Pumps & Systems, November 2008
One of the major problems facing industry today is the limited number of people with sufficient skill and experience to diagnose and rectify the basic problems plaguing centrifugal pumps. Another difficulty is that the same lack of skill and experience is creating many of these problems in the first place.
A detailed evaluation of a pump problem requires a depth of knowledge which usually surpasses that to which most people are ever exposed. Most pump engineers, operators and maintenance people develop their knowledge base from the same "school of hard knocks." While this on-the-job type of training has much to commend it, unfortunately it exposes the pupil to the opportunity of learning the mistakes and misconceptions of others. At best, it only teaches what is necessary to execute a particular job function in exactly the same manner as it was previously performed-good or bad!
The ramifications are generally imposed on the maintenance department, where the training is usually limited to the physical change-out of the parts when a breakdown occurs. As the underlying cause of pump failure often extends beyond the failed item, these maintenance methods will effectively reinstall the same old problem.
This is particularly concerning when we realize that over 80 percent of all pump failures tend to manifest themselves at the mechanical seal or the bearings, which then act in a manner similar to a fuse in an electrical system.
When a fuse in an electrical system fails, it does not mean there is anything wrong with the fuse. In fact, we understand that the problem is almost always somewhere else in the system. Despite this, when a seal or bearing fails, we rarely look for the real problem. Instead, we simply replace the offending part. While that will occasionally solve the problem, simply replacing a seal or bearing rarely provides long-lasting relief from the problem.
The extent to which this happens varies from industry to industry, as some are more aware of the root causes of pump failure than others.
As there are only a few symptoms with which to recognize a troubled pump, the key to failure analysis lies in understanding how the combinations of symptoms identify the underlying cause of the problem.
Speed of Problem Occurrence
An effective troubleshooting tool will always begin with the question, "When did this start?" If the problem has only suddenly appeared, it is likely to have a different cause than a similar problem that has been developing over time. It is also fairly obvious that a sudden appearance of the problem is probably caused by a sudden change in the condition that created the problem. Therefore, it is highly unlikely that such a problem can be attributed to normal wear and tear. It is much more probable that an inappropriate action has been initiated.
It can be argued that the exception to that concept is where wear gradually takes place until the point at which failure suddenly occurs. In this case, however, the wear is usually indicated by a gradual reduction in performance until the breaking point is reached, which provides some prior notification of imminent failure. This type of condition underscores the need for constant performance measurement as it relates to temperature, pressure, flow, vibration and power draw.
Frequency of Problem Occurrence
A typical example of this problem is when a mechanical seal in a particular pump fails every six months, regardless of the type of seal used in that pump. Maintenance may have tried many different models, types and face material combinations, but the seal fails with the same frequency every time. As it is logical to expect different seals to last different periods of time between failures, it becomes evident that this is a situation where the seal is simply acting as the "fuse" in the system. Consequently, the underlying problem is obviously elsewhere in either the pump or the system.
Skill and experience come in to play here. The above condition is one where the experienced troubleshooter would immediately consider either the pump shaft or the piping arrangement, depending on the pump model in question.
Hydraulic Imbalance in a Double Suction Pump
A horizontal, double-suction pump may be fitted with a 90-deg elbow mounted on the suction nozzle in such a way that the line leading to the elbow is parallel with the axis of the pump shaft (see Figure 1). When the liquid sweeps round the elbow, it is centrifuged out towards the long radius and feeds the eye on one side of the impeller, effectively starving the opposing eye. This creates an imbalance of the liquid in the pump casing that can cause an excessive axial thrust to be imposed on the impeller.
The normal outcome of such an arrangement is a consistently frequent failure of the mechanical seal or bearing (when packing is fitted) at the end of the shaft closest to the suction source. Such failure will normally occur at approximately 6-month intervals regardless of the type of seal or bearing installed.
Undersized Shaft in an End Suction Pump
In a horizontal, end suction centrifugal pump, frequent and regular seal failure with different seals indicates an undersized shaft subjected to excessive deflection.