Wear bands

Problems with impellers, casings, and wear bands. 

With the pump di.sa.ssemblcd in the shop, with open impellers, the damage is seen on the leading edge of the impeller blades toward the eye of the impeller, and on the blade tips toward the impeller s OD. With enclosed impellers, the damage reveals itself on the wear bands between the impeller and the volute casing. See the illustration (Figure 3-2). 

Some pump eompanies will design replaceable wear bands for the OD of the impeller eye and the bore of the pump housing. It s said that the pump loses 1.5% to 2% efficiency points for every one thousandths wear in a wear band beyond the factory setting. Therefore, by changing wear bands, the pump is returned to its original effieiency. Becau.se of this, the term wear band is a misnomer. A better term would be effieiency band (Figure 6-19). 

The solution is Identify the dominant and weak pump should they exist. To do this, take pressure gauge readings with the pumps running at shut-off head. Verify that the impellers are the same diameter, and that the wear bands and motor speeds are equal. If you can identify one pump in the pair as dominant, ahvays start the weak pump first and then add the dominant pump in parallel with the weaker. The dominant pump coming on stream w ill push open the check valve. It may be necessary to override a sequential starter. 

A - Impeller OD B - Wear Band C - Restriction Bushing D - Seal Posterior... 

Stationary wear band bores on enclo.sed impellers. 

After being in line for six months, this pump is disassembled and it is noted that the tolerance in the wear bands has opened 0.004 inch from... 

Pistons may be of segmented construction to permit the use of one-piece wear bands. One-piece wear bands are a requirement in API 618. Pistons have a problem in common with humans—a weight problem. Weight in a piston contributes directly to the compressor shaking forces and must be controlled. For this reason, aluminum pistons are often found in larger low pressure cylinders. Hollow pistons are used but can pose a hazard to maintenance personnel if not properly vented. If trapped, the gas will be released in an unpredictable and dangerous manner when the piston is dismantled. 

Figure 3-8. Piston rings and wear band. (Courtesy ofNuovo Pignone)...

The rod drop monitor is recommended for all non-lubricated compressor applications, because there is no lubricant to act as a buffer to prevent piston-to-cylinder contact on the loss of the wear band. Lubricated com pressors handling gases with traces of water or gas components that can degrade the local lubricant are candidates. Hydrogen compressors should be considered for monitors because hydrogen is a difficult gas in it sc It and may contain trace quantities of water. While sweet gas compressor-, as are found in pipeline service, would normally not be considered a problem, the rod drop monitor may be used to signal a loss of lubricant anti the compressor can be shutdown before damage can occur. 

To reduce friction and improve compression efficiency, the piston will be provided with segmented compression rings as shown in Figure 11-11. To prevent piston-to-bore contact, the piston may also be provided with removable wear bands that are in continuous contact with the cylinder wall. The compression rings and wear bands are replaced at regular intervals and typically made from soft materials such as brass, Micarta, Teflon, and the newer thermoplastics. 

Figure 11-11. Piston rings and wear bands are made of material that is softer than the c inder wall with which they are in constant contact, so they must be replaced regularly. [Courtesy of Dresser-Rand Company.]...

Piston. A wide variety of piston constructions and materials is available. Depending on the operating condition, we select pistons, sealing rings, wear bands, and piston rods accordingly. See Pig. C-72. 

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