The shaft deflection in your pump directly affects mechanical seal reliability. Reducing wear ring clearance is an easy upgrade to minimize shaft deflection.
Consider the faces of your mechanical seals–lapped flat to within one or two light bands of flatness, designed to run with precise alignment and minimal leakage across the faces. Excessive shaft deflection at the seal prevents proper alignment of the faces, potentially allows particles between the faces, and can lead to higher leakage and faster wear of critical components.
Reducing the clearance at your pump wear rings will help reduce shaft deflection and improve your seal life.
What does API say?
API 610 11th Edition (section 188.8.131.52) states: To obtain satisfactory seal performance, the shaft stiffness shall limit the total deflection under the most severe dynamic conditions over the allowable operating range of the pump with maximum diameter impeller(s) and the specific speed and liquid to 50 μm (0.002 in) at the primary seal faces.
The same section goes on to state the variables pump designers can manipulate in order to achieve this target:
- Shaft diameter
- Shaft span between bearings or shaft overhang
- Casing design including dual volutes or diffusers
Finally, there is a provision for the wear rings: For one- and two-stage pumps, no credit shall be taken for the liquid stiffening effects of impeller wear rings. For multistage pumps, liquid stiffening effects shall be considered and calculations performed at both one and two times the nominal design clearances.
Why does wear ring clearance matter?
That last phrase highlights a major role of pump wear rings. Why does the standard insist that the designer calculate the stiffness effects at one and two times clearance?
The reason is the Lomakin Effect–the bearing effect generated by the differential pressure across the wear rings and throttle bushings in your pumps. The stiffness from the Lomakin Effect is inversely proportional to clearance. If your wear ring clearance doubles, you lose half the stiffness generated by the wear rings.
Conversely, if you use non-seizing composite wear rings from materials like DuPont™ Vespel CR-6100, you can reduce the clearance by up to 50% and double the stiffness generated by the wear rings. Increased stiffness from the wear rings helps to reduce shaft deflection. Field results have shown that pumps running with reduced clearance exhibit lower vibration and fewer seal leaks.
New Pumps Only
It is important to note that the current API standard applies to new pumps. For pumps which already exist in your plant, upgrading the wear rings and reducing clearance is an easy upgrade; whereas, there is little you can do to modify the design of the shaft or volute without a major pump upgrade or replacement.
This is particularly important because the refineries in North America and Europe (along with older plants around the world) continue to operate large populations of pumps from the 60’s 70’s and 80’s. One- and two-stage pumps built during that period frequently have long, thin shafts which suffer from excessive shaft deflection. Multi-stage pumps, including those built to the current standard, also have flexible shafts and rely upon the wear rings to limit shaft deflection.
Upgrading older and multi-stage pumps with Vespel CR-6100 wear rings and reducing the clearance is one of the fastest and easiest ways to improve the reliability of these older pumps. The upgrade will also produce a significant increase in pump efficiency.
Reducing wear ring clearance will help reduce the shaft deflection in your pumps and help improve your mechanical seal performance. Vespel CR-6100 has proven reliable in a wide range of services from cryogenic to 260 C (500 F) in thousands of applications around the world.
Contact Boulden today with your application details, and we can discuss whether Vespel CR-6100 is a good fit for your pump. We have a huge inventory of stock sizes available for immediate delivery almost anywhere in the world.
For application and installation details, download the Boulden Installation Guide for Vespel CR-6100.