The hot external surfaces of the cylinders are lagged, i.e., covered with a thick layer of thermal insulation. There are several reasons for this:
- The loss of heat from the steam is reduced.
- The thermal stresses in the outer casing are substantially reduced.
- Danger to personnel is reduced.
The insulation consists of up to 150 mm thickness of mineral fibre which is applied either as fibre 'mattresses' or sprayed on as loose fibres, with a binder to set them in position. The thickness is largely dependent on the temperature of the surface to be covered, enough insulation being used to reduce the outside temperature to about 50°C. Asbestos was used extensively for sprayed insulation in the past, but all modern insulation is asbestos-free, because of the health risk of inhaled asbestos fibres.
The mineral fibre is held in position by wire mesh which covers it and is attached to studs welded to the outer casing and passing out through the mineral fibre (Fig 1.103). If the mineral fibre is thicker than 100 mm it is applied in two layers with an additional wire mesh support between them. The outside wire mesh is then covered with 6-8 mm of cement followed by a thin coat of oilproof and waterproof sealant. A glass reinforced plastic may be used as the sealant to provide suitable mechanical strength and life.
The insulation extends over the horizontal joint covering the bolt heads and has to be chipped off before the casings can be separated: the joint relagged on reassembly.
It is essential that oil is not allowed to enter the lagging through gaps or damage to the covering, as hot oil-soaked lagging has been responsible for some severe turbine fires.