Leakage only at shutdown may indicate the bolt was inelastically elongated during
operation by greater thermal expansion of the flange, or possibly creep of the bolts
– Bolts should be replaced
Leakage, and the need for hot bolting, after achieving an elevated operating temperature may mean the bolts loosened due to greater thermal expansion than the flange
Further complicating the problem are the different temperatures (hence thermal growth) seen by the flange and the bolts and the potentially different materials (with different expansion coefficients) used for the two components. For example, stainless steel bolts will expand much more than a low chrome or carbon steel flange.
During large start-up temperature changes, the flange heats first and the thermal expansion is greater than that of the cooler bolt, potentially resulting in permanent bolt stretch. When the bolt later heats and expands, the growth is additive to any permanent bolt stretch, causing a lower gasket pressure and possible flange leakage. Hot bolting may retighten the bolt and restore the seal.
Hot bolting is helpful if there is additional elastic strain capacity, i.e., more strain results in more force or if the bolt has thermally expanded and loosened. A bolt inelastically strained during startup (e.g., due to uneven heating of the flange system) will behave elastically when the temperatures equalize, though permanent elongation (strain) is present. If inelastic strain is present due to greater steady state expansion of the flange than the bolt, the bolt is still in the inelastic range and hot bolting will further strain the bolt but will not increase the stress or force.
Continued “tightening” of the bolt will not improve the seal, but will strain the bolt, eventually to failure.
EDS-2004/FL-116
Leakage Correction
(continued)
If the flange heated first and yielded the bolts, then the bolts expanded and loosened, hot bolting may correct a leak present at or near startup
Hot bolting is not effective in other cases of bolt yielding because the additional strain (due to tightening) will not result in a significant bolt force increase
EDS-2004/FL-117
Leakage Correction
(continued)
Hot bolting may be avoided by using proprietary products such as Belleville Washers, or spring washers, to provide a constant bolt stress over wide temperature ranges, elongation of the bolt relative to the flange from thermal expansion or creep, and vibration exposure
Belleville washers look like an umbrella and act like a spring between the nut and the flange. They maintain bolt tension and seal pressures when the bolts creep at higher temperature or are stretched by differential expansion between the flange and the bolt. They are effective when the bolt elongates slightly more than the flange.
A small increase in bolt length can greatly decrease the bolt strain and, therefore, the force. The washers keep the strain in the bolt. They do not help for the reverse situation unless the situation is temporary and permanently stretches the bolt resulting, in effect, in an elongated bolt. Of course, the washer must not be crushed during the time the flange expands more than the bolt.
The washers give an effective spring force that compensates for different expansion rates in a joint to maintain a high tension load in the bolts. The spring effect allows load compensation to account for deflection of the bolts.
Strain is a dimensionless value defined as the change in length (deformation) divided by length. Using extra long bolts and a sleeve greatly increases the overall length. The change in length due to thermal expansion is nearly the same as for a shorter bolt because the uninsulated extended portion of the bolt is much cooler than the portion between the flanges and expands little. Therefore, the numerator (change in length) increases a small amount and the denominator increases appreciably resulting in a reduced total strain change. In the elastic range, strain is directly proportional to stress and, as a result, the bolt stress change due to thermal expansion within the flange is also reduced. The bolt stress necessary to seat and seal the flange, provided by tightening the bolt, must still be provided. This will require a greater overall bolt extension to achieve the necessary strain.
For a reasonable case where the thermal growth of the sleeved bolt is 50% greater than the non-sleeved bolt, but the length between the nuts is 4 times as long:
Strain change (normal bolt length) = thermal change in length between flanges bolt length between flanges
<is greater than>
Strain change (with a sleeve) = (thermal change in length between flanges) X 1.5 bolt length between flanges X 4
The effects of creep may also be modified. There must be sufficient stress in the bolt to seat and seal the gasket. If part of the bolt is in the creep range, creep will still occur. One benefit is that differential thermal expansion will add less strain and stress to a long bolt. This lower stress will reduce the effects of creep compared to what would otherwise occur. Another benefit is that creep occurs only over the portion of the bolt hot enough to be in the creep range. The extended part of the bolt is probably not hot enough. For the reasons described above, the creep will relieve less strain than for a non-extended bolt.
EDS-2004/FL-118
Leakage Correction
(continued)
Use of a sleeve and extra long bolts reduces the relative strain differential due to thermal expansion variances or creep
– The bolt extension is cooler and elongates less than the portion between the flanges
– The total strain change averaged over the entire bolt length is, therefore, less
– A lower change in strain means a lower stress change
• This reduces the chance of exceeding the bolt’s yield strength and reduces the creep rate (if in the creep range)
EDS-2004/FL-119
Solution
– Consider the use of a sleeve around the bolts to increase the effective bolt length
– or consider the use of conical spring washers to eliminate force losses over wide temperature ranges
PPF-R01-33