SUBJECT: Is there a reliable method of introducing a centrifugal pump predictive maintenance program? 6-11

Probably not! But if you want to try you are first going to have to define what you mean by predictive maintenance. If you mean that you're going to inspect the pump and based on your observation, you're going to accurately predict future life, you're going to have a problem.

The relationship between life to date and future life is generally accepted as valid. As an example:

These are items that tend to "wear out" so life to date is a valid measurement. The problem with centrifugal pumps is that seals and bearings account for over 90% of premature pump failures and neither of these items ever "wears out". Seals should run until the sacrificial carbon face has worn away, but a close look at used seals will demonstrate that wear is actually a minor problem. In excess of 85% of mechanical seals leak with plenty of wearable face still visible.

Bearings do not "wear out" like mechanical seals. They have a predictive fatigue life that is based on load and cycles. Properly loaded they could last a hundred years, but like seals, they experience a very high premature failure rate. All of this means that the measurements you are taking today are no indication of what is going to happen tomorrow. It's like trying to predict an automobile accident. There are precautions you can take, but accidents still happen.

Most companies base their predictive maintenance programs on vibration analysis or interval timed, visual inspection. and that is why we find "reactive maintenance" the norm in most plants. How many times have we heard the expression "I did not have time to do the job correctly (realignment, dynamic balancing, etc.) because I had to get the pump back on stream".

A more sensible approach to predictive maintenance is to monitor the equipment for changes that could be destructive in the future, but allow you to correct them before the destruction starts. I spent my formative years in nuclear power. If, as an operator, you did something wrong that would be harmful to the atomic reactor it would "scram" and shut down immediately. But if you took an action that could be potentially dangerous, the reactor would start an "insertion" that would start to slowly shut down the reactor and give you time to correct what ever it was you did.

Medical people use a predictive maintenance program when they:

Pumps also "scram" and give "insertion" signals." Unfortunately vibration analysis indicates that destruction has already started (scram). Let's look at some of the "insertion" signals:

The stuffing box temperature is increasing. If it gets too hot you're going to have a problem. You had better correct the condition if you do not want to experience a premature seal failure. What can happen if the stuffing box temperature gets too hot?

What could be causing this high heat? If you take no corrective action one of the above will occur.

A change in the stuffing box pressure can cause:

If you are monitoring temperature and pressure in the stuffing box area you will note the changes mentioned and depending upon your knowledge of the above, you will have time to react before seal failure occurs.

An increase in the bearing case oil temperature is significant because the life of bearing oil is directly related to the oil temperature. Lubricating oil has a useful life of thirty years at thirty degrees centigrade (86°F) and its life is cut in half for every ten degree centigrade (18°F) increase in temperature. You can figure the temperature in the bearing is at least ten degrees centigrade (18°F) higher than the oil sump temperature. At elevated temperatures the oil will carbonize by first forming a "varnish like" film that will turn into a hard black coke at these higher temperatures. It is these formed solids that will destroy the bearing.

What is causing these elevated temperatures? There are a number of possibilities:

Oil sampling is always a good idea. It can tell you:

If you monitor pump suction and discharge pressure and coordinate this information with flow and motor amperage readings you can come up with a lot of useful information such as:

It goes without saying that constant monitoring is the most sensible answer to predictive maintenance. It is the same logic you use with your automobile. You believe that the extra expense of installed gauges is a cheap investment for longer engine life.

There is nothing wrong with vibration analysis (an E.K.G. is still part of taking a physical) but do not substitute it for sensible monitoring. The "scram " is too expensive in this very competitive world of ours.

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