Wear/Corrosion Rate on Carbon Steel Casing of Boiler Feed Water Pumps 2

[amjp]

Please share the monitored data on "Wear/Corrosion Rate" in the CS casing of a BFW Pump relative to the "Quality of Feed Water" plus internal failure and casing leaks experienced due to the increasing/decreasing water quality used on the BFW pumps and preventive measures taken to solve the problem.

 

[JJPellin]

My information is more anecdotal rather than hard data. But we have seen serious problems with our large boiler feed water pumps that have carbon steel cases. We have seen corrosion that has eaten through the volutes and allowed holes as large as 3/4" diameter between stages. We have seen substantial metal loss on the volute lips and other high velocity areas in our carbon steel cases. In response to this, we changed our specifications and now we only purchase CA6NM (12-14% chrome) cases for BFW. We are changing out our existing carbon steel cases to chrome as they need repair. I don't have data to correlate this to water quality. And water quality is not my area of expertise. But, I am told that we have very good water quality. The one area where we may occasionally slip in quality is oxygen content. The operation of our deaerators may not be consistent. And some of the worse damage we have seen is on pumps close to surface condenser systems where slight air leaks could be introducing oxygen.

Johnny Pellin

 

[amjp]

Johnny,

Are you injecting any Oxygen Scavengers in the Boiler Feed Water.

Have you considered SS or Chrome cladding inside the casing to control corrosion.

It may be much cheaper to clad the casing internals (wetted areas) with Chrome Alloys or with Stainless Steel rather than replace the entire CS Casing with a Chrome Casing.

 

[JJPellin]

I believe that oxygen scavengers are being used at our main boiler house. But I don't know if they are injected at our many deaerators. When we reach the point of replacing our cases with chrome, they are already badly damaged and in need of major weld repairs. We usually cannot do without one of these pumps long enough for this repair and coating. The risk is too great. We can purchase a new chrome case for a little bit more than new carbon steel and re-install our damaged case as an emergency spare while awaiting delivery.

Johnny Pellin

 

[amjp]

My thinking was that a new CS casing with SS cladding would be much cheaper to a new whole Chrome casing.

Any idea about cost difference between a CS and Chrome casing.

I was given to understand that cost split between material and labour for large pumps is about 45% and 55% and a new chrome casing is 1.5 to 2.5 times more expensive compared to a CS casing.

 

[JJPellin]

I have never heard of a clad option for a BFW pump case. The BFW pumps I am referring to are axial split, multi-stage horizontal API volute pumps. I don't know of anyone who would make a clad case for this style. In the few instances when I have looked at quotes for CS vs CA6NM, I seem to recall a much lower differential cost. I would expect the chrome to cost 25% to 50% more. But, my memory of this is poor and the cost of steel has gone up a lot since then.

Johnny Pellin

 

[insult2injury]

I believe that amjp is referring to barrel-style feed pumps. SS cladding is very common for this service. I would also recommend a forged barrel over a cast one.

I2I

 

[JJPellin]

Our highest pressure BFW system is only 600#. We don't have any barrel style BFW pumps. We do use big barrel pumps in coker cutting water. This water is much nastier than an BFW could ever be (400 ppm Chlorides). These pumps use carbon steel barrels with 309L SS overlay in the sealing surfaces and high velocity areas. We are in the process of ordering a new coker cutting water pump that will be much larger than our existing. It is designed for foul service and has SS overlay throughout. I should note, that for all three of these pumps, the internals are all CA6NM or Duplex SS. If had a barrel style BFW pump and was concerned about water quality resulting in corrosion, I would overlay the high velocity areas and internal sealing surfaces with SS. Sorry if I took this discussion down the wrong path. I should have been clearer about the configuration of my pumps.

Johnny Pellin

 

[insult2injury]

400 ppm! At what temperature?

I2I