VacTran 3 Manual (779748), страница 27
Текст из файла (страница 27)
The most common type of permeation element used in vacuum systems isthe o-ring. Commonly found in the design of vessel feed-throughs, windows, bolted flanges, and pipe connections,o-rings are key to preventing leaks in vessel penetrations.O-rings are elastomer sealing devices used to prevent gas flow across joints in vacuum system components.
An oring is usually a circular cross section, ring shaped seal, which is squeezed in a precise way between matingsurfaces of a clamped or bolted joint. O-rings are often confined in a groove of rectangular cross section, and aregenerally molded from a variety of elastomers, such as Buna-N, silicone, and Viton, depending on the application.This manual is not intended to be a design guide to the selection and application of o-rings. Major industrysuppliers produce a large range of o-rings in many materials, and have complete data pertaining to the detailing andfabrication of o-ring joints.Standard o-ring sizes are used throughout the hydraulic, pneumatic, and vacuum industries for various applications.In order to make standard o-rings readily available when modeling a vacuum system, VacTran provides the o-ringlibrary utility to store and retrieve o-ring information on disk.See also:Why use an o-ring library?CaveatsCreating o-ring librariesO-ring library dialog descriptionO-ring compressionDialog commandsOpening existing o-ring librariesO-ring library file format14.1Why use an o-ring library?The o-ring library is handy when creating or modifying a system model.
When adding an o-ring/permeation gas loadto the system model, using an o-ring library to choose standard sizes can preclude the need to look up the dataeach time from a handbook. System models, shows how to add an o-ring to a system model.An o-ring library consists of a list of o-ring descriptions and their corresponding dimensions.
Like all otherdimensional data in VacTran, the o-ring dimensions can be entered in most common length units, and changed atany time.Any number of O-ring libraries can be saved, up to the capacity of disk storage. It is most useful to put related oring data into a common library. For example, an o-ring library saved as "Standard 4 inch" could contain all o-ringsof 4 inches in diameter with various cross section diameters.14.2CaveatsAlthough the o-ring permeation rate is both material and geometry dependent, the library only contains geometryinformation. Permeation data is stored in separate permeation libraries. This way, any combination of o-rings andpermeation materials can be combined in the system model.The same general warnings for permeation libraries apply to o-rings.
These specifically pertain to the problemsassociated with the characterization of the proper material to be modeled.© 2011 Professional Engineering ComputationsO-Ring Libraries14.3253Creating o-ring librariesTo create a new O-Ring Library: Under the File menu, select the New... command, or click on the icon as shown:In the New Document dialog that appears, select O-Ring Library and click on OK.(click to expand)© 2011 Professional Engineering Computations254VacTran 3The following dialog allows o-ring data entry and editing.© 2011 Professional Engineering ComputationsO-Ring Libraries14.4255O-ring library dialog descriptionFour input fields are given at the top of the dialog, one for a material description and the remaining for o-ringgeometry.
Initially, the data list is shown empty, waiting for data. The input fields are filled with default values. Atthe bottom of the dialog are pull down menus for length units. This can be used at any time, and all data will beimmediately converted.The data list in the center of the dialog contains a scrolling window of permeation data. In a new o-ring library, thislist will be initially empty. If more data is in the list than can fit in the window, use the scroll bar to move up anddown the list.
Clicking on one row of the list will update the input fields at the top of the dialog. To change the valueof this data, click in each input field and edit the number as you would with a word processor. The number you editin the input field will not change the list values below until you either add it to the list using the Add command, orreplace the selected list value with the Replace command.Underneath the data list, a pull down menu is provided to change the number format of the displayed data. Theformat menu can be changed at any time, an all data in the list will be immediately updated.Buttons on the bottom left of the dialog are provided for the basic editing functions. These Add, Replace, andDelete commands are also available under the Model menu at the top of the screen.© 2011 Professional Engineering Computations25614.5VacTran 3O-ring compressionTo provide effective sealing of two mating surfaces, o-rings are usually compressed by a specific amount.
This isoften expressed as a percentage of the cross section diameter, and is typically between 20-30%. In other words, a0.3-inch diameter cross section o-ring that has a compression factor of 0.2, will compress in height by 0.06 inches.Its installed section height will be squeezed from 0.3 to 0.24 inches. Simultaneously, the section depth willexpand by a factor 0.2 to roughly 0.36 inches, as shown in the figure below:Before squeeze0.3"0.3"After squeeze0.240.360.36When specifying the cross section diameter of the o-ring in the o-ring library dialog, use the nominal diameter beforecompression.
Specify the compression as a fraction of the nominal diameter. For an o-ring that needs to becompressed by 20%, specify 0.2 compression.The actual shape of a squeezed o-ring is somewhat more complex than the preceding assumptions indicate. Someerror is introduced during permeation calculations because VacTran approximates the o-ring as a rectangular crosssection.14.6Dialog commandsAdd (Ctrl+A): Adds the data in the input fields to the list. Since this is the default command, pressing the Returnkey has the same effect as clicking on Add.The Replace, and Delete commands are available only if there is data in the list.Replace (Ctrl+R): Replace the currently highlighted list selection with the data in the input fields.resorted according to the current sort option.
This option will be dimmed if the model is empty.The list isDelete (Ctrl+D): Delete the currently highlighted list selection. This option will be dimmed if the model is empty.14.7Opening existing o-ring librariesTo open an existing O-Ring Library: Under the File menu, select the Open... command, or click on the icon asshown:© 2011 Professional Engineering ComputationsO-Ring Libraries257In the Open Document dialog that appears, select O-Ring Library and click on OK.(click to expand)You will then be presented with a file selection dialog from which you can choose the directory and name of the ORing library file.© 2011 Professional Engineering Computations25814.8VacTran 3O-ring library file formatThe o-ring library file is stored as a text file, and can be edited with most word processors.
So if you prefer to use aword processor to enter all the data, save the file as "TEXT". VacTran will not read word processor files in theirnative formats, but will read text files. Change the extension of the file from "TXT" to "VTRNG". When editing yourown files outside VacTran, be sure to follow the formatting guide for o-ring libraries or VacTran will report an errorreading the file. A badly corrupted o-ring library file may cause unpredictable results.The following format is required for o-ring libraries.
ID is the inside diameter of the o-ring. SectionD is the crosssection diameter of the o-ring.ID and Section D are in cm units. Compression has no units.example:[General]Total=3[0]Description=2-001ID=0.07366SectionD=0.1016Compression=0.2[1]Description=2-002ID=0.10668SectionD=0.127Compression=0.2[2]Description=2-003ID=0.14224SectionD=0.1524Compression=0.2© 2011 Professional Engineering ComputationsPipe libraries15259Pipe librariesStandard pipe sizes are used throughout different industries for various applications, and vacuum systems are noexception.
In order to make standard pipes readily available when modeling a vacuum system, VacTran providesthe Pipe library utility to store and retrieve pipe information on disk.(click to expand)A pipe library consists of a list of pipe descriptions and their corresponding inside diameters. The pipe diametercan be entered in any length units of measure, and changed at any time.See also:Why use a pipe library?Pipe library creationPipe Library Dialog descriptionDialog commandsOpening Existing Pipe LibrariesPipe library file format15.1Why use a pipe library?The Pipe library is handy when creating or modifying a system model.
Any addition of pipes, elbows, apertures,and pipe bends can use a pipe library to choose a standard pipe size, and avoid the need to look up the data in ahandbook.Any number of pipe libraries can be saved up to the capacity of disk storage. It is most useful to put related pipedata into individual libraries. For example, a pipe library saved as "Sched 40" could have all common sizes ofschedule 40 pipe.© 2011 Professional Engineering Computations26015.2VacTran 3Pipe library creationTo create a new Pipe Library: Under the File menu, select the New... command, or click on the icon as shown:In the New Document dialog that appears, select Pipe Library and click on OK.(click to expand)The following dialog allows pipe data entry and editing.(click to expand)© 2011 Professional Engineering ComputationsPipe libraries15.3261Pipe Library Dialog descriptionTwo input fields are given at the top of the dialog, one for a pipe description and one for the inside diameter.
Initially,the data list is shown empty, waiting for data. The input fields are filled with a default value. At the bottom of thedialog is a pull down menu for length units. This pull down menu can be used at any time, and all pipe data will beconverted.The data list in the center of the dialog contains a scrolling window of pipe data. In a new pipe library, this list willbe initially empty.
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