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Pumps & Systems, May 2008

Failures of submerged vertical pumps often occur without warning from the installed sensors and monitors  because of the lack of transducers mounted where they can reliably sense the condition of the pump-down in the hole! Since there are no industrial standards that require sensors at the bottom of the pump, regardless of the power rating, the sensors are most often mounted on the motor, some distance from the pump. In the past, reduced efficiency and pump failure have not proven enough incentive for operators to properly instrument the system.

Vertical Pump Monitoring

The monitoring methodologies in ISO 10816-1: 1995 Mechanical vibration - Evaluation of machine vibration by measurements on non-rotating parts recommend measuring at points that "significantly respond to the dynamic forces and characterize the overall vibration of the machine." However, most end-users consider it unreasonable to monitor every cutlass rubber bearing on the riser tube, and 10816-3: 1998 (E) recognizes the practical considerations that cause the points to usually be on "exposed parts of the machine that are normally accessible."

In fact, the measuring points suggested by 10816-1 (see Figure 1) are shown on only the bearing housings at the top of the machine; because the pump bowl/impeller region is not easily accessible, it is omitted. Therefore, many users of vertical pumps are unaware of the machinery information available "down the hole" and ignore, for what was formerly justifiable economic reasons, an essential source of data. In the case of vertical cooling water pumps, where the intake, pump bowl, impeller(s) and bearings that ISO strongly advocates monitoring, are all under water, the data must be collected by other than the usual methods.

While the ISO 10816-1 standard only addresses measurements taken on non-rotating parts (the casing), there are good reasons to use proximity measurements of the shaft. For example, it is impossible to determine the amount of wear on the journal bearing caused by suspended particulate matter by using case-mounted transducers alone. No matter which transducer is selected to provide the data needed to manage the machine, it is likely that an effective, reliable installation package can be designed to simplify the process, reduce costs and economically justify monitoring machinery that, up to now, has been pumping some profits down the drain.

The machinery management regime selected by the user will dictate the need for permanent online systems for critical units or a portable data collector (PDC) on small pumps or multi-spare installations. Whether the data is collected via a PDC, or via permanent monitoring and management systems, there are methodologies available to help collect data from those previously inaccessible places.

Information Is Missing or Attenuated

There are certain conditions and configurations where the pump vibration data can be picked up at the top of the motor. However, this "tail-wagging-the-dog" methodology has limitations; the signal will be attenuated by the different machine components between the pump and the motor and will be overlaid with the signal of the motor. For example, the detection of cavitation can only be carried out in the impeller/pump bowl region. Farther up the riser tube, flow-generated noise from the intermediate bearings adds to the cavitation signal and confuses the diagnosis. Since higher frequency signals attenuate faster than lower frequency signals, the data may not even reach the transducers (see Figure 5).

Results from several installations have shown that bearing and impeller condition data is readily available to transducers installed under water, close to the source of the data.

Why Would This Data Be Needed?

For a pump to be maintained at its optimum performance, regular inspection is recommended. Downstream indicators of the pump may exist that can show if the pressure or flow is below the required norm; if trended, they will give an indication that the pump should be withdrawn from service before unacceptable performance or failure occurs. The vibration data can be used to indicate the mechanical condition of the submerged bearings, what parts might need to be replaced and an appropriate maintenance schedule to minimize the cost of downtime. 

The removal of the submerged section of a large vertical pump requires a great deal of preparatory work and the use of heavy mobile cranes. In some cases, the high cost of crane hire has been a deciding factor in the decision to instrument the submerged section of the pumps; the cost of instrumentation and monitoring is typically less than the charge to hire a heavy crane for one week. With monitoring installed, the decision to lift the pump for maintenance can be based on real data rather than on guesswork.

Considerations for Bearing Wear Monitoring

When XY proximity probes are installed to monitor pump bearing vibration and wear, a special probe protection package and modifications are required to make a successful installation. The proximity probe has to be packaged in a watertight outer casing that includes a ceramic cap. This cap provides a nonmetallic window that prevents the erosion of the probe tip. The thickness of the cap, 0.25- to 0.50-mm dependent on the transducer, reduces the effective linear range of the transducer. 

The reduction of linear range means that less wear can be detected. With a 5-mm proximity transducer, the thickness of the ceramic tip reduces the range for diametral wear allowance to 1.5-mm or less. It may be appropriate to use a larger 11-mm probe, which offers a diametral wear allowance of up to 3.8-mm. In both cases, the probe setup is critical because the thickness of the end cap, while it decreases the physical range of the probe, does not lower the OK limit in the monitor. It is therefore possible for the shaft to contact the ceramic tip before this limit is reached. If this can occur, then a modification to the monitor may be required.

Mechanical modifications to the shaft and casing will also be significant; for example, a probe target ring , usually a nickel-based superalloy steel, must be attached to the shaft to minimize corrosion of the track that the probe views. Where the probe cable has to pass through the pump waterways, the conduit pipe for the probe cable should be attached to the leading edge of fixed spokes or the cable carried in drillings within these webs. Secure methods of attachment are mandatory.