Inlet steam pressure around 60 bar, outlet around 3 bar. Expansion down to lower pressures would require excessively large casings and would lead to problems of centrifugal stresses due to the larger blading required.
Carbon seals are used instead of labyrinths for simplicity and to keep the length of the unit down to a minimum. Due to the different coefficients of expansions between the carbon and the steel a madrill must be used to set the correct running clearance.
For multistage pumps an extra end bearing is required and hence additional packed gland.
The pump is dynamically balanced by means of the balance chamber leak off to the suction eye, and the dam edges on the back of the impeller
Feed pumps of this type of balance are best started against either a closed or spring loaded discharge v/v to ensure rapid build up of pressure
Bearings operate at 115oC with water supplied at 5.5bar 70oC.
The bearings should also be replaced if less than 75 % shows on the surface.
Constant speed ( electric )- Below 10% turbulence becomes so great that the pump operates very inefficiently and must not be worked in this range. Hence, a recirc valve must be used. At full flow pressure is only just sufficient to feed the boiler
Pressure governed- Pressure droop designed in to give stable control.
Flow + Speed and/or Pressure- The extra element must be added with the flow otherwise the system becomes unstable. Characteristic can be level or slightly rising giving low pressure at low flow rates conserving energy and preventing water being forced at high pressures through partly open feed control valve.
Materials
The impulse Curtis wheel ( velocity compounded ) rotates at speeds of around 7000 rpm. Velocity compounding means that there is very little pressure drop across the stages reducing the need for fine clearances. This allows the turbine to be run up quickly from cold.
The balance chamber must have a diameter greater than the suction wear ring. If the pump is designed to be supercavitating hard metal inducers are fitted which screw into the water. Any cavitation occurs on this which is made sacrificial.Balancing of two stage pumps
Bearings
Water lubricated bearings- Steel backing onto which is sintered a layer of porous bronze impregnated with PTFE (0.025mm thick). This PTFE is transferred to the shaft so providing very low coefficients of friction
Bearing clearance- 0.15 mm
Max- 0.25mm must be replaced
Danger- 0.3 mm severe damage will occurFlow characteristics of controlled feed pumps
Gland
Can be operated at very low flow rates due to reduced speed