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The selection of seal gasket The selection principle of seal gasket:for not high request condition,can choose by experience,change it when not suitable.But for strict request condition,for example ,easy explode、poison and flammability gas and corrosive liquid equipment、retort and delivery pipe system etc,should choose according to work pressure、work temperature、seal medium's causticity and combine the form of seal face. General speaking,at normal temperature condition,chose non metal soft rubber gasket seal ;mid pressure high temperature,choose non metal and metal assembled seal gasket or metal seal gasket;at temperature、pressure have big wave condition,choose good flexibility or self close seal gasket;at low temperature、causticity medium or vacuum condition should consider seal gasket's special capability.Need to explain specially is the effect of flange condition to choose gasket.
（1）The effect of flange form
The different of flange form,the different require to gasket.Glossy surface flange use at low pressure generally,mix soft thin seal gasket;In high pressure,if flange's strength is enough,can use glossy flange also,but should use deep soft gasket,or use in and out strength ring's spirotallic gasket.At this occasion,metal gasket is also not suitable,because require very big compaction power,cause bolt big transfiguration,made flange not easy to seal tight.If use metal gasket,will reduce the glaze,make it and gasket's contact area reduce.Like this,at the same bolt strain condition,after reduce narrow glaze's compaction power will increase.
（2）The effect of flange surface's coarseness degree
The flange's coarseness degree have very big effect to seal impact,specially when adopt non-soft gasket,seal face's coarseness degree is the main reason to create leak.For example,turning flange face's grain is screw,when use metal gasket,if coarseness degree is big,gasket will couldn't stop up this screw which create by grain,at the application of pressure,medium will leak through this groove.The requirement of soft seal gasket to flange's coarseness degree is more low,because it's easy to transfiguration,could stop up machining grain,to avoid seal leak.To soft gasket,it's disadvantageous of flange is too smooth,because the arises interface's leak resistance is lessening.So,different gasket,require different flange face's coarseness degree.The table 29.3-15 are list the experience data of all kind of seal gasket require the flange coarseness degree.
（3）The hardness of Flange gasket is different
The purpose of use gasket is make gasket create flexible or model transfiguration to fill in flange face's little uneven,avoid leak's create.Thus should make gasket's material's hardness id lower than flange's material's hardness,the different is more big between them,to create seal is more easy.When use metal gasket,For assure seal create, should be try the best to choose soft material,you'd best make the metal gasket's hardness lower than beyond 40HB flange's hardness.
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