Operation practices that cause frequent seal and bearing maintenance problems 6-7
Wouldn’t it be wonderful if the plant operation and maintenance departments could work independently? The fact of the matter is that there are three types of problems we encounter with centrifugal pumps and poor operation is only one of them. If you’re curious, the other two are design problems and poor maintenance practices.
Seals and bearings account for over eighty-five percent (85%) of premature centrifugal pump failure. In the following paragraphs we will be looking at only those operation practices that can, and will cause premature seal and bearing failure. Design and maintenance practices will be discussed in other papers in this series.
When pumps were supplied with jam packing, the soft packing stabilized the shaft to prevent too much deflection. In an effort to save flushing water and to conserve power, many of these same pumps have since been converted to a mechanical seal and the radial stabilization the packing provided has been lost.
The bad operating practices include:
Running the pump dry will cause over-heating and excessive vibration problems that will shorten seal life. Here are some of the common reasons why a pump is run dry:
- Failing to vent the pump prior to start-up.
- Running the tank dry at the end of the operation cycle.
- Emptying the tank for steaming or introduction of the next product.
- Running on the steam that is being used to flush the tank.
- Starting the standby pump without venting it. Venting a hazardous product can cause a lot of problems with the liquid disposal. Many operators have stopped venting for that reason.
- Tank vents sometimes freeze during cold weather. This will cause a vacuum in the suction tank, and in some cases could collapse the tank.
- Sump fluids are often dirty, corrosive or both. The control rods for the float switch will often “gum up” or corrode and give a false reading to the operator. He may think that there is an adequate level, when in fact, the tank is empty.
Dead heading the pump can cause severe shaft deflection as the pump moves off of its best efficiency point (bep.). This translates to excessive heat that will affect both the seal and the bearings as well as causing the seal faces to open, and the possibility of the impeller contacting the volute when the shaft deflects.
- Starting the centrifugal pump with a shut discharge valve is standard practice with many operation departments. The concern is to save power without realizing the damage that is being done to the mechanical seal, impeller, wear rings and bearings.
- Some pumps are equipped with a recirculation valve that must be opened to lessen the problem, but many times the valve is not opened, or the bypass line is clogged or not of the correct diameter to prevent the excessive head. Another point to remember is that if the bypass line is discharged to the suction side of the pump the increased temperature can cause cavitation.
- After a system has been blocked out the pump is started with one or more valves not opened.
- Discharge valves are shut before the pump has been stopped.
Operating off of the best efficiency point (bep.) causes shaft deflection that can fail the mechanical seal and over-load the bearings.
- Starting the pump with the discharge valve closed to save power.
- The level in the suction tank is changing. Remember that the pump pumps the difference between the discharge and suction heads. If the suction head varies, the pump moves to a different point on its curve.
- Any upset in the system such as closing, throttling or opening a valve will cause the pump to move to a new point on the curve as the tank fills.
- Pumping to the bottom of a storage tank will cause the pump to move to a different point on the curve as the tank fills. Some systems were designed for a low capacity positive displacement pump and have since been converted to a centrifugal design because of a need for higher capacity. Centrifugal pumps must discharge to the top of the tank to prevent this problem.
- If the discharge piping is restricted because of product build up on the inside walls, the pump will run throttled. This is one of the reasons that it is important to take periodic flow and amperage readings.
- Increasing the flow will often cause cavitation problems.
Seal environmental controls are necessary to insure long mechanical seal life. It is important that operations understand their function and need because many times we find the controls installed, but not functioning.
- Cooling-heating jackets should show a differential temperature between the inlet and outlet lines. If the jacket clogs up, this differential will be lost and seal failure will shortly follow.
- Barrier fluid is circulated between two mechanical seals. There may or may not be a differential temperature depending upon the flow rate. If a convection tank is installed, there should be a temperature differential between the inlet and outlet lines. The line coming out of the top of the seal to the side of the tank should be warmer than the line from the bottom of the tank to the bottom of the seals, otherwise the system is running backwards and may fail completely. The level in the tank is also critical. It should be above the tank inlet line or no convection will occur. Some convection tanks are pressurized with a gas of some type. Many original equipment (O.E.M.) seal designs will fail if this differential pressure is lost.
- Some seal glands (A.P.I. type) are equipped with a quench connection that looks like the seal is leaking water or steam. If there is too much steam pressure on this quench connection, the excessive leakage will get into the bearings causing premature failure. The steam is often used to keep the product warm to prevent it from solidifying, crystallizing, getting too viscous, building a film on the faces etc. Operating people frequently shut off the quench to stop the condensate from leaking.
- Flushing fluids are used for a variety of purposes, but most of the time they are used to get rid of unwanted solids. The flush can be closely controlled with a flow meter or throttling valve. The amount of flush is determined by the seal design. As an example, those designs that have springs in the product require more flush.
- It is important to check that the stuffing box has been vented in vertical pumps. The vent should be coming out of the seal gland and not the stuffing box lantern ring connection.
There are some additional things that all operators should know to insure longer rotating equipment life. As an example :
- Mechanical seals have an 85% or more failure rate that is normally correctable. This is causing unnecessary down time and excessive operating expense. Seals should run until the sacrificial carbon face is worn away, but in more that 85% of the cases the seal fails before this happens.
- There are five different causes of cavitation.
- You should know where the best efficiency point (bep.) is on a particular pump, and how far it is safe to operate off the bep. with a mechanical seal installed.
- You should be aware that washing down the pump area with a water hose will cause premature bearing failure when the water penetrates the bearing case.
- Learn about the affect of shaft L3/D4 on pump operation.
- Know how the pumped product affects the life of the mechanical seal and why environmental controls are necessary.
- If you are not using cartridge seals, adjusting the open impeller for efficiency will shorten the seal life. In most cases the seal will open as the impeller is being adjusted to the volute. Flowserve (Durco) pumps are the best example of the exception to this rule. The popular Durco pumps adjust to the back plate causing a compression of the seal faces that can create mechanical seal “over heating” problems.
- Cycling pumps for testing will often cause a mechanical seal failure unless an environmental control has been installed to prevent the failure.
- Mechanical seals should be positioned after the impeller has been adjusted for thermal growth. This is important on any pump that is operated above 200°F (100°C) or you will experience premature seal failure.
- Some elastomers will be affected by steaming the system. A great deal of caution must be exercised if a flushing fluid such as caustic is going to be circulated through the lines or used to clean a tank. Both the elastomer and some seal faces (reaction bonded silicone carbide is a good example) can be damaged. If the elastomer is attacked, the failure usually occurs within one week of the cleaning procedure.
- The stuffing box must be vented on all vertical centrifugal pumps or otherwise air will be trapped at the seal faces that can cause premature failure of many seal designs.
- Most original equipment seal designs cause shaft damage (fretting) necessitating the use of shaft sleeves that weaken the shaft and restrict pump operation to a narrow range at the bep.
Here are a few common misconceptions that cause friction between maintenance and operation departments
- Shutting the pump discharge valve suddenly, will blow the seal open.
- All ceramics cold shock.
- High head, low capacity consumes a lot of power.
- The pump must come into the shop to change a mechanical seal.
- If you use two hard faces or dual mechanical seals in slurry applications, you will not need flushing water with its corresponding product dilution.
- If you use metal bellows seals for hot oil applications, you will not need the stuffing box cooling jacket operating.
- It’s okay to use an oversized impeller because throttling back will save power.
A few more thoughts on the subject
- Operators should receive proper schooling on the trouble shooting and maintenance of pumps. In the military and many modern plants, the operator and the maintenance mechanic are often the same person. If the operator knows how the pump works he will have no trouble figuring out the solution to his problem. Too often he is told to keep the flow gage at a certain point, or between two values without understanding what is actually happening with the equipment. If the operator recognizes cavitation he can tell the maintenance department and help them with their trouble shooting.
- As you wander around the plant, look out for painters that paint the springs of outside and double mechanical seals. There is a trend to putting two seals in a pump for environmental reasons and the painting of springs is becoming a common problem.
- If someone is adjusting the impeller make sure he is resetting the seal spring tension at the same time.
- If the pump is getting hot or making excessive noises, report it immediately. After the failure, it does no good to tell maintenance that it was making noise for two weeks.
- If you are the floor operator it is common knowledge that taking temperature and pressure readings is very boring, especially on those gages that are located in hot or awkward locations. Avoid the temptation to “radio” these readings. From hot to failure is a very short trip.
- Maintenance’s favorite expression is “There is never time to do it right, but there is always time to fix it.” Try to keep this in mind when the pressure is on to get the equipment running again.
- Do not let cleaning people direct their “wash down” hoses directly at the pump. Water entering the bearings through the lip or grease seals is a major cause of premature bearing failure. Most water wash downs are used to dilute and wash away seal leakage. Stop the leak and you have eliminated the reason for the hose.
- A great many motor and electrical problems are caused by these same wash down hoses.
- Cooling a bearing outside diameter will cause it to shrink, and the bearing will get hotter as the radial load increases. Keep the water hose and all other forms of cooling off of the bearing casing.