The advantage of barrel type feed pump is it has smaller number of joints to be sealed against the high pressure action and this minimise leaking of pumps which is an added feature for power plants.
As the stages multiply, the boiler feed pump has an increased length and to attain radial rigidity, bigger shaft is designed and installed for reliability. Although, in these pumps where the stages are less, the reliability is high but efficiency is affected the reason being increase in diameter of the shaft.
In view of reliability, the bearing used in these boiler feed pumps is ‘Kingsbury Thrust’ or ‘Journal’ type bearing because roller contact bearings are not believed to be of substantially dependable.
Categorically, In low pressure installations, horizontal single stage volute type centrifugal pumps are more in use. For medium pressure below 100 bars, split case multistage twin volute diffuser pumps are common because of the difficulties found in sealing the case joint surfaces.
The seals used in boiler feed pumps are generally stuffing box type and injected condensate to avoid leakage of feed water. Asbestos gland packing is out dated due to health hazards and off late are replaced by non-asbestos braided gland packings which include Graphite, PTFE, GFO, Aramid or in combination due to their self lubricating properties. Hence the scoring of shaft is avoided, leakages are reduced and the life of shaft is also enhanced. The selection of gland packing depends on the temperature, flow rate etc. In power plants, especially in nuclear, mechanical seals are also used but rubbing speed is always a problem due to the large diameter and high speed shafts.
The required spares whose inventory should always be maintained for boiler feed pumps are set of casing wearing rings, balancing device which is usually made of stationary and rotating parts, set of diffuser bushings, 2-4 sets of shaft sleeves, gland packing rings in several sets and thrust bearings.