Some pump rebuilding tips to prevent future vibration problems
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
- 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
- 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
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
- 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
- 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.
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