Page 366 - Improving Machinery Reliability

332 Improving Machinery Reliabiliv

should not have been the least bit surprising, since all a vendor has to do to make it
2X or 3X is to provide a couple of braces or stiffeners. Recognizing the popular
demand for the 2X or 3X baseplate, the API formally adopted it to its pump stan-
dards. Since the sixth edition of API Std-610, the allowable has been increased to a
level that makes the 2X and 3X specification no longer necessary. In other words,
the strength of the whole pump assembly has become fairly uniform and no addition-
al allowable can be squeezed out without adding substantial cost. Unfortunately, at
present this philosophy has not been shared by the manufacturers of most other fluid
machines. For example, the 1956 NEMA turbine allowable load is probably the most
unreasonable of its kind. The API 617 centrifugal compressor and ASME/ANSI
B73.1 pump standards are not far behind. API Standard 617 uses 1.85 times the
NEMA allowable, and ANSI B73.1 vendors often use 1.30 times the NEMA values
for the allowables. Figure 7-4 shows the comparison of the pipe strength, the allow-
able API Std-610 piping load, and the NEMA allowable piping load. The pipe
strength curve is based on a 7500 psi bending stress. It should be noted that the
allowable pipe stress against thermal expansion can be as much as three times higher
than 7500 psi.
Figure 7-4 clearly shows that the piping load that can be applied to equipment is
much smaller than the strength of the pipe itself. Therefore, in designing the piping
connected to an equipment item, the equipment allowable load is the controlling fac-
tor. For low-allowable items, such as a large-size steam turbine, an extensive expan-
sion loop and a restraining system are generally required. This is a fact that should
be understood by all parties concerned.

14 -'

10 -

5 ..

01
2 4 6 8 10 12 14 16
Pipe Sizes, inches

Figure 7-4. Allowable piping loads.