Gas and Steam flow metering!
When we are engaged in monitoring consumption or generation of gaseous materials, many people in management, or even among technical people, have the natural impulse of resorting to metering of the substance for purposes of budget monitoring and control. This in effect, results to either the purchase of sophisticated measuring facility with some computerized calculating functions for changes in the parameters on line, or inaccurate readings given by supposedly monitoring instruments that are put on line, but does not have the capability to measure instantaneous changes in parameters, like temperature and pressure.
We know for a fact that when temperature fluctuates, or when pressure does have changes, the fluid density also changes. Ordinary orifice meters and rotating-meters or displacement meters can measure volume, and mass based on fixed conditions as assumptions. These meters, in the absence of pressure and temperature corrections therefore are useful only for monitoring mass flow at fixed conditions. They become less useful and inaccurate with frequent changes in temperature and pressure over long periods , and can create confusion in the interpretation of metered data.
It is not easy to explain this to some management people. In this regard, other means of measurement have to be done in cases, where computerized systems for temperature-pressure corrections are not available. Usually, a competent operations man would measure fluid usage by indirect means, like measuring the tare-weight of containers or tare liquid level of tanks holding the gases or the water usage in the case of steam systems. These methods are far more reliable and accurate, than simply putting in the pipelines a non-correction capable meter.
Understandably, these appear crude to top management, but technically, it is more accurate. There is no sense in putting on line, a beautiful meter that will just create a confusion and a lot of inaccuracy and subsequent controversy. This explains, why you seldom see a metering device in a compressed air system, and a steam system output pipeline. If they are there, they are used more for short period experiments and tactical observation purposes, wherein temperature and pressures are monitored to be constant, but seldom used as an integrating system for a long period of measurement, unless corrective computational capability is also put in place. So, the next time you see a non-correction capable metering device in a gas or steam line, be cautious , be aware and knowledgeable of the limitations and manner of their usage or applications. An inaccurate device is no better than a seemingly crude method that provides accurate measurements.
Written by: Sanoy C. Suerte, RME /MBM; Http://www.linkedin.com/in/sannysuerte