PUMP CONDITION MONITORING GUIDELINES

Process pump users often seek a method of determining the optimum time for overhaul of a pump to avoid unplanned downtime, realize energy savings or other considerations.  Once developed, such a tool should be widely used by maintenance engineers and managers in their role of managing assets to provide capacity for production, and energy efficiency to save operating expense or even to minimize greenhouse impact.  This optimization approach can also be applied to any item where deterioration results in equipment breakdowns or loss of efficiency.

Vibration Basics

All rotating machines, including pumps, vibrate to some extent due to response from excitation forces, such as residual rotor unbalance, turbulent liquid flow, pressure pulsations, cavitation, and/or pump wear.  Further, the magnitude of the vibration will be amplified if the vibration frequency approaches the resonant frequency of a major pump, foundation and/or piping component.  The issue of interest is not whether or not the pump vibrates, but:

• is the amplitude and/or frequency of the vibration sufficient to cause actual or perceived damage to any of the pump components, or
• is the vibration a symptom of some other damaging phenomenon happening within the pump.

Various industry organizations, such as the Hydraulic Institute (Table 1, per ANSI/HI 9.6.4), and the American Petroleum Institute (API-610) have set vibration limits to help guide users to avoid excessive levels of vibration in centrifugal pumps.  However, since neither of these standards is intended for moderate-speed positive displacement pumps, one can often rely on other, experience-based values.  As an example, the bearing life vs. vibration approximation for general-purpose machinery, Fig. 1 is of interest here.

Table 1:  Allowable pump field-installed vibration values (Per Hydraulic Institute Standard ANSI/HI 9.6.4, 2000)

 

Fig. 1:  Approx. bearing life vs. vibration relationship for general purpose machinery.

Another experience-based guideline was developed by ISO, the International Standards Organization, Fig. 2.  PeopleFlo EnviroGear pumps fit best into “Class ll, medium machines” and therefore vibration velocities are acceptable as long as they don’t exceed 0.11 in./s (2.8 mm/s).

 

Fig. 2: Vibration classification chart (after ISO 10816)

Where to measure vibration

PeopleFlo teamed up with Emerson Process Management to identify the optimum locations for taking vibration readings.  The two preferred locations are shown in Figs. 3 and 4 for radial and axial readings, respectively:

• Radial bearing loads are best measured at the top and side of the bearing housing, essentially at locations between the bearings.

Fig. 3: Radial vibration monitoring points in the vertical and horizontal directions

• The front of the pump head is the best location to measure axial vibration excursions and to pick up misalignment and/or pumping element problems.

 

Fig. 4: Axial vibration monitoring point on pump head

PeopleFlo’s EnviroBase coupling guard (Fig. 5) is designed with access holes to accommodate vibration monitoring of bearings at both of these optimum locations while the guard is in place. 

Fig. 5: EnviroBase coupling guard resists deflection and accommodates all recommended means of predictive maintenance monitoring while the guard is in place

Infrared thermography and stroboscopic examination

PeopleFlo’s EnviroBase coupling guard is also designed to accommodate two prominent types of predictive maintenance measurements, infrared thermography and strobe lights.  Thermography monitors and displays equipment and component temperatures by infrared techniques.  Using either an infrared camera or “thermo-gun” aimed at the bearing housings and coupling guard allows viewing deviations from normal operation.  Typically, temperatures exceeding baseline values by more than 15°F (8°C) are high-probability indicators of problem development.  Capturing baseline data and subsequently spotting these temperature deviations will allow safe shutdown of the pump train.  Early detection will avoid more extensive and potentially much more costly repairs.

PeopleFlo also designed its coupling guards to facilitate temperature measurements on the low-speed coupling.  Of course, the drive motor bearing housings are readily accessible for these measurements.  Slotting the coupling guard allows stroboscopic inspection of coupling condition by making the running shaft “stand still” while operating.  Any potentially excessive deflection or wear-related degradation of the elastomeric block separating the two metallic coupling hubs can thus be observed.  However, with good alignment, it may take years for such degradation to occur.