Pump preventive maintenance
Pump and seal preventative maintenance – what is it all about? 2-6
Smart shop maintenance can be approached several different ways:
- Reactionary Maintenance – The equipment has failed and you have to fix it right now! If you have an installed spare it helps, but you must fix it immediately because you can’t afford to run without a spare. This is the “norm” in most plants.
- Preventative Maintenance – You’ll take appropriate actions and thereby prevent the unit from failing . Most companies are still trying to figure out what those appropriate actions are.
- Predictive Maintenance – By taking selected readings we hope to be able to predict an impending problem and calculate how much longer the unit will run before failure. A lot of information is being collected, but the concerned parties are still trying to figure out how to use it. Most predictive maintenance calls for shutting down the equipment when some arbitrary time limit has been reached and this puts you back to reactive maintenance again.
- Continuous Diagnostic Maintenance – You’ll take constant readings and note any significant change in these readings. Hopefully you’ll then be able to predict impending failure. This is very similar to reading the instruments on the dashboard of your automobile.
- Machinery History – By keeping good records we hope to predict the life of the unit or its individual components. This system assumes that the life of the previous unit somehow relates to the life of the present one.
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.
- Heat – Especially in the seal chamber and bearing case. A changing reading at the pump suction would be helpful in predicting cavitation. Volute casing readings could indicate internal recirculation and minimum flow problems as well as an indication of impeller rubbing.
- Pressure – You can take readings at the pump discharge, suction and stuffing box to determine where you are on the pump curve and see if you’re within the operating range of your mechanical seal.
- Speed – To see how it affects pump curve data. The pump curves were generated with a variable frequency motor at a speed different than your induction motor.
- Noise – To indicate cavitation, rubbing, location on the pump curve, bad bearings, or some other abnormal condition.
- Flow – To check the status of wear rings, impeller adjustment and the discharge recirculation system.
- Strain – To anticipate rubbing and stress corrosion problems.
- Liquid level – To anticipate npsh,bep and air ingestion problems.
- Leakage and Fugitive Emissions – To check the seal performance in both the stuffing box and bearing case locations.
- Product contamination- To monitor the performance of dual seals and flushing controls.
- Functioning of stuffing box environmental controls – To anticipate seal failure.
- Power Consumption – To check pump efficiency and to anticipate heat problems.
- Vibration – At multiple locations in the system to indicate that a failure has already started.
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.
- Evidence of rubbing.
- Product attaching to the hardware.
- The presence of foreign objects.
- Missing parts.
- A wrong part.
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:
- Clearances – At the wear rings and bearing fits.
- Dynamic balance – of the entire rotating assembly or the individual components
- Alignment – Between the pump and the driver as well as the piping and the pump flanges.
- Settings – For the seal face loading and impeller clearance.
- Shaft deflection – To insure that rotating parts will not contact stationary parts.
- Shaft axial movement – Especially equipment with sleeve or babbitt bearings. Both impellers and mechanical seals are sensitive to this movement.
- Oil analysis – To learn if we are experiencing excessive wear or if our lubrication is breaking down. An 18 degree Fahrenheit (10 C.) increase in oil temperature will cut the service life of the oil in half.
- X-Ray – To detect cracks in metal, especially at the welds or to indicate evidence of Stress Corrosion cracking.
- Thermal imaging – To detect rubbing and heat losses.
- Magnetism – Especially in the bearing area. Magnetized bearings or seals attract the metal particles found in worn lubricating oil.
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 :
- How to read a pump curve
- How to make a system curve and how to relate it to the pump curve.
- The causes of Cavitation and how to stop it.
- How to align the pump and motor.
- How to prevent pipe strain.
- Good piping practices to prevent liquid turbulence.
- Troubleshooting pumps and seals.
- How to set impellers.
- How to install a bearing.
- How to install a Mechanical Seal and still be able to adjust open impellers for thermal growth and wear.
- How to install wear rings.
- And the list goes on……
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?
- Use “C” ( Metric uses “D”) frame adapters to simplify pump/motor alignment.
- Use Centerline wet ends if the operating fluid temperature exceeds 200° Fahrenheit (100° C)
- Balance all shaft assemblies and check they are straight.
- If you’re using open impellers keep them adjusted to the correct “hot” setting.
- Maintain the correct oil level and change bearing oil frequently. If you’re using grease lubrication it’s more difficult to change the grease, but it has to be done. Two thousand hours (83 days) should be a maximum unless you can guarantee there was no moisture ingestion or the lubricant was not overheated. Be careful not to over lubricate the bearings.
- Use labyrinth, or positive face seals to keep moisture out of the bearing lubrication and to prevent shaft fretting damage.
- Do not use shafts with an L3/D4 ratio above 60 (2,0 metric)
- Try to keep Suction Specific Speed numbers below 8500 (10,000 metric) and never above 15,000 (16,500 metric)
- Maintain the correct clearance between the impeller and the pump cutwater or tongue. It should run between 4% and 6% of the impeller diameter. Use 4% for impeller diameters up to 14″ (355 mm) or less and 6% over 14″ (355 mm).
- Use corrosion resistant solid shafts only. Sleeves do not add strength to shafts. Sealed pumps shouldn’t need sleeves, unless you’re using the type thet frett and groove shafts.
- Make sure you have enough Net Positive Suction Head Available (npsha) to prevent cavitation.
- Do not let air enter into the system. Air comes in through shaft packing, flanges, and valves above the water line. Vortexing, aerating the incoming liquid, and pumping the suction dry are some other common causes.
- Replace wear rings any time the original specified clearance doubles.
- Pay attention to parts storage.
- As an example, Buna “N” rubber has a shelf life of only one year because of ozone attack. Proper packaging can increase this life considerably.
- Many pump power ends are already rusted internally at the time of installation because of poor storage policies and lack of internal corrosion protection.
- Lapped seals should be packaged in such a way that they can survive a one meter (39 inch ) drop without injuring the lapped faces.
- Use only hydraulically balanced seals for all of your sealing applications. They’ll be able to handle fluctuating system and flush pressures.
- Use only non-fretting seal designs to prevent costly shaft damage. All real seal companies have them available.
- If possible, bore out your present stuffing box, or install a commercially available large bore sealing chamber to give the mechanical seal room to move and centrifuge dirt and/or solids from the faces.
- Use universal seal materials to prevent material mix up and lower inventory costs.
- Grade 316 stainless steel seal metal components can usually be used in any pump manufactured from iron, steel, stainless steel or bronze. CAUTION do not use stainless steel springs or metal bellows because of Chloride Stress Corrosion. Hastelloy “C” would be the best choice for these locations.
- Use only unfilled carbons. They’ll be chemically compatible with any chemical except an oxidizing agent. Oxidizing agents combine with carbon to form carbon monoxide and carbon dioxide
- Silicone carbide is the best universal hard face material. Two versions are available, reaction bonded and alpha sintered. Alpha sintered is the preferred one
- Most of the chemical in this world can be sealed by either Viton® or Ethylene Propylene elastomers. Dupont’s Kalrez ®, Green Tweed’s Chemraz or a similar product should take care of the rest.
- Unless you’re pumping a fluid at, or close to its vapor point, connect a recirculation line from the bottom of the stuffing box back to the suction side of the pump, in place of the line installed from the discharge side of the pump to the stuffing box..
- To insure good seal life, be sure that the installed environmental controls are working.
- Cooling/heating jackets can become clogged with calcium and become inoperative. If your cooling water is too hard or dirty, use condensate instead.
- Flushing pressures can vary, or flushing lines can become clogged. You may have to install a separate system. A flow meter will help yo to be sure that you’re flushing the correct amount.
- Quenching must be regulated to prevent water entering into the bearings. (Another good reason to use labyrinth or face type bearing seals).
- Convection tanks can run backwards, make sure the piping is installed correctly and the rotating components are centered in the stationary gland.
- Install cathode protection where ever practical and possible.
- Use o-ring seal designs whenever possible. O-rings seal iboth vacuum and pressure, and can flex to compensate for minor shaft vibration and movement. Remember also that the o-ring is a natural vibration damper.
- Avoid pipe strain problems by piping from the pump to the pipe rack and use a “centerline” wet end any time the pumping fluid temperature exceeds 200° Fahrenheit. (100 C.)
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.
- You can use either tandem, face to face, or “two way balanced seals”, but never rotating, “back to back” designs. A convection tank can be installed between the seals and the level / pressure in the tank will indicate which seal has worn out or failed first.
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.
® DuPont Dow elastomer
- On February 17, 2018