PUMP MODIFICATIONS (CENTRIFUGAL) P043
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 money; 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 designed 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 to below 60 (2 in the metric system).
- 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
The stuffing box bore is too small for mechanical seals. In most cases there is not enough material to bore out so you will have to make or purchase a replacement part. Most of these standard stuffing boxes were designed for 3/8″ or 10-mm. packing. You need at least 1″ (25 mm.) radial clearance to take advantage of centrifugal force throwing solids away from the seal faces. The more room you can get, the better off you will be or you can install an oversize bore stuffing box to give the mechanical seal more room.
- Avoid tapered designs that direct solids to the lapped seal faces.
- If you install a large sealing chamber in place of the narrow packing stuffing box that came as original equipment, you should be able to eliminate almost all need for clean flushing liquid in the seal area. 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. Try to tap the bottom of the stuffing box as close to the seal faces as you can. Many of the quality cartridge seals supplied today have this connection already installed. Needless to say this will not work all the time. Here are four instances where suction recirculation would not be a good idea:
- Duriron (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 into 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.
- If you are going to use grease lubrication. Hand pack the bearings but do not put grease into the bearing housing. In other words, no grease guns!
Replace the bearing grease or lip seals that damage the shaft and allow moisture to 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 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.
- Do not use a vent on the top of the bearing case. Each time the pump stops the vent will allow moisture to enter the bearing cavity as the oil cools down (this is called Aspiration). You are much better off positively sealing the casing and installing an expansion chamber on the top of the bearing casing to allow for air expansion.
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.
- Most high-speed pumps ( greater than electric motor speeds ) come equipped with this inducer feature.
Specify double volute designs any time the impeller diameter is 14″ (356 mm.) or greater to prevent shaft deflection. Smaller size pumps do not lend themselves to this modification.
Convert to a “Vortex” pump volute any time you are pumping liquid that contains lots of solids. Although the pump efficiency will be lower than conventional designs, the increased service life will more than compensate.
At overhaul time substitute a medium or heavy weight power end for the lightweight version that came with the pump, and get most of the features we have discussed. Medium and heavy weight power ends are available for most popular pump brands.
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 and 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.
In addition to the modifications mentioned, here are some more recommendations to help insure good seal and bearing life.
- Since a seal failure is the most common reason for shutting down a pump, Install a back up seal and convection tank to prevent unexpected shut downs.
- Change the bearing oil on a regular basis. Contact your favorite oil supplier for his recommendation and then follow his advice. If the inside of the bearing frame has been coated with a protective material to prevent rusting, avoid synthetic oils as their detergent action can damage these protective coatings.
- Maintain the proper oil level. Too much is just as bad as not enough.
- Trim the impeller to obtain operation at the best efficiency point (B.E.P). Throttling the pump discharge is not the same thing.
- If you are using open impellers, keep them adjusted to the correct volute or backplate clearance.
- If you intend to use a closed impeller end suction centrifugal pump, try to convert to a design that has adjustable wear rings.
- Install bearings by expanding the bore with an induction coil. Heating the bearing in a pan of warm oil is not a good idea because the oil can easily be contaminated.
- Install pressure gages on the suction and discharge of the pump. This is the only way to tell if the pump is running near its best efficiency point (BEP).
- Do not specify canned or magnetic pumps if:
- The pumping fluid contains solids.
- You might pump the tank dry.
- The pumping fluid is a poor lubricant.
- You need a wide operating window.
- Your product is sensitive to small increases in temperature.
Pumps equipped with a repeller and some sort of static seal can usually be converted to a good mechanical seal.
- The problem with most repeller designs is that the seal faces are designed to open when the pump is running and then close on any solids as the pump stops.
- The rule with mechanical seals is a simple one, “Keep the seal faces together”, do not open them on purpose.
- In negative stuffing box applications these repeller seals also have a habit of pumping air into the product because of the open seal faces.