Consider Top Mounting DP Flowmeters in Steam Applications

by | Aug 23, 2010 | Measurement Instrumentation

Jim Cahill

Chief Blogger, Editor

I connected with Emerson’s Don Verhaagen and Emily Vinella and got my hands on a whitepaper on DP (differential pressure) flowmeter installations in steam applications. The whitepaper, Top Mount Installation for DP Flowmeters in Steam Service, describes conditions where top mounting these devices helps to reduce installation costs, eliminates the need and expense for heat tracing, improves accuracy at low range DP measurements, and increases overall reliability.

The traditional installation method is below-pipe mounting, 45-75 degrees below horizontal. This installation method is used to prevent the heat from the process from overheating and inducing transmitter failures. This installation method creates columns of condensate or “wet legs” in the impulse lines running from the process pipe to the pressure sensor ports on the flowmeter.

The whitepaper notes the issues associated with the traditional approach. On startup, the wet legs must be manually filled or else flow measurement errors will occur until the steam condensate refills the legs. Workarounds such as low-flow cutoff configurations in the transmitter are required to avoid these measurement errors that can be 1% of full scale at low range measurements.

Another issue for cold weather installations is the need for steam or electrical heat tracing on the impulse lines to prevent freezing. These are costly to install and difficult to maintain. Also, in corrosive, superheated steam applications, corrosion at the sensing element interface can result.

The whitepaper shares guidance for top mounting these devices by Rosemount DP flowmeter product line and steam application temperature maximums. The Rosemount Annubar products should be mounted 45-75 degrees above horizontal to allow condensate to efficiently drain back into the process pipe. These products are rated for steam applications up to 400degF/205degC.

This compares with 500degF/315degC for those mounted below the process piping. The tradeoff in temperature maximum is improved operation and maintenance. Manifolds and transmitter remote mount options increase the temperature maximums, all the way up to 1200degF/649degC with the Rosemount 585 Severe Service Annubar.

The whitepaper provides similar recommendations on mounting angles and temperature maximums with the 405 Compact Orifice Flowmeter. The 405 can be top-mounted anywhere north of 45 degrees from horizontal, even up to 90 degrees.

The whitepaper authors recommend the top-mount installation method for DP flowmeters used in steam applications where possible to gain the cost savings, improved measurement, and increased reliability.

I recommend giving this whitepaper a read if your plant has steam flow measurement applications.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.