Page 367 - Improving Machinery Reliability

Equipment Reliability Improvement Through Reduced Pipe Stress 333

Because of the elaborate design of the piping system attached to sensitive equip-
ment, engineers may sometimes get too trapped in the computer maze and overlook
engineering fundamentals. Typical examples that can cause unreliable operation are
discussed next.

Excessive Flexibility

Adequate piping flexibility is required to reduce the piping load at the equipment
nozzle to the acceptable value. However, a good design should take into account the
flexibility of the support structure and make proper use of protective restraints.
Without properly located restraints, a piping system, no matter how flexible it is, has
difficulty meeting the allowable load imposed by the equipment. Figure 7-5 shows a
pump piping system that was designed without any restraints installed. This is a
common mistake made by inexperienced engineers who think that a restraint can
only increase the stiffness, thus increasing the load. It is true that a restraint will tend
to decrease the flexibility of the system as a whole and will increase the maximum
stress and force in the system. However, a properly designed restraint can shift the
stress from the portion of piping near the equipment to a portion further away from
the equipment.
Although extensive loops are used in the piping shown in Figure 7-5, the piping
load still may not meet the equipment allowable due to the lack of a restraining sys-
tem. Excessive flexibility makes the system prone to vibration, because it is easily

'* i f L~~~~~~ BEAM
INSTALLED IN
'I \ THE FIELD

Figure 7-5. Too much flexibility.