Predicting oncoming failure is a snap with sealless pumps

You can predict incipient failures in this class of fluid handling equipment.

By Julien Le Bleu, Jr., Arco Chemical Corp., and James Lobach, Chempump Division of Crane Pumps and Systems

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Centrifugal sealess pumps, both canned motor and magnetic drive, should be monitored to determine mechanical condition. In sealess pumps, the pumped fluid is the cooling and lubricating medium for the pump bearings. If intermittent monitoring is used, then the chance of detecting pump damage caused by process changes is very small.

Vibration monitoring techniques as applied to sealed pumps have been unreliable for detecting problems. The effectiveness of conventional monitoring techniques is limited by the time interval between measurements, the relative isolation of the inner pump rotor from the outer measuring location, and by the pumped fluid. Other factors such as fluid affects and process noises can make interpretation difficult.

This paper presents the synergistic combination of two relatively new methods of sealess pump monitoring. These methods considerably enhance the range and magnitude of mechanical problems that can be identified on this type of pump.

One of the goals of predictive maintenance is the reduction of maintenance costs by use of condition monitoring. Identification of off-design operating conditions or mechanical damage at an early stage enables one to correct the conditions before damage occurs, or to optimally schedule repairs. Continuous monitoring of sealess pumps reduces equipment maintenance costs and facilitates root cause analysis of mechanical failures and operational problems. This cost reduction is accomplished by the use of the monitoring system to immediately identify conditions that can lead to failure.

This article presents the results of experiments that attempted to determine reliable predictive condition measurement tools for sealess pumps. These experiments measured mechanical and operating parameters as the pump operating points varied. The data from individual measurements was examined for correlation with other measurements and also to mechanical condition and damage. Monitoring of mechanical condition can be used to schedule maintenance intervals, but the best use of monitoring is to allow the detection of the conditions that lead to equipment damage. The monitoring system should provide information early enough to allow the potentially damaging conditions to be changed. Thus, the possible cause for the response and potential for failure are eliminated.

The parameters recorded during lab tests were: power in watts, overall high frequency tracking (overall high frequency tracking), rotor position, suction pressure, and capacity in gpm. The testing was done on a canned motor pump with a commercially available rotor position monitor supplied by a major pump manufacturer. A "truth" table was generated for various conditions and the table was tested using a closed pumping loop.

Sealess centrifugal process pumps fall into two major categories. One is the magnetically driven design and the other is the canned motor design. The two types have certain similarities. For example, they both use the process fluid for cooling the drive mechanism and lubricating of the internal bearings. The designs differ in how rotation is induced in the impeller and rotor.

The synchronous magnetic drive utilizes two sets of magnets, one on either side of a containment shell made of non-magnetic material (usually stainless steel or Hastaloy). An electric motor moves the outer magnets and oil lubricated bearings supported them. The inner magnets follow the outer magnets by the attraction through the containment shell. A second non-magnetic covering protects the internal magnets from the process fluid. The internal magnets are attached to a shaft that drives the pump impeller. The shaft is supported on bearings that are lubricated by the process fluid.

The canned motor pump uses a single rotating element that is essentially the rotor of an electric motor but with an impeller mounted on the shaft. The motor and pump casings are sealed eliminating the shaft penetrations common to conventional pumps with mechanical seals. A non-magnetic containment shell protects the stator and motor rotor from the process fluid. The rotor is supported generally using fluid lubricated film bearings. A portion of the pumped fluid is circulated to the motor to provide cooling and bearing lubrication. Canned motor mechanical construction is less complex than magnetic drive designs, but catastrophic failures of the stator assemblies are expensive to repair and difficult to decontaminate.

We sought a method of getting reliable and timely information about the condition of sealess pumps. The ideal condition monitoring method should:

  • be non-intrusive, the monitoring devices should not penetrate the liquid containing parts of the pump;
  • be able to indicate process changes that influence pump operation as well as measure mechanical wear;
  • be reliable and proven technology;
  • be readily available;
  • the results should be easy to interpret;
  • contain enough parameters to indicate a problem so "false trips" are not an occurrence and detecting failures are assured;
  • be easily retrofitted to existing sealess pumps;
  • not require the monitoring sensors to be sacrificial or consumed during normal wear of the equipment;
  • be upgradable so that new pumps do not need to be purchased just to have the improved monitoring technology; and
  • be able to withstand a chemical plant environment.

Several clear and simple patterns indicate problems with sealess pumps. However, the sensitivity of the detection system generates many indications. Not all indications can be resolved and not all indications imply an immediate problem. Improved diagnostics based on multiple parameters should be able to determine a healthy or unhealthy pump and pumping environment. This thinking lead to the collaboration between a pump manufacturer and an end user to instrument a pump and manipulate many normally occurring operating parameters to get a known response. The collaboration resulted in the generation of a truth table. This truth table lets pump users determine the cause of conditions leading to the response measured by the monitoring systems.

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