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An Engineers Guide to the Instrument Selection Process

Conventional Liquid Level Gauges and Modern Instrumentation



Liquid Level Instrumentation - a topic often relegated to the last in project order, has the potential to inflict high cost due to disruption in project commissioning, manufacturing, or maintenance activity. Many technically strong engineers often underestimate complexity in the selection of type and specifications of the instruments for an application, may it be modern state-of-the-art instruments or conventional Instruments/gauges.


With the advent of new non-intrusive #ultrasonic, radar and gamma-ray technologies, it was thought that conventional liquid level instruments like #tubular, #reflex – transparent, or #magnetic indicators would close shops and would get replaced in their entirety. Undoubtedly, there are numerous advantages of modern instruments, particularly when it comes to communicating with DCS / SCADA in the control room and automation. However, as these state-of-the-art instruments started being deployed, the application-specific limitations were noticed.


The application-specific limitations to look out for:

  • High cost,

  • Complex specifications,

  • The requirement of skilled/ qualified maintenance staff with the entailing cost,

  • Unsuitability for very high temperatures high pressure or high vacuum,

  • The dielectric constant of liquids,

  • Presence of suspended solids,

  • Deposition on sensing surface,

  • Condensation on the probe, solidification of vapors,

  • Availability of corrosion-resistant materials of construction and cost implications,

  • Human safety concerns in the case of Gamma-ray technology.


The high cost of Intrinsic safety or Ex-proof Instruments was compounded by the equally high cost of safe communication hardware like double compression cable glands, armored cables, junction boxes necessary for hazardous/ flammable liquids also added to cost. Conventional instruments continued to remain in vogue because simpler applications involving water, non-flammable edible/ mineral oils or comparable liquids could not justify the high costs.


Need for redundancy in Hazardous Applications - One of the important drawbacks of Radar / Ultrasonic / PT-DPT / RF-Capacitance type instruments noticed was - for hazardous processes. directly exposed operators are unwilling to take chance with failure of power supply, loose contact, or failure of the display, since it meant possible uncontrolled temp/ pressure built up not getting detected and posing an extremely high risk to human lives. The cost of an additional instrument or redundancy is also prohibitively high. Most engineers preferred to have simple mechanical gauges, not only for calibration but also as regular standby for cross-checking, to be doubly sure.


In one such case, operators refused to work when modern Torque Tube type Instrument failed to indicate. The engineer had to get an additional Magnetic Indicator which would work even during total power failure/ display failure.


Instrument / Process engineers are often not given sufficient time to interact, collect process data, select an appropriate instruments, and finalize specification sheets keeping in mind all aspects of individual applications discussed above. This results in high costs often due to wrong selection of instruments/ technology or mistakes in understanding properties of chemicals that have an impact on finalizing specification and rush orders to replace the wrong selection. In various sectors like Heavy / Fine Chemicals, Pharmaceuticals, Petrochemicals, Fertilizer, Agrochemicals, Dyes & Intermediates, Adhesives, Textile & Leather Chemicals, the range of #liquids and process blends we come across is mind-boggling and so is the complexity of #selectinginstruments.


In subsequent chapters, we shall discuss, what are different factors and how they affect the instrument selection.


R K Dutt Concerns, the pioneers in manufacturing of conventional gauges are now also Chanel Partners to Siemens India Ltd. We are now able to offer both conventional and modern instruments under one roof, giving the user best blend of most appropriate selection. The qualified and trained Application Engineers guided by experienced Senior Managers with over 100 years of cumulative experience are more than willing to offer free consultation on selection of these instruments or finalizing specifications.


The article shall underline the complexities of selection on the backdrop of historical developments and help engineers have some insight in to selection process, before they request free consultation.


Simple low-risk applications: Low cost, Noncorrosive, non-flammable, non-toxic, mostly open atm. Or pressures < 8 bar, temperature mostly ambient or < 100 liquids, which are not injurious to humans and stored in small vessels.


Liquids: Water, Potable, Utility, BFW, Edible oils, Lube Oils, Dil. Acids, Alkalis, DoP, Glucose solution, Dextrose solution, Fruit Juices, Aerated drinks, milk, effluent,


Level Instruments: Mechanical - Tubular Gauges, Reflex / Transparent Gauges, Magnetic Indicators, Float and Board Indicators (for open tanks only). Electronics – PT/DPT, RF Capacitance, Ultrasonic, Radar, #Displacer Transmitters


Dimensional Specs: Side Mounted - 20-50 NB Flanged connections x up to 3 m CC Distance. Top Mounted: 20-50 NB Float & Board for taller Storage Vessels, Min 100 mm flange for Magnetic Float Indicators x 3.5 m depths.


Materials of Construction: PP/PTFE Lined for acidic liquids and CS, SS 304/316 for others

Other Factors: Cost of liquids (slight leakage may cost hundreds of Rupees), Dielectric Constant, Toxicity, Turbidity, Colour, Consistency, Suspended Particles/slurry, Vessel Dimensions


Accessories: Add-on Level #switches with step-less adjustment of set points without Electrical Supply / process shut down. Add-on #Transmitters- Magnetic Reed Switch and #Resistors type.


Contact us for a free consultation with us.



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