Page 147 - Pipeline Rules of Thumb Handbook

134 Pipeline Rules of Thumb Handbook

Hydrostatic testing for pipelines

After construction of a pipeline is completed, or if pipe has data, will be used to determine the length and number of
been replaced or relocated, it is necessary to hydrostatically test sections. Figure 1 shows a typical pipeline elevation
test the pipeline to demonstrate that the pipeline has the gradient.
strength required to meet the design conditions, and to verify
that the pipeline is leak free.
The U.S. Federal Safety Regulations for Pipelines require
that pipelines used to transport hazardous or highly volatile
liquids be tested at a pressure equal to 125% of the maximum
allowable operating pressure (MAOP) for at least four con-
tinuous hours and for an additional four continuous hours at
a pressure equal to 110% or more of the MAOP if the line is
not visually inspected for leakage during the test. A design
factor of 72% of the specified minimum yield strength
(SMYS) of the pipe is used to determine the maximum allow-
able operating pressure. The requirement to test to 125% of
the MAOP will therefore cause the pipe to be tested to a pres-
sure equal to 90% of the SMYS of the pipe. See Section 3—
Pipe Design for additional information on calculating the
MAOP.
The Regulations for gas lines specify design factors based
on the class location of the pipeline. The class location is
determined by the number of buildings in a specified area on
either side of the pipeline. Refer to Part 192.111 of the
Minimum Federal Safety Standards for Gas Lines for the
details on how to determine the class location. See Section 3,
Figure 1. Pipeline elevation profile.
Pipe Design for a listing of the design factors for the differ-
ent class locations. The Regulations specify that the test pres-
sure must be maintained for at least eight hours and must be
equal to at least 125% of the maximum allowable operating Where the pipeline traverses hilly terrain, the elevation gra-
pressure. The Regulations should be consulted for the spe- dient must be carefully considered in selecting the pipeline
cific testing requirements as the Regulations are subject to test segments. Different companies have differing philoso-
change. phies on how to do this. Some limit the amount of elevation
Usually, operators will specify a test pressure range from difference while others may specify a range of allowable per-
90% to 95% of the SMYS of the pipe. Some will allow test centages of the SMYS of the pipe—i.e., 90 to 95% or 90 to
pressures as high as 100% of the SMYS of the pipe and some 100% of the SMYS of the pipe. In any case, the test gradient
will test to slightly beyond the SMYS of the pipe. Specify- should be plotted to be sure the test pressure falls within the
ing a test pressure at least equal to 90% of the SMYS of specified pressure limits.
the pipe will qualify it for the maximum allowable operating The test gradient must be based on water head in feet, if
pressure. In some cases, the test pressure will be based on water is used as the test medium. Water pressure may be con-
the minimum yield strength determined from the mill test verted to head by dividing the pressure by 0.433. If sea water
reports. is being used as the test medium, a factor of 0.445 should be
Hydrostatic testing a pipeline is certainly a major operation used. This assumes a specific gravity of 1.02 for sea water. In
and should be carefully planned. Most companies have hydro- any case, the factor of 0.433 should be modified according to
static test manuals that detail the procedures to be followed the specific gravity of the test medium.
to complete the test. Usually, this work is performed by a Let’s assume that we have a line that is to be tested and the
hydrostatic testing contractor hired by the pipeline owner, or elevation at the test site is 1,000 feet and the elevation at
hydrostatic testing may be included as a part of the main the end of the line is 1,200 feet. Fresh water will be used
construction contract. as the test medium and it is desired to test the pipe to a
One of the first steps in planning the hydrostatic test oper- minimum of 90% and a maximum of 96% of the SMYS of
ation is to examine the elevation gradient. The gradient, along the pipe. The pipe is 30≤OD ¥ 0.390≤wt API 5LX X60. There
with the location of the water source, and the pipe design is a difference in elevation of 200 feet and this is equal to