Page 369 - Improving Machinery Reliability

Equipment Reliability Improvement Through Reduced Pipe Stress 335

equipment. If friction is ignored in the design calculations, the calculated reaction
at the equipment is often very small. However, in reality, friction acting at the stop
surface will prevent the pipe from expanding in the positive X-direction. This fric-
tion effect can cause a high X-direction reaction at the equipment. Calculations
including the friction will predict this problem beforehand. A proper type of
restraint such as a low-friction plate or a strut would then be used.
* An ineffective support member is another problem often encountered in the protec-
tive restraints. Figure 7-6 (b) shows a popular arrangement to protect the equip-
ment. The engineer’s instinct is to always put the fix at the problem location. For
instance, if the computer shows that the Z-direction reaction is too high, the natural
fix is to place a Z-direction stop near the nozzle connection. This may be all right
on the computer, but in reality it is very ineffective. For the support to be effective,
the stiffness of support member A has to be at least one order of magnitude higher
than the stiffness of the pipe. Here, the pipe stiffness is very high due to the rela-
tively short distance from the nozzle to the support.
A gap is generally required in the actual installation of a stop. Therefore, if a stop
is placed too close to the nozzle connection, its effectiveness is questionable due to
the inherent gap. As shown in Figure 7-6 (c), the pipe has to be bent or moved a
distance equal to the gap before the stop becomes active. Due to the closeness of
the stop to the equipment, nozzle stresses will often reach severe levels even
before the pipe reaches the stop. This configuration is not acceptable because the
equipment generally can only tolerate a much smaller deformation than the con-
struction gap of the stop.
e Choking is another problem relating to the gap at the stop. Some engineers are
aware of the consequences of the gap at the stop mentioned above and try to solve
it by specifying that no gap be allowed at the stop. This gives the appearance of
solving the problem, but another problem is actually waiting to occur. As shown in
Figure 7-6 (d), when the gap is not provided, the pipe will be choked by the stop as
soon as the pipe temperature starts to rise. We generally remember to pay attention
TO the longitudinal or axial expansion of a pipe, but we often forget that the pipe
expands radially as well. When the temperature rises to a point when the radial
expansion is completely choked by the support, the pipe can no longer slide along
the stop surface. The axial expansion will then move upward, pushing the entire
machine upward.

Expansion Joints

An alternative solution to keeping allowable nozzle loads in check involves the
use of bellows expansion joints. Bellows expansion joints are popular in the exhaust
systems of steam turbine drives which typically have extremely low allowable pipe
loads for pipes 8 inches and above. Bellows joints are also often used for fitting units
coming off a common header, as shown in Figure 7-7 (b). A properly installed and
maintained bellows expansion joint should have the same reliability as other compo-
nents, such as flanges and valves. However, in real applications, expansion joints are
often considered undesirable due to anticipated maintenance problems. For instance,
when covered with insulation, the expansion joint looks just like thickly insulated