Article 66: Metallurgy Examined - Use the Right Materials for the Wear Rings
The Duke is Back - part 3 of 4
(This is a continuation of Parts 1 and 3 (articles 55 and 56)
In July, - Bob Sellers from the Duck Pump company visited Jim Makem, a maintenance manager at the Nasty Creek municipal water plant. By doing some testing, they discovered that the pumps operated at reduced efficiency, and Bob recommended to repair the pumps. After Bob sent a quotation, Jim and his boss Charlie Beans, went to the store room to inspect the rotating assembly of this large double suction split case pump. Storeroom manager, Grady Cricket, and a plant mechanic Rusty Hammer, brought out the rotating assembly, and they did a basic inspection, discovering the entire rotating assembly non- magnetic (magnet would not stick), with the exception of the wear rings. They became concerned that what they thought should have been a completely stainless steel (316ss) rotor, had in fact regular carbon steel wear rings. That, they felt, could be causing rusting and wear, leakage across the rings, and lowered efficiency. Also concerned, Bob requested Sandy Mixup, from the Duck Pump engineering department, to come to the meeting at the plant in September, answer the plant’s questions, and help clarify the situation.
“Hi Sandy!”, - Charlie Beans greeted her as she and Bob came out from their car, - “Good to see you. I am glad Bob got some heavy artillery to help us troubleshoot this pump problems we seem to be having”
“Sure thing Charlie. That’s why I am here. Bob said you had some questions on metallurgy of the pump rotors. Can we see the rings?”
“Of course. We got them at the conference room – both the impeller ring and casing ring, along with the magnet – it does not stick to it, so we suspect it is not a 316ss stainless, as compared to the rest of the rotor”
Sandy touched the rings with the magnet:
“You are right guys, it does not stick, but it does not mean it is plan carbon steel. According to our specs, these are 416 stainless steel. The 400 series is called martensitic stainless, and it is not magnetic. The 300 series is also stainless but austenitic, and it is not magnetic. Both types are very good against corrosion, but 300 series is especially good against cavitation, which is why your impeller is 316ss, while the rings are 416ss.”
Rusty was not convinced:
“But look, Sandy, - the impeller is soft! – I just scratched it a bit with a knife blade, and it cuts a grove in it like butter! How can it rest cavitation? I have seen cavitated impellers – nasty!”
“True, Rusty. 316ss is soft, - but that is only initially”, - Sandy was looking thru her files, - “Once in service, and as it begins to get bombarded by the cavitation (which is essentially a high energy of the imploding water bubbles against the impeller blades) the impeller material work-hardens, and any cavitation cannot damage it from then on. Check it at the reference site:
But the 400-series is better for the wear rings. Cavitation is not really an issue there, but they need to be hard enough to withstand occasional contact, or wear by particulates, such as sand, grinds, etc.”
“But Bob did this testing in the field and told me we are losing efficiency!” – Jim pointed at Bob’s report, - “He told me it is costing us lots of money because of the rings wear! If they are so hard, how come they wear so quick?!”
“That’s a good question, Bob”, - Sandy handed him a copy of an article from Pumps & Systems magazine, - “Here is a good technical article on the effect of clearances on efficiency. It was published in March 2007, but did not become less relevant over the years! (ref. ). I got you a copy. Also, here is a chart on recommended clearances. So in your case, the rings are at 12.125” diameter? That means the clearance should be about 0.023”, according to a chart:”
“Thanks Sandy,” – Charlie Beans was examining the chart, - “So, what do we actually have? Can we measure them impeller ring OD and subtract form the casing ring ID?”
“Sure. But we got another meeting in a few minutes. Can we do the measurements and email you results? Thanks Sandy. Appreciate you and Bob getting all this information for us, helps to learn about pumps more”
“You are welcome, Charlie. Thanks for your time also. Talk to you later”
The next month (October issue), find out what did the measured wear ring clearances turned out to be, and did it explain the problem with efficiency as measured by Jim in June?
1. L. Nelik, Pumps & Systems magazine, March, 2007, page 18, "How much Energy is wasted When Wear Rings are Worn to Double Their Initial Value?"
2. PREMS-2A Pumps Reliability and Efficiency Monitoring System, rev. 2A, March 2015:
www.pumpingmachinery.com/pump_school/pump_school.htm (PVA modules 10 and 11)