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Example 1 - Inspection Interval Determination

An NPS 16 piping system has been in operation for 10 years and has been takenout of service for its first thorough inspection. The following information is given:

• Pipe service - Gas with 3.5% H2S

• Minimum required thickness - 0.28 in.

• Originally installed thickness - 0.375 in.

• Thicknesses measured at five locations: 0.36, 0.32, 0.33, 0.34, 0.32

Based on the information provided, what maximum thickness measurementinterval should be used for this system?

Solution:

The pipe service places this system into Class I. Therefore, the maximuminterval cannot be more than 5 years based only on the service. Now check theremaining life criterion.

CR/Maximum = 10

32.0375.0 − = 5.5 x 10-3 in./yr.

Available corrosion allowance = (0.32 - 0.28) = 0.04 in.

Maximum Interval = 310x5.5x2

04.0−

= 3.6 years < 5 years

∴ Maximum thickness measurement interval is 3.6 years.

MAWP Determination

The MAWP of a piping system must be determined based on the requirements ofthe applicable piping code (i.e., ASME B31.3 in the case of process plant pipingsystems). The MAWP of the system is that of the weakest component within thesystem. Thus, in addition to the pipe itself, all other system components must beconsidered (e.g., flanges, valves, etc.). If the pipe material is unknown, theMAWP calculations must be based on the lowest grade (i.e., weakest) materialand lowest weld joint efficiency that would be permitted by the code.

The MAWP calculation is based on:

• The actual thicknesses determined by inspection.

• Double the estimated corrosion loss until the next inspection is done.

• Additional allowances that might be necessary in specific cases to account forapplied loadings other than pressure.

The following examples illustrate calculation of the MAWP. Note that in bothcases, only the pipe thickness is considered.

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Example 2 – MAWP Determination

Design Pressure 500 psig

Design Temperature 400°F

Pipe Material A 106 Gr. B

Pipe Size NPS 16

Allowable Stress 20,000 psi (from B31.3)

Longitudinal Weld Efficiency 1.0 (A 106 Gr. B is seamless pipe)

Thickness Measured During Inspection 0.32 in.

Observed Corrosion Rate 0.01 in./year

Next Planned Inspection 5 years

Estimated Thinning Until Next Inspection 5 x 0.01 = 0.05 in.

MAWP = D

EtS2(from B31.3)

MAWP = ( )

16

05.0x232.0x1x000,20x2 −

MAWP = 550 psig > 500 psig

Since the MAWP exceeds the system design pressure, the system may remainin service at the design pressure without repairs, replacements, or rerating.