Pump standards, the problem
A quick check of existing pump standards will reveal that there are a variety of them. The list includes:
- Hydraulic Institute Standards
- American National Institute Standards for Chemical Pumps :
- B73.1 for Horizontal type.
- B73.2 for Vertical Inline
- API 610 for centrifugal Pumps
- API 674 for Reciprocating Pumps
- API 675 for Controlled Volume Pumps
- API 676 for Rotary Positive Displacement Pumps
- ISO aimed at the medium duty single stage pumps ( Metric)
- DIN. West German standard
- VDMA West German standard for pump seals.
- They were written for pumps equipped with jam packing. Most of the standards were written in the nineteen fifties at a time when mechanical seals were not popular. In those days we had a lack of the modern materials that make mechanical seals practical. As an example Viton® was not invented until 1958 and did not come into general use until the sixties. Kalrez® did not come out until 1975 and in the eighties the duplex metals came into their own.
- The customer believes that by purchasing a standard design he’s getting a good pump. Customers have the same problem with pump efficiency. They believe there is a correlation between efficiency and the quality of the pump. Needless to say there is none! Problems caused by these standards are reflected in continual poor seal performance. The fact of the matter is that these standards reflect only an attempt to standardize envelope (outside) dimensions, nothing more!
Unfortunately standardizing the length of pumps ,prevented manufacturers from designing short shafts that were not prone to the bending problems associated with low cost A.N.S.I. and I.S.O. design pumps, operating off of their best efficiency point (B.E.P).
Here is a list of some of the modifications you should make to your standard A.N.S.I. or I.S.O. pump, if you want to get good mechanical seal and bearing life. Unless you are prepared to upgrade the pump, seal and bearing life will always be less than desirable
- The stuffing box bore is too small for mechanical seals. In most cases there’s not enough material to bore out, so you’ll 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.
- When using mechanical seals, install a recirculation line from the bottom of the stuffing box back to the suction of the pump. Try to tap the box as close to the face as possible to insure good circulation. Most high quality cartridge seals come with this connection already installed in the gland.
- Because packing needs lubrication, the pump came equipped with a recirculation pipe from the discharge side of the pump to the stuffing box lantern ring connection. 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. One exception to this is if you’re pumping a fluid close to its vaporization point. In that instance you don’t want to lower stuffing box pressure because of the possibility of vaporizing the fluid in the stuffing box and possibly blowing open the seal faces
- Convert to cartridge or split seals to insure correct seal installation and allow you to make important impeller settings in “back pull out,” or other types of pump designs.
- If you’re using single stage centrifugal pumps, convert to solid shafts with a low L3/D4 ratio to resist shaft bending. The back pull out design was made for easy sleeve removal. If you’re using good mechanical seals, corrosion resistant shaft materials and labyrinth oil seals, there should be no need to replace pump shafts.
- Pump manufacturers are not required to provide L3/D4 ratio numbers that would predict shaft bending problems with their pump. The relationship between shaft size and shaft diameter is expressed in the ratio L3/D4. Try to keep tghis number below 60 (2.5 Metric)
- “L” is the distance from the center of the inboard bearing to the center of the impeller (inches).
- “D” is the diameter of the shaft (under the sleeve if there is one), in the stuffing box area.(inches).
- Substitute labyrinth or positive face seals for the lip or grease seals that are installed in the bearing case. They will not only do a better job of keeping contaminants out of the bearing oil/grease, but they will not damage the expensive shaft. These labyrinth seals also make sense in the motors to eliminate moisture from damaging the windings and contaminating the lubricating grease.
- Use only non- fretting mechanical seals. Shafts are too costly not to pay attention to this.
- The easiest way to get pump/motor alignment is with a “C” (inch) or “D” (metric) frame adapter. If you elect not to use the adapter you’re in for a long process aligning the pump and driver correctly, and unless you are using split mechanical seals, you are going to have to go through the procedure each time you change seals. You should be able to get the C or D frame adapter as part of your next power frame change or upgrade.
- Convert to a “center line” wet end if you’re pumping liquids in excess of 200 degrees Fahrenheit (100 Centigrade) It’ll allow the suction flange to expand without causing pipe strain and wear ring damage.
- Do not use a vent on the bearing cavity of the pump. 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’re much better off positively sealing the casing with a mechanical seal and installing an expansion chamber on the top of the casing to allow for air expansion.
- If you intend to use a closed impeller, end suction, centrifugal pump, try to convert to a design that has adjustable wear rings.
- Install a sight glass to be sure that the oil level is at the correct height. Too much oil is as bad as not enough. If you have a positive pressure oil mist system, be sure that it does not vent to atmosphere. Oil mist systems require mechanical seals outboard of the bearings to prevent atmosphere contamination. If you have installed labyrinth seals, they will almost guarantee the correct oil level because excess oil will spill out of the labyrinth.
- Coat the inside of the bearing case with a suitable protective covering to prevent rusting and the leaching out of harmful substances from the bearing casting.
- Install magnetic plugs into the bottom of the bearing casing to attract loose metal shavings that would damage the bearings.
- 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 light weight 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.
In addition to these modifications, here are some recommendations that will help to 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 often 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 B.E.P. Throttling the pump discharge is not the same thing.
- If you are using open impellers, keep them adjusted to the correct clearance.
- 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 B.E.P.
- Avoid canned or magnetic drive pumps if the pumping fluid contains solids, or if it is a poor lubricant.
Pumps equipped with a “repeller” and some sort of static seal can usually be converted to a good mechanical seal. The problem with the repeller design is that in most of the configurations, 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.” Don’t open them on purpose.
® E I Dupont Dow elastomer
- On February 17, 2018