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%

0.007" (.18 mm)

0.013" (.34 mm)

0.026" (.67 mm)

20%

0.006" (.15 mm)

0.012" (.30 mm)

0.023" (.59 mm)

40%

0.005" (.12 mm)

0.009" (.24 mm)

0.018" (.47 mm)

60%

0.003" (.09 mm)

0.006" (.15 mm)

0.013" (.33 mm)

80%

0.002" (.05 mm)

0.004" (.10 mm)

0.008" (.21 mm)

100%

0.001" (.03 mm)

0.003" (.07 mm)

0.005" (.14 mm)

120%

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.