Getting steam out of the ground is the first crucial step in the conversion of geothermal steam into electricity. Getting it to the turbine and making the steam flow measurement is likewise important. It is at the turbine or, more precisely, just before the turbine, that many measurements are made. Performance Acceptance Tests (PAT's) are conducted here to ensure that the equipment provided by the turbine manufacturer is performing as specified. The need for accurate steam flow measurement is critical on the main steam line (MSL). Historically, orifice plates, venturies and averaging pitot tubes have been used with greater or lesser success and with associated challenges. Orifice plates typically have high permanent pressure losses and averaging pitot tubes have small holes on their leading edge which can plug. A different solution is needed - one that provides an accurate and repeatable measurement with low maintenance requirements. In recent testing spanning over a year of operation, it was found that an Annubar flowmeter, installed in the top mounted position, provided the low permanent pressure loss, and met the accuracy, repeatability and maintenance requirements of the end user. Typically, in steam applications with Annubar primary elements, conventional wisdom dictates that the Annubar primary element and transmitter be positioned below the pipe. The previously recommended orientation has a number of issues particularly with geothermal steam and its constituents (silica scale, non-condensable gases,) plus the likelihood that pockets of condensate will form and lead to metallurgical issues including stress corrosion cracking, thus shortening the life of the primary element. The installation can be further enhanced with an automated nitrogen purge system that blows nitrogen through the Annubar primary element to ensure the ports remain clear. The MSL flow measurement is critical for regulatory compliance and performance monitoring. Knowing with precision and repeatability the volumetric steam flow rate is critical. The top-mounted Annubar flowmeter with nitrogen purge system, which has been in service for more than a year, has proven to be reliable and has provided precise, repeatable measurement of steam flow.
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