6.1.3 Low pressure casings
LP cylinders (Fig 1.96) are often of double-casing construction with an inner casing containing diaphragm supports, bled-steam and water extraction belts, and an outer casing directing the exhaust to the condensers and providing structural support for the inner casing (Fig 1.96).
This is not always so, however, particularly with pannier condensers where a single casing may be employed. The large size of LP outer casings combined with their low pressure loading favours a fabricated rather than cast construction. Inner casings which are more complex can be fabricated or cast depending on economic considerations. All casings have bolted horizontal joints.
LP cylinders are virtually all of double-flow design but vary greatly in layout due to the different condenser configurations. The latest practice for condensers is similar to early 4mderslung' designs with separate condensers underneath each cylinder, tubed transversely, allowing easy access to both condenser and LP cylinder. Foundation columns, bearing and cylinder supports are located between condensers. However, during the development of 500 MW machines, variations of the turbine and condenser arrangement were tried, both to simplify the LP casing support arrangements, to increase the area and shorten the exhaust connections. The four arrangements used were:
- The axial condenser, which permits the LP turbines to be supported on two parallel walls. Space has to be allowed beneath the generator for tube withdrawal.
- The bridge condenser, which bridges the gap between the IP turbine block and the generator. It is constructed in the form of two deep girders, which carry two parallel rigid soleplates for the LP turbine support. The condenser tubes are mounted transversely. This construction uses a lot of steel.
- The pannier condenser, here the tubes are mounted axially with the condensers themselves mounted on either side of the turbine, so that the steam is exhausted horizontally into the condensers. The advantage of this arrangement is that the exhaust steam flow from the turbines is facilitated, which cuts down the hood loss; in addition, construction savings are made by having a reduced basement depth.
- The integral condenser, which is a development of the pannier type; the condensers and LP turbine outer casings are combined in a single structure containing internal ribs to give it adequate stiffness. Access to the bearings is obtained via wells, and access to the turbine inner shells is obtained by removing the top cover plates. Here again, the basement depth is small and there is direct support beneath the bearings.
The LP exhaust casings are designed to give the maximum possible area of flow for the exhaust steam, so that the pressure drop is kept to a minimum. To turn the huge volume of steam through 90° without 'bunching', curved vanes are employed and, by careful design, these can be made to have a diffusing action (i.e., they can make use of the kinetic energy of the steam to produce a pressure difference) so that the pressure at the blading exhaust annulus is slightly lower than that in the condenser.
To minimise the losses in the flow to the condenser, the entire volume of the outer casing may be used, steam from the top sections of the exhausts passing down the side of the casing and exiting to the condenser in the centre of the lower casing in an underslung condenser design.
In many designs of LP cylinder the outer casing also provides support for the LP bearings; if not, then the bearing may be supported on a pedestal located in a recess at the end of the outer casing, in order to keep the rotor length between bearings as short as possible. The outer casing contains structural members to transfer the weight to the support points at the ends, sides, or corners of the casing and to withstand the atmospheric pressure, as the inside of the outer casing is entirely at condenser vacuum conditions.
Watersprays are provided in the exhaust area of the casings to cool the exhausts during prolonged periods of light load or poor vacuum running.
To prevent overpressurisation of the LP outer casing, 'bursting disc' pressure relief valves are fitted at the top of the casing.