PREDICTIVE MAINTENANCE P027
Is there a reliable method of introducing a centrifugal pump predictive maintenance program? 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 are going to inspect the pump and based on your observation you are going to accurately predict future life, you are going to have a problem.
In making a judgment about a piece of rotating equipment the relationship between life to date and future life is generally accepted as valid. As an example:
- Measure the depth of the tread on your automobile tires, record the distance driven on the tires, and if you do not change your driving habits, you can accurately predict the life remaining.
- Do the same thing with the shoes you are wearing and you will come up with a similar result.
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 is that the measurements you are taking today are no indication of what is going to happen tomorrow. It is 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:
- Monitor your cholesterol level. If it exceeds some preset number (two hundred in the U.S.) it means that your arteries are in danger of clogging, so you should change your diet before it becomes serious. (insertion)
- If your blood pressure is too high you could get a stroke. (insertion)
- A high fever indicates a need to get medical attention before destruction starts. (insertion)
- Some types of pain initiate an immediate operation. (scram)
- You do the same thing with your automobile:
- A high engine water temperature is a sign of engine failure in the future. You better check the fan belt and look for water leaks or a failed thermostat. You will not be shut down immediately, but you should react to the warning signs. (Insertion)
- A non-charging alternator could mean the alternator has failed or possibly a wire has come loose. You will only be able to run until the battery discharges.(insertion)
- High fuel consumption indicates a need for an engine tune-up. (insertion)
- A loss of oil pressure means shut off the engine and react immediately. (scram)
Pumps also “scram” and give “insertion” signals”. Unfortunately vibration analysis indicates that destruction has already started. Let’s look at some of the “insertion” signals:
The stuffing box temperature is increasing. If it gets too hot you are 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?
- The product can change state.
- It can stop being a lubricant and quickly become a destructive solid.
- It can vaporize, expand and blow the seal faces open leaving solids between the faces.
- It can become viscous, interfering with the free movement of the springs and bellows.
- It can solidify, gluing the lapped faces together or making the moveable components inoperable.
- It can crystallize and interfere with the moving parts of the seal.
- It can cause the product to build a film on the faces (hot oil as an example) and sliding components making them inoperable.
- Corrosion increases with increasing temperatures.
- Temperature causes materials to expand. Seal faces can go out of flat, and pressed in carbon faces can loosen in their holder. Bellows vibration dampers can stick to the shaft sleeve opening the faces.
- Some seal faces can be damaged by high heat. Plated materials and filled carbons are two such examples. Air pockets in some carbon faces can expand causing pits in the lapped faces.
- Elastomers can experience compression set problems, causing them to leak or in some cases fail completely at higher heat levels.
What could be causing this high heat? If you take no corrective action one of the above will occur.
- A loss of flushing fluid. There are multiple reasons why this could happen and I am confident you can think of many of them.
- Loss of barrier or buffer fluid between two mechanical seals or the convection of the barrier fluid has stopped for some reason. Keep in mind that petroleum products need forced lubrication or a pumping ring because of the petroleum low specific heat and poor conductivity.
- Loss of the quench in an API (American Petroleum Institute) gland.
- Loss of the discharge recirculation line because of a clogged filter, cyclone separator or heat exchanger.
- Loss of suction recirculation because of solids in the fluid.
- Loss of cooling in the stuffing box cooling jacket because the circulating water was “hard” and has deposited an insulating layer of calcium on the inside of the cooling jacket.
- The seal is running dry because the stuffing box was not vented in a vertical application.
- The seal was installed incorrectly. There is too much spring load on the faces.
- You need a hydraulic balanced seal. The unbalanced design cannot compensate for the high stuffing box pressure.
- Thermal shaft expansion is over compressing an outside seal design, or one of the seals in a dual seal application.
- The open impeller adjusting technique can over compress some seal designs.
- The stuffing box is running in a vacuum because the supply tank is not vented properly or cold weather is freezing the tank vent.
- Water hammer, pressure surges and cavitation will all alter seal face loading.
A change in the stuffing box pressure can cause:
- The product to vaporize opening the lapped faces.
- O-rings and other elastomer designs to extrude and jam the sliding components.
- Lapped seal faces to distort and go out of flat.
- A stuffing box vacuum can blow open unbalanced seals.
- A differential pressure across the elastomer can cause ethylene oxide to penetrate into the elastomer and destroy it as it expands in the lower pressure side.
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:
- Loss of circulation in the stuffing box cooling jacket.
- Loss of cooling in the bearing case cooling sump.
- Someone is cooling the outside of the bearing casing causing the outside diameter of the bearing to shrink, increasing the load.
- The bearing was installed incorrectly.
- The bearing is over lubricated. The oil level is too high or there is too much grease in the bearing.
- The lubricating oil is contaminated with water.
- The shaft is overloaded because the pump is operating off of the B.E.P., misalignment, unbalance, etc.
- There is too much axial thrust of the shaft.
Oil sampling is always a good idea. It can tell you:
- If water is getting into the oil.
- If the oil additives are still present and functioning.
- If the oil is carbonizing due to high temperature.
- If there are solids due to corrosion, bearing-cage destruction, or some other reason.
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:
- You can tell if you have the right size pump.
- You can estimate where you are in respect to the best efficiency point (BEP) and know if the shaft is deflecting, or is about to deflect.
- You can tell if the motor is close to an overload condition.
- You will know when the impeller needs adjusting or the wear rings need replacement.
- You can spot poor operating practices if you have a chart recorder installed, instead of pressure and temperature gages.
- You can tell if the tank you are pumping from is holding the proper level or if the suction lines are clogging.
- You can tell if you are getting close to cavitation.
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. Many of us believe that the extra expense of installed gauges is a cheap investment for longer engine life.
There is nothing wrong with using vibration analysis techniques to determine bearing problems (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.