How the hydraulic forces generated at the wear rings can make your pump more reliable.

Next time you are working on a pump in your repair shop, consider the role of the wear rings and throttle bushings. They create stabilizing forces in the pump due to the Lomakin Effect. When you reduce your wear ring clearance using DuPont™ Vespel® CR-6100, you can maximize these forces and make your pump more reliable.

What is the Lomakin Effect?

The Lomakin Effect is a force created at the wear rings and throttle bushings within a centrifugal pump. The force is a result of an unequal pressure distribution around the circumference of the component during periods of rotor eccentricity or shaft deflection.

Figure 1 shows how shaft deflection creates the unequal pressure distribution. As the fluid enters the clearance between the rotor and wear component, it accelerates as it passes from the high pressure end to the low pressure end. Due to the eccentricity of the rotor, there is more clearance on one side of the wear ring than the other. There will be more flow and a locally higher velocity on the side with more clearance and lower velocity on the side with less clearance. Higher velocity results in lower pressure; lower velocity results in higher pressure, creating a net corrective force which acts in the direction opposite of the shaft deflection.

Lomakin Effect


Figure 1: The unequal pressure across the wear ring due to the Lomakin Effect.

In other words, when your pump experiences shaft deflection, there is a hydraulic "stiffness" (Lomakin Stiffness) which is generated to counteract the shaft deflection. The Lomakin stiffness coefficient (kLomakin) for a wear ring or throttle bushing is represented by the following equation:

kLomakin = (RL∆P/c)K (N/mm)
R = Wear Ring Radius (mm)
L = Wear Ring Length (mm)
∆P = Differential Pressure(MPa)
c = Radial Clearance (mm)
K = non-dimensional stiffness coefficient

The lone variable you can easily manipulate in this equation is the clearance. If you reduce the wear ring clearance by 50%, you double the Lomakin stiffness.

Which pumps benefit the most? Any pump where the shaft is long and thin. Typical examples are older overhung pumps with high L3/D4 ratios and multi-stage horizontal pumps like boiler feed water pumps and product shipping pumps. Also, any pump where clearance was increased in the past to avoid pump seizure. When you upgrade to Vespel® CR-6100, you minimize the risk of pump seizure and can confidently run tighter clearances, which also improves the pump efficiency.

Boulden has a large inventory of standard sizes of Vespel® CR-6100 in the U.S., Europe, and Singapore to help you make the upgrade today.  Vespel® CR-6100 can be used in almost all chemicals and temperatures from cryogenic to 500F (260C). Contact Boulden today for details.