3.3 MAWP Calculations for In-Service Piping

3.3 MAWP Calculations for In-Service Piping

The Maximum Allowable Working Pressure (MAWP) of a piping system is the maximum internal pressure permitted in the system for continuous operation at the design temperature. Unlike the original design pressure established during construction, the in-service MAWP must be periodically recalculated by the inspector or piping engineer to reflect the actual corroded state of the piping. Evaluating the in-service MAWP ensures that the pipe wall remains thick enough to contain the operating pressure safely, taking into account future metal loss expected before the next scheduled inspection.

The In-Service MAWP Formula (Barlow's Equation)

API 570 allows the use of the simplified Barlow's formula to calculate the MAWP of in-service piping systems. The formula is written as:

$MAWP = \\frac{2 S E t}{D}$

Where:

• $MAWP$ is the Maximum Allowable Working Pressure, in pounds per square inch (psi) or megapascals (MPa).
• $S$ is the allowable stress value for the pipe material at the design temperature, in psi or MPa, as specified in the applicable construction code (e.g., ASME B31.3 Table A-1).
• $E$ is the longitudinal weld joint quality factor (or weld joint efficiency), which ranges from $0.60$ to $1.00$ depending on the pipe fabrication method and radiography.
• $t$ is the thickness used for in-service MAWP design, in inches or millimeters.
• $D$ is the nominal outside diameter of the pipe, in inches or millimeters.

How Thickness (t) is Defined for In-Service MAWP

A common mistake among candidates is using the actual measured thickness ($t_{actual}$) directly in the MAWP formula. To ensure safety over the next operating period, the thickness used in the calculation must be adjusted to account for the corrosion that will occur before the next inspection.

API 570 defines the thickness ($t$) for in-service MAWP calculations as: $t = t_{actual} - 2(CR)(I_{next})$

Where:

• $t_{actual}$ is the actual thickness measured at the CML, in inches or millimeters.
• $CR$ is the controlling corrosion rate, in inches/year or millimeters/year.
• $I_{next}$ is the interval to the next scheduled thickness inspection, in years.

The Logic Behind the "2" Factor

Why does the formula subtract twice the estimated corrosion loss? This factor is tied directly to the half-life rule. Recall that the thickness inspection interval ($I_{next}$) is set at half the remaining life ($RL / 2$). If the piping is inspected at the maximum allowed half-life interval, the time to reach its retirement thickness ($t_{min}$) is equal to $2 \cdot I_{next}$. Over this remaining life period, the estimated metal loss is: $\\text{Metal Loss} = CR \\cdot RL = CR \\cdot 2(I_{next})$

Subtracting $2(CR)(I_{next})$ from $t_{actual}$ effectively reduces the measured thickness down to the minimum required thickness ($t_{min}$). If the calculated MAWP using this corroded thickness is greater than or equal to the design operating pressure, the piping is safe to operate at that pressure for the duration of the next inspection interval.

Allowable Stress and Temperature Ratings

The allowable stress ($S$) of the piping material is a function of temperature. As the operating temperature of the piping system increases, the mechanical strength of the metal decreases, resulting in a lower allowable stress. For example, ASTM A106 Grade B carbon steel has an allowable stress of:

• $20,000\ \text{psi}$ from $-20^\circ\text{F}$ to $400^\circ\text{F}$
• $18,900\ \text{psi}$ at $500^\circ\text{F}$
• $17,300\ \text{psi}$ at $600^\circ\text{F}$
• $14,300\ \text{psi}$ at $700^\circ\text{F}$

Therefore, if the piping system's operating temperature is increased, the allowable stress must be downgraded, which in turn reduces the calculated MAWP.

Weld Joint Quality Factors (E)

The weld joint efficiency factor ($E$) accounts for the type of longitudinal weld seam and the degree of non-destructive testing performed during manufacturing. Some common joint efficiency values from ASME B31.3 include:

• Seamless Pipe (e.g., ASTM A106): $E = 1.00$
• Electric Resistance Welded (ERW) (e.g., ASTM A53 Gr. B ERW): $E = 0.85$
• Furnace Butt-Welded: $E = 0.60$
• Double-Welded Spiral Seam: $E = 0.85$

Forgetting to apply the joint efficiency or using $1.00$ for a welded pipe is a major source of calculation errors on the API 570 exam.

The Weakest Link Rule (System MAWP)

Under API 570, the MAWP of an entire piping system is limited by the weakest component in the system. The inspector must not only calculate the MAWP of the straight pipe wall but must also verify the pressure-temperature ratings of other components, such as:

• ASME B16.5 Flanges: The rating class (e.g., Class 150, 300, 600) and material group determine the maximum pressure a flange can contain at a specific temperature.
• Valves and Fittings: Must comply with ASME B16.34 or manufacturer pressure ratings.
• Branch Connections and Weldolets: Must be evaluated for reinforcement limits.

If the pipe wall calculation yields an MAWP of $850\ \text{psi}$, but the system contains a Class 150 carbon steel flange rated for $285\ \text{psi}$ at ambient temperature, the MAWP of the entire piping system must be restricted to $285\ \text{psi}$.

Rerating Requirements

If the calculated MAWP is lower than the current design pressure of the system, the piping system must be derated (or rerated to lower values). API 570 Section 8.11 outlines strict requirements for rerating piping systems. Rerating is considered an alteration and requires:

1. Calculations performed by a piping engineer or the inspector.
2. A leak test (pressure test) if the new pressure rating exceeds the original test pressure.
3. Approval from the piping engineer and the authorized inspector.
4. Documentation verifying that all components (valves, flanges, fittings) are rating-compatible with the new pressure-temperature design. The piping system must be tagged, and the records updated to reflect the new MAWP.

Worked Calculation Example

Calculate the in-service MAWP for a carbon steel piping system (ASME B31.3) with the following details:

• Pipe Material: ASTM A53 Grade B ERW ($E = 0.85$)
• Nominal Pipe Size: NPS 8 ($D = 8.625\ \text{in.}$)
• Design Temperature: $400^\circ\text{F}$ (Allowable stress $S = 20,000\ \text{psi}$)
• Actual Measured Thickness ($t_{actual}$): $0.280\ \text{in.}$
• Controlling Corrosion Rate ($CR$): $0.010\ \text{in./yr}$
• Next Inspection Interval ($I_{next}$): $5.0\ \text{years}$

Step 1: Calculate thickness ($t$) to use in the MAWP formula: $t = t_{actual} - 2(CR)(I_{next})$ $t = 0.280\\ \\text{in.} - 2(0.010\\ \\text{in./yr} \\cdot 5\\ \\text{years})$ $t = 0.280\\ \\text{in.} - 0.100\\ \\text{in.} = 0.180\\ \\text{in.}$

Step 2: Calculate the pipe wall MAWP: $MAWP = \\frac{2 S E t}{D} = \\frac{2 \\cdot 20,000\\ \\text{psi} \\cdot 0.85 \\cdot 0.180\\ \\text{in.}}{8.625\\ \\text{in.}}$ $MAWP = \\frac{6,120}{8.625} \\approx 709.6\\ \\text{psi}$

If all flanges and valves in the system are Class 300 (rated for $740\ \text{psi}$ at $400^\circ\text{F}$), then the weakest link is the pipe wall itself, and the system MAWP is $709\ \text{psi}$ (rounded down).