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vjr0512 (Mechanical)
19 Jan 12 22:22
Hi all,

Is Wear rings in Centrifugal pumps is meant to increase the efficiency of the pump? My understanding is "Reciruculation or loss (liquid escape to Seal chamber) of Pumping liquid reduces the net output of the pump. Thus reducing the efficiency. For that, the escape path (impeller to casing clearance) is to maintained as close as possible (that depends on pumping liquid) It is tougher to manufacture the impeller and casing to maintain such clearance(which requires perfect machining of the parts). For this purpose only we are going for the wear ring design. Also in case of any wear (due to misalignment or something). the wear ring will take the care, thus we can replace the wear ring only. Pl Clarify whether my understanding is correct. Pls correct if i am wrong.
Thanks in advance.
Helpful Member!(2)  bimr (Civil/Environmental)
20 Jan 12 0:22
Wear rings are sacrificial components installed on the casing and impeller to inhibit fluid from recirculating back to suction from the discharge. They provide a renewable restriction between a closed impeller and the casing. Wear rings are often installed on both the front and back of the impeller. When wear rings are installed on the back of the impeller, another set of rings is installed in the backcover.

Wear rings act as the seal between the casing and the impeller.

Pump manufacturers know that decreasing wear ring clearance leads to improved efficiency.

http://www.pump-zone.com/topics/pumps/pumps/power-wear-rings-part-two-efficiency

 
JJPellin (Mechanical)
20 Jan 12 7:50
That is an excellent description by bimr.  There is one other purpose that can be served by wear rings that was not mentioned.  The close clearance of the wear rings tends to support the shaft and reduce shaft loads and vibration.  This occurs because of the Lomakin effect.  Each wear ring behaves like a bearing that provides force to hold the shaft close to the center of the clearance.

There are alternatives to wear rings.  Non-API pumps often use open or semi-open impellers that rely on axial clearance to limit the leak-back of discharge pressure to the suction.  For some configurations, this reduces the required precision since the axial clearances are likely much larger than the tight radial clearances common in standard wear rings.  Possible damage from contact can be reduced with the used of non-metallic wear ring materials.

Johnny Pellin

DubMac (Petroleum)
20 Jan 12 9:10
Yes, very good descriptions. We can also note that while "closing up" the clearances will improve the efficiency, it can definitely come at the expense of reliability.

The closer the rings are to each other, the greater the chance they "touch off" and begin rubbing each other. In the best case, this will just open the clearances back up without much harm (other than efficiency loss). In more critical cases, this can cause severe galling, seal failures, excessive vibration, and catastrophic failures of several types.

A balance needs to be struck for a particular pump/service between efficiency and reliability.
1gibson (Mechanical)
20 Jan 12 16:49
Minimum API clearances are a good place to start, tighten them up for non-metallic materials. If fluid is >500*F or the materials have "high galling tendency" like 316, then add 0.005"

ANY leakage from a high pressure to low pressure area in a pump, will reduce efficiency.

If you are adding a replaceable wear ring to a part with an integral wear surface, then you only need to balance the manufacturing tolerances to end up at the same diameters.

Also don't forget that you will eventually need to remove the wear ring. Consider small steps in your part to allow a pry bar to "catch" the wear ring and theoretically be able to remove without machining it off.

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