SUBJECT: Pump and seal preventative maintenance - what is it all about? 2-6

Smart shop maintenance can be approached several different ways:

The problem with most of these systems is that we collect more data than the operator or any one else can deal with. The result is that Reactionary Maintenance is a "reality" in most plants today.

Since the taking of readings is part of most of these programs let us take a look at the type of information we can gather for analysis.

You can monitor :

The monitoring of vibration is confusing to many people. We hear about frequency, amplitude, velocity, acceleration, I.P.S. and all sorts of technical jargon. Probably the system verbalized the most, is the reading of acceleration ( in./sec2 or mm/ sec2). The problem with this system is that it is dependent upon the frequency of the vibration. Other companies use decibels as a method of measurement with a decibel defined as:

20 log10 input /reference

In this system everyone uses a different reference except the people measuring sound who have agreed upon background noise as their reference. Since this is a logarithmic scale it allows you a big range to compute change in levels. In fact each 6 db is equivalent to a two times increase in vibration level.

The bottom line is, regardless of the method you are using, only a relative number. Most people agree that a two times increase in reading is cause for concern and the equipment should be shut down for a visual inspection.

The transducers that pick up this vibration can be either permanently mounted or portable, with permanent being the preferred method. Be sure to install the transducers on a flat, clean surface and be careful how you screw them down. To insure good contact it helps to place a small amount of silicone grease under the transducer to fill in irregularities that might trap air and give a false reading.

If you are going to use the portable type of vibration analyzer you should drill a small recess at the location you wish to monitor and lubricate it with silicone grease to prevent corrosion. This recess should match the curvature of the probe. Be sure the area is clean before placing the probe in the recess and be sure to hold the probe in a vertical or horizontal position, never upside down. If it must be at an angle you must try to duplicate the same angle each time you take a reading. Your readings will be relative readings so they will have no meaning outside of your own organization and this particular piece of equipment.

Many problems become visible when we look at the disassembled hardware.

An inspection of individual components is still one of the best methods of troubleshooting. You can see :

Be sure to note the order in which the parts came out to determine an improper assembly.

There are things you can measure as well as things that can be monitored or observed:

Lists like the one above could keep a maintenance staff busy forever, and no one could deny that the information would be valuable. The real question, however, is how practical would it be to do those things? A human being could be wired to give constant readings of his blood pressure, pulse, E.K.G., cholesterol etc.. but no one would think of doing it unless he were in terrible health and in intensive care.

Most maintenance programs start with the false assumption that the life to date is some how related to how much service life is left in the equipment. In other words; if half of the seal wearable face is still left then the seal can be logically expected to run the same amount of time as before. The problem with this logic is that it only works if the components are wearing out. In the case of seals and bearings, failure is the most common condition with "wearing out" taking place less than fifteen percent of the time.

You only have to look at the mechanical seals that have been removed from your pumps to verify this statement. The only sacrificial part of any mechanical seal is the carbon face and an inspection of used seals will show that in better than 85% of the cases, the used seals have substantial face material left. Normally fatigued bearings are even more rare than worn out seals.

Some years ago the U.S. Navy contracted for the building of K (Killer) Class submarines. They were super SONAR (listening) ships with the capability of detecting enemy submarines from a long distance. They did an excellent job of detecting enemy submarines, but were too slow to catch and destroy them. The result was that they recorded only the passing of ships and were eventually scrapped. I see this as the same problem with most of these maintenance programs. We are recording the data, but the seals and bearings are still failing at the same rate.

I have no problem with people who want to monitor equipment, but I do have a problem with people who want to substitute monitoring for good maintenance practices. Unfortunately these two groups are often composed of different people operating under different budgets.

Lecturing to maintenance groups, I find very little concern with sensible maintenance practices and a growing concern for monitoring. The common complaint among maintenance people is that there's no time to do the work correctly because of the pressures of production. I also find a lack of training in the basics such as :

Most experienced people, and almost all pump manufacturers agree that the main cause of premature pump shutdown is seal and/or bearing failure. What then would be minimum good maintenance practices for seals and bearings?

Stop shaft deflection. It'll cause problems with packing, mechanical seals, bearings and will change critical dimensions such as impeller clearances, wear ring clearances, seal settings etc.

Other good practices :

The most sensible thing you can do to prevent unexpected pump shut down is to install a "back up" mechanical seal in each of your pumps. Since the seal is the most likely component to fail. and you want to maximize the seal life, the "back up" seal will allow you to run to failure and will give you time to schedule a shut down at your convenience.

The only other sensible solution to an unexpected costly shutdown is a split mechanical seal that can get you back on line, usually in less than an hour.

Once these maintenance practices have been initiated and back up sealing provided, a well thought out monitoring system can be of great value. If given a choice I would vote for a constant monitoring type of system, but the fact of the matter is that any of them would be of value.

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