A BFP or boiler feed pump is used in boiler feed water system to increase the water pressure; high enough so it can be pumped into the boiler drum. It helps maintain proper working of a boiler providing continues feed water supply. A continues feed water supply is essential for steam boilers; as it not only avoid overheating but any further damage to the boiler.
There is no universal fix criteria for BFP pumps; as it depends on need, capacity and boiler type. Traditionally auxiliary boilers have a positive displacement feed pump; with more and more replaced with in-line multistage centrifugal pumps in recent times. While an large boiler such as on steam ship and power plant; have multistage centrifugal pumps. These centrifugal feed pumps are then further divided into two sub groups; steam driven and motor driven.
It is essential for feed pumps to withstand axial trust; generated due to pressure difference at suction and discharge side. Boiler feed pump must not only be able to supply feed water requirement; but also work as a condensate and makeup water pump when required. To ensure proper operation of these pump; feed water must be free from steam, air and insoluble solids.
Parts of Boiler Feed Pump
1 ) Casing
A casing is the air tight shell or cover surrounding pump impeller; which covers, protect and provide support for the pump assembly. This watertight casing acts as a seal; avoiding leakage and assist in pressure generation. Apart from supporting pump assembly, shaft and bearings; it also convert kinetic energy into pressure energy. Out of different casing types in centrifugal pumps; a diffuser casing is used in boiler feed pump.
2 ) Impeller
An impeller is the rotating part of the centrifugal pump which has vanes attached to it. They are coupled together with the shaft to form the rotating part of the pump. It takes energy from the shaft and add velocity to the fluid. Shaft energy is transfered to the fluid upon impeller rotation; it is this energy that force the water outwards radially.
This generate a negative pressure at the pump inlet while a positive pressure and kinetic energy at pump discharge. The impeller blades along with volute / vortex or diffuser casing produce a high pressure high flow discharge. Based on the design an impeller can be axial or radial. An axial impeller throws the water outwards axially; while a radial impeller pump out radially.
3 ) Shaft
A pump shaft is the main part of a centrifugal pump; that transfer mechanical energy from motor or turbine to the impeller blades. Other than that it also supports and house moving parts of a pump. It provide housing to impeller, shaft sleeve, bearings and couplings; that together form the rotor part of a pump. A pump shaft is chosen not just based on the maximum torque it can provide; but also taking shaft deflection and torsional vibration into consideration.
4 ) Shaft Sleeve
Pump shaft is protected from wear, corrosion or erosion by a hollow cylindrical tube placed over the shaft known as shaft sleeve. The main purpose of shaft sleeve is to protect shaft wear due packing and mechanical seat at stuffing box. While shaft sleeves are an important part of single stage pumps; some multistage feed pumps still have them.
Since multistage in-line boiler feed pumps have shaft made of alloy steel with added chromium. The pump shafts are immune to major corrosion due to feed water; this reduces the need for such shaft sleeve which can be proved disadvantageous at certain condition.
5 ) Balance Chamber
Multistage centrifugal pump as in boiler feed pump have a acute pressure difference between their suction and discharge side. This increase the risk of the whole rotor assembly of the pump; to be pushed toward the suction side. This extreme force that push the rotor assembly ( shaft, impeller and bearings ) is called Axial thrust.
This axial trust if unchecked can lead to bearing damage and shaft misalignment / bending. To nullify this effect a balance chamber is used in boiler feed check. A balance line is taken from the discharge side of the pump and connected to the balancing drum in the balance chamber. In many pump design an additional recirculating line is provided to avoid pumps; to run dry or at low flow rate.
These in line multistage boiler feed pump require a minimum flow rate to work properly. In case the net flow rate go below the continues stable flow of the pump; the control valve actuates the opening of recirculation line to stabilize the pump.
6 ) Wear Rings
A rotating impeller is contained inside the solid casing in centrifugal pump. This provide a minimum clearance area for the free movement of impeller. To reduce any fluid leakage and improve pump efficiency the clearance must be as minimum as possible. This lead to catch 22 situation where any misalignment of shaft can damage the whole pump.
To avoid such situation a wear ring is provided in each pump. The main role of these wear rings is to wear out in event of contact; and thus protect the pump assembly. This is why they are designed in a way that; they can be easily and periodically replaced during the pump lifetime.
7 ) Mechanical Seal
Traditionally packing material is used to seal the pump shaft. But for high speed multistage pumps they are not enough; and thus additional sealing arrangement is required. A mechanical seal is that additional sealing arrangement which allow the shaft; to pass through a wet area without allowing the fluid to pass through.
It consists of two key parts stationary and rotatory. The stationary part is clamped to the pump housing; while the rotary part is attached to the shaft. Stationary part contains simple gasket or o-ring as a sealing medium; while rotary part seal on the shaft using a series of o-ring and spring loaded bearings attached to the flat surface.
These sealing surface are pushed together using the hydraulic pressure and spring tension from the spring loaded bearings. To learn more about mechanical seal and why it is preferred over traditional packing material; you can *refer to this article .
Working Principle of a Boiler Feed Pump
The work of a boiler feed pump is to discharge the condensate boiler feed water from hotwell to the boiler drum. With the rotation of shaft and impeller the hot feed water is thrown outwards radially across the walls of the pump; along the path of the flow. This increase the speed of the fluid so does the pressure.
Now when the fluid is directed to the next impeller blades along the flow path; it is again accelerated and thrown radially outwards towards the casing. Such gradual increase in the speed of flow and pressure; with each impeller we finally meet the pressure at the pump outlet. The final outlet pressure of the pump is higher than the normal working pressure of the boiler.
A multistage boiler feed pump have two or more impeller in series to produce such high pressure. Here the pressure generated by each impeller is added upon by the other. The overall capacity of the pump is determined based on the distance of fluid passageway between each stage. While the pressure for each stage depends largely upon the size of impeller; with overall pressure depending upon number of impellers in series.
It is an integral part of feed pump operation to maintain the temperature difference; between the suction and discharge side of the feed pump. Under no circumstance the temperature difference be more than 60-70 degree centigrade; to avoid damage due to thermal shock. In feed water system the feed pump take suction from the hotwell through strainer; while discharge to the boiler through orifice plate and feed check valve.
Boiler Feed Pump Starting Procedure
A boiler feed pump is one of the easiest on-board ( on ship ) to start but need regular attention for prolonged life. To start boiler feed pump we need to line up the system; and follow the following steps:-
- Make sure the hotwell have at least minimum feed water to start the pump.
- The temperature difference between the feed water in hotwell and boiler water must always be within 0 to 60 degree Celsius.
- Check whether the pump is primed or not; and in not take proper steps to prime the pump.
- Open the suction side of the pump
- Close the discharge side using discharge valve.
- Now open the air cock to allow air inside to escape.
- Ensure there is proper flow of lubrication available to the seals.
- Start the pump.
- Close the air cock once all the air inside the pump escape the pump system.
- Monitor pump suction pressure.
- Slowly open the discharge valve once the pump reach its optimal speed.
- Now monitor discharge pressure for some time.
- Check for bearings temperature.
- At last check for the pump flow rate.
Maintenance Required In Boiler Feed Pump ( BFP )
Boiler feed pump are subjected to both preventive and breakdown maintenance on ship and power plant. These pumps are mainly subjected to wear and tear, float lever problem, damage to balancing drum and increased bearing clearance. Under preventive maintenance the quality / state of wear rings are checked and replaced if necessary. Pump is dismantled and cleaned along with comparing performance parameters of the pump; with manufacturer recommendation as given in pump manual.
Along with that vibration and sound is also monitored for any fault or issue. Break down maintenance is done on pump failure and often required more time and effort. The damaged parts are replaced, and alignment and clearance are checked to be within limits. Once satisfied then the pump is reinstated for further use. In case the problem can not be fixed on-site these pumps are often send to the manufacturer for repair.
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