Dupont Vespel CR-6100 wear rings

Pump designs with long, slender shafts run better with
Vespel® CR-6100 and reduced
wear ring clearance.

Turn Old, Tired Pumps into Better Pumps with Vespel® CR-6100

Pump Problem and using L/D Ratio

Many pump designs, typically older pumps such as the single-stage overhung pump shown in Figure 1, have shafts that are too long and thin. This leads to a range of problems:

How do you know if the shaft is too long and thin? One common method for overhung pump types is to calculate the L/D ratio, using the following formula:

L/D Ratio = L3/D4
L = shaft length from center of radial bearing to centerline of the impeller
D = diameter of shaft under the mechanical seal

Using inches, the L/D ratio should be less than 60; or less than 2 using mm.

Shaft deflection is driven by the radial load on the impeller. At BEP, this load is at a minimum; increasing as the flow moves away from the BEP. Table 1 shows a model of how a typical overhung pump with a 10” (250 mm) long shaft behaves at different flow rates depending on the L/D ratio. The version with an L/D ratio of 60 (2”/50 mm diameter shaft) has relatively low shaft deflec

Table 1: Shaft deflection at the mechanical seal as a function of L/D ratiotion until very low flow rates. The version with an L/D ratio of 120 (~1.75”/44 mm shaft) begins to show excessive deflection across the operating range. The version with an L/D ratio of 240 (~1.5”/38 mm shaft) has excessive shaft deflection all the time.

Flow % of BEP

L/D ratio of 60

L/D ratio of 120

L/D ratio of 240


0.007" (.18 mm)

0.013" (.34 mm)

0.026" (.67 mm)


0.006" (.15 mm)

0.012" (.30 mm)

0.023" (.59 mm)


0.005" (.12 mm)

0.009" (.24 mm)

0.018" (.47 mm)


0.003" (.09 mm)

0.006" (.15 mm)

0.013" (.33 mm)


0.002" (.05 mm)

0.004" (.10 mm)

0.008" (.21 mm)


0.001" (.03 mm)

0.003" (.07 mm)

0.005" (.14 mm)


0.002" (.05 mm)

0.004" (.10 mm)

0.075" (.19 mm)

Considering that mechanical seal manufacturers recommend keeping shaft deflection to less than 0.002” (0.05 mm), it is easy to see that high L/D ratio pumps can be expected to have inadequate seal life.

What to do when Replacing with Vespel® CR-6100:

Typical Results when Replacing with Vespel® CR-6100:

  • Reduced shaft deflection
  • Lower overall vibration levels
  • Longer seal life
  • Reduced wear at the wear rings
  • Higher efficiency
  • Reduced risk of pump seizure 

Older Pumps? Contact Boulden today to learn more about the Vespel® CR-6100

If you have older pumps with long, slender shafts which are causing you headaches, contact us or request a quote to upgrade the pumps with Vespel® CR-6100. Also get updates from Boulden by following our Newsletter! Turn those old, tired pumps into new, better pumps.