Good and tight for the right tight, tighter is better.
Bolts clamp two steel flanges together. A rubberlike gasket fills the void; it is chosen for its media and temperature suitability. The initial bolting load elongates the bolts- an axial load in tension. The elongated bolt clamps the flanges, an axial load in compression.
When a pressure load acts on the joint tending to separate the flanges, the compressive force in the flanges will react to the pressure first. When the compressive force drops to zero, the joint will open up and the bolt in tension will react to the pressure. Now, the axial load on the bolt is both the initial bolt load plus the pressure load.
The bolt should have adequate strength to handle both loads. The initial bolt load should be sufficiently high to compresses the flanges and seat the gasket but it should not be too high. If the initial bolt load is too high, the bolt will not have any remaining strength (stress) to handle the pressure load.
Flange design and assembly is a complex mechanism of transferred loads. The bolt can break if the structure is ill- proportioned, the bolt is undersized, the bolt is of insufficient strength to handle the sum of loads, or the bolt is overtightened during the initial assembly. Undersized bolts, or bolts of inadequate strength exacerbate the assembly problem. Brittle bolting materials or materials with an unpredictable non-linear region may not meet the leak before break criteria.
Some considerations are 1) gasket load - produce an adequate load to seat and maintain a gasket seat when pressure is applied – a tighter joint produces higher friction between the flange face and gasket, 2) effects of cyclic conditions - a tighter joint is less likely to have premature bolt failure, 3) distribution of compressive forces - the cone shaped distribution of compressive forces beneath the bolt holes in a raised face joint means the bolt is likely carry both the initial load and the pressure load, 4) the bolted flanged joint is an analysis of preload and elastic modulus phenomena.
When selecting and assembling components for standard ASME B16 flanged joints, choose bolting wisely then Get it good and tight for the right tight, tighter is better.