Tag Archives: CR-6100

Upgrading Pumps With Composite Wear Components Part: 10

Upgrading pumps with composite wear componentsPart 10: Vertically Suspended Pump Installation Tips

Review

We have discussed how upgrading your pumps with Vespel® CR-6100 helps to eliminate pump seizures, allowing you to reduce wear ring clearance, which improves pump efficiency and improves pump reliability by increasing the Lomakin Effect in the pump.

Quality repair and installation practices are an essential counterpart to the success of upgrading pumps with Vespel® CR-6100. This is true of all pump types–horizontal and vertical. When you are finished with the overhaul, the rotor should turn freely.

Vertically suspended pumps with their multiple fits and pilots require some additional consideration. To ensure the best possible results in these pumps, below are some tips which have been passed on to us from our customers.

Vespel CR-6100 for LPG Pumps

Vertically suspended LPG pumps fitted with Vespel® CR-6100.

 

Mechanical Alignment of Pump Components

Multi-stage vertical pumps pose a challenge for the shop performing the overhaul because there are multiple fits and pilots. Keeping the whole pump assembly concentric and square will give you the best results with your upgrade.

Good practices should be followed from the machine shop through the final alignment in the field.

In the machine shop:

  • Ensure all pilot fits within the pump are 0.002″ (0.05 mm) or better.
  • Ensure all mating faces of assembly elements are square.
  • If possible, assemble the pump in a vertical position.
  • Install Vespel® CR-6100 shaft bearings, bowl bearings, and wear rings, then final machine with the lathe indexed to the pilot fit of the part-this will ensure all bores at wear interfaces are concentric within the assembly. (Alternatively, all wear part fits can be machined concentric to the pilot fits prior to the installation of the Vespel® CR-6100 components.)
  • Install the Vespel® CR-6100 shaft bearings with the same clearance as the original design for the pump. If the original clearance is not available, See Table 3a (imperial) or 3b (metric) in the Boulden Installation Guide for recommended minimum clearances for vertically suspended pump shaft bearings.
  • When the assembly is complete, make sure the rotor turns freely within the pump with no hard rubs. If there are hard rubs, disassemble, try to find the source of the rub and correct the concentricity of the misaligned component. If clearances are very tight, consider a slight increase of the bushing clearance and re-check to make sure there is no hard rub.
  • Our recommended clearance for Vespel® CR-6100 wear rings in vertically suspended pumps is the shaft bushing clearance plus 0.002″ (0.05 mm) or 50% of the API minimum clearance for metal parts-whichever is larger.
  • If the pump is operating in very cold liquid (temperature below 0 C), increase the clearance at the shaft bearings by 0.002″ (0.05 mm) above the minimum values shown in table 3a or 3b.

In the field:

Generally, vertically suspended pumps incorporate a rigid coupling and the pump does not have its own rolling element bearings. The purpose of the rigid coupling is to make the pump shaft and motor shaft act as one unit with the pump relying upon the rolling element bearings in the motor. When dealing with a rigidly coupled vertical pump, traditional alignment methods can introduce misalignment. You also cannot rely upon the register fits from the motor to motor mount to the pump to be concentric.

Here are some tips for aligning a vertical pump with line shaft bushings and no rolling element bearings. (The motor bearings carry the axial load and coupling is rigid)

  • The pump must hang as close to plumb (vertical) as possible. This requires inspection of the base plate at the sump to ensure it is flat and level, and inspection of the mounting plate on the pump to ensure it is also flat and square to the assembly. If the pump is hanging “at an angle” the shaft will bend as it tries to hang plumb and pump life can be reduced.
  • Install the pump without the seal installed
  • Verify that the pump is level on the base
  • Install the motor on the pump.
  • Mount a dial indicator on the motor shaft, reading the ID and face of the seal chamber
  • Correct any radial misalignment by moving the motor and/or motor mounts in their fits. Squareness should be corrected by machining mounting faces (shims are sometimes used).
  • Lock the motor in position (installing 2 dowel pins is a proven method).
  • At this point, you may want to couple the pump and check for any run-out.
    • Any run-out that shows up after the alignment is likely due to a fault in the coupling
    • If resistance is still encountered after alignment and run-out are corrected, the source of rubbing is likely eccentric pump internals, which will need to be corrected back in the shop.
  • Install the seal (if the motor must be removed to install the seal, care must be taken to ensure motor returns to aligned position)
  • Install the rigid coupling (Coupling should be dimensionally checked and checked for trueness in the lathe before installation)
  • Measure the shaft run out between the coupling and the seal. This should be as close to zero as possible. The purpose of the rigid coupling is to make one shaft out of the driver and driven shafts. The end of the motor shaft is the zero point, so just a small run out at 15 cm below the coupling translates into huge side loads on the shaft bushings 1-2 meters down the assembly.

Side note: if you experience misalignment of the rigid coupling, the evidence will likely be wear of the bushing and/or shaft concentrated at the top bushing in the pump.

If you have anything to add to the above notes, please contact us. We’d love to hear your thoughts.

Conclusion

We hope you have found this series on how to upgrade your pumps with Vespel® CR-6100 helpful. In future weeks, we’ll have a couple of bonus sections on special topics. Until then, if you need any Vespel® CR-6100, contact Boulden. We have whatever size and quantity you need in stock and ready for immediate delivery.

For information on how to install Vespel® CR-6100 into nearly any centrifugal pump type, download the Boulden Installation Guide.

The Sydney Opera House , just out of frame is the Sydney Harbor Bridge

The Sydney Opera House, just out of frame is the Sydney Harbor Bridge

 

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

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

 

 

 

 

 

 

 

 

 

 

 

 

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Upgrading Pumps With Composite Wear Components Part: 2

Part 2: Reduce Clearance–The Lomakin Effect

Welcome back to our series on upgrading pumps with composite materials. In part 1, we discussed how using composite materials like Vespel® CR-6100 in your pumps allows you to eliminate the metal-to-metal contact points in the pump and minimize the risk of pump seizure:

  • In the shop during assembly
  • In the field during alignment
  • During slow-roll, start-up, and shut down
  • During off-design events like dry-running or low flow

Part 2: Reducing Clearance-The Lomakin Effect

Minimizing the risk of seizure in your pump sets the stage for reducing the clearance at the wear parts in your pump. Reducing clearance can be a significant pump reliability upgrade due to a phenomenon known as the “Lomakin Effect“.

Your Wear Rings are Bearings

During pump operation, the flow created by differential pressure across the wear parts in the pump (wear rings, throttle bushings) creates a force called The Lomakin Effect. The force arises from an unequal pressure distribution around the circumference of the component during periods of rotor eccentricity. This force counteracts shaft deflection in the pump.

Figure 1 shows how shaft deflection creates this force. 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 part than the other. There will be more flow and a locally higher velocity on the side of the wear ring with more clearance and lower velocity on the side of the ring 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. In other words, when your pump experiences shaft deflection, there is a hydraulic “stiffness” (Lomakin Stiffness) which is generated to counteract the shaft deflection.

Figure 1: The Lomakin Effect

Figure 1: The Lomakin Effect

Using Vespel® CR-6100 you can typically reduce the clearance at the pump wear rings by 50% compared to the API recommended minimum for metal wear parts. The Lomakin Stiffness is inversely proportional to clearance; therefore, a 50% reduction in clearance doubles this force. Potential benefits for your pumps include:

  • Less shaft deflection
  • Lower vibration levels
  • Fewer mechanical seal leaks
  • Longer bearing life

Which Pumps?

The Lomakin Effect is generally beneficial to all centrifugal pumps, but some pump types often show significant vibration reductions and reliability improvements with reduced clearance:

  • Multi-stage horizontal pumps
  • Older overhung pumps with long, thin shafts (high L/D ratios)
  • Two-stage overhung pumps

Conclusion

Reducing the clearance at the wear components can be a major reliability upgrade for your pumps. The reduced clearance increases The Lomakin Effect which improves pump rotor stability. The net result is a pump which runs with potentially lower vibration, fewer seal leaks, and longer bearing life.

Reducing the clearance also increases pump efficiency, which we will discuss in Part 3.

Until then, if you are working on a pump with a long, thin, flexible rotor, contact Boulden to discuss upgrading the wear parts to Vespel® CR-6100 and reducing the clearance. We have a huge stock of Vespel® CR-6100 standard sizes in the USA, Europe, and Singapore available for immediate delivery to your workshop.

For application and installation details, download the Boulden Installation Guide for Vespel® CR-6100

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.