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Our Spring Discs Keep Flanges Secure Against Unexpected Loads.

Spring Discs Resist Looseness Due To Thermal Expansion And Contraction.

Live Loading with Fan Disc Flange Washers

It is estimated that 50% to 80% of all flange leaks are caused by insufficient bolt preload. Typically,the bolts are all tightened to a given aim preload. At this original load the bolt has a certain amountof stretch and the gasket yields (takes a set). At start-up, the process fluid heats up the flange andgasket causing them to expand more than the bolts. This leads to an increase in gasket stress causingmore yielding. As the bolt temperature rises, preload may fall substantially. This is because the boltstretch is insufficient to "make up-'for what is lost when the gasket yields. For example, if the boltstretch was .005" and the gasket yield is .005", then all of the original preload will be lost.

Fan Disc Flange Washers are eff ective because they add deflection to the fastening system. This isknown as "Live Loading". Flange washers are Belleville springs that are typically very elasticcompared to the bolts they are used with. The increase in elasticity leads to dramatically less loss of

preload when joint components yield. Using the previous example, if the joint employed two flangewashers with .025" of deflection each, then preload would fall by only 9% rather than 100%.

Examples of candidates for live loading using flange washers are:

A. Flanges where failure could cause a safety problem.

B. Joints that see large temperature fluctuations.

C. The length to diameter ratio of the bolt is less than three.

D. One or more of the flange components are subject to yielding such as gasket creep, bolt creep,vibration, or elastic interaction.

E. The flange has a history of maintenance problems.

Live loading will not correct all flange leaks. However, maintaining more of the original preload willsignificantly reduce the risk of failure. It has been shown through various studies by a number ofpetrochemical companies, that the use of flange washers has a positive overall impact on theperformance of a live loaded flange joint.

Selecting a material

The obvious selection would be to use the same material used for the bolt. However, sinceBellevilles are normally made from materials which make good springs, this is not always possible.Fan Disc Corporation produces flange washers in three different materials. H-13 Tool Steel, 17-7PHStainless Steel and Inconel 750 Nickel Alloy. The material you select should be based on the designand environment of the flange joint being live loaded. The description and characteristic of each ofthese steels can be found in the catalog pages of our technical catalog.

Selecting the right size

Fan Disc's flange washers are designed so that the OD of the spring matches the points on a standard

nut. This is done to prevent any interference between the flange washer and the flange and so thespring will not overhang the flange joint.

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Next the flat load of the flange washer must be selected. ldeally,the flat load should equal thecalculated preload based on the gasket design and criteria. In other words, the preload calculationsshould not be any different than when no flange washers are used. Multiple washers can be stackedin parallel to achieve varying flat loads. The calculated preload may not always equal the flat load ofa single spring. If this is the case choose the spring with the flat load closest to the aim preload. Theflange washer will not be damaged if the flat load is exceeded. On the contrary, completely flatteningthe Belleville optimizes it's benefit.

There are applications where flange washers should not be completely flattened, such as cyclical andwhen flange washers are employed to reduce the increase in preload that occurs during a temperaturecycle by using a Belleville with a flat load that is higher than the assembly preload. Contact Fan Discfor more information.

How many flange washers per bolt?

A good rule of thumb is to use two: one on each end of the bolt. This will increase the deflection ofthe fastening system by a factor of 7 to 15 for atypical 3001b flange joint. This increase should beample for almost any bolting application.

Excessive movement in joint materials, short active bolt length, and high differential thermalexpansion are some examples where more than two flange washers should be used. Fan Disc'sengineering staff can assist you when these problems occur.

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Material: STAINLESS: 17/7 PH (Armco) precipitation hardened Hardness: Rc-38-43TEMPERATURE RANGE: -220 C to +300 C For higher temperatures, to +600 C we can supplyInconel X-750 (non standard) Note: For normal operating temperatures, we can supply ChromeVanadium SAI 6150.

Tolerance: OD - Plus .000/Minus 1.5% x OD (designed for standard Flange spot face diameter) (D-Minus .000/Plus/Minus 1.5% ID THICKNESS - Plus/Minus 5% x normal thickness shown LOAD -Plus/Minus 20% of nominal shown (when ordering, ALWAYS specify materials after part number)

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FAN DISC CORPORATION8127 Clybourn Avenue Sun Valley CA 91352

(818) 767-1886 (818) 767-7415 FAXCopyright 1997 Fan Disc Corporation


26/02/2010http://www fandisc com/sdi htm

spring disc was her

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