SUBJECT: A sensible preventative maintenance program for centrifugal pumps, cheat sheet 9-5

TRY TO INSTALL CONSTANT MONITORING. With proper instrumentation you can tell:

• If the fluid in the stuffing box is about to change state and fail the mechanical seal.
  • It could vaporize.
  • It could solidify.
  • It could crystallize.
  • It could become viscous.
  • It could build a film on the sliding surfaces and between the lapped faces.
• The differential pressure between the suction and the discharge can tell you how far you are operating the pump from its best efficiency point (BEP).
• The suction pressure can tell you if cavitation is about to begin.
• Comparing flow to amperage can tell you if you need an impeller adjustment.
• Comparing flow to amperage can tell you if the wear rings need replacing.
• A probe in the bearing case or an infra red light can tell you if the bearing temperature is too high.
• A drop in capacity with no change in amperage consumption could tell you the open impeller needs adjusting

GOOD MAINTENANCE PRACTICES

• Be sure the pump and motor pedestal is at least five times the mass of the hardware sitting on it.
• Be sure there are ten diameters of pipe between the pump suction and the first elbow in the suction piping to prevent cavitation problems.
• Be sure piping reducers have not been installed upside down. They will trap air that will eventually enter the pump suction
• Dynamic balance the rotating parts of the pump.
• Do your pump to driver alignment at operating temperature.
• Eliminate pipe strain.
• Be sure the initial impeller adjustment is made at the pump operating temperature.
• Be sure the vertical pump stuffing box is vented to a low point in the system.
• Use suction recirculation for most seal applications.
• Insure the bearing oil is being changed on a regular basis.
• Insure that no water or solids are getting into the bearing case. Replace the grease or lip seals with labyrinth or positive face seals.
• Stagger pipe hangers.
• Do not use hardened shafts, the seal set-screws can slip.

GOOD OPERATING PRACTICES WILL LOWER MAINTENANCE PROBLEMS

• Do not pump the supply tank dry.
• Be sure the tank vent is clear and will not freeze in cold weather.
• Run as close as possible to the pump B.E.P. You may have to reduce the impeller diameter to do this.
• Be sure to keep any environmental controls functioning when the pump is stopped.
• Remember that constant running is easier on bearings and seals.

RECOMMEND SENSIBLE DESIGN FEATURES TO LESSEN MAINTENANCE PROBLEMS

• Make sure that the pump shaft L³/D⁴ is less than 60 (2.0 in the metric system).
• Use the double volute design whenever possible.
• Be sure the suction specific speed number is less than 8500
• Specify a C or D frame adapter.
• Specify a Centerline design pump.
• Look for designs that adjust the open impeller from the wet end of the pump.
• Specify a positive bearing retention method. A simple snap ring is not good enough.
• Specify face seals for the bearing case, with an expansion chamber installed on the bearing case vent.
• Install a sight glass or dip stick to check the oil level in the bearing case.
• Do not use grease fittings on the bearing housing. If you are going to use grease, hand pack the bearings.
• Use only a solid pump shaft for mechanical seals.
• Specify a duplex metal for impellers to get the combination of wear and corrosion resistance.
• Be sure there is adequate N.P.S.H. for the application.
• Specify the large diameter stuffing box for mechanical seals. Do not use tapered versions.
• Select the correct diameter impeller to stay within 10% of the best efficiency point.
• Be sure the impeller to cutwater clearance is no less than 4% of the impeller diameter to avoid cavitation problems.

IF YOU MUST OPERATE OFF THE BEP

• Stabilize the shaft
  • Be sure the shaft L³/D⁴ is less than 60 (2.0 in the metric pumps).
  • Try to specify as double volute if possible.
  • Stabilize the shaft with a suitable non sparking bushing
• Move the seal close to the bearing. You can do this with a stuffing box extension.
• Provide bearing oil cooling if the application is hot and you are concerned about soak temperature through the shaft.
• Watch out for cavitation problems if you are operating on the high capacity side of the pump curve.
• Throttle only the discharge side of the pump, never the suction side unless you are certain that the extra heat caused by discharge throttling will flash the product.

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