On
February 8, 2011, PHMSA published several updates to the pipeline integrity
FAQ’s for hazardous liquid pipelines. The revised FAQ’s are listed below.
7.4 What is an ‘immediate repair condition’?
An immediate repair condition is a detected anomaly involving:
- Metal loss greater than 80% of nominal wall regardless of dimensions.
- Predicted burst pressure less than the maximum operating pressure at the location of the anomaly. (Where burst pressure has been calculated from the remaining strength of the pipe using a suitable metal loss strength calculation, e.g., ASME/ANSI B31G).
- Dents on the top of the pipeline (above 4 and 8 o’clock position) with any indicated metal loss, cracking, or a stress riser.
- Dents on top of the pipeline with a depth greater than 6 percent of nominal pipe diameter.
- Significant anomaly that in the judgment of the person evaluating the assessment results requires immediate action.
Repairs must be made as soon as practicable. An operator must
reduce pressure (to a level calculated using the formula in section 451.6.2.2
(b) of ASME/ANSI B31.4 as applicable (see FAQ 7.15)) as soon as safety allows
and operate at or below that pressure until a repair can be made.
7.15 The rule requires that an operator temporarily reduce pressure if an
immediate repair condition is discovered (195.452(h)(4)(i)). With respect to
this requirement:
a. Can the temporary reduction in operating pressure be based upon previous
maximum operating pressures?
No. A reduction in operating pressure is intended to provide an additional
safety margin until the defect can be remediated. To assure that additional
margin is provided, the pressure reduction must be based upon pressures that
the pipe has actually experienced, with the defect present (i.e., pressures for
which safety has been demonstrated). These may be well below the “maximum
operating pressure” for the pipe.
The rule requires that the pressure reduction must be calculated using the
method in section 451.6.2.2 (b) of ANSI/ASME B31.4 if that method is applicable
and the information needed is available. If that method cannot be used, the
operator is responsible for determining an appropriate basis for assuring
additional safety through a reduction in pressure. A reduction of 20 percent
below the highest operating pressure actually experienced at the location of
the defect within the two months preceding the inspection may provide the
necessary additional safety margin.
b. Can the temporary reduction in operating pressure be based on
calculations other than those defined in section 451.6.2.2 (b) of ASME/ANSI
B31.4?
The method described in section 451.6.2.2 (b) of ASME/ANSI B31.4 is required by
the rule and must be used for all circumstances for which it is appropriate
(e.g., corrosion). There are anomalies defined by the rule as immediate repair
conditions for which the method of section 451.6.2.2 (b) is not applicable
(e.g., dents). These are addressed in c. below. PHMSA Pipeline Safety is
considering a change to the rule to recognize that section 451.6.2.2 (b) is not
applicable to all immediate repair conditions, and may also allow alternative
methods for calculating the required reduction in pressure. Until the rule is
changed, however, the specified method must be used in all instances in which
it applies.
c. Is section 451.6.2.2 (b) of ASME/ANSI B31.4 applicable for calculating
the temporary pressure reduction required for top-side dents with metal loss
(195.452(h)(4)(i)(C)) and dents greater than 6% of the pipe diameter
(195.452(h)(4)(i)(D))?
No. The calculation in Section 451.6.2.2 (b) of ASME/ANSI B31.4 is applicable
to determining the remaining strength of pipe with corrosion defects or grind
repairs (i.e., loss of wall thickness). Pressure must be reduced for other
types of immediate repair conditions, but operators must develop appropriate
engineering justification for the amount of pressure reduction. A reduction in
operating pressure is intended to provide an additional safety margin until the
defect can be remediated. To assure that additional margin is provided, the
pressure reduction must be based upon pressures that the pipe has actually
experienced, with the defect present (i.e., pressures for which safety has been
demonstrated). These may be well below the “maximum operating pressure” for the
pipe.
A reduction of 20 percent below the highest operating pressure actually
experienced at the location of the defect within the two months preceding the
inspection may provide the necessary additional safety margin.
7.20 Is a 20 percent reduction in pressure an adequate interim measure for
immediate repair conditions?
A reduction of 20 percent below the highest operating pressure actually
experienced at the location of the defect within the period immediately
preceding the inspection (e.g., two months) may provide the necessary
additional safety margin. Operators should evaluate each situation to determine
if additional reduction, or line shutdown, is needed. Operators must use
Section 451.6.2.2 (b) of ASME/ANSI B31.4 to calculate the required pressure
reduction for all situations in which it applies (as required by 452(h)(4)(i)).
See FAQs 7.15 and 7.22 for more information.
7.22 Section 195.452(h)(4)(i) requires that I temporarily reduce pressure in
response to an immediate repair condition. The same paragraph also requires
that I must calculate the reduction using the formula in section 451.6.2.2 (b)
of ASME/ANSI B31.4. If using that formula results in a calculated safe pressure
that is higher than my original operating pressure, must I still reduce
pressure? To what?
Yes. A pressure reduction is required to provide additional safety margin until
an immediate repair condition can be addressed. PHMSA Pipeline Safety expects
that situations in which the calculated safe pressure using the formula in
section 451.6.2.2 (b) of ASME/ANSI B31.4 is higher than the original operating
pressure will be rare. Nevertheless, if the calculated pressure is greater than
the existing operating pressure, pressure must still be reduced to provide the
necessary margin. Operators should determine the amount of such reduction based
on their particular circumstances.
Operators should also note that the specified formula only applies to metal
loss anomalies (i.e., corrosion). It does not apply to immediate repair
conditions that do not involve metal loss, nor does it apply to dents with
metal loss. For those circumstances, operators must determine an acceptable
method for calculating an acceptable reduced operating pressure. A reduction of
20 percent below the highest operating pressure actually experienced at the
location of the defect within the two months preceding the inspection may
provide the necessary additional safety margin.