Not a question of taste – Position of valves in flowmeters!

21/06/2010

  • Position of control valve in gas flowmeters crucial
  • With the “valve at the outlet” version, the loss in pressure caused does not affect measurement
  • The position of the control valve in flowmeters can be crucial when measuring gas flow, regardless of whether the valves are built into the device or separate. If a valve is used to reduce the flow at the inlet of the measuring device, the pressure in the measuring tube behind the valve is reduced. Because gases are compressible, this change in operating pressure results in a significant measurement error.

    Krohne DK 800 flowmeter featuring built-in control valve at the inlet or outlet

    For example, a pressure reduction from 5 to 4 bars of absolute pressure in the measuring tube of a variable area flowmeter results in a measurement error of almost 12 %. Therefore, more or less gas flows unnoticed through the device than is displayed. However, when the valve is positioned at the outlet, the drop in pressure does not occur until after the measuring tube and thus does not affect measurement.

    For this reason, KROHNE offers the “control valve at the outlet” version for its DK series. The DK series consists of a range of low volume flowmeters with glass or metal cones used in many different applications ranging from inlet pipes to gas chromatography and gas inertisation measurements in chemistry to the measurement of gases used for anaesthesia in hospitals.

    In some instances, the control valve can be in front of the measuring device, for example when a gas is measured against a constant pressure (e.g. atmospheric pressure). In this case, the measuring device is not calibrated based on the primary pressure but instead based on the constant output pressure prevailing in the measuring tube. The position of the valve is not crucial when measuring liquids, as the compressibility of liquids is negligible even at high pressures.


    Final control elements and other stories

    12/05/2010


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    Douglas Control  & Automation

    Loose Insert: Metrology Systems & Services

    The April/May 2010 issue of Read-out, Ireland’s journal of instrumentation, control and automation, highlights final control elements.

    Steriflo’s Mark 96 pressure regulator, marketed by Manotherm, is used in sanitary applications. Emersons Fieldvue digital valve controllers are used in an Australian chemical plant “saving us thousands of pounds,” according to the instrument technician on the site. Also featured is Festo’s range of ultra-fast jet valves and Tyco’s EBCO valves to provide full flow replenishment to storage tanks in, for example high-rise buildings.

    The front page article gave details of the new marketing strategy of Irish company Biotector Analytical, who have appointed Hach as exclusive distributor in the US, Canada, Mexico, Brazil and Europe for their range of on-line liquid analysers. Another company with a presence on the North American continent, Qumas, has won the Deloite Best Managed Company Award. This company is a provider of compliance solutions.

    There is a report on the Ireland Section of the International Society of Automation visit to the Blanchardstown Institute of Technology where a large assembly of first and second year students participated in a talk on combined heat and power. These students are hoping to qualify with a BSc in Sustainable Electrical and Control Technology. The purpose of this course is “to equip students with the skills and knowledge to embark upon a rewarding career in sustainable engineering within the construction and manufacturing sectors.”

    The National Instruments scheme to support micro and SMEs in embedded development is discussed. This is in the form of training and grants of up to nearly €30,000 in software, support and training. “National Instruments…is committed to supporting innovation!”

    John McAuliffe, in the InSide Front article, “Cracking the Safety Code“, discusses the poractical applications of the new European Machinery Directive (SI 407/08). that came into force in January. John is Managing Director of Pilz Ireland.

    Among the new products highlightes in this issue is Yokogawa’s DXAdvanced DAQSTATION range, Phoenix Contact’s PSI-Bluetooth ProfiBus set, E+H’s Liquiphant M density meter and Blue-White’s new junction box and connector arrangement on their Flex-Pro A3 peri-pump.

    Read-out is published every two months and distributed throughout Ireland. Advertising rates, which have maintained their 2004 levels are on the website in Euro, Pounds Sterling and US Dollars.

    The next issue for June/July will concentrate on Flow measurement & Control.

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