Formulas - How Fast Should Steam Mains be Warmed Up?
The table shows linear expansion in inches per 100 feet of 24” pipe
from 0 – 1000°F. It can be noted that for heating up the pipe to
1000°F an expansion of 9.4” is encountered. Rapid heating of such
lines would destroy the system and it is, therefore, important to reduce the
expansion and temperature stresses as much as possible. An extremely important
factor is the ability of the steam traps to discharge condensate as it is formed
in the relatively cool pipe. Water hammer must be avoided since it can rupture
the pipe or cause valve failure, particularly at the end of the line. Closed
gate valves are most susceptible to damage at the end of the line because they
can crack at the seat ring when struck by a slug of water. Proper calculating
of the heat losses and condensate loads will avoid the backing-up of condensate,
and most of the difficulties will be eliminated; however, it is recommended,
if the header must be ready for service on short notice, to heat the header
with a steam flow through a bypass or a 3/4”, 1” valve first, and
only after the pipe has been heated to a certain minimum temperature of approximately
250°F should high pressure steam be admitted to the line. Velan traps operate
from 0 to maximum pressure and can handle this job, however, bypasses are recommended,
and here the Velan Piping King can offer great savings. The stressing of pipes
under such conditions are enormous, and it is important to provide proper expansion
loops and expansion joins to keep the effective stress forces within allowable
limits.
Stresses can be calculated from a formula:
S = C x E x TD psi
where : S =
Stress (PSI)
E = tensile elastic modulus (psi)
C = Linear coefficient
of expansion of pipe material (in/∆°F)
TD = The difference between
the initial and final temperature of the pipe
Temperature Expansion of Pipes per 100 Feet (inches)
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