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Vespel® CR-6100 Solutions for Pump Cavitation

Noise. Excessive energy use. Life-shortening pump damage. If you’re experiencing these things, pump cavitation may be the problem.

What is Cavitation?

Cavitation happens when bubbles form in liquid and then implode. The force created by the implosions can lead to a lot of problems, like excessive vibration, decreased flow rates, damaged impellers and bearing failures. Any one of these problems can take your pump offline. All of them together can shorten the life of your pump considerably.

Replacing standard wear rings with Vespel® CR-6100 wear rings and reducing the running clearance in your pump can help solve this problem. One of the causes of cavitation is excessive leakage past the wear rings, as liquid recirculates from pump discharge back to suction. Reducing the clearance at the wear rings using Vespel® CR-6100 minimizes this flow and improves the flow conditions at the pump suction.

What Are Vespel® CR-6100 Wear Rings?

A composite material made from Teflon® PFA resin andlong carbon fibers, Vespel® CR-6100 can seamlessly replace metal, carbon and composite parts. They provide numerous benefits, including:

• Improved survival of pump run dry events
• Greater centrifugal pump efficiency
• Better rotor stability and reduced pump vibration*
• Improved mechanical seal reliability*
• Temperature capabilities from cryogenic to 550 F
• Broad chemical compatibility

How to Install Vespel® CR-6100 Pump Wear Rings

This video will show you how Vespel® CR-6100 pump wear rings fit. Installing wear rings is quick and easy.

Contact Boulden

For more information about how Vespel® CR-6100 wear rings can help minimize cavitation, contact Boulden today to speak with one of our engineers.

Replace PTFE Mixer and Agitator Bearings with Vespel® CR-6100 for Maximized Performance and Service Life

Mixers and agitators typically have shafts 5 to 7 meters (15-20 feet) long with large propellers designed to mix the process fluid. The shafts are often supported by only two bearings—one at the top and one at the bottom.  These bearings must withstand significant radial loads, in addition to a wide range of temperatures and chemicals.

Mixers and agitators are frequently built with PTFE or filled-PTFE shaft bearings. PTFE-based materials are used because they offer excellent chemical resistance and low friction. Unfortunately, these materials have significant shortcomings in their dimensional stability.

Vespel® CR-6100 offers similar chemical and frictional performance, combined with excellent dimensional stability.

The Weakness of PTFE

PTFE agitator and mixer bearings wear prematurely for several reasons. It is well-known that PTFE-based materials tend to cold-flow and creep. So, when radial loads are applied to the bearings, the PTFE will deform under load. Over time, the bearing clearance increases, allowing the shaft to move, increasing radial loads and accelerating wear.

Compounding the problem, PTFE-based materials typically have a very high coefficient of thermal expansion. In elevated temperature service, the clearance at the bearing will change with temperature, making the bearing clearance dependent upon operating temperature.

Finally, PTFE can be abrasive to the mating metal surface, causing wear of the metal part.

The Strength of DuPont™ Vespel® CR-6100

Vespel® CR-6100 mitigates all of the above issues. Vespel® CR-6100 does not cold flow or creep and experiences minimal deformation under load. The mixer or agitator bearing acts like a bearing and keeps running. The material can withstand very high loads with minimal wear and continue running.

Beyond that, Vespel® CR-6100 has an extremely low coefficient of thermal expansion. When a mixer or agitator bearing with Vespel® CR-6100 is properly designed, the clearance at the bearing remains constant over a wide temperature range, allowing the bearing to function as designed in any temperature up to 260 C (500 F).

Furthermore, Vespel® CR-6100 generally does not cause damage to mating metal parts, allowing the bearing to maintain minimal clearance for a very long run.

Sulfuric Acid Mixer Bearings

Example

Consider the photo above from a sulfuric acid mixer. The top shafts were running against glass-filled PTFE mixer bearings. The bottom shaft was running against Vespel® CR-6100. After 3.5 years, there was no damage to the shaft under the Vespel® CR-6100. The shafts running against glass-filled PTFE were severely worn.

Conclusion

For many years, Vespel® CR-6100 has been used for mixer and agitator bearings with excellent results. Process fluids have included, acids, bases, aromatics, hydrocarbons, and specialized chemicals. If you are experiencing premature wear of any mixer or agitator bearings in your plant, contact your local Boulden representative to discuss an upgrade to Vespel® CR-6100.

Use DuPont Vespel CR-6100 in Cryogenic Ethane and Methane Pumps

Ethane and methane are the lightest light hydrocarbons. In liquid form, they are most commonly found at cryogenic temperatures down to -160⁰C. The cryogenic temperatures combined with the low viscosity and flashing nature of the products makes cryogenic ethane, methane, and LNG pumps a challenging pump application.

Limitations of traditional materials

The wear rings in a typical cryogenic LNG pump are often 316L stainless steel, and shaft bushings are often carbon-filled PTFE. Both
materials are fine with the product and low temperatures, but they require tough design trade-offs.

316L parts require generous clearances to avoid seizure. Larger clearances increase the risk of cavitation and vaporization of the product at suction. Carbon-filled PTFE is a good cryogenic material, but it lacks dimensional stability. If a pump runs dry in this service, the carbon-filled PTFE bushings will be destroyed quite quickly.

Vespel® CR-6100 is an ideal material for cryogenic LNG service

Vespel® CR-6100 is a composite material made from long, oriented carbon fibers and Teflon® PFA resin. It provides excellent performance in cryogenic temperatures, but also delivers exceptional dimensional stability. Using Vespel® CR-6100, wear ring clearances can be reduced to less than the API 610 minimum standard for metal wear rings, and shaft bushings will run for many years. If the light methane or ethane gas evaporates at suction, the pump will not have metal-to-metal contact points, and the risk of seizure will be minimized.

Photos 1 and 2 show the shaft bearing and case wear ring from a single stage liquid methane pump running at -82⁰C. The pump was removed for inspection after 4 months of running because it appeared as though the pump had run dry due to operational issues. Upon inspection, the parts were found in “as installed” condition and the pump returned to service.

Consider DuPont™ Vespel® CR-6100 for API Separator Bearings

API Separator Chain

One of the first waste treatments steps in a refinery or petrochemical plant is the API Separator, which uses differences in specific gravity to separate oil, water, and suspended solids. The oil rises to the top, the solids settle at the bottom, and a layer of water is in between. The oil is typically returned to the plant for reprocessing, the sediment is scraped off the bottom, and the water is sent onward for further waste treatment.

Several plants have used Vespel® CR-6100 to improve the life of the bearings on the shafts which drive the chain and flight scraper which scrapes the oil off the bottom of the separator. Typically, these bearings are plain Babbitt/bronze bearings which can experience significant wear over time. When the wear becomes excessive, the chain driving the flight scrapers can fall off, creating a maintenance headache. In extreme cases, the shafts can fall into the bottom of the pit requiring a shutdown of the unit.

The application is tough because the bearings are lubricated by the product, which is a mix of water, some suspended solids, and trace amounts of any kind of chemical which might exist in the plant. In addition, the bearings are under massive loads due to the long, large-diameter shafts which drive the chain and flight scraper.

In the early 2000’s, a refinery in California was having problems with the original Babbitt/bronze bearings. They would wear excessively, resulting in the chain driving the flight scrapers falling off. They decided to try two potential solutions. On one end of each shaft, they installed Vespel® CR-6100 bearings, by machining out some existing bronze bearings and installing a sleeve of Vespel® CR-6100. On the other end of the shaft, they installed a bearing made from nickel impregnated graphite.

After 4 years, the unit was shut down and the bearings were inspected. The Vespel® CR-6100 bearings had 0,6 mm of wear (0.024” of wear). The impregnated graphite bearings did no better than the previous Babbitt/bronze bearings and had about 10-12 mm of wear (0.4-0.5” of wear). All of the bearings were subsequently upgraded to Vespel® CR-6100.

After 4 years of service:

Nickel Impregnated Bearing and Bearing Made from Vespel CR-6100

              Nickel Impregnated Graphite Bearing              Vespel® CR-6100 Bearing  

If you are experiencing excessive wear of your bearings in this service, consider Vespel® CR-6100 as an upgrade for your next overhaul.

How Much Does it Cost to Run a Pump?

Best Wishes to all of our customers, colleagues, and friends for 2014. We look forward to working with you in the New Year.

Over the past few months, we have discussed the efficiency gains which can be attained from using Vespel® CR-6100 wear rings with reduced clearance in centrifugal pumps. One of the common questions we have received is “what is the payback period for the investment?” The answer to this question depends on how much you are paying for power, and that varies widely around the world. Below is a table showing the average cost of industrial power around the world and the cost of operating a 100 horse power pump continuously for one year.

Country Cost $/kWh Cost for 100 HP for a year
EU Average  $ 0.16  $ 101,319
Germany  $ 0.16  $ 105,724
U.K.  $ 0.14  $ 91,628
Spain  $ 0.16  $ 102,200
France  $ 0.11  $ 71,364
Italy  $ 0.23  $ 147,133
Belgium  $ 0.16  $ 101,319
Netherlands  $ 0.12  $ 79,293
U.S. Average  $ 0.07  $ 45,683
Gulf Coast  $ 0.06  $ 39,157
California  $ 0.12  $ 78,314
New Jersey  $ 0.10  $ 65,262
Singapore  $ 0.19  $ 123,998

Middle East

In the Middle East, it is not unusual for plants to play $0.02/kWh for power. However, this does not reflect the true cost of power. In Saudi Arabia, power is often produced by burning oil. Whatever they do not burn can be exported for more than $100/bbl. This means the true cost of power to Saudi Aramco is $0.07/kWh. A similar situation exists in other regional countries which export natural gas. The value of the gas as an export is approximately $0.05/kWh (depending on the spot price of gas).

Payback Period

As we discussed in recent months, the % of the power which can be saved is a function of pump specific speed. For the payback period, we will look at a typical process pump with a specific speed of 2000, and assume the pump service has 2 pumps (100% fully spared operation). Using the above assumptions, the payback period from energy savings alone for upgrading a pump with Vespel® CR-6100 is as follows:

Country

Payback Period (Months)

EU Average

9

Germany

9

U.K.

10

Spain

9

France

12

Italy

6

Belgium

9

Netherlands

11

U.S. Average

20

Gulf Coast

23

California

11

New Jersey

14

Singapore

8

Hot Water Condensate Pumps: Upgrade with DuPont™ Vespel® CR-6100

Condensate return pumps often cause reliability problems. Hot water can vaporize, causing pumps to run dry. Hot water is also a notoriously poor lubricant, resulting in excessive wear of traditional materials. Vespel® CR-6100 has been used in dozens of condensate pumps over the years with excellent results. The first known pump to be upgraded to Vespel® CR-6100 was a condensate pump in a refinery, and this simple upgrade increased the MTBR of the pump from 4 months to 9 years.

The first pump upgraded with Vespel® CR-6100

In 1997, the condensate return pump at a California refinery was unreliable, failing due to run-dry events. Plant personnel had tried all types of different materials—the original bronze, metal alloys, plastics, and even ceramic bearings. Nothing worked. The pumps would fail three times per year.

The plant engineers decided to try Vespel® CR-6100 in this application due to its low coefficient of thermal expansion, believing this could help the pump survive run dry events. They installed Vespel® CR-6100 shaft bearings and stationary wear rings.

Because this was likely the first pump in the world to use Vespel® CR-6100, they ran a test. While monitoring motor amps and vibration, they deliberately ran the pump dry for progressively longer periods of time. When the pump survived running dry for 30-minutes, they turned the pump over to operations, and shortly thereafter upgraded the other pump in the service.

For several years after this installation, the pumps ran without failure.

Click here to see a video that quickly demonstrates where Vespel® CR-6100 parts are used in this pump. 

Epilogue

At a turnaround 3 years after the initial installation, one of the pumps was pulled for inspection. The Vespel® CR-6100 remained in excellent condition with nearly zero wear. The pump was reassembled and returned to service. In 2006, the refinery engineers took a look at the maintenance records of the pumps and found that neither pump had been overhauled since the original installation of Vespel® CR-6100. The MTBR of the pumps had increased from 4 months to 9 years.

Over the past 15 years, thousands of pumps have been upgraded with Vespel® CR-6100 with excellent results. Due to successful applications around the world, Vespel® CR-6100 is now included in API610 11th Edition—the global standard for centrifugal pumps in the hydrocarbon processing industry.

Use DuPont™ Vespel® CR-6100 to make hydrofluoric acid pumps last longer

Hydrofluoric acid—also known as HF acid or hydrogen fluoride—is a strong acid found in some refinery alkylation processes along with specialized chemical processes. DuPont™ Vespel® CR-6100 can help hydrofluoric acid pumps last longer.

Limitations of traditional materials

Traditionally, pump wear parts in hydrofluoric acid pumps are made from Monel, which is very good for corrosion resistance. The downside of this arrangement is that both rotating and stationary wear components need to be made from Monel, requiring large clearances at the wear rings to avoid pump seizure. These large clearances compromise the mechanical seal life, exposing the seal to higher vibration and larger shaft deflections.

Hydrofluoric acid also tends to form crystalline deposits. Several plants using hydrofluoric acid have reported pumps being seized due to crystalline deposits accumulating while the pump is in stand-by, ultimately eliminating the clearance at the wear rings. When Operations tries to switch to the spare pump, it proves to be unavailable.

Wear rings made from Dupont CR-6100

Replace the stationary Monel rings

The way to upgrade your hydrofluoric acid pumps is to install DuPont™ Vespel® CR-6100 stationary wear rings and continue using Monel rotating rings. This small change essentially eliminates the risk of seizure between Monel parts and allows tighter clearances at the wear rings.

Vespel® CR-6100 is a composite material made from oriented carbon fibers and Teflon® PFA resin, resulting in broad chemical resistance combined, excellent dimensional stability, and a temperature range from cryogenic to 260⁰C (500⁰F). Vespel® CR-6100 has been used in a wide range of hydrofluoric acid services for more than 10 years with excellent results.

Because Vespel® CR-6100 is non-seizing, wear ring and bushing clearances can be reduced to 50% of the API minimum values for metal rings. The reduced clearance increases the hydraulic forces which stabilize the pump rotor. With a more stable rotor, shaft deflection is reduced, vibration is reduced, and seals and bearings will last longer. Furthermore, even if crystalline deposits build up when the pump is in stand-by, it is unlikely they will cause the pump to seize at start up.

The end result of this upgrade is greater reliability and availability. Life cycle costs will be lower with fewer repairs and avoiding seizures of the Monel parts.

 

Keep 2-Stage Overhung Pumps Running Smooth by Upgrading to DuPont™ Vespel® CR-6100

The 2-stage overhung pump is a common design from the 1960’s through the early 1980’s. Most plants built during that era will have at least a few of these pumps still running, and most likely causing problems. Upgrading the internal wear components with Vespel® CR-6100 can mitigate the most common issues with these pumps.

2-stage-overhung-pump

Why 2-Stage Overhung Pumps Fail

The common failure modes for 2-stage overhung pumps are seal failures and pump seizure. Both are caused by the same fundamental design flaw. The shaft is too long, too thin, and too flexible to handle the loads associated with two impellers. The shaft bends, the wear rings or the bushing between the stages rub and potentially seize, the pump vibrates, and the seal fails.

Worse yet, changing the pump in the field is almost never easy. From the introduction of API610, 7th Edition in 1991, this design no longer complies with the standard. New 2-stage pumps require a between-bearings design. In the plant, this requires piping modifications, a new foundation, and significantly more space, which is often not available. This kind of modification can also easily cost more than $100,000 per pump. Unless the pump is a safety hazard or restricts production, obtaining funding for this project can be quite difficult. So, in most situations, the pump remains.

A Simple Improvement

Installing DuPont™ Vespel® CR-6100 case rings and inter-stage bushings in this pump will help to address the excessive shaft deflection in these pumps. Vespel® CR-6100 is a composite material made from oriented carbon fibers and Teflon® PFA resin. It will not seize like a metal wear ring, minimizing the risk of pump seizure. Because the risk of seizure is minimized, wear ring and bushing clearances can be reduced down to 50% of the API minimum values for metal rings. The reduced clearance increases the hydraulic forces which stabilize the pump rotor. Importantly, these stabilizing forces act on the rotor at the impellers where they have the most impact. With a more stable rotor, shaft deflection is reduced, vibration is reduced, and seals and bearings will last longer.

Reduced clearance also augments the pump performance and efficiency and mitigates cavitation. In many situations, the wear ring clearance on these pumps has been increased to avoid seizure. Reducing this excessive clearance to 50% of the API minimum for metal rings has shown efficiency gains of over 10% in some examples and has dramatically reduced the pump noise and vibration.

Side Note:

Vespel® CR-6100 can be used in continuous process temperatures up to 500⁰ F (260⁰ C). What if your pump runs hotter? Consider DuPont™ Vespel® SCP-5050, which can operate in continuous temperatures up to 600⁰ F (315⁰ C). Contact Boulden for more details. 

How to Achieve World Class Pump Reliability

In 2004, a refinery in North America was struggling with pump reliability. The MTBR (mean time between repair) for pumps was stuck at 39 months, which was about the same level as the plant had achieved in 1996. In a dramatic turnaround, by the end of 2011, the refinery MTBR for pumps had improved to 85 months (figure 1).

Figure 1: Refinery Pump MTBR 1996—2011

8 stage boiler feed pump

This is the story of how they did it.

A Simple Program

There were two aspects to the program: better repair practices and reduced internal clearances using Vespel® CR-6100. Over the 7 years of the program, more than 400 pumps were upgraded using Vespel® CR-6100 wear components—nearly half the refinery pump population.

The Repair Practices

One of the core issues the refinery faced before 2004 was a repair schedule dictated by Operations. Pump repair was based on timeliness, not quality. Operations would put pressure on Maintenance because they did not want to run without a spare pump in place. Most pumps were repaired “as is” or “to the original specification” without systematically implementing root cause failure analyses or
best practices. Many pumps were repaired with pump cases left in the field. The refinery had all the necessary personnel and skills; they simply needed to use their resources effectively.

In 2004, the Maintenance department began to change the repair process such that every repair was viewed as an opportunity for improvement.

The first step was to gain more time to execute a proper pump repair. The plant used the existing condition monitoring program to verify that the running spare pump was healthy. This information was communicated to Operations and over time they became more comfortable with pumps running for longer periods of time without the spare pump in place.

With enough time to perform a thorough inspection and repair, the Maintenance department was able
to implement a much more thorough procedure. Some of the changes implemented were as follows:

• Pulling all pumps and cases into the workshop for repair. No more field repairs (except seal changes on deep well pumps not showing signs of internal wear).
• Root cause failure analysis of the pump and seal for each pump repair
• Measurement and correction of fits, concentricity, and squareness of all pump components—whether new from the OEM or used
• Improved lubrication practices to ensure clean lube oil
• Correcting of seal flush plans, and bringing piping within the pump flanges so the whole assembly could be tested prior to leaving the shop
• Installation of Vespel® CR-6100 wear components : wear rings, throat bushings, throttle bushings, and vertical pump shaft bushings with clearance of wear rings set to 50% of the API minimum for metal parts.
• After assembly, but before mechanical seals were set, pump rotor turned by hand to ensure there were no internal rubs—verifying that the fits and concentricity were good
• Air testing of the pump before being sent to the field for installation

Vespel® CR-6100 with reduced clearance

A major catalyst for the program was the use of Vespel® CR-6100 for the stationary wear components in the pumps. Vespel® CR-6100 components can be installed with much tighter clearance than metal wear components—typically 50% of the API minimum values for metal wear rings. The tighter clearance provides the impetus for better fits, superior concentricity, and improved squareness of mating surfaces. Furthermore, Vespel® CR-6100 does not gall or seize like metal parts, giving mechanics the courage to reduce the internal clearances, knowing that the pump will not stick during alignment or start up and return to the shop.

Vespel® CR-6100 is a composite material with exceptional dimensional stability. It is chemically compatible with all of the typical process fluids and chemicals in a refinery with a temperature range from cryogenic to 260⁰ C. This broad application window made it an ideal choice for a standard repair material. By standardizing on one material for upgrades, the plant was able to have one set of installation and machining procedures and one material on the shelf.

The Vespel® CR-6100 was installed as the stationary wear components—wear rings, throat bushings, throttle bushings, and vertical pump shaft bushings. In general, wear ring clearance was reduced to 50% of the API minimum, throat bushing clearance was set to function with the mechanical seal flush plan., and throttle bushing clearance and vertical pump shaft bushing clearance were set to 50% of the API minimum plus 0.002” (0,05 mm).

The small change of reducing internal clearance is a major reliability upgrade. Reduced clearance increases hydraulic damping and stiffness, reducing vibration and shaft deflection—mechanical seals and bearings last longer. Reduced clearance also reduces NPSHR, reducing the potential for cavitation. Because internal leakage is reduced, pump performance and efficiency are improved, reducing energy consumption or boosting throughput. And even in the case where a pump suffers from an off-design process event such as running dry or running blocked in, Vespel® CR-6100 has proven that it can withstand significant abuse, helping the pump to survive these events with minimal damage.

Conclusions

The results at this plant are just one more data point in a growing list of plants which have derived very large reliability gains from the use of Vespel® CR-6100 with reduced clearance combined with superior repair practices.

Over the 7 years of this program, the refinery was able to reduce their annual pump repair costs by 30% and save millions of dollars in maintenance cost.

Nearly all of the original Vespel® CR-6100 wear components remain in service today