# PIPING CODES
A piping system is designed and constructed based on codes and standards. Therefore, it is important that we have a good understanding of the applicable codes and standards.
Engineers have always strived to build the safest plant at the lowest cost. Because it is impossible
for them to build an absolutely safe plant at any cost, they have to settle for a reasonably safe plant at
a reasonable cost.
A sufficient level of protection for both the
investor and the general public shall be achieved by a consensual opinion from all related parties. On
behalf of the piping industry in the United States, the ASME took the lead in forming action committees, consisting of experts from engineering companies, academic institutions, government agencies,
equipment manufacturers, plant owners, insurance companies, and independent consultants.
These committee members represented different and sometimes opposing interests, and their recommendations resulted in a set of specifications now referred to as the Piping Code, or the Code.
In the United States, the piping code is divided into two main categories:
(1) nuclear power plant
piping, which is governed by the ASME B&PV Code, Section III [25];
(2) non-nuclear piping, which
is governed by ASME B31 [26]. The organization header for B31 code has gone through American
Standard Association (ASA), American National Standards Institute (ANSI), ANSI/ASME, ASME/
ANSI, to the current ASME over the years. Many engineers still call it ANSI B31 to this day. The
same situation also applies to many standards for piping components.
1. Power Piping (B31.1)
The piping systems in a power plant include main steam, reheat steam, feed water, condensate
water, and some utilities. Compared to the cost of heavy equipment (e.g., turbine, boiler, pumps,
heat exchangers, and pollution control facility), the cost of piping is just a small part of the total
cost of the plant.
Because of this low cost (in proportion to the rest) and the fact that an unexpected
plant shutdown can create public chaos, it is logical to make the piping system as safe and reliable as
possible.
The safety factor used is about 3.5 against the ultimate strength of the pipe.
Due to the lack
of extreme corrosive fluids involved, the corrosion allowance is considered only in the calculation
of wall thickness.
All other calculations are mostly based on the nominal wall thickness.
The long
service life of a power plant also warrants a more conservative approach in the design and construction. B31.1 also opts to use simpler and more conservative formulas in calculating pipe stresses.
The
resultant moments are used for all categories of stress calculations, and the stress intensification
factors are applied to all components of the moment including torsion moment, which is generally
not applied with a stress intensification factor.
2 Process Piping (B31.3)
A process plant, such as a petrochemical complex, normally constitutes many processing units
spread out in a very large area. The interconnecting piping is also necessarily spread out all over the
area. Because the cost of the piping can be as high as 35% of the cost of the entire plant, and also
because the public does not pay as much attention to shutdowns at process plants, the safety factor
can be reduced somewhat to pare down the overall cost of the plant.
The safety factor used is about
3.0 against the ultimate strength of the pipe.
Because some of the fluids in a process plant are highly
corrosive, the Code requires that the corrosion allowance, as well as the manufacturing under tolerance, needs to be included in all calculations involving sustained loadings.
The calculation of
pipe stress is more precise in B31.3, which has different stress intensification factors for in-plane
and out-plane bending moments, and does not apply any stress intensification on torsion moment.
Some process plants have to deal with toxic fluids. These fluids require special treatment and
are classified as category-M piping.
Process piping involve very cold,
very hot, and very high-pressure applications. Special stipulations are provided for these cases.
3. Gas Transmission and Distribution Piping Systems (B31.8)
This piping system has many similar characteristics to B31.4 liquid transportation systems. However, in gas transmission, due to its explosive nature and the necessity of routing through highly
populated areas, the piping is divided into four location classes:
ranging from Location Class 1 (areas where any 1-mile section has ten or fewer buildings intended for human occupancy) to Location class 4 (areas where multi-story buildings are prevalent, where traffic is heavy or dense, and where
there may be numerous other utilities underground).
Higher allowable stress, thus lower safety factor, is used for areas with lower location classes. Location Class 1 is further divided into Division
1 and Division 2, two types of construction depending on the pressure used in the hydrostatic test.
Higher allowable stress is permitted for Division 1 for testing at higher pressure. Stress calculations
are mainly based on nominal wall thickness of the pipe, and the allowable stresses are mainly based
on the SMYS.
