Page 371 - Improving Machinery Reliability

Equipment Reliability Improvement Through Reduced Pipe Stress 337

one of many steam turbine exhaust configurations installed in petrochemical plants,
The expansion joint layout scheme appears to be sound, but the construction may not
be done properly. When the base elbow is anchored, the tie-rod loses its function as
soon as the pipe starts to expand. In this case, the pipe expands from the anchor
toward the bellow joint, making the tie-rod loose and ineffective. The large pressure
thrust force pushes the turbine, often causing shaft misalignment and severe vibra-
tions. Figure 7-7 (b) depicts a similar situation. In one plant, the bellow expansion
joints were used solely for fitting up the connections. The tie-rods were supposed to
be locked; however, before start-up, an engineer had loosened the tie-rod nuts,
apparently thinking the tie-rods defeated the purpose of the expansion joint. The tur-
bine encountered serious vibration and it took quite a while before it was discovered
that the problem was caused by the loose tie-rods. When the nuts are loose, the pres-
sure end force simply pushes the machine out of alignment.

Other Practical Considerations

As can be seen, pipe stress reductions are not always easy to achieve. Especially
when dealing with the low allowable nozzle loads specified for some equipment, the
technique can become tricky and very often works only on paper. Other practical
approaches may have to be explored to further improve overall reliability. One very
important resource not to be overlooked is the experience found in operating plants.
We have seen good, simple working layouts changed to complicated and question-
able layouts only because a computer liked it that way. Undoubtedly, computers are
important tools, but they are only as good as the information we give them. Since
there are parameters such as friction, anchor flexibility, etc., that cannot be given
accurately, computer results need to be interpreted carefully. It is time to realize that
if something works well in a plant day in and day out, it should be considered good,
regardless of whether or not the computer predicted it to be good. The process of
examining and incorporating field experience is very important in designing a good,
reliable plant.
Other solutions such as the use of sliding supports, spring supports, and more
compact in-line arrangements as shown in Figure 7-8 also merit serious considera-
tion. It is understood that engineers do not feel too confident about movable assem-
blies, but it is important to understand the difference between the movement of the
whole assembly and the movement of only the pump or turbine. When the whole
assembly moves, shaft alignment can still be maintained, provided the distortion of
the equipment is not excessive. This pre-supposes that the piping load is still within
the allowable range. It should be noted, however, that movable assemblies are just
potential alternatives. One should not be oversold on the idea and blindly use sliding
or spring-supported schemes in a plant. To make the sliding base or the spring-sup-
port scheme work, an extra strong baseplate is required. Then again, if we have that
strong of a baseplate in the first place, it may well be possible to substantially
increase the allowable piping load.