Pump partnering 13-11
With all the current talk about seal partnering, can pump partnering be far behind? As with mechanical seals the concept is simple; call in several pump companies, talk to them about a lot of lofty ideals and then learn which of them will give you the best price on a standard ANSI (American National Standards Institute) pump if you promise to give them all of your business.
This is often called convergence and is recognized as a clearly defined trend in industry..
If you are getting good life out of your present pump, and if every one was selling the same thing, then that would be a good idea, but that is not the way it is. The fact is that the ANSI (American National Standards Institute) pump is not giving reasonable trouble free life and to standardize on it would be foolish if you were looking for long service.
There are two major problems with this pump:
- You will get poor mechanical seal life. A seal should run until the carbon face wears down and in better than 90% of the cases the seal will begin to leak with plenty of carbon face left untouched.
- The bearings have a L10 rated life of from 100 to 300 years and you know that is not happening.
The problem simply stated is that the ANSI (American National Standards Institute) pump was made for conventional packing and you are trying to use a mechanical seal because leakage is no longer acceptable for a variety of reasons that include:
- Pollution problems cause hassles with the government and you do not need these problems.
- Expensive products cost too much to have them leaking on the floor.
- Fugitive emission laws legislate against all leakage.
- The packing is using too much water flush.
Let’s look at the cause of the seal problem:
- Your inital seals were supplied by the pump manufacturer who, in many instances, was forced to use Teflon® and other non-elastomers in the seals because the pump was shipped to a distributor that did not know where the pump was going to be used other than in a product that was compatible with the pump metallurgy. The Teflon® caused fretting problems (damage) on the shaft so the shaft diameter was reduced and a sleeve was installed to accept the damage and the resultant diameter reduction weakened the shaft.
- The shaft was made long to accommodate at least five rings of packing, a lantern ring or seal cage, a follower to tighten the packing and enough room had to be left to fit your hands into the pump to install this hardware. To provide all of this room the impeller was moved too far away from the bearings.
- When the packing was replaced with the mechanical seal, the shaft became unstable causing excessive radial movement.
- The long shaft additionally added a lot of unnecessary cost to the pump.
- The narrow stuffing box made sense when packing was installed but made no sense when the packing was replaced with an expensive mechanical seal. There is not enough clearance between the seal outside diameter and the stuffing box inside diameter to provide proper cooling and allow solids to settle out when the pump is stopped. It took the pump companies forever to make the large stuffing box available as an extra cost accessory.
- The double volute design was always the answer to shaft deflection caused by operating off the pump’s best efficiency point (BEP). This feature was just about eliminated in the smaller impeller sizes (less than 14 inch or 355 millimeters) to accommodate the consumer’s request for more efficiency. The pump manufacturer should have explained to the consumer that the small decrease in efficiency would be offset by a major decrease in seal failure, but the pump companies kept silent. The whole subject has proven to be academic because over 90% of the present pumps in industry currently run throttled as a result of safety factors imposed when the pump was sized and purchased requiring the manufacturer to supply an oversized pump.
- The pump should have come equipped with a “C or D” frame adapter to eliminate the need for making an alignment between the pump and the motor. Unlike the packing, the seal is very sensitive to misalignment problems and most mechanics complain because there is never time to do a proper alignment.
- A vortex impeller would have reduced the amount of solids sticking on the impeller in heavy slurry applications. The attached solids destroyed the dynamic balance of the rotating assembly, causing seal problems. Again the desire to appear efficient took priority over reliability.
- With the exception of oil refineries most pumps sold in the United States are of the open impeller type. This means that the impeller has to be adjusted to either the wet end volute or back plate to maintain the pump’s efficiency. In all cases the ANSI (American National Standards Institute) pump adjusts the impeller from the power end causing the rotating part of the mechanical seal to move every time an impeller adjustment is made. The open impeller clearance should have been adjustable from the power end and this problem never would have occurred. To adjust the open or semi-open impeller without disturbing the seal face load requires that an expensive cartridge version of the seal be purchased that will consume most of the small radial clearance that is available in the stuffing box.
- The mechanical seal should be designed to be installed in the space between the stuffing box and the bearing case where there is plenty of radial room and the seal is physically located closer to the bearings. Pusher gland bolts could be used to eliminate the problems caused by the present gland bolt’s variety of bolt circle diameters.
The overall problem is that the pump manufacturer did not want to alter his packed pump design to accommodate a mechanical seal. Since he held all the power over which seal design was going into his pump, he insisted on a set of criteria that reduced his cost and guaranteed premature seal failure. The consumer who possessed little pump knowledge and even less knowledge about mechanical seals went along with a “child like faith” that all was well because he was dealing with reputable manufacturers.
The ball or roller bearing problems are just as easy to identify:
- There is not enough oil in the bearing case sump causing the oil to get too warm. There should be enough room to accommodate at least two liters of oil when the lubricant is at the proper level of half way through the bottom ball when the pump is at rest. Oil has a useful life of 30 years at 30°C (86°F). The life of oil is cut in half for every 10°C (18°F) rise in temperature. This means that oil has a useful life of only 90 days at 100°C (212°F) because of “coking” problems.
- Grease fitting make no sense for grease lubricated bearings. The fitting will guarantee that the bearings will be over lubricated causing a heat problem. The bearing must be hand packed and that is a real inconvenience.
- Grease or lip seals should never be approved for use in centrifugal pumps or any other rotating equipment. The pump company provides them as a standard
- These seals have a designed life of less than 2000 hours (83 days at 24 hours per day)
- They will cut (frett) the shaft because they remove the corrosion resistant shaft’s protective oxide coating.
- Labyrinth or face seal are a much better choice. The problem is they cost more than grease seals.
- During operation, the shaft will thrust towards the pump volute. Too often the radial bearing is being retained by a simple snap ring that can bend or loosen as the snap ring groove wears.
There are other features that can be incorporated into the standard pump that would eliminate many of the current seal and bearing failures:
- A centerline design would compensate for normal thermal growth of the volute causing the shaft to run non-concentric with the stuffing box.
- A low L3/D4 shaft would resist much of the deflection caused by:
- Operating off the best efficiency point (BEP).
- Misalignment between the pump and driver.
- A shaft that is not dynamically balanced.
- Severe vibration. Cavitation as an example.
- The manufacturer should make various specific speed number impellers available to the consumer. Too often the impeller sold is a compromise, because the correct specific speed number was not available.
- An impeller inducer should be available if needed. Some times it is the most practical way to get the NPSH you need.
- A “C” or “D” frame adapter would eliminate the argument that “we do not have time to do an alignment”.
Equally as important as the pump design is the knowledge you need to troubleshoot piping and installation problems. A very high percentage of troubleshooting time is spent on those two subjects. If the consumer is going to save the cost of this service in the form of a pump distributor discount (pump manufacturers seldom get involved in seal and bearing failures other than to blame some one in operation or maintenance as the cause of the premature pump failure) then the consumer is going to have to provide his own service in these areas.
In this age of multi-craft mechanics and non-specialized mechanical engineering, I’m not very confident about the probability of their success. Think about it this way:
Two people coming towards each other from opposite directions will meet somewhere in the middle as they converge, but they are clearly going in opposite directions.