Pump modifications you can make to increase the life of mechanical seals and centrifugal pump bearings 11-1
If you learn that you have the wrong pump in your application, and your company will not buy you the correct pump because it costs too much, you may still have the option of modifying the pump you have to increase its performance, and increase the life of the seal and bearings.
- Mechanical seals should run leak free until the sacrificial faces is worn away. Over 90% of mechanical seal leak excessively long before they wear out.
- Pump bearings should run trouble free through out their L10 life. The L10 life of a radial bearing in a Durco Mark II pump, size 3 x 2 x 10 is 300 years.
Here are a few modifications you can make to your centrifugal pump to extend the life of both the bearings and seals :
Stabilize the pump shaft. Any time you reduce shaft deflection you will increase both seal and bearing life.
- Reduce the L3/D4 number.
- Shorten the shaft.
- Replace the sleeved shaft with a solid shaft.
- Install an upgrade new power end with a larger diameter shaft.
- Install a support bushing in the end of the stuffing box to support a deflecting shaft.
- Carbon or any other bearing material that is chemically compatible with the pumpage should work.
- A 0.002 inch per inch (0.002 mm/mm) of shaft diameter is a typical diametrical clearance between the shaft and the bushing.
Install an oversize bore stuffing box to give the mechanical seal more room.
- Avoid tapered designs that direct solids to the lapped seal faces.
- Make the stuffing box bore as large as possible.
- You can make an oversize stuffing box by welding a pipe to the volute back plate after the old stuffing box has been cut away.
- Most pump manufacturers stock special back plates with the oversize stuffing boxes for their popular pump designs.
Replace the stuffing box discharge recirculation line with a suction recirculation line connected from the bottom of the stuffing box to the suction side of the pump, or any other low pressure point in the system. Needless to say this will not work all the time. Here are four instances where suction recirculation would not be a good idea:
- Flowserve pumps that adjust the semi-open impeller to the volute back plate. In this design the stuffing box is very close to suction pressure.
- Any time you are pumping a product close to its vapor point. Suction recirculation usually lowers the stuffing box pressure.
- Any time the solids in the pumpage float. In other words they have a specific gravity or density lower than the liquid they float on.
- Double ended pump designs where the stuffing box is at suction pressure.
Go to a centerline design wet end any time the pumpage exceeds 200°F (100°C).
- If you are using a closed impeller pump this is the best protection for the wear rings.
- The center line design will insure better centering for the mechanical seal, and possibly prevent the rotating face from sliding off the stationary face.
- A centerline design will prevent some pipe strain on the suction side of the pump.
Use a “C” or “D” frame adapter to insure the best alignment between the pump and the motor.
- The adapter performs the same function as the bell housing on an automobile that aligns the engine to the automatic transmission.
- The adapter eliminates the complaint that there is never enough time to do a proper pump/driver alignment.
- The adapter equalizes the temperature between the pump and motor casings. This is a better idea than trying to equalize temperature through the shaft.
Install a sight glass in the bearing housing.
- The oil level should be through the center of the bottom ball in the bearing when the pump shaft is not turning.
Replace the bearing grease or lip seals that damage the shaft and allow moisture o penetrate into the bearing housing.
- Positive face seals should be your first choice, with an expansion chamber installed in the housing vent.
- Labyrinth seals are an alternative, but they only work when the shaft is turning. They are not very good on intermittent service pumps.
- Grease or lip seals have a design life of less than 2000 hours (84 days) and they will cut and damage the expensive shaft.
Install a better thrust bearing retention method.
- Above 65% of its efficiency, a centrifugal pump is thrusting towards the volute and in many pump designs it is being retained by a simple and inexpensive snap ring.
Check to be sure that the pump’s suction specific speed number is less than 8500 (5200 in the metric system)
- It is possible to change the impeller to a different shape if you need to meet this number.
Install an impeller inducer if you do not have enough NPSH available to prevent the pump from cavitating.
- Most high speed pumps (greater than electric motor speeds) come equipped with this inducer feature.
Install a split mechanical seal to fix a leaking pump. It doesn’t make any sense to remove and disassemble a pump just to fix a leak.
- In many cases insulation would have to be removed and then replaced.
- Pump disassembly means that you will have to do the alignment again.
- Both rotating a stationary versions of split seals are available.
- The seals can often be installed on shafts or sleeves that have been damaged by packing or mechanical seals that frett shafts. The split seals are installed between the stuffing box and the bearing case where we seldom find any damage.
- Split seals are located closer to the bearings making them less sensitive to radial movement of the shaft.