Impeller maintenance

Hi = Inlet head, or the losses expressed in feet that occur in the suction throat of the pump up to and including the eye of the impeller. These losses would not be registered on a suction pressure gauge. They could be insignificant, or as high as 2 feet. Some pump manufacturers factor them into their new pumps, and others don t. Also, changes occur in maintenance that may alter the Hi. If you don t know the Hi, call it a safety factor of 2 feet. 

The Hi, inlet head, is simply a. safety factor of 2 feet. Some pumps have an insignificant Hi. Other pumps have inlet losses approaching 2 feet. The Hi is losses to the fluid after it passes the suction pre,s,sure gauge and goes into the impeller eye. In a maintenance function, you can t be precise about what s happening to the fluid in this part of the pump. Just call it 2 feet. 

As we ve di,seussed, system design is responsible for much of cavitation. Yet, the maintenanee mechanic is responsible for stopping and preventing eavitation. And certainly, it s the maintenance mechanic who has to deal with the results of cavitation, the constant changing of bearings, meehanical seals, damaged impellers, wear rings and other pump parts. 

Vane Pass Syndrome Design / Maintenance Oversized impeller, inadequate aftermarket parts, repair or rebuild with incorrect specifications and measurements. 

Many clients specify and buy pumps with the back pullout option, and they -----take advantage of the option. This is like buying a car with an air conoi. -r never turning it on. Many engineers, operators, and even pump sale . i t the back pullout feature is designed to facilitate maintenance. This is wr<. . c back pullout pump exists to facilitate the rapid and frequent impeller change -- the pump to the ever-changing needs of production. The back pullout pump exists to facilitate production. 

Very few industrial pumps come out of service and go into the maintenance shop because tlie volute casing or impeller split down the middle, or because the shaft fractured into four pieces, d he majority of pumps go into the shop because the bearings or the mechanical seal tailed. 

If a coupling hub does not have to be removed for maintenance, seal removal, or impeller removal, it should be integral with the shaft,... 

If a plant had periodic inspections, the impeller corrosion in No. 5 might be detected before it became a significant problem, thereby altering the failure mode from catastrophic to a degraded or an incipient failure. In a plant with routine maintenance, it is possible that Nos. 1 and 5 may be eliminated completely by routine seal and impeller changes. 

Figure 7.9. Some types of centrifugal pumps, (a) Single-stage, single suction volute pump, (b) Flow path in a volute pump, (c) Double suction for minimizing axial thrust, (d) Horizontally split casing for ease of maintenance, (e) Diffuser pump vanes V are fixed, impellers P rotate, (f) A related type, Ihe turbine pump.

Double volute the liquid leaving the impeller is collected in two similar volutes displaced 180° with a common outlet radial thrust is counterbalanced and shaft deflection is minimized, resulting in lower maintenance and repair, used in high speed pumps producing above 500 ft per stage. 

The impeller within a large sodium chloride brine pump in a closed loop system of a chlorine plant needed to be replaced. The job was not assigned to the regular maintenance crew, but... 

After the maintenance foreman searched for the appropriate titanium impeller and could not find one, he decided to improvise. The maintenance foreman located a stainless steel impeller that was available for other pumps of the same size and manufacturer. He thought the stainless steel would be satisfactory, but would have a shorter life. The mechanics completed the job and the maintenance foreman went to eat an overtime supper at the plant cafeteria where he joined the operating foreman. The maintenance foreman just happened to mention the problems he faced securing the specified impeller and his clever temporary approach to restore the pump. 

After listening intently, the operating foreman explained that mere traces of chrome salts in the brine system could create an explosive situation within the electrolytic chlorine cells. Traces of chrome salts in the feed brine to the chlorine cells liberate hydrogen gas in the chlorine cell gas. Hydrogen in the chlorine cell gas has a very wide explosive range. Installation of stainless steel equipment in sodium chloride brine systems has devastated chlorine processing equipment within other similar chlorine manufacturing plants. The maintenance foreman had the improper pump impeller removed immediately before any problems occurred. 

Appendix A contains a materials selection guide for aerated freshwater systems. As indicated in Note 27 of Appendix A, in freshwater systems, admiralty brass should be limited to a maximum pH value of 7.2 from ammonia and copper-nickel alloys and should not be used in waters containing more sulfides than 0.007 mg/L The materials selection guide is also satisfactory for seawater, although pump cases and impellers should be a suitable duplex stainless steel or nickel-aluminum-bronze (properly heat treated). Neoprene-lined water boxes should be considered. For piping, fiber-reinforced plastic (up to 150 psi [1,035 kPa] operating pressure) and neoprene-lined steel should also be considered. Titanium and high-molybdenum SS tubes should be considered where low maintenance is required or the cost can be justified by life expectancy. 

For vessels greater than 38 m (10,000 gal), the economic attractiveness of side-entering impellers increases. For vessels larger than 380 m (100,000 gal), units may be as large as 56 kW (75 hp), and two or even three may be installed in one tank. For the suspension of slow-settling particles or the maintenance of uniformity in a viscous slurry of small particles, the diameter and rotational speed of a sideentering agitator must be selected on the basis of model tests or experience with similar operations. 

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