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The problem with pump standards 2-4

A quick check of existing pump standards will reveal that there are a variet. The list includes:

Hydraulic Institute Standards

American National Institute Standards for Chemical Pumps :o B73.1 for Horizontal type.o 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.

There are at least two problems with these standards:

They were written for pumps equipped with jam packing. Most of the standards werewritten in the nineteen fifties at a time when mechanical seals were not popular. In thosedays 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 thesixties. 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. Thefact 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

WHAT TO MODIFY

The stuffing box bore is too small for mechanical seals. In most cases there's not enoughmaterial 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 solidsaway from the seal faces.

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When using mechanical seals, install a recirculation line from the bottom of the stuffingbox back to the suction of the pump. Try to tap the box as close to the face as possible toinsure 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 pipefrom the discharge side of the pump to the stuffing box lantern ring connection. If youinstall 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 itsvaporization 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 makeimportant 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 L 3/D4ratio 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 oilseals, there should be no need to replace pump shafts.

Pump manufacturers are not required to provide L3/D4 ratio numbers that would predictshaft 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)

o "L" is the distance from the center of the inboard bearing to the center of theimpeller (inches).

o "D" is the diameter of the shaft (under the sleeve if there is one), in the stuffingbox area.(inches).

Substitute labyrinth or positive face seals for the lip or grease seals that are installed inthe 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 pumpand 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 degreesFahrenheit (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 ventwill 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 adesign that has adjustable wear rings.

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Install a sight glass to be sure that the oil level is at the correct height. Too much oil is asbad as not enough. If you have a positive pressure oil mist system, be sure that it does notvent 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 rustingand the leaching out of harmful substances from the bearing casting.

Install magnetic plugs into the bottom of the bearing casing to attract loose metalshavings that would damage the bearings.

Specify double volute designs any time the impeller diameter is 14" (356 mm.) or greaterto 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 ofsolids. 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 weightversion that came with the pump and get most of the features we have discussed. Mediumand 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 backup seal and convection tank to prevent unexpected shut downs.

Change the bearing oil on a regular basis. Contact your favorite oil supplier for hisrecommendation 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 totell 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 apoor 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 thepump 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

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