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The boiler feed pump is a multistage centrifugal pump used to increase the discharge pressure of the feed water to the boiler. To deliver the feed water into the boiler steam drum, it has to raise the pressure beyond the boiler steam drum pressure. Because of the high pressure and temperature involved, boiler feed water pumps are made up of cast steel casings and shafts along with an impeller made up of stainless steel.
To achieve this condition, the multistage centrifugal pumps are used where the discharge of the first stage becomes the suction of the second stage. Such pumps are provided with the diffuser rings around each impeller that serve to increase discharge pressure. This type of pump is thus often referred as a "turbine pump."
The shaft is protected in the way of bearings by stainless steel sleeves, where we normally use a mechanical seal to the high pressure for liquid handling. So we provide a separate lubricating arrangement. The oil within the seal chamber is maintained constantly one bar above the discharge pressure by an accumulator containing a plunger which is operated by air or nitrogen pressure.
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Mechanical Seals are used in Boiler Feed Water Pumps
It is preferable on a vertical pump to have shaft sealing at the pump upper end only. Mechanical seals are spring loaded to hold the sealing faces together. It is important that cooling and lubrication liquid is led to the mechanical seal from the lowest point on the pressure side of the pump to ensure that some liquid reaches there even when priming.
Special header tanks have been installed for seal lubrication in some applications. They must not run dry, and care must be taken to prevent ingress of foreign matter. Many mechanical seals incorporate a carbon face, and there is a possibility of electrolytic action in the presence of sea water. Normally this does not take place in boiler feed water pumps because we use the fresh water treated dosed with corrosion preventing chemicals as the boiler feed.
In the case of sea water pumps, we do not use the mechanical sealing arrangement- instead we use a soft packing arrangement. Stuffing box type glands may be packed with soft or metal foil type packing. The pump internal bearings may be lubricated and cooled by pumping liquid that is always available when the pump is running. A lubricator for the application of grease is also fitted in some circumstances.
The image below shows a mechanical seal arrangement. (Click to enlarge and then use the Back button in your browser to return to the same place in this article.) In the image, elements d1 and a1 bear and slide on each other, creating a seal at their interface. One group of parts is connected to the rotating shaft and the other to the machine's case. The spring keeps the elements tight against each other, maintaining the seal and allowing for wear.
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Repairing Boiler Feed Water Pump - Mechanical Seal Replacement
Tools Required for Dismantling of the Boiler Feed Water Pump:
- Open end spanner, box spanner, and ring spanner of required size.
- Allen key set
- Bearing puller
- Vernier caliper
Precaution to Be Taken Before Opening the Boiler Feed Pump:
- Switch off the power and put a board on electrical banner “work in progress."
- Drain all the water inside the pump.
- Close the suction and discharge side valves.
Dismantling Procedure of the Boiler Feed Pump:
- By using spanner, slacken all the bolts.
- Then open all the bolts of the outer casing diagonally.
- Remove the bearing by using the bearing puller.
- Take out the impeller of the first stage and remove the diffuser ring of the first stage.
- Remove the other impeller along with its diffuser rings.
- Place the shaft on shaft holder.
- Open the mechanical seal using the special tool. Care must be taken while removing the spring arrangement of the seals.
- Check for the impeller clearance using the vernier caliper.
- Check the wear ring is in good condition or not.
- Check the trueness of the shaft.
- Shaft should be clearly examined especially in the area of mechanical seal arrangement if excessive wear has taken place the quality of packing used should be investigated and a larger amount of lubricant should be used while running.
- Check for bushes and bearing of the pump.
- Check for pump casing any traces of corrosion or pitting.
Assembling Procedure of Boiler Feed Pump:
- Put back the new bearing if needed.
- Replace a new wear ring for the impeller blades.
- Replace a new mechanical seal on the shaft.
- Assemble the impellers and diffuser rings, check for impeller clearances.
- Then place a wear ring in place.
- Next place the outer casing of the pump.
- After that tightening the casing of the pump by using spanner diagonally.
Checks to Be Made Before Staring the Boiler Feed Pumps:
- First check the priming of the pump is done or not.
- Check the lubrication for the seals.
- Open the suction valve and air cocks. After all the air has escaped close the pump’s air cock.
- When the pump has reached the correct speed, open the discharge valve.
- Check all gauges to see that a proper pressure is being developed.
And this is how the boiler feed water pump is repaired and the seals are replaced.
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