SUBJECT: The relationship between the pump L3/D4 and premature seal failure. 11-6

Some pump and mechanical seal sales people talk about L3/D4 . How important is the number when it comes to selecting a pump? Well that's what this paper is all about, but keep in mind that any discussion of L3/D4 is limited to single stage, end suction centrifugal pumps.

The frame #1 pump is commonly supplied with a six inch impeller and turns at 3500 rpm (150 mm at 3000 rpm) The pump is used in applications that require a high head (pressure) and modest capacity.

If we compare the L3/D4 numbers of some shafts that are used in this very popular and competitively priced U.S. frame I pump, we'd find the following:

U.S. BRAND NAME
L3/D4 NUMBER

Duriron Mark II Group I solid shaft

129

Duriron Mark II Group I with a sleeve

353

Goulds 3196 ST with a solid shaft

64

Goulds 3196 ST with a sleeve

146

Worthington CNN frame 1

216
If you looked at the European and Asian versions of this same design you'd find that their L3/D4 numbers are in the range of 3 to 5. At my schools I teach that the number should be less than 60 ( 2 in the metric system). Does this mean that these models are not acceptable as good quality process pumps?

Of course not. It means that these pumps are designed for different purposes, in the same way a Porche sports car is designed differently than a Mazda Miata. They are both two door sports cars, but they sell for radically different prices.

If you want the feel of a sports car as you drive around town, the Miata is a good choice, but if you intend to drive down the German autobahn at 200 kilometers per hour, the more expensive Porche would probably be a more sensible selection.

Pumps are like that. If you're going to run a pump twenty four hours a day, 365 day a year, and not open and close system valves, these lower cost pumps would be a logical choice. All you're required to do is size the pump correctly. The shaft displacement, at the best efficiency point (B.E.P.), would then be negligible.

If you're going to do any of the following, a pump with a shaft L3/D4 number less than 60 (2 in metric) would make a lot more sense.

The conventional automobile water pump is attached to a vibrating engine. The shaft is pulley driven and the service is intermittent. At best, a very difficult application for the mechanical seal we find on all of these applications.

What kind of a L3/D4 number do we find on the shaft of this water pump? Less than fifteen is typical in the imperial system.

Check with your pump supplier to learn the L3/D4 number of the pump you're about to purchase. Often you can get the correct L3/D4 by specifying the pump with a solid shaft rather than with a sleeve, but in other cases you may have to go to a more expensive heavy duty model.

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