2 replies to this topic

[rsk]

pressure rating of pipe is by schedule no.
For eg A 40 schedule, 4inch pipe will have same OD but ID will vary with material strength.
Now various fittings are rated by pressure class 125 ,150,300,600 psi, For these ratings various pressure & temperature ratings are given
I have following doubts
1. If for a specific application i select a CS pipe of 4inch SCH40, then is there any straght thumb rule which rating fitting i shall choose.
2. Pressure ratings of these fittings 125,150,300 is specified at what temperature.
please enlighten me on my fundmentals
3. is there any relation between these two different types of ratings for pipes & fittings

[fallah]

Sch. No. defines the required thickness for a pipe and rating defines the characteristic of the flanges and some fittings.
1.while pipe of several schedules is available, fittings are not manufactured in all schedules,and usually specified as st. weigt,extra strong, sch. 160 and double extra strong.It is usually advantageous to specify the fitting thickness of the next higher available weight if the pipe wall thickness is not standard, extra strong,....
2.Relation of temp. and press. that could be considered for a special rating of flanges (among 150,300,400,600,...) and limitations of their applications are presented in some standards such as ASME B16.5
3.Sch. is specified for pipe and fitting (some fittings such as threaded ones have specified rating) and rating usually specified for flanges.

Hope this helps,
Regards

[djack77494]

rsk,
You appear to be under the mistaken impression that there is a one to one correspondence between pipe schedule and pressure rating and/or between pipe schedule and fitting pressure class. That is not true.

As one proceeds with the design of a system, you will identify fluids, pressures, and temperatures that must be handled. At some point, a piping engineer will develop piping specifications that define the materials of construction, corrosion allowances, and specific hardware (pipe and fittings) that must be used for the service. This nearly complete definition of components is the piping spec for that service. Any piping component added to the system but not in the piping spec is a specialty item.

For most systems I have seen, flanges are included in the piping spec AND the flanges are "the weakest link in the chain". Flanges are defined by their rating and materials of construction. Knowing these two parameters, you can refer to tables found in ASTM/ASME B16.5 and obtain the allowable working pressures for the flanges as a function of temperature and class. The piping class is commonly abbreviated as 150#, 300#, 400#, etc. Piping of adequate thickness (schedule) is added to the piping spec such that the piping is stronger than the flanges. The concept is that you want a (presumably) less disasterous flange leak to develop rather than have a more disasterous pipe rupture in the event that the system is overstressed. In a similar fashion, all fittings in the spec should be more robust than the flanges.

Note that you might have Schedule 40 piping in a 300# piping system and Schedule 80 piping in a 150# piping system. There are many other factors involved, including the pipe size, design temperature, materials of construction, etc.