Tag Archives: Ring clearance

Upgrade to DuPont™ Vespel® CR-6100: 100%

Upgrading pumps with composite wear componentsThere is great satisfaction in doing a job all the way.

100%

In South Africa, when you make a statement that someone agrees with, they say “100 %”–similar to how other English speakers say “absolutely.” With that in mind, we want the pumps upgraded to Vespel® CR-6100 to be upgraded 100% whenever possible.

 

Review

 

When you upgrade your pumps to Vespel® CR-6100, there are two steps:

  • Eliminate the metal-to-metal contact points in the pump
  • Reduce the running clearance (of wear rings, center bushings, and throttle bushings)

Upgrading your pumps to Vespel CR-6100 Step #1

Our recommendation when upgrading to Vespel® CR-6100 is to convert all of the stationary wear parts to Vespel® CR-6100. All of the rotating parts remain metal, thus eliminating all of the metal-to-metal contact points in the pump. This essentially eliminates the risk of pump seizure.

 

Yet, once in a while, customers try to make a small change instead of fully upgrading the pump. The three partial upgrades we run into are:

Overhung Pumps

Older pumps with long slender shafts (high L/D ratios) create problems for mechanical seals due to excessive shaft deflection. If you want to increase the rotor stability of these pumps using the wear parts, you will want to upgrade the wear rings to Vespel® CR-6100 and reduce the clearance.

 

The reason is that the Lomakin Effect-the hydraulic force which stabilizes the rotor-is driven by differential pressure and the surface speed at the differential pressure interface. Wear rings have significant differential pressure and high velocity, creating a lot of stability from the Lomakin Effect. Throat bushings? Not so much.

 

Horizontal Multi-Stage Pumps

If a horizontal multi-stage pump like a boiler feed water pump seizes, it will usually occur at the center bushing or throttle bushing, depending on the pump type. These two components generally have the tightest clearance in the pump and will be the first points of contact. In these services, there is a temptation to only upgrade the one or two components which seized.

 

While this approach has been successful in reducing pump seizures, there are some limitations. Metal-to-metal contact points remain and thus there is still a possibility of seizure. If all of the wear parts are upgraded, the risk of seizure is essentially eliminated. Furthermore, the wear rings also add to the rotor stability and efficiency of these pumps. Upgrading the wear rings as well as the center and throttle bushings will make for a much better pump. Especially if you use the Boulden PERF-Seal™ design (patent-pending).

Two-stage kerosene pump with all the components upgrade to Vespel® CR-6100 and the PERF-Seal™ design

 

 

Center Bushing of a 2-stage pump.

 

Vertically Suspended Pumps

Vespel® CR-6100 is a great material for vertically suspended pump shaft bearings in LPG, butane, natural gas liquids (NGL), and other flashing products. It can survive running dry at start up with limited wear. It doesn’t break like a carbon part. This application for Vespel® CR-6100 is so common that Boulden carries a huge inventory of standard stock sizes for the dimensions typically used for shaft bearings.

 

What some users miss is the opportunity to also upgrade the case rings of these pumps and reduce the clearance. This addition to the upgrade eliminates the other potential seizing points in the pump. Furthermore, reducing the clearance increase the pump efficiency and reduces the NPSHR–all of this making the pump easier to operate.

 

In Summary

The only partial upgrade above which we do not recommend is trying to stabilize a rotor with a throat bushing. In our experience, this approach is marginally successful at best. The other partial upgrades have worked and there are situations where they are necessary. But, given the choice, why not do the job 100%?

 

The main point is to recognize that the maximum improvement in reliability, safety, and efficiency will be achieved if you upgrade all of the wear parts (Table 1) in your pump to Vespel® CR-6100.

Table 1: Components to upgrade to Vespel® CR-6100

Overhung Pumps Between Bearings Pumps Vertically Suspended Pumps
Case Wear Rings Case Wear Rings Case Wear Rings
Throat Bushings Throat Bushings Throat Bushings
Inter-Stage Bushings Line-shaft bearings  
Center-Stage Bushings Bowl bearings  
Throttle Bushings Bottom bushings

Contact Boulden Today for your Vespel® CR-6100 Needs!

If you have a pump operating at less than 500 F (260 C) where you want to improve the reliability or efficiency, contact Boulden today. We can provide you all of the details required for your upgrade and have the Vespel® CR-6100 material required for the upgrade in stock in a wide range of sizes available for immediate delivery.

 

Helpful Links:

Standard Stock Sizes of Vespel® CR-6100

Boulden Installation Guide for Vespel® CR-6100

Vespel® CR-6100 Product Data Sheet

Vespel® CR-6100 Machining Guide

3MW Boiler Feed Pump Case Study

 

Stay Connected

Upgrading Pumps With Composite Wear Components Part: 8

Upgrading pumps with composite wear components

Part 8: Vertically Suspended Pumps (API Types VS1–3, VS6, VS7)

Review

To date, we have addressed how to install DuPont™ Vespel® CR-6100 into the various horizontal pump types to eliminate metal-to-metal contact points in the pump and minimize the risk of pump seizure. This allows a reduction of clearance which improves efficiency and rotor stability.

This segment will discuss how to use Vespel® CR-6100 in vertically suspended pump types (API types VS1-VS7). These types can be further broken down as “discharge through column” (VS1, VS2, VS3, VS6, VS7) and “separate discharge” (VS4 and VS5).

Which Parts?

In vertically suspended pumps, we can upgrade the same components as horizontal pumps (wear ringsthrottle bushingsthroat bushings) for the same reasons-to eliminate the metal-to-metal contact areas in the pump and reduce the clearance resulting in improved reliability and efficiency.

Today we will focus on the components which are unique to vertically suspended pumps-the vertical pump shaft bearings: line shaft bearings, bowl bearings, and bottom bearings. Vespel® CR-6100 is ideally suited to this application, particularly in services which suffer from a lack of lubricity or may run dry at startup. Vespel® CR-6100 does not seize like metal alloys, it can survive running dry, and it can withstand mechanical impacts and thermal shocks so it doesn’t break like carbon or graphite.

Multi-stage LPG pump bowl assemblies being upgraded to Vespel® CR-6100.

Multi-stage LPG pump bowl assemblies being upgraded to Vespel® CR-6100.

One thing of note is that vertical pump shaft bearings do not have differential pressure across the parts, therefore, they do not impact pump efficiency. Because the clearance of these components tends to be rather tight, to begin with, a further reduction in clearance can easily lead to assembly issues with a limited upside associated with the tighter clearance. Therefore, our recommendation for these parts is to install the Vespel® CR-6100 into the spiders or bowl assemblies with the press fit shown in our installation guide, then final machine to the original design clearance.

In our installation guide, you will find two clearance charts–one for horizontal pump types, one for vertical pump types. Because we don’t want the wear ring clearance tighter than the shaft bushing clearance, we simply recommend making the wear ring clearance 0.002″ (0.05 mm) larger than the shaft bushing clearance in these pumps. In short, we highly recommend that you download the Boulden Installation Guide for Vespel® CR-6100.

Discharge Through Column

Vertically suspended pumps are often selected because the service offers poor suction conditions such as light hydrocarbon service or condensate. In a long-term study, a refinery upgraded 22 vertical pumps to Vespel® CR-6100 wear parts. The pumps were operating in light hydrocarbons, flare knockout drum, and several stop/start services like product transfer. In the 5 years prior to upgrading the pumps, this population of pumps comprised many “bad actors” with poor reliability. In the 5 years after upgrading the pumps to Vespel® CR-6100, there were only 8 repairs on the entire population of pumps and the MTBR of this group of pumps increased to more than 10 years!

The combination of excellent reliability with ease of machining and installation along with immediate stock availability has made Vespel® CR-6100 the material of choice in these applications.

Vertically suspended pumps assembled with Vespel®CR-6100 shaft bearings, wear rings, bowl bushings and bottom bearings

Vertically suspended pumps assembled with Vespel®CR-6100 shaft bearings, wear rings, bowl bushings, and bottom bearings

Vespel® CR-6100 can handle significant periods of dry running with minimal wear, making it easier to bring the pump online. When you upgrade your wear rings to Vespel® CR-6100 and reduce the clearance, you might also find the pump easier to start due to a reduction in the NPSHR (Net Positive Suction Head Required).

To be continued…

In the next two issues, we will address vertically suspended pumps with a separate discharge (API Type VS4) and then we will discuss installation and assembly issues unique to vertically suspended pumps.

Until then, if you have a vertical pump which is giving you headaches, consider upgrading the wear parts to Vespel® CR-6100. Contact us today. We have the Vespel® CR-6100 in stock in a wide range of sizes available for immediate delivery to nearly anywhere in the world.
For details on installing Vespel® CR-6100 into nearly any centrifugal pump type, download the Boulden Installation Guide.

Contact Us Today To Learn More About Vespel and Boulden Company!

Contact Us Today To Learn More About Vespel and Boulden Company!

 

 

 

 

 

fb

tw

yt

Boulden on Google+

 

Today’s Photo

Gardens by the Bay, Singapore

Gardens by the Bay, Singapore

Reduce Shaft Deflection by Upgrading Your Wear Rings

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 6.9.1.3) 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.

Conclusion

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.