10f

Gaskets

In this blog I will briefly present gasket design criteria and bolt torque calculation. For this purpose spreadsheet has been made, where yellow fields are required data that will be entered. All equations, units and description are given in the figures. Calculation is conducted for 4″ flanges class 300 having Flexitallic spiral wound gasket. Bolt torque calculation and checking gasket design criteria is accomplished in the five steps.
Step 1
Calculate gasket effective gasket width and diameter of gasket load reaction as per ASME DIV.2.
1-effective-gasket-width
Step 2
Calculate required bolt torque assuming that bolt preload tension stress is 0.65 of yield stress. Note that bolt torque must be in the range recommend by supplier. Sometimes bolt torque is given as percentage of yield stress (e.g bolt preload stress must be equal to 65% of yield stress).
2-bolt-torque
Step 3
Calculate equivalent pressure using Kellog’s method.
3-kellog-equivalent-pressure-method
Step 4
Calculate minimum required bolt load for gasket: seating condition (Wm2), operating condition (Wm1) and hydro test condition (Wmt).
Calculate: bolt preload tension force (Ft), bolt load for operating condition (Fm1) and bolt load for seating condition (Fm2).
Note that calculation is based on given design conditions: operating pressure is 150 Psi and design temperature is 350 degree F.
4-gasket-design-criteria
Step 5
Check gasket design criteria for:
1. Gasket seating condition (bolt load for gasket seating condition must be less that the bolt preload tension force).
2. Operating condition (bolt load during operating condition must be less than allowable bolt force).
3. Hydro test condition (bolt load during hydro test must less than the bolt preload tension force).
5-checking-gasket-design-criteria
Summary:
Bolting and torque value recommended by Flexittalic is adequate for piping system. Please see the figures below.
6-verifying-gasket-design-criteria
This calculation will only ensure that bolting and bolt preload tension force (or bolt torque) is adequate for operating and hydro test condition. Flange supplier usually recommends bolt torque range and in the most cases it will be adequate for hydro test condition.
Flange leakage occurs due to overstressing flange and stress on flange along with flange rotation (or flange rigidity) must be checked. There are two methods for flange leakage calculation:
1. Pressure Equivalent method based on ASME B 16.5 pressure temperature table.
2. ASME BPVC Sec VIII Div 1 Appendix 2 method.
A good tutorial on flange leakage calculation utilizing Caesar II can be found on the site What is Piping.
Normally, flange leakage calculation is specified as a mandatory in stress specification. Bolt torque along with bolt tightening sequence is specified in installation instructions and should be calculated (checked) as per procedure describe above. Procedure describe above is PRVC method which is based on ‘m’ and ‘y’ values. New method ROTT (Room Temperature Tightness Test) developed by ASME SWG (Special working Group) is still in development phase.