| Title | Investigation and Application of Flashing Flow Nozzles to Mitigate Geothermal Turbine Life Reduction at Te Mihi Power Station |
|---|---|
| Authors | T. Misa, K. Harwood, C. Morris |
| Year | 2020 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | steam impurities, turbine, flashing nozzle, steam separation, steam scrubbing |
| Abstract | Because flash steam is not completely dry after processing in the separation equipment, pipelines and demisters are used to remove the remaining moisture together with condensation formed during transmission. If they are not fully effective, the result is scaling of turbine inlet nozzles, erosive wear of turbine internals, corrosion of alloy blade paths and rotor forgings. This significantly reduces the economic life of steam turbines. This issue of poor steam quality and purity is common to many geothermal plants worldwide, and prevalent in stations with separators nearby. Investigation determined that significant moisture travels downstream of the separators as droplets less than 10 micrometers in size. Preventing the formation of these small sized droplets results in a significant reduction of mist borne-impurities that are resistant to conventional methods of gravity, centrifugal or impingement separation. Using the TRIZ (Theory of Inventive Problem Solving) methodology, a systematic review of 34 engineering options to improve the steam purity at Te Mihi Power Station was conducted. Importantly, this drew on knowledge from outside the geothermal industry. The main solution selected was the application of flashing flow nozzles instead of orifice plates, or control valves, to minimise the creation of droplets < 10 micrometers in diameter when producing flash steam. Plant tests following installation measured a significant reduction in the total mass of mineral impurities flowing through the Te Mihi Power Station low pressure steam system. This was a low cost but effective improvement, avoiding invasive modifications to the steam system. Minimising the creation of droplets smaller than 10 micrometers is an important design consideration for future steam field and station developments around the world. Furthermore, the internal design of cyclone separators and transmission systems should avoid sharp edge flow shedding which can increase creation and entrainment of small droplets. |