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Reliable Turbine Condenser Level

Marchwood Power operates a £380million natural gas-fired combined cycle (CCGT) power station on Marchwood Industrial Park, adjacent to the River Test near Southampton. The station generates almost 900 MW of electricity for the national grid, enough to supply over one million homes. This is equivalent to the needs of Southampton, the New Forest and Winchester.
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 Marchwood 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 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.
When originally designed and commissioned, the plant installation consisted of 3 Differential Pressure transmitters, using a 2 out of 3 voting method and a single magnetic float gauge on a bypass chamber. However, there were ongoing issues with DP measurement, mainly due to the condensate filled impulse lines, and vacuum within the condenser, which meant they often had an unreliable level measurement on at least one, increasing the risk of nuisance trips. A DP 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 with balancing the condensate in pots and the wet legs, as well as regular recalibration of the DP transmitters themselves. Marchwood Power’s requirement was three devices that would remain accurate in relation to each other and the process, enabling higher plant availability.

Figure 2: Just a part of the complex valve and condensate pot system needed for the original DP level system

VEGA recommended Marchwood opt for additional interconnecting pipe work where the magnetic level gauge is installed, so that they could fit 3 VEGAFLEX GWR transmitters into bypass tubes, using the same 2 out of 3 voting method, which would give them reliable accurate level measurement and control. VEGAFLEX use guided wave microwave pulses to detect the level, these are virtually unaffected by temperature, pressure, vacuum. 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 head room for installation. The measuring range required was over 1.7 m and the temperatures were much lower than the process at 40 degrees C, (even though VEGAFLEX can go to 450ºC and 400 Bar if needed).

Marchwood installed each GWR in a 50mm 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 back up 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 2 out of 3 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 that Marchwood Power were looking for.

Contact info.uk@vega.com 01444 870055 ask for the sales team.

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