POWER UP A WINNER - Plant Services

Reliability / Piping
rations that can reconcile these conflicting Pressure counts
criteria for more sustainable piping systems.
Industry has made great strides in
reducing the number of leaks from
flanged joints, and some plants have even
eliminated them. Because reliable piping
system performance, including pumps,
valves and other equipment, is critical to
plant sustainability both operationally
and environmentally, it’s vital to pay careful
attention to these flanged joints and
the gaskets that keep them from leaking.
Before replacing a leaking gasket, make
sure you’ve identified the root cause of the
leak. A good place to start is to verify that
the proper gasket was installed in the first place based on the
operating parameters of the system, notably temperature, media
and pressure. The first consideration should be temperature
because it can eliminate many unsuitable gasket materials from
consideration. Next, look at the media’s chemical compatibility
with the gasket, including any secondary media to which the
gasket might be exposed, such as fluids that are intermittently
present during chemical or steam/hot-water flushing. Then
consider pressure. Most systems operate at relatively consistent
pressure, but it’s important to take into account severe spikes or
surges that might occur.
However, selecting the right gasket requires an understanding
of the application that goes beyond these criteria.
Flanged joints rarely leak because chemicals attacked the
gasket. Most people understand that gasket materials must
be compatible with system media. Likewise, it’s rare to find a
piping system that exceeds a gasket’s maximum pressure or
temperature ratings.
Figure 2. This compressed sheet gasket
installed in a flat-faced flange failed
because of insufficient compression.
flanges for vacuum service (Figure 2).
The available stress for these types of
flanges varies widely. For example, an
8-in., 150# raised-face flange and a ring
gasket having a contact area of 32.2 sq.
in., a bolt stress of 60,000 psi and a bolt
torque of 200 ft.-lbs. produces 4,800
psi of gasket compression. The same
size flange with a flat face, fitted with
an 80-sq.-in., full-face gasket under the
same bolt stress and torque, results in
gasket stress of only 1,813 psi, about
38% of that from the ring gasket with its
smaller contact area.
As a general rule, therefore, the best
choice for raised-face flanges are ring gaskets made of compressed
sheet, PTFE, graphite or metal. Use full-face rubber
gaskets or gaskets with molded-in sealing rings with greatly
reduced contact area for flat-face flanges.
Valves and pumps with flat flanges and standard ASME
drilling can produce a stress between 1,000 psi and 2,000
psi; glass-lined flanges produce between 2,000 psi and 3,500
psi; flat-faced, non-metallic flanges from 150 psi to 500 psi;
and angle-iron flanges around 100 psi to 300 psi. The bolting
on some flanges used in vacuum service produces almost no
compressive load.
Even within these low-load flanges, there’s considerable
variation in available load. Not all nonmetallic flanges are flat
faced; many have raised ridges to concentrate the load. Some
are stub-end designs in which the contact surface is confined
to within the bolt circle and the bolted flange slips on behind
Technological rescue
Compressive load key to performance
More often than not, flanged joint gasket failures result
from the mechanics of the application, usually from incorrect
compressive load. Eliminating such a simple cause
requires knowing the type of flange in which the gasket will
be installed, including the material and bolting information,
to determine the compressive force. This is extremely important
because nearly 70% of gasket failures are attributable to
insufficient load (Figure 1).
This root cause might be uneven bolt loads, but it’s far
more common that achieving proper loading was impossible
given the flange design and available bolting. Under the
same bolting scheme, a flat-faced flange with a full-face gasket,
for example, won’t apply the same compressive force to
the gasket as a raised-face flange with its smaller compressed
area. Flange types that produce low compressive gasket
loads include flat-faced cast iron flanges commonly found in
valves and pumps, flanges on glass-lined equipment, nonmetallic
flanges, rolled angle-iron flanges for ductwork, and
Figure 3. This gasket’s raised, molded-in rings allow it to seal
with 75% less surface contact. The lower seating stress makes it
suitable for nonmetallic flanges.
36 APRIL 2009 www.PLANTSERVICES.com