L3/d4 and premature seal failure

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

Some pump and mechanical seal sales people talk about L3/D. 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.

  • L = the length of the shaft measured from the center of the impeller to the center of the radial or inboard bearing. This measurement must be in inches or millimeters.
  • D = the outside diameter of the solid shaft measured beneath the sleeve, if one has been installed on the shaft. The measurement is in inches or millimeters.

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:

Duriron Mark II Group I solid shaft
Duriron Mark II Group I with a sleeve
Goulds 3196 ST with a solid shaft
Goulds 3196 ST with a sleeve
Worthington CNN frame 1

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.

  • Start and start the pump frequently. Batch operations as an example.
  • If tank levels are going to change. Loading and unloading pumps often experience this.
  • Operate the pump with a variable speed motor and the application is not a circulating system, or a system where the head is predominately system head.
  • Run the pump throttled because it was purchased too large for the application, anticipating the need for a larger pump in the future.
  • Let inexperienced people size the pump and add in safety factors because they do not trust their skill in pump selection.
  • Operate at different points on the pump curve by opening and closing valves in the system. Some applications require the isolation of parts of the system as a normal routine. The valves are opened, closed or throttled to satisfy the local demand.
  • Start the pump with the discharge valve throttled or shut to save power.
  • Experience occasional cavitation problems.
  • Fill a tank from the bottom instead of the top. This is a common occurrence if the pump is putting a head on the system.

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.