Tag Archives: reducing pump seizure

Case Study: Repeat Failures of Boiler Feed Water Pumps

Vespel®CR-6100 centrifugal pump components

Boiler Feed Pump Seizing Problems

The low lubricity of boiler feed water along with operational challenges can lead to pump seizures. How do you avoid the problem? Or, if it happens, what is the best way to solve the problem? This case study is about repeat seizures of two, new, 1.2 MW boiler feed water pumps operating in a refinery. The pumps were fitted with metal wear parts, ran at 2980 rpm, and experienced failures immediately after commissioning.

The Tale of the Terrible Boiler Feed Pumps

Here is a brief summary of the problems this site faced with their boiler feed pumps:
  1. The first failure was due to pump seizure, the original failure analysis suggested the pumps failed due to sand or debris lodging into the close clearance between rotating and stationary wear parts. The filters on the suction strainer were changed to a finer mesh to limit particle size which could enter the pump.
  2. Soon after, one of the pumps seized again–this time, the metal parts had galled before start up and the pump could not be rotated by hand. The clearances at the wear rings, center bushing, and throttle bushing were increased.
  3. The pumps seized again.
  4. Repeated start-up attempts against seized pumps had also damaged the pump motors.

The failure analysis led the engineers to re-check the entire design and installation of thepumps:

  • Re-checking the design basis of the pumps
  • Re-checking the materials of construction
  • Re-checking assembly and rotor run-out
  • Re-balancing the rotor
  • Performing a new pump rotordynamic analysis
  • Verifying soundness of foundation and that pipe strain within limits
  • Evaluation of operational practices and function of the minimum control valve

Minimum Flow Valve Problem

Ultimately, the site realized the minimum flow valve was not functioning properly, leading to low flow rates at start up. The low flow rates caused localized flashing inside the pump and the metal parts would seize.

Their corrective action was to change the minimum flow valve, increase the clearance at the center bushing, throttle bushing, and wear rings, and “upgrade” to a “non-seizing” metal alloy. Because they increased the clearance at all of the internal parts, they ran a new rotor dynamic analysis to verify that the rotor would remain stable.

How Much Did It Cost?

Between repeated pump repairs, motor overhauls, engineering resources to troubleshoot and re-verify the design, the problem undoubtedly cost hundreds of thousands of dollars. If start up was delayed or the site lost production, the losses were probably in the millions.
The solution was also extremely expensive. The clearance at the pump wear parts was permanently increased. We’ll generously assume the increased clearance resulted in a 2% efficiency loss. Assuming a power price of $0.12/kw-hr, this loss of efficiency will cost about $50,000/year. Over the life of the pumps, the site will lose another million dollars or more!

Avoid Pump Seizure By Using Vespel® CR-6100

If the site had specified Vespel® CR-6100 wear parts when the pump was ordered, the pump would not have seized. Most likely, the pumps would have made it through the low flow transients without issue and all of the efforts above could have been avoided. The motors would not have been damaged, and they would not have had to increase the internal clearances. Even after the first seizure occurred, they could have easily converted to Vespel® CR-6100 and saved time, effort, and cost. The small adder to specify Vespel® CR-6100 would have saved several hundred thousand dollars at a minimum.

Furthermore, because Vespel® CR-6100 does not seize, clearance at the wear rings, center bushing, and throttle bushing could have been reduced, resulting in an efficiency gain instead of an efficiency loss. So, instead of losing $50,000 per year in operating costs, the site could have saved at least $50,000 per year–a net $100,000 annual savings from using Vespel® CR-6100. Combined with the Boulden PERF-Seal® design, the pump would be more reliable, easier to operate, and more efficient than a pump with metal parts and increased clearance.

Conclusion

If you are buying or overhauling a boiler feed water pump, specify Vespel®CR-6100 for all of the stationary wear parts (the rotating parts will remain metal). You’ll have a better pump that is easier to operate with a lower life cycle cost due to higher efficiency. If you have an existing pump that seizes, contact Boulden today. We can help you solve this problem in just about any pump service.

If you simply have an urgent repair and need a great material fast, we have a large inventory of material in stock and can supply raw material or machined parts with very short lead times. If you have dimensions, quantities, and basic service conditions, simply request a quote. We’re here to help you!

How to drill a hole into a solid Vespel® CR-6100 rod

A quick tip for the machine shop

Recently, a pump repair shop bought some 2″ solid rods of Vespel® CR-6100. This was their first purchase of solid rods and they asked for instructions to machine the I.D. to size. Below is the method we use in our shop at Boulden:

Step 1: Drill Pilot Hole 0.375–0.500″ (10-12 mm) through the center of the bar

How to drill a hole into a solid Vespel CR-6100 rod

Step 1: Drill Pilot Hole 0.375–0.500″ (10-12 mm) through the center of the bar

Step 2: Use a spade blade (or larger drill bit) to enlarge the hole

How to drill a hole in a solid Vespel rod

Step 2: Use a spade blade (or larger drill bit) to enlarge the hole

 

 

 

 

 

 

 

 

 

 

 

Step 3: Final machine with boring bar

How to drill a hole in a solid Vespel rod

Step 3: Final machine with boring bar

 

 

 

 

 

 

 

 

 

 

 

Until Next Time

If you have a pump in your shop where you would like to improve reliability, efficiency, avoid seizing, or reduce vibration, consider an upgrade to Vespel® CR-6100 wear rings with the Boulden PERF-Seal® design. Whatever the pump geometry, Boulden can help you fit the parts into the pump, make drawings and machined parts for you, or simply help you with tips and tricks to use in your shop. Contact us today.

 

Helpful Links:

Boulden Installation Guide for Vespel® CR-6100

Standard Stock Sizes of Vespel® CR-6100

Vespel® CR-6100 Product Data Sheet

Vespel® CR-6100 Machining Guide

2MW Boiler Feed Pump Case Study

Amine Stripping Pump Case Study

 

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Kebab grill in Turkey

The master of the kebab grill, Ankara, Turkey

 

 

 

Upgrading Pumps With Composite Wear Components Part: 9

Upgrading pumps with composite wear componentsPart 9: Vertically Suspended Pumps with Separate Discharge (API Type VS4)

Review

Last week, we looked at vertically suspended pumps with the discharge through the column. This week, we take a look at vertically suspended pumps with a separate discharge (API Type VS4).

Vertically suspended pumps with separate discharge are generally used for sump pumps or wastewater pumps. It seems that these pumps cause headaches at most plants. The shaft bushings wear out leading to chronic repairs. Over the years, Vespel® CR-6100 has been used in a lot of these pumps, sometimes the pump life has been extended from a few months to several years; sometimes the pump life has not improved. Therefore, it is important to identify the failure mode before making the upgrade. We have used our experience to create the roadmap below.

Define the Service

The first step is to clarify “sump pump” or “waste water pump”, which are generic terms encompassing a wide range of services. Some services are pumping primarily chemicals mixed with water, and some of them are pumping primarily water mixed with dirt. Some of the sumps are so dirty, that the pump suction strainer seems to be immersed in mud. Another variable is that the shaft bearings are flushed with different arrangements. Some of them are flushed with the process fluid, others with clean water, and others are greased.

Given the range of service conditions, there are several different problems which can cause the shaft bearings to fail. However, there are some commonalities. First, by design, the shaft bearings are along the column of the pump with a separate discharge for the process fluid. The bearings can run dry at start-up until the flushing fluid arrives. Sometimes, the external water flush is turned off; sometimes the flush lines from the process fluid are plugged; sometimes the grease supply runs out. In a chemical sump, the mix of chemicals can cause corrosion with metal wear parts. Finally, in very dirty sumps where the bearings are flushed with the process fluid, abrasive wear tends to be the main problem.

The Road Map

Considering the above, we have created the following table to guide your selection of where to use Vespel® CR-6100 in sump pump services:

Vespel CR-6100 sump pump services.

Vespel CR-6100 sump pump services.

*Some of the alternatives to consider are switching to a clean water flush or looking at an abrasive resistant, non-seizing combination for the shaft bushings and sleeve. This generally entails a hardened sleeve and an abrasive resistant bushing material. If you have any doubts about whether Vespel® CR-6100 is a good fit for your service, contact Boulden to discuss.

Vertical Pump Conclusion

To finish our discussion of vertically suspended pumps, we will talk about assembly and installation issues which impact pump reliability. Until then, use the above information as a guide on how to use Vespel® CR-6100 in your pumps with a separate discharge. If you need any material, 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.

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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Late Night Snack, Thailand

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Boulden Company

Conshohocken, PA, USA

1-610-825-1515

 

Boulden International, S.ar.L

Ellange, Luxembourg

+352 26 39 33 99

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