Recommended Mounting for Process Steam Measurement

Differential Pressure (DP) flow meters have been used almost as long as flow has been measured.  An obstruction in a process, like an orifice plate, will create a differential pressure that can be correlated to flow (the square of the flow to be exact).  There has been a recent increase in the demand for steam flow measurement.  Process, industrial and commercial business want to know where there steam is going.  It is expensive to produce and the proper end-user needs to pay for it.  So, where do I install my meter on steam?

There has been a lot of discussion regarding the installation of DP flow meters for steam.  Impulse lines have commonly been used to remotely mount the transmitter and protect it from the process temperature.  It is a best practice to install a liquid flow meter on the bottom to ensure the pipe is full and vent gas.  In the same way, a gas flow meter should be on top to eliminate any liquid.  While steam is a gas, it was considered a best practice to mount a steam flow meter on the bottom.  That best practice has now changed.  A steam measurement should be made with the transmitter on top.

When impulse lines were used making a steam measurement took advantage of the condensate in the process.  Keeping the impulse lines full of condensate protected the instrument from the process temperature.  They also maintained a balance of head pressure on both the high and low sides of the sensor.  With the creation of integrated flow meters (the transmitter is directly connected to the primary element) the standard convention was adopted and the instrument was mounted on the bottom.  It was discovered, however, that condensate directly on the sensor could have a negative impact, especially in demand only applications.

There are a number of processes that are not used continually throughout the year.  Steam tracing is one of those applications.  As the weather warms, steam is no longer used to maintain equipment temperature.  Residual condensate would remain in the line and on the process sensor.  If the weather were to cool down, the condensate would freeze including the condensate on the transmitter.  This can, and usually does, damage the sensor.  While this is great repair business for the transmitter manufacturer it costs money that doesn’t need to be spent.

Transmitters are capable of handing higher temperatures.  While an integrated flow meter is mounted directly in a process, the primary element will act as a heat sink.  In high pressure steam applications, an extension is available to keep the transmitter as close to the process as possible, which is another best practice.  Integrated flow meters are less expensive to purchase, install and maintain.  They are more accurate and are available as a multivariable device.

Steam applications are, once again, considered a gas which means you should get your instrument to the top of your process.

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