for production. Flow meters installed on these lines will allow you to ensure adequate feed during peak demands and note reduced flows that are indicative of the end of the batching. This often requires an instrument capable of high turndown ratios in order to capture the minimum and maximum process flow conditions.
Controlling tank blanketing. The use of a nitrogen blanket on chemical storage tanks is a common practice used to avoid the emission of toxic or combustible fumes. A low pressure blanket of inert gas, usually nitrogen, is injected into the vapor space of a storage vessel. A flow meter is often utilized to monitor and control the low flow rates of nitrogen into the tank as the level of the liquid changes. In addition, single point switches may be used to monitor the condition of relief valves in order to indicate an unsettled condition or potential leak.
Vapor recovery measurement. Environmental demands result in the need to recover vapors that result from batch processing. The measurement of these off-gases requires a suitable technology for varying low flow rates. Selection of an instrument can be dependent upon the gas composition and a requirement for larger turndown rations. Thermal technology is well adapted to this by providing a mass flow output for the gas being measured with common turndown capabilities of 100:1.
Matching Flow Instruments to the Process
The selection of any flow measurement device goes beyond just the process conditions for the media being monitored. Other factors include, but are not limited to:
• Accuracy
• Line size
• Calibration requirements
• Installation environment
• Physical installation constraints
• Maintenance requirements
• Instrument life
• Life-cycle costs
It is important to determine the proper flow range, accuracy and repeatability of any flow meter or switch to ensure it meets your process requirements. For example, a standard small line thermal flow meter measures gas flows in line sizes from 6 to 51mm over a wide range from as little as 0.01 NCMH with accuracies from +/- 1% to 2% of reading and 0.5% of full scale. Switches have the capability of detecting flow rates as low as 0.15 cc/sec with +/- 0.5% of reading.
There are times that calibrating a meter in the actual process media will improve your measuring accuracy, especially in gases. There can be significant differences in calibration integrity when a theoretical air equivalency method is used in place of an actual gas, resulting in costly post installation conditions. Only a handful of flow sensor manufacturers maintain their own flow calibration laboratories in order to provide more exact calibrations to specific gases and multiple gas compositions to achieve the most accurate flow measurement for your process.
The wetted materials of construction and design principle can play a major role in chemical compatibility and service life of any instrument. Acidic or caustic fluids can rapidly erode components of mechanical flow elements, rendering them inoperable. This can be very critical for switches that may only operate in unbalanced process conditions. In addition to the process media, do not overlook compatibility with any cleaning fluids that may be utilized within the lines. Instruments that utilize no moving parts, expensive chemical seals and chemically compatible materials such as Hastelloy, Monel and Tantalum can significantly improve the service life and provide lower initial costs.
The matching of the instrument enclosure to environmental requirements is also important. Fiberglass or plastic housings may be suitable for indoor, clean, climate controlled environments. However, in plant conditions that may be dirty, have airborne particles or gases, exposure to extreme ambient temperatures, and subject to potentially hazardous fluids, an enclosure with the appropriately rated and certified Ex/IP should be utilized to ensure proper operation, reduce maintenance, and increase safety. Physical installation requirements also affect the packaging of the instrument you select for your process. Consider how much space is available, accessibility for maintenance, high ambient temperatures and vibration, and whether there are devices that create flow disturbances at the point of metering such as elbows and valves. Most flow instruments require an adequate amount of upstream and downstream straight pipe run to ensure a well developed flow profile for measurement accuracy.
Some manufacturers, such as FCI, are designing new flow meters for small line sizes that integrate flow conditioning devices into the product. A flow conditioner will reduce swirl and develop a repeatable velocity flow profile, allowing for a reduction in straight-run by up to 70% without any degradation in accuracy or additional pressure drops in the line. In many processes where there is insufficient real estate, the space-savings achieved with a reduction in piping materials and labor, in addition to improved accuracy, can result in significant total cost savings.
Optimizing Your Process
The careful selection of flow instrumentation can result in improvements to product quality, consistency, throughput and the lowest installed cost. The value of raw materials, material waste, energy, and even end-customer satisfaction can all be influenced by the precise measurement of your process air, gas, steam and liquid. By Art Womack, Sr. Applications Engineer..
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