Pressurised Lubrication requirements for BFP thrust brg

[MickusB80]

Hi all,

The company I work for has purchased a number of Boiler Feed Pumps for retrofit in our boiler plant. BFP selected has journal radial brgs & a tilting pad thrust brg. The OEM is requesting pressurised oil supply in the order of 20L/min at 250kPa for the thrust brg.

Our previous pumps are of similar design & our measurements indicate we deliver very little flow to their brgs at about 75kPa. We've operated these pumps for 40 years and get around 8-10years life before they need a rebuild. In terms of pump life, this is good as we run no demin water and we do get axial imblances in the pump due to BFW quality.

It was my belief that the high oil flowrates the OEM is specifying is to ensure that cooling is sufficient across the full operating range (highest thrust loads), and at around BEP, the thrust loads should be minimal and therefore this figure could be reduced if we keep the pump around BEP. As to the required oil pressure, I dont have enough understanding of tilting pad type bearings to understand why this is so high. I have conveyed my thoughts to the OEM with very little information forthcoming on their reasons for high oil flow & pressure.

Can anybody enlighten me on oil flow & pressure requirements for tilting pad thrust brgs? At the end of the day I need to know whether we can utilise the existing lubrication system, or we need to procure another lube system to meet OEM requirements. With an increase in orifice plate size, I believe I get get almost the required flow rate, but the pressure will not be anywhere near 250kPa.

Regards,
M

 

[Artisi]

You can only assume that the OEM knows what is right, and consider that any failure resulting from lack of any warranty. However, I would still push the OEM for a clearly defined answer to your question.

 

[JJPellin]

Many pump companies don't manufacture their own tilt-pad thrust bearings. They may be using bearings from Kingsbury or another bearing company in their design. If you could get information on the exact bearing that they are using, you might be able to bypass the pump OEM and contact the bearing company directly. I didn’t spend the time to do all the conversions for Metric/English units. But a typical 7" Kingsbury thrust bearing in a pump at our plant might require only about 2 gpm at perhaps 15 or 20 psi. We have bearings of this type running with oil pressures as high as 50 psi and as low as 10 psi. If they are requesting higher pressure, it really only suggest that they are using smaller orifices to get the required flow. Most bearings of this type run flooded but don't really have oil pressure within the bearing. A lower pressure with larger orifice might delivery the same performance. The key is really the flow rate needed to remove heat, as you already noted. Some designs use a higher oil pressure only because they want the oil to be at a higher pressure than the cooling water. This is done so that a cooler failure will result in oil leakage into the water rather than water leakage into the oil. For API 614 oil systems, I believe it is a requirement that the oil is at the higher pressure in the cooler. If you have specified your pump to meet this API specification, that could be the reason for the higher pressure.

 

[MickusB80]

Thanks for both responses to date.

I agree going away from OEM requirements is fraught with danger. The OEM will specify figures that cover the whole range of operation and in our application I believe our control system prevents the worst cases that they would be catering for. Having enough information to make that decision is what i'm after, and at the end of the day, its our call if we believe the risk is acceptable. I will continue to attempt to extract information from them.

Re: lube oil pressure being higher than cooling water pressure, excellent idea as we have had cooler failures causing shaft damage to our turbines. The OEM specifies however CW pressure between 2 & 6 bar i.e. ~30-90PSI, & this exceeds the lube oil pressure. Note: our original documentation on the tender did not specify lube oil pressure, but rather flow only, so I believe req'd pressures is still up for negotiation. I'm still no closer to understanding the reason wh pressurised oil lubrication is specified in instances, apart from when a hydrostatic bearing is specified..

thanks

 

[ccfowler]

MickusB80,

I agree that it may be nice to know the manufacturers reasoning for the pressure and flow requirements, but the potential costs of lost production and physical damage are assuredly staggering in comparison to the costs satisfying the requirements. I don't see the point of pursuing the minor savings available from cutting back on the lube oil flow when the potential risks are so great.

Am I missing something here?

 

[MickusB80]

ccfowler,

Real estate (or lack of) is my main problem. Existing lubrication is piggybacked off the pumps variable speed coupling. Unfortunately the system we run has been expanded upon over the years, and this has lead to very little space between our 5 pumps, not to mention the other rotating equipment in the vicinity.

By the sounds of the responses, there doesnt appear to be a hard&fast reason as to why pressures are specified for thrust pad lubrication, apart from it being a design requirement. If this is the case, then this forum has answered my question. Im still interested in the thoughts of others. Any previous experiences/problems with ring section Boiler feed pumps also appreciated.

cheers

 

[sms]

The lube supply pressure will be set based on the psi loading on the thrust bearing. Small bearing with a high load = higher pressure lubrication. The bearing design itself also will determine the requirement. How is it lubricated? What is the expected thermal load, and the resulting flow required? Are the pads plain steel, or are they copper backed? The thrust bearing size will be set in the pump design. You might ask the OEM if a different bearing would reduce the lube supply requirement, but if this is not an engineered pump but rather a more or less off the shelf design, you may be stuck.

-The future's so bright I gotta wear shades!

 

[MickusB80]

Well I received a response from the OEM re: lubrication pressure & flow requirements. The high flow rates are based on a worn balance device, which in turn produces the maximum thrust imbalance. Flow rate indicated is ultra conservative. The pressure they originally quoted was also incorrect and it seems typical pressures (7-10PSI) is all thats req'd. Perserverance with the OEM has paid off.