Pump rebuilding

Some pump rebuilding tips to prevent future vibration problems 15-8

Installing a new key in a coupling

  • If a balancing problem is suspected, check the length of the key protruding from the couplings.
  • Insure that the keys are not sitting at a 90-degree angle to each other across the coupling.

Rotating elements, that require setscrews positioned at different angles to each other (such as at 90 degrees), require the following procedure to be performed during installation or balancing:

  • Always tighten the setscrew located over the keyway before tightening the second setscrew. This procedure should also be used during final assembly. When setscrews are located at only one angle, such as two setscrews over the key, there is no need for any special procedure.
  • All machined surfaces should be checked for squareness of their mating surfaces. This can be confirmed on pumps by checking the total indicator runout (TIR) of the shafts when the coupling bolts are slackened off slightly, and then again when fully tightened. All machined surfaces should be checked out to ensure that surfaces are as flat as possible.
  • The peripheral edge of screwed holes on flat surfaces, especially those that will secure pump and motor feet to a bedplate, must be checked to ensure the metal pullout from the thread does not form a lip that will cause soft foot. Soft foot describes the tendency for one of the feet to rise when its hold down bolt is loosened.
  • Nicks, burrs, and hammer dings should be eliminated around bolt holes. To ensure that dirt and other foreign matter do not get pushed into bolt holes, or between mating surfaces when the bolts are inserted, clean the holes and bolts as carefully as the mating surfaces.
  • Never put threadguard or other thread lubricant into blind bolt holes. The thread lubricant should always be applied lightly along the bolt’s thread to prevent the lubricant from being pushed into the bottom of the blind hole and, as the bolt is screwed down into the hole, create a hydraulic jacking effect preventing the bolt from tightening properly.

Split case double suction pumps

  • Some of these pumps use “sleeve nuts” to lock the sleeves in place on the shaft. When mounting mechanical seals on these sleeve nuts be sure to check the sleeve nut outside diameters for concentricity after assembling the rotating element. Also check to insure that the sleeve is sealed to either the impeller or the shaft. It is easy to forget to check this, and it always results in a leak.
  • The impellers on these pumps are usually held centered in the pump case via the position of the sleeves. If the impeller is not properly centered in the pump casing there will be a hydraulic imbalance that could result in excessive thrust loads on the pump bearings.
  • Impeller wear rings should have their final outside diameter machined after the wear rings have been installed on the impeller if possible. This eliminates any “out of round” conditions due to set screw distortion, welding or rough handling.
  • When purchasing casing rings, check them for concentricity before installing them. These parts are often damaged in shipping and handling.

Installing a centrifugal pump

  • The mass of the pump concrete foundation should be five times the mass of the pump, base plate and other equipment being supported.
  • The foundation should be three inches (75 mm) wider than the base plate, all around, up to 500 horsepower (375 KW) and six inches (150 mm) above 500 horse power (375 KW).
  • Imaginary lines, extended downward 30 degrees to either side of a vertical through the pump shaft, should pass through the bottom of the foundation and not the sides.

Common sense dictates that any loose bolts or hardware is going to cause a problem with vibration, so be sure to check for the use of lock washers and all tightening torque specifications. Here are a couple of more things you can check while you are at it:

  • Be careful of torquing the bearing retainer nuts after you make an impeller sdjustment. This uneven clamping force can distort the bearing and cause overheating problems. In many cases torquing to the manufacturers recommended specification can cause the problem.
  • Do not let anyone hold or rotate the pump shaft using a pipe wrench. Not only can you create an unbalance problem, but you will have future problems installing cartridge mechanical seals and many types of bearing seals.
  • Do not use shaft centers when you use a bearing puller, it is too easy to damage the soft centers. Use a piece of flat plate instead, or a hardened insert into the shaft center.

Category

Posted

  • On February 18, 2018