Selection of Boiler Feed Pumps
Boiler Feed Pump is selected depending on the capacity of the boiler. The operation of the pump may be continuous or intermittent and depends on the liquid level control of the boiler. A float type switch is provided in boilers with capacity 10,000 lbs/hr or lesser which ensure the programmed level of water in the boiler and this is a classic intermittent operation.
The boilers having capacity higher than 10,000 lbs/hr will have modulating feed water regulator to feed the water continuously at diverse flow rates to satisfy the need of maintaining designed water level in the boiler and this is a continuous operation.
Now when we discuss about the capacity of the boiler feed pump which is nothing but the quantity of water to be pumped, it is designed taking into account the evaporation rate of the boiler. The on-off operation would be 2 times of the evaporation rate of the boiler considering the safe factor whereas with modulating level control, a factor of 1.3 times of the evaporation rate of the boiler plus recirculation is suggested.
Boiler Feed Pumps can handle temperature upto 220°F but higher temperature is made possible with the help of external water cooling. The deaerator pump used in power plants is also designed for higher temperatures.
Boiler Feed Pumps in Power Generation
Most common pump is a multistage centrifugal in power plants for feeding water to boilers. The main parts of this pump are impeller, stainless steel 2-piece twin volute and stage pieces, combination of balancing drum & disc, shaft and gland packing. A heavy cylindrical forged steel barrel is an integral part for the stages to be stacked and closed at one end by a bolt on the head. The stages inside barrel casing are split radially. In few pumps, the stages are also split axially.
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.