Does your pump have an alignment problem? Part 2
by Jim Elsey
April 24, 2019

Last month, I discussed why a pump should be aligned nine times and declared that four of those alignment adjustments should be made before the pump is started up. The prevailing, yet erroneous, assumption is that the pump is pre-aligned to a high level of precision in the factory and the initial factory alignment will hold firmly in place during the transit and installation processes.

Driver-to-pump misalignment will cause many expensive issues, and the subsequent cost of operating the pump will be higher. Pay to align the pump upfront, or you will end up paying more later. Field data has consistently shown that without a proper alignment, you may expect reliable operations of one year or less without issues. If you take time to properly align the units, the runtime can be extended to eight to 10 years.

Thermal Rise Due to Expansion

Just because the manufacturer said that some piece of equipment will have a certain thermal rise due to expansion does not mean it will actually grow that much. My experience is it could be substantially more or less. That is why the hot alignment checks are necessary. Note: this comment also applies to cryogenic applications (shrinkage).

How much a pump or driver will rise depends on many factors including how it is mounted and the material involved. Two common mounting methods are foot mounted and centerline. As an example, typical ANSI pump (foot mounted) will grow about 0.001 inches per 100 degrees of fluid temperature above the ambient temperature. So, a pump with a fluid temperature of around 180 to 200 F with an ambient of 70 F will grow less than 0.002 inches and probably closer to 0.0013 inches.

The material of construction will change the total thermal growth a small amount depending on the C value for the material, where C value is the coefficient of linear expansion. Cast iron, carbon steel and austenitic stainless steels in the 300 range all have similar C values. Stainless will grow more than cast iron with carbon steel in the midrange between them.

There are several technical methods and associated instruments for accurately measuring the alignment at the operating temperature. I cannot go into detail here, but the point is to make you aware they exist.

Having worked for two large companies that manufacture turbines, I know that steam turbines, being foot mounted, grow up with both ends fairly parallel to each other. Gas turbines do not grow at the same rate per end and will present an offset. You will see angular differences at the coupling face in lieu of parallel.

Machine temperature is not the only thing that affects change to the alignment during machine operation. Note that foundation design, operating pressure, shaft torque and machine location will also be factors. A major and commonly overlooked factor is the pipe system design. Understand that if the unit is involved in a drive train of several pieces of equipment, that will be a major factor, and I suggest you consult specialists in this area.

Caution note: pipe strain can change substantially depending on if it is full of liquid or not. An alignment with the associated piping empty will likely change when the piping is full.

The acceptance criteria for alignment specifications is directly proportional to the machine speed along with the coupling gap distance. The higher the speed, the smaller the acceptable tolerances. Be careful with variable speed applications and make sure your criterion is for the highest speed that the unit will experience. On laser alignment equipment, one of the setup criteria in the base program is speed. Be careful that you don’t set the base specifications at 1,200 revolutions per minute (rpm) for a 3,600 rpm unit.

Having spent many years in the U.S. Navy submarine service, I know and understand how much the U.S. Navy’s practice of using tapered dowel pins to preserve equipment alignments has carried over to the civilian world. Please note two things:

  • First, question if you really need to use a dowel pin (probably not).
  • Second, when you take the unit apart, never reuse the same hole and dowel pin. It is a common field practice to drill and taper ream a larger hole in the same spot, which I suppose is an acceptable practice, but I still question the taper pin practice outside of warship applications in the first place.

Checking for Pipe Strain Effects

To check for pipe strain effects on the equipment alignment, an easy method is to place one or two dial indicators to read off of the coupling surface. If you have a laser unit, that also works fine. Offset the two dial indicators by 90 degrees (one at the midnight position and the other at either the 9 or 3 o’clock position) and zero them on the coupling. Release the bolts on one of the pump flanges and look for potential movement on the dial indicator. More than 0.002 inches movement is not acceptable. Repeat the test for the remaining pump flange.

You should be able to duplicate the final alignment readings to test that the procedure you used was valid.

Pages