14 Nov Kelly & Lewis Mather Platt 1490kW 10 Stage Boiler Feed Pump Balance Cover Upgrade
Most older Boiler Feed Pumps incorporate asbestos based gasket seals throughout the pump, given the high pressures and more so the temperatures and thermal changes Asbestos based material was an obvious choice. Obviously in modern times we will not utilise asbestos based materials so most seal manufacturers have opted for graphite based gaskets. The common type for boiler feed pumps use is Klingersil C4500, this gasket type is fine in most applications, there are some occasions though where the gasket is at its limit with regard to the diameter versus face width versus pressure and the torque required.
For many years now we have been upgrading boiler feed pumps to take 316/graphite spiral wound gaskets, this is a great option as they can handle the pressures and the temperature savings of boiler feed pump easily. This is also a good option to replace O-Rings in some cases.
The Kelly & Lewis – Mather Platt 6″/8″ H.S. 10 stage 1490kW boiler feed pumps at the power station have been fitted with asbestos based gaskets and various v-seals and o-rings.
Recently we have seen a spate of temperature related failures of these pups, caused by pumps being run below the minimum speed recommended by OEM thus causing flashing off of the pumped medium, very high temperatures and thrust loads are then experienced particularly in the balance chamber. The result in the worst case is the balance devices wear heavily and cause an axial shift of the rotor position which results in major failure of the pump, we also see o-rings becoming a liken to a piece of carbon due to the high temperatures.
Recently we have had a pump dismantled for major overhaul due to a drop in performance, from early inspections of the parts we could see that the major reason for this drop off in performance was recirculation within the balance chamber due to the failure of the balancing ring to end cover o-ring seal. This failure lead to major water cutting across the sealing area and past the balance device, being caused by the water passing through from the full pressure discharge side of the pump to the balance chamber area which only marginally above suction, this pressure drop combined with high temperature water at high velocity creates these water cuts which worsen and create a vicious catch 22 situation, this causes the hydraulic axial balancing of the pump to work ineffectively causing excessive thrust loading towards DE and heavy wear to balance components and ultimately leading to early and usually catastrophic failure of the pump, especially with this type of balance device design. We believe that from the discolouration of the balancing ring that there has been very high temperatures in and around this component which had caused this failure of the o-ring. We would assume that once again running the pump below recommended minimum speed has caused this high temperature and subsequent failure.
We also have a 2nd pump opened up at NDE to check and adjust its axial running position, we noted heavy wear on this balance devise also, but thankfully the o-ring although very hard had not yet failed so we had no damage to the sealing area.
Previously we have upgraded the gasket seal on the balance chamber from a gasket to an o-ring, this was done due to the fact that the sealing area was outside that of which a reliable seal could be achieved with modern day graphite type calf gaskets.
We recommended that we upgrade this and most of the other major gaskets within this pump to allow fitting of a spiral wound gaskets 316/graphite Type “r”.
The following components could easily be upgraded to accept this superior gasket type:
- Balance Chamber – machine a groove in the sealing face
- Balancing Ring – machine a groove on the inboard face which abuts to the end cover (giving a back-up if the o-ring should fail)
- Casing Elements – machine grooves on the stepped sealing faces
This gasket type can easily handle temperature fluctuations as opposed to graphite calf alternatives, pressure and temperatures are well within range of any boiler feed pump. We have upgraded many boiler feed pumps to this gasket type over the years with great success. You cannot over compress the gasket or nip and damage it as is common with other types and o-rings.
It is a simple upgrade – A square groove is machine into the component this groove is machined to the correct depth to allow exact tolerance compressibility of the spiral wound gasket thus ensuring that provided the fees are pre-machine concentric then the gasket cannot offset though incorrect torque procedure and disturb pump set up.
In an ideal world we should not experience there very highly elevated temperatures in the balance chamber, however given the ease by which this upgrade is possible we believed it was a viable and necessary upgrade.
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The unit was suffering from a large amount of vibration and was unable to adjust its speed.
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Due to recent pump failures and the findings of both mechanical component and gasket / o-ring condition a change of seal type is recommended.
Failing of mechanical seals can have major implications on the performance of pumps. Here we look at a client whose power station has 12 booster pumps that feed their Sulzer HPT Pok30 boiler feed pumps where they have suffered numerous seal failures.
Swirl breaks are designed into high energy centrifugal pumps (especially modern Boiler Feed Pumps) to dampen vibrations especially at higher speeds caused by rotor dynamic forces.
Pump types come in two main categories – Centrifugal Pumps and Positive Displacement Pumps as classified according to the method of how the energy is imparted to the fluid – Kinetic Energy or Positive Displacement and again each of these categories having many pump types….