International Rotating Equipment Conference 2012, Düsseldorf

Session Name:

Session:

Pipe Load Reductions on Load Sensitive Equipment

Author:

Klaus P. RedmannDirector of Quality ManagementDisc Spring Technology, LLCRichland, WA 99352 U.S.A

Co-Author 1:

James O. TaylorDirector of FabricationDisc Spring Technology, LLCRichland, WA 99352 U.S.A

Co-Author 2:

Bharat V. Makadia, P.E.Director of EngineeringDisc Spring Technology, LLCRichland, WA 99352 U.S.A

Co-Author 3:

Kirk V. LeonardiDirector of Product Development Disc Spring Technology, LLCRichland, WA 99352 U.S.A

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International Rotating Equipment Conference 2012, Düsseldorf

Summary:

In general, any excessive loads on rotating equipment will lead to safety and operational issues. For those reasons, all engineering efforts and rotating equipment designs focuses on minimizing the piping loads on the equipment.

Engineering evaluations have shown that the highest load reduction can be achieved by adding a spring support below the rotating equipment nozzles. Unfortunately the placement of the conventional helical spring support system is not possible due to available heights or space restrictions. Consequently, this geometry of the equip-ment nozzles requires a compact spring support system.

Through the availability of a new innovative device, using “Belleville” springs, it is now possible to support excessive nozzle loads in these confined spaces. The primary advantage for the use of the new device is the application of heavy equip-ment loads subject to small movements.

With the use of Belleville spring support systems in space restricted areas, excess nozzle loads can be reduced up to 60%, which improves the overall safety and per -formance of the rotating equipment.

Corrosion deteriorated support systems often fail to meet the initial engineering design function of the device, which would impact the loading on the rotating equipment. Helical coils support systems made out of stainless steel or other corrosion resistant materials are very costly and rarely used. Due to the use of the Belleville springs, the entire support systems can easily be made of corrosion resistant materials.

In summary, by the use of the Belleville spring support system, the rotating equipment is subject to reduction in excessive loads and minimizing undesired equipment vibrations. These contributing factors warrant proper alignment of the rotating equipment, which results in lower maintenance and operational cost of the equipment.

Description:

In today’s economic climate, it has become increasingly important to improve process safety performance and reduce overall processing costs in order to increase the margin of profits.

One of the biggest contributors for high maintenance and plant shut downs is the insufficient control of pipe loads induced on rotating equipment nozzles. Federal and state regulations are defined in multiple Industry and regulatory codes and standards.

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International Rotating Equipment Conference 2012, Düsseldorf

For example, the ASME B31 codes require consideration of the effects that piping system displacements have on connected equipment. However, these code requirements give wide latitude regarding the specific analysis to be done, and no guidance at all regarding acceptable nozzle loads.

Therefore, controlling the piping loads imposed on load sensitive rotating equipment is essential to satisfy and meeting the nozzle allowable loads. These allowable loads are called out in U.S. applicable industry standards such as: API 610 - Centrifugal Pumps, API 617 - Centrifugal Compressors, and NEMA SM-23 - Steam Turbines.

Further, for non-rotating equipment, similiar industry standards exist. (i.e., WRC-107 and WRC-297 - pressure vessels and shell and tube heat exchangers, API 662 - plate and frame heat exchangers, API 661 - Air-cooled Heat exchangers and API 650 and API 620 - API storage tanks).

Excessive nozzle loads essentially originate from sustained gravity sources and restrain of free thermal displacement of the attached piping. Excessive piping loads in these cases can cause higher stresses on flanges, machine vibrations, shaft misalignment and coupling failures. These problems result in increased maintanance cost and can lead to equipment and plant shut down.

In summary, the main contributing factors and causes for excessive pipe loads are:

Misalignment Pipe Strain Due to Thermal Expansion Pipe Lift-off Poor Engineering Practices and Design Errors Unsupported Pipe Weight Insufficient Plant Space availability to install required Pipe Supports Equipment Vibrations

Costly unscheduled shutdowns and accidents which occur in power, chemical and petrochemical plants are greatly diminished by the reducing strain or excessive loads on rotating equipment. In many cases, situations that involve unscheduled shut-downs result from chronic flange leaks and overloaded equipment nozzles.

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International Rotating Equipment Conference 2012, Düsseldorf

According to numerous sources, such as Popular Mechanics, refineries and indus-trial plants that experience frequent unscheduled shutdowns are more prone to acci-dents, down time and costly safety fines.

In the article, Refinery Asphalt Pump Seal Failure and Fire, “A small refinery that pro-duced asphalt and ink oil experienced a serious fire under the atmospheric and vac-uum bottom towers” (Intertek Group Plc.). “The fire caused damage to the plant and resulted in one year of business interruption. An investigation by the refinery deter-mined the cause to be accidental ignition of light oil that was spraying out from a leaking pump shaft seal” (Intertek Group Plc.).

In another example, time and production where lost when a pump seal failure caused a fire in a California refinery. It was in the FCC unit which caused a 16 day shutdown, cutting the plant’s gasoline production by 2.3 million gallons a day.

During another refinery incident, a bearing problem caused a pump seal to fail, which ultimately spilled a chemical, also resulting in a refinery fire. No one was injured in the fire blaze, but the 40-foot flames fried overhead instrumentation cables and crip-pled the FCC.

There are countless examples cited in industry literature and internet sites, which describe catastrphic failures of rotating equipment due to excessive pipe load conditions.

In order to prevent these accidents and meet code and standard requirements, many engineers try to address these problems by designing pipe configurations that will reduce these excessive forces. This approach of pipe stress analysis is time consuming and costly.

The common practice is to design a series of horizontal or vertical loops to introduce some degree of flexibility to control thermal expansion to the piping system. In addition, in some incidents a spring support system is added below the the elbow portion of the piping close to the pump. Despite of these efforts, un-resolved conditions during the design process, misalignment during installation, and change in operating conditions can create loading that exceeds the pump manufacture’s specifications.

”Unacceptable pipe strain can be defined as any forces from unanchored piping that will cause equipment deformation of more than 0.002-inch.”(Pump Zone, 2009). “Misalignment may also be the result of high nozzle loads. If the suction or discharge piping needs to be mechanically leveraged into position, in order to be bolted to the pump, there is a chance the resultant forces and moments acting on the pump will be sufficient to move the seal chamber out of alignment with the pump shaft” (Andrews, 2005).

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International Rotating Equipment Conference 2012, Düsseldorf

Over time, a pump flange that initially experiences excessive loads due to misalignment and thermal expansion will continue to be problematic and will eventually cause premature pump failure. “Pump bearing housings, shafts, and seal chambers are manufactured with close tolerances.

As a pump ages, there tolerances open as a result of wear, corrosion, erosion and, in some cases, improper maintenance. “Misalignment resulting from excessive clearance in each of these areas, compounds to create an ever increasing misalignment at the seal faces, accelerating seal wear, resulting in premature seal-pump failure” (Andrews, 2005).

What is needed is a means to, effectively, remove loads and moments from the pump nozzle. One way to accomplish this, is to drill out the flange bolt holes so they align with the bolts of the mating flange. This technique is widely accepted but since the two mating flanges still have a skewed surface between them, the potential for seal leakage continues to be a possibility. This possibility increases substantially when the flanges undergo thermal expansion, otherwise known as hot misalignment.

It is imperative that loads be limited to meet applicable codes. Another way to effectively reduce or even remove loads and moments from a pump’s flanges is to use a spring support located below the mating flanges.

Currently, helical coil type springs are utilized to allow for thermal expansion within piping systems in order to meet requirements of code and industry standards. Coil spring supports have been proven throughout different industries to solve large movements. However, for small movements, coil springs have not proven to be effective.

Design engineers are reluctant to use a coil spring support where thermal pipe expansion and/or equipment displacements are small (e.g. 1/16 inches). The need to satisfy thermal and gravitational loads on sensitive equipment in a piping system is just as viable for movements measured in thousands of an inch, as it is for large movements of several inches.

Space limitations are often controlling factors near sensitive equipment. The helical coil spring support is too large of a device to be typically installed under any flange connecting pipes. This limitation is also magnified by piping and HVAC insulation requirements that must be recognized as well.

Solution:

Since a pipe support system is required to significantly reduce piping loads, the challenge becomes to have a device which physically fits under the pipe flange location. The majority of the piping and rotating equipment connections are physically bound by limited hight availability.

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International Rotating Equipment Conference 2012, Düsseldorf

A U.S. - Pipe Support Engineering Company, utilizing common Belleville springs, has patented, engineered and tested a series of spring supports small enough to fit into confined spaces as small as 4 inches.

The compact design was achieved by the use of commonly known - Belleville springs, which have been used for a wide range of applications throughout various industries.

Their load carrying capacity varies with the numerous ways the spring’s diameter, height and thickness can be manipulated and the manner in which the springs are stacked. Detailed Engineering efforts are required to manage the attributes to select the proper spring arangements.

These innovative Belleville spring supports are designed to mimic the variable spring rate and load range of a coil spring support for smaller movements. The use of an additional spring support directly below the mating flanges, while placing a second spring support at a fulcrum point, away from the flanges, reduces dead weight and thermal loading on equipment flanges from 20 to over 60%.

With the use of Belleville springs, the number of load and deflection possibilities varies widely. This is accomplished by manipulating the spring’s inner and outer diameters, height and thickness with respect to one another. Engineering calculations and studies determined the appropriate relationships between Belleville springs to accomplish the desired load reductions.

These springs are engineered to match the variable spring rate and load capacities of helical coil springs. In comparison, noumerous engineered spring support sizes match support loads from 50 to 50,000 lbs. Due to the use of the compact Belleville springs, these devices require approximatly 25 to 50 % less space than the conventional helical coil spring supports.

By using the compact Belleville support system, the following additional advantages of the device can be summarized as follows:

1. The spring support system was designed to maintain the 25% variability require-ments of the MSS-SP58 Standard, as required by the mandatory ASME - Piping Codes. The 25% safety margin has significant impact on longevity and reliability of the rotating equipment.

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International Rotating Equipment Conference 2012, Düsseldorf

2. Excessive forces and moments on connected rotating requipment are minimized meeting nozzle load requirements set by the equipment’s manufacturer. Further, excessive stresses on adjacent supporting and restraining elements are also minimized.

3. The reduction of excessive load forces results in an increase of equipment life, as well as significant savings in overall maintenance and operational costs. Loads and moments are effectively transferred from the equipment’s flange to the spring support. The relief the equipment receives allows it to operate as it is intended for the duration of the equipment’s life cycle.

4. Compared to conventional helical coil support systems, the Belleville spring support systems allows to change desired deflection and load characteristics, by stacking disc springs in series, parallel, or combinations of series and parallel. This is ideal for unique situations where loads and movement requirements demand a stiffer or softer spring than is normally offered.

5. In regard to corrosion or process enviromental concerns, the entire spring support system can be made using stainless steel springs and housing, which require no maintenance during the life time of the support.

6. Due to the additional spring support, the effects on the pump nozzle are the con-tinuous reduction of forces, and the decreased possibilities of leaking pipe joints and equipment flanges.

7. By reducing the excessive nozzle loads on rotating equipment, the shaft and bearings of the equipment are allowed to rotate freely without any directional forces or vibrations.

CAESAR II Stress Analysis Testing:

In order to verify and confirm engineering calculations and expected stress load re-ductions, a CAESAR II stress computer modal analysis was performed.

The performed stress analysis compared two separate pump / pipe configurations. The differences in the configurations were the presence or absence of the spring support system below the pump flange. The goal was to show the reduction of loads and moments on the flange of a pump.

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International Rotating Equipment Conference 2012, Düsseldorf

The CAESAR II analysis results showed a dead weight load of 933 lbs, and a moment of 2,531 ft-lbs, for the configuration without the spring support under the pump flange.

For the configuration with the spring support on the pump flange, the analysis showed a dead weight load of 406 lbs, and a moment of 2,297 ft-lbs, which indicated a significant reduction in the dead weight load.

This reduction in forces and moments also assures that required equipment allowables will be met. The overall reduced pump strain further warrants a reduction in maintenance and an extended life time of the rotating equipment.

Bibliography:

Douglass, Elizabeth (2003, July 28). A Refinery’s Fever PitchRetrieved May 25, 2010, from The Los Angeles Times Web site: http://cwd.grassroots.com/energy/nw/?postId =3188&pageTitle=A+Refinery%27s+Fever+Pitch%3B

Popular Mechanics - 2005, September 14 What Went Wrong: Oil Refinery Disaster.Retrieved May 27, 2010, from http://www.popularmechanics.com/technology/ gadgets/news/1758242

Intertek Group Plc. - 2009, August 7Refinery Asphalt Pump Seal Failure and FireRetrieved May 27, 2010, from http://www.intertek.com/servicesdetail.aspx?id=8296

Pump Zone – March 2009How Do Plant Pipe Strain Problems Affect My Pumping Systems?Retrieved May 27, 2010, from http://www.pump-zone.com/topics/seals/how-do-plant-pipe-strain-problems-affect-my-pumping-systems

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