Keep 2-Stage Overhung Pumps Running Smooth by Upgrading to DuPont™ Vespel® CR-6100
The 2-stage overhung pump is a common design from the 1960’s through the early 1980’s. Most plants built during that era will have at least a few of these pumps still running, and most likely causing problems. Upgrading the internal wear components with Vespel® CR-6100 can mitigate the most common issues with these pumps.
Why 2-Stage Overhung Pumps Fail
The common failure modes for 2-stage overhung pumps are seal failures and pump seizure. Both are caused by the same fundamental design flaw. The shaft is too long, too thin, and too flexible to handle the loads associated with two impellers. The shaft bends, the wear rings or the bushing between the stages rub and potentially seize, the pump vibrates, and the seal fails.
Worse yet, changing the pump in the field is almost never easy. From the introduction of API610, 7th Edition in 1991, this design no longer complies with the standard. New 2-stage pumps require a between-bearings design. In the plant, this requires piping modifications, a new foundation, and significantly more space, which is often not available. This kind of modification can also easily cost more than $100,000 per pump. Unless the pump is a safety hazard or restricts production, obtaining funding for this project can be quite difficult. So, in most situations, the pump remains.
A Simple Improvement
Installing DuPont™ Vespel® CR-6100 case rings and inter-stage bushings in this pump will help to address the excessive shaft deflection in these pumps. Vespel® CR-6100 is a composite material made from oriented carbon fibers and Teflon® PFA resin. It will not seize like a metal wear ring, minimizing the risk of pump seizure. Because the risk of seizure is minimized, wear ring and bushing clearances can be reduced down to 50% of the API minimum values for metal rings. The reduced clearance increases the hydraulic forces which stabilize the pump rotor. Importantly, these stabilizing forces act on the rotor at the impellers where they have the most impact. With a more stable rotor, shaft deflection is reduced, vibration is reduced, and seals and bearings will last longer.
Reduced clearance also augments the pump performance and efficiency and mitigates cavitation. In many situations, the wear ring clearance on these pumps has been increased to avoid seizure. Reducing this excessive clearance to 50% of the API minimum for metal rings has shown efficiency gains of over 10% in some examples and has dramatically reduced the pump noise and vibration.
Side Note:
Vespel® CR-6100 can be used in continuous process temperatures up to 500⁰ F (260⁰ C). What if your pump runs hotter? Consider DuPont™ Vespel® SCP-5050, which can operate in continuous temperatures up to 600⁰ F (315⁰ C). Contact Boulden for more details.