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C-42
Bolting and Flange
Information
The gasket's function is to seal two different surfaces
held together by one of several means, the most common
being screw-threaded devices such as bolts. Sometimes
the fastener itself must be sealed, as in the case of a steel
drum bung.
The bolt is a spring. It is an elastic member that has
been stretched to develop a load. The more spring pro-
vided by the bolt, the better the retention of stress on the
gasket to maintain a leakproof joint. It must not be over-
elongated (over-strained), or the elastic limit of the steel
will be exceeded. The bolt then deforms and, with contin-
ued loading (stressing), may rupture.
To avoid such problems with bolt tightening, the use
of a torque wrench is recommended. The torque tables
on page C-44 show the recommended torque values for
Garlock compressed sheet, GYLON® and GRAPH-LOCK®
gasketing materials in 150 lb. and 300 lb. raised face
flanges. The equipment designer may specify the recom-
mended torque to prevent damage to the equipment from
overtorquing. Garlock's recommended assembly stresses,
page C-43, may help the equipment designer determine
the maximum allowable torque per bolt. The load will be
retained better by using a bolt with a longer grip, thereby
ensuring a leakproof joint.
There are limits on the degree of flange surface
imperfection that can be sealed successfully with a gasket.
Large nicks, dents, or gouges must be avoided, since a
gasket cannot properly seal against them. The surface fin-
ish of a flange is described as follows:
1. Roughness : Roughness is read in millionths of an
inch (or meter) as the average of the peaks and valleys
measured from a midline of the flange surface. This is ex-
pressed either as rms (root mean square) or AA (arithmetic
average). The difference between these two methods of
reading is so small that they may be used interchangeably.
Roughness is also expressed as AARH (arithmetic aver-
age roughness height).
2. Lay: Lay is the direction of the predominant surface-
roughness pattern. Example: multidirectional, phonograph-
ic spiral serrations, etc.
3. Waviness: Waviness is measured in thousandths or
fractions of an inch. Basically, it is the departure from
overall flatness.
Typical roughness readings can be from 125 to 500
micro-inches for serrated flanges and 125-250 micro-
inches for non-serrated flanges. Fine finishes, such as
polished surfaces, should be avoided. Adequate "bite"
in the surface is required to develop enough friction to
prevent the gasket from being blown out or from extruding
or creeping excessively.
The lay of the finish should follow the midline of the
gasket if possible. Take, for example, concentric circles
on a round flange, or a phonographic spiral. Every effort
should be made to avoid lines across the face, such as
linear surface grinding, which at 180° points will cross the
seal area at right angles to the gasket, allowing a direct
leak path.
Waviness is seldom a problem under normal condi-
tions. There are two areas that must be watched, however,
since excessive waviness is very difficult to handle.
The first area is glass-lined equipment where the
natural flow of the fused glass creates extreme waviness.
Often the answer here is to use thick and highly compress-
ible gasketing.
The second area of concern is warped flanges. If
warpage is caused by heat or internal stresses, re-ma-
chining is generally sufficient. However, warpage due to
excessive bolt loads or insufficient flange thickness results
in what is generally called bowing.
The solution is to redesign for greater flange rigidity.
Sometimes backer plates can be added to strengthen the
design without having to replace the parts. Another step
would be to add more bolts. When this is done, usually
smaller bolt diameters are possible, thus adding more bolt
stretch and better joint performance.
Questions? Call Gasket Applications Engineering
at 1-800-448-6688
®
Gaskets
Garlock