Increasing Pump Reliability and Life (Part Two)


Written by:
Michael Mancini, Mancini Consulting Services

Editor's Note: This is part two of a two part series about the 'commandments' of increasing pump reliability and life.

Last month, we discussed Commandment 11, Thou shalt always operate centrifugal pumps at their best efficiency point.{C}

This article will explain the reasoning behind the second unknown commandment, Commandment 12: Thou shalt avoid prematurely opening pump wear component clearances.

Commandment 12

The premature opening of wear components clearances is especially damaging to a pump's life and reliability. The wear components act as water lubricated bearings and provide stiffness and damping to the pump rotor. The wear components also limit internal recirculation and directly impact overall pump efficiency.

Opening wear component clearances geometrically reduces rotor stiffness and damping, intensifying the relative motion between the rotating and stationary elements caused by normal pump external forces. Additional contact of the wear components caused by this motion increases the clearance, further reducing rotor damping and increasing relative motion. This vicious cycle continues until high vibration, loss of performance or other issues lead to the pump being removed from service.

Causes

Wear ring clearances open at an increased rate when the stationary and rotating wear surfaces come into contact. The most common causes of contact are high vibration amplitudes, increased shaft orbit and non-centerline compatibility of stationary close-clearance bores (wear rings, sleeve bearings, balance sleeves) to the true shaft centerline.

In addition to the causes discussed in Part One, some of the major factors that influence the premature opening of the wear ring clearances are described below.

Impeller and Coupling Imbalance

Balance tolerances are typically expressed as a function of the component weight and the pump operating speed (see Equation 1).

Best-in-class processes achieve a rotor balance < 1W/N for high energy services and < 4W/N for other services. To achieve 1W/N, certain conditions must preexist:

  1. All keys must be fitted to keyways with no excessive stock or unfilled areas (as would occur when using square instead of full-radius keys)
  2. The impeller must have an interference fit-up to the shaft
  3. Shaft concentricity (total indicated runout, T.I.R.) cannot exceed 0.001-in
  4. Impeller hub turn T.I.R. cannot exceed 0.002-in
  5. Split rings, when used, must be manufactured to maintain circularity of the bore with exact fit-up of the split ring halves

 

Angular or Parallel Pump-to-Driver Misalignment-Horizontal Pumps

Angular and parallel pump-to-driver misalignment in horizontal pumps is usually the result of unevenness of the pump mounting pads and baseplate, distortion due to thermal growth or poor maintenance processes. Routine maintenance and inspections are often completed on the pump internal elements while the baseplate, barrel or discharge head remain unchecked. As installations age, it is important to periodically check that the baseplate and pump mounting surfaces have not been distorted through external forces, deteriorating grout or other factors. Misalignment due to thermal growth relates predominantly to foot-mounted pumps in high temperature services. In these instances, centerline mounting of the pump should be considered.

Many techniques are used to achieve proper pump-to-motor alignment. State-of-the-art laser systems can provide information for both cold and process temperature conditions.

Angular or Parallel Pump-to-Driver Misalignment-Vertical Pumps

Angular misalignment in vertical pumps occurs as a result of mating faces that are not parallel to each other and perpendicular to the true shaft centerline (see Figure 2). Primary among these conditions are the misalignment of the motor-to-pump shaft, distortion of the discharge head over time and build-up of FME (foreign material exclusion) that can change the relationship between mating faces.

Parallel misalignment in vertical pumps occurs as a result of eccentricities and/or excessive clearance in register fits, non-adjustability of the motor to the discharge head and poor assembly practices (see Figure 3).

Assembling vertical pumps horizontally will also lead to parallel misalignment, even when critical fits are within tolerance, and especially when many column/casing sections are involved. Each component will sit on the bottom of its register fit, resulting in a stack-up of the individual clearances and eccentricities. Better practices include assembling the pump in the vertical orientation where the centerline relationship of each added component can be checked during assembly, or rotating the assembly 180-deg after the installation of each stage piece.

Static Shaft Bow

Shaft straightness is a function of material selection, the process to produce the raw bar and the manufacturing process to achieve the final product. Preferred shaft run-out is maintained at or below 0.001-in and should not exceed 0.002-in. Many specifications allow 0.001-in total indicated runout (T.I.R.) for each foot of shaft length. This is unacceptable, and manufacturers should be encouraged to meet the more stringent criteria.

In addition, neither mechanical nor thermal straightening should be used to restore shaft straightness since internal stresses will relieve themselves under load and the shaft will assume its former shape. Shaft straightening should be prohibited from internal and sub-vendor processes.

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See also:

Upstream Pumping Solutions

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