gasket compression

[picasa]

When a gasket between two flanges or between the heat exchanger partition plate tip and the partition plate groove on the tubesheet gets compressed, it exerts some opposing force. How to calculate this force? Does this force die down over time?

 

[arto]

Contact the gasket manufacturer & ask for their compressibility curves. Ask about Creep & Relaxation too.

Mark's Handbook has a chart "Compressibility of Gaskets" [get 9th ed. Fig 8.6.37 they left off the legend on the 8th edition Ch.8,Fig. 37]

 

[jte]

Picasa-

You already have the m and y factors for the gasket right? If so, presuming the partition gasket is the same mat'l as the outer ring, you have all the info you need. Take a close look at Div. 1 App. 2-5.

You're calculating bolt loads based on gasket seating or operating conditions (Wm1 and Wm2). In these formulas wherever you see a pi*diameter (3.14G) term substitute in (pi*dia+length of partition).

The Wm1 term is basically (pressure*area or force on the bolts due to pressure) + (gasket area *pressure*multiplier or the additional load req'd to maintain gasket compression). Read the definition of m.

The Wm2 term is determining how much force it will take to mash (seat) the gasket into the micro-grooves in the flange with no internal pressure. This is simply area of gasket (pi*b*G) times the gasket seating stress (psi). Read the definition of y.

By substituting in the total length of the gasket (dia + any partition length) you're compensating for the existence of the partition. No, its probably not perfect... but then just about nothing is, and this approach is reasonable.

Good paper topic for someone: Say I have a channel with a horizontal partition. Should the four bolts on each side of the partition (two above, two below) be torqued higher since the stiffness of the flange is influenced by the partition and these bolts will have to "do more work" to compress the partition gasket?

jt

 

[johnnymist2003]

Hi Picasa,

also see TEMA RGP-RCB-11.7

john

 

[gasketguru]

Most people tend not to worry too much about the sealing performance across the bars compared to the main gasket element. The load-compression characteristics of most common gaskets tend to take the format of an exponential decay curve (e.g. they get progressively harder). The load-leak curves tend to be similar also – sealing performance increases quickly, then greater improvements require far greater loads.

Most gaskets will see some degree of stress relaxation – graphite based gaskets tend to be best in terms of their stress retention properties.

When calculating bolt load requirements I usually include the bar area in the gasket contact area and of course subtract it from the hydrostatic area. Be careful if you are relying on ASME VIII “m” and “y” factors as you will almost certainly end up with less load than you actually require in practice, due to the “effective width” part of the calculation. You might typically require as much as 50 to 100% more load than the calculation gives (appendix S of the code has a few notes about this). Note that there is no valid test for these factors - they are more of an “engineering opinion” than anything else.

Also be careful with how you actually achieve and measure the final bolt tension. – There is little reason to try and load the bolts higher nearer the bars – most ASME designed flanges tend to be rigid enough to distribute the load anyway – besides, how are you measuring the tension in the first place (torque is relatively inaccurate). Again, a lesser tightness parameter across the bars is generally acceptable anyway.