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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

Centrifugal Pump Problems & L/D Ratio

Many centrifugal pump designs, typically older pumps such as the single-stage overhung pump shown in Figure 1, have shafts that are too long and/or thin for their application. This causes shaft deflection, which can lead to a range of problems:

How do you know if the shaft on your centrifugal pump is too long or thin? One common way to check 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

Measured in inches, the L/D ratio should be less than 60; in millimeters, less than 2mm.

What is Shaft Deflection?

 

If a centrifugal pump is operating at its best efficiency point (BEP), the bending forces caused by its operation will be evenly distributed around its impeller. However, BEP is difficult to achieve and maintain, and shaft deflection in centrifugal pumps, caused when bending forces are unevenly distributed, is quite common.

 

A bent and re-bent paper clip is a simple analogy for pump deflection. If one bends a paper clip back and forth, it will become brittle and weak, and eventually break. The same thing can happen to shafts on centrifugal pumps if shaft deflection is not corrected, especially for high RPM pumps or those that run continuously for extended periods.

 

Ultimately, shaft deflection is driven by the radial load on the impeller. At BEP, this load is at a minimum, and increases 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 deflection, while those with greater L/D ratios experience greater and greater shaft deflection.


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.

Vespel® CR-6100: The Solution for Shaft Deflection in Centrifugal Pumps

 

There are two options for alleviating shaft deflection in centrifugal pumps. First, you can replace the shaft with one that is shorter and stiffer. However, this method is often prohibitively expensive, costing as much as $50,000 (or more) per pump.

 

An easier and more cost-effective solution is to replace your centrifugal pump’s existing wear rings and throttle bushings with those manufactured from DuPont™ Vespel® CR-6100. For a fraction of the cost of a new shaft, flexible, durable Vespel® CR-6100 will improve performance and extend the life of your pump.

 

Steps to Take When Upgrading Your Centrifugal Pump with Vespel® CR-6100

 

  1. Replace the case wear rings and throttle bushings with Vespel® CR-6100 material
  2. Reduce the wear ring clearance to 50% of the API minimum for the wear rings
  3. Use good repair procedures to ensure rotor concentricity

 

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

 

Typical Results After Upgrading to 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 

Running Older Pumps? Contact Boulden to Learn More About Vespel® CR-6100

If you have older centrifugal pumps with long, slender shafts that are causing you headaches, contact us or request a quote to upgrade your pumps with Vespel® CR-6100. We'll help you turn those old, tired pumps into new, better pumps.

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