Marchwood Power operates a £380 million natural gas-fired combined cycle (CCGT) power station near Southampton in the UK. The station generates 842 MW of electricity for the national grid, enough to supply nearly one million homes.
The facility uses the latest technology to ensure maximum energy efficiency and minimal impact on the environment. It is currently one of the most efficient power stations in the UK. The under-slung condenser vessel sits directly under the steam turbine in the power station, condensing the exhausted steam and returning it to an aqueous state, which is then circulated for reuse in the generation cycle. A reliable, accurate level control is critical in this part of the process; if the level in the vessel gets too high, it could impinge on the turbine itself causing untold damage and loss of generation. The correct condensate level needs to be maintained to ensure the condenser back pressure is at the optimum value, lowering the flash point of water and preventing wet steam from contacting the low pressure turbine blades, assisting steam flow through the turbine and increasing efficiency.
Differential pressure measurement proves unreliable
When originally designed and commissioned, the plant installation consisted of three differential pressure transmitters, using a two out of three voting method and a single magnetic float gauge on a bypass chamber. However, there were ongoing issues with differential pressure measurement, mainly due to the condensate filled impulse lines and vacuum within the condenser, which meant there was often an unreliable measurement on at least one, increasing the risk of nuisance trips. A differential pressure-based level system for this type of measurement is complex and requires a lot of interconnecting pipe work, valves and condensate pots. There was much ongoing work undertaken, balancing the condensate in pots and the wet legs, as well as regular recalibration of the differential pressure transmitters themselves.
Vegaflex GWR transmitters provide the solution
Marchwood Power’s requirement was three devices that would remain accurate in relation to each other and the process, enabling higher plant availability. VEGA recommended that Marchwood opt for additional interconnecting pipework where the magnetic level gauge is installed, so that they could fit three Vegaflex GWR transmitters into bypass tubes, using the same two out of three voting method, which would give reliable accurate level measurement and control. Vegaflex use guided microwave pulses, which are virtually unaffected by temperature, pressure or vacuum, to detect the level. There were no issues with the bypass tubes sharing the same vessel connection ‘manifold’, as it is a ‘clean’ process. The units were mounted to the side of the boiler, alongside the existing MLG, where there was plenty of headroom for installation. The measuring range required was over 1,7 m and the temperatures were much lower than the process at 40°C, (Vegaflex can go to 450°C and 400 bar if required).
Marchwood installed each GWR in a 50 mm diameter bypass tube configuration, with isolation valves at the top and bottom. Once in place, it just needed the GWR units to be cabled and they were ready to commission. Marchwood Power engineers carried out the setup themselves using PACTware software, so they could have a full configuration backup and calibration record. As well as reducing maintenance costs, the installation of the new sensors was a more reliable, simpler system and the cost was much lower than the original arrangement.
Since installation, no outages due to unreliable levels have been recorded; there is very good correlation and repeatability between devices, which makes the two out of three system work as it should. With no moving parts, and unaffected by density, pressure or vacuum changes, the Vegaflex GWR sensors have delivered the reliability required by Marchwood Power.
For more information contact Chantal Groom, VEGA Instruments SA, +27 (0)11 795 3249, [email protected], www.vega.com
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