Upgrading your centrifugal pumps to composite wear parts is one of the easiest, fastest and most reliable ways to make your pumps better. While this upgrade is easy and reliable, there are a few pitfalls to avoid in order to ensure success.
The normal rule of thumb for upgrading to composite materials like Vespel® CR-6100 or Boulden B-Series is to reduce the clearance by 50% compared to API values for metal parts. This can increase pump efficiency and increase the Lomakin Effect in your pump, making the pump more reliable.
#1 Rotor Concentricity
However, reduced clearance requires that the pump be rebuilt to a good standard with a concentric rotor. Just because the Vespel® CR-6100 parts will not seize does not mean that the shop can do a sloppy job. The standard check used for metal parts–that the rotor must turn freely after assembly and coupling in the field still applies. Friction from rotor sag is acceptable, hard mechanical interference from poor alignment of internal parts is not.
#2 Clearance within Vertically Suspended Pumps
As mentioned in item #1, you can reduce the internal running clearance for wear rings. For vertically suspended pump shaft bushings, however, reducing the clearance doesn’t offer much benefit. There is generally no significant differential pressure across these parts. Therefore, reducing clearance at these parts will not increase pump efficiency or do much for rotor stability. If you reduce the clearance of these components too much, you will gain very little and make it very hard to keep the rotor concentric.
Download the Boulden Installation Guide and note the different tables for vertical and horizontal pump wear parts to avoid this issue
There are a lot of problems you can solve when you upgrade your pump to composite wear parts. You can help your pump survive dry running, avoid seizure, reduce cavitation, reduce vibration, help your seals last longer, and improve efficiency. In general, however, the composite materials used for wear parts are not the best choice for services that cause severe erosion of metal parts. The rule of thumb is that if standard metal parts do not show signs of abrasive erosion, you can consider composite materials.
#3 Expecting to Solve Abrasive Wear Problems
To put it in perspective, refined products, boiler feed water, chemicals, and utilities are a great place to use Vespel® CR-6100 and Boulden B-Series. For vertical pump shaft bearings in dirty service like wastewater and river water intake pumps, Boulden B-1050 is a good choice. If it is a true slurry service, composites are not the right choice.
Several times over the years skeptical customers have asked us: “We tried black plastic in our pumps 10 years ago. It didn’t work. What makes you think your material will be any different?”
#4 Using a Material with a High Coefficient of Thermal Expansion
Almost invariably, what we find is that the site had used a material with a high coefficient of thermal expansion. One of the reasons Vespel® CR-6100 has proven incredibly reliable is that the coefficient of thermal expansion (CTE) is about 60% lower than carbon steel. The earlier generation of composite materials on the market had CTE values around 3X higher than carbon steel. In practice, a high CTE will make problems in the pump worse–because the material will tend to “grow into” the rubbing or contact in the pump, reduce the clearance, and rapidly fail. Because Vespel® CR-6100 has a low CTE, it does not do this. It remains dimensionally stable until normal operation is resumed.
Boulden believes that CTE is so important, we developed our entire B-Series line of materials to have CTE values very close to or less than carbon steel.
After 20 years and over 10,000 pumps upgraded with composite materials, problems have been few and far between. Using our experience in application and design, we want to be sure all of your upgrades are successful. We can supply raw material from our inventory, CAD drawings, and machined parts. Boulden has a huge inventory with the best lead times and service in the business. Contact us to upgrade your next pump today.