# Codes for boiler control systems and pressure vessels

02/13/2014

A-479 316L stainless steel

Pressure limited by thread shear strength

Assuming the female thread material is at least as strong as the filling, the shear area of threads at the pitch line (tight) is: Ashear: = π ÷ 2 • (E0 + E1) • Leff ÷ 2 = 0.3in2

Cross-section area on which pressure acts (use pipe OD for conservatism): Ashear: = π ÷ 2 • (E0 + E1) • Leff ÷ 2 = 0.3in2

If the allowable shear stress is 0.6*S, then the allowable pressure limited by the fitting thread strength is: Ptd: = 0.6 • A shear • S1 ÷ Ap = 13136 psi

Thread allowable test pressure at yield: 0.6 • A shear • S1.tst ÷ Ap  = 58994 psi

Closure (membrane) calculations: Paragraph 304.4.1

Table 304.1.1 specifies the use of ASME Section VII, Div 1 Paragraph UG-34 for the calculation of the thickness of a flat closure. The 2010 Edition, 2011a Addenda is used for these calculations.

Assuming the head to shell joint is equivalent to Fig UG-34(h): Ch: = 0.33

Part number

Membrane thickness tolerance: Tol: = 0.001 • in

Membrane nominal thickness: tm.noma: = 0.008 • in

Minimum thickness: tma: = tm.noma – Tol – ca = 0.007 in

Membrane maximum allowable pressure: Pma: = S1 • Es • tma2 ÷ Ch • Di2 = 25.8 psi

Maximum allowable working pressure: Pa: = min (Ps, Ptd, Pma) = 25.8 psi

Use a maximum allowable working pressure of: MAWPa: = 25 psi at temp = 300 °F

Membrane allowable test pressure at yield: Pma.tst: = S1.tst • Es • Tma2 ÷ Ch • Di2 = 116 psi

Maximum test pressure: Pa.tst.max:= min (P1.tst.A, Ptd.tst, Pma.tst, 1.5 • Rr1 • Pa) = 38.7 psi

Minimum test pressure: Pa.tst.min:= min (Pa.tst.max, 1.5 • Rr1 • MAWPa) = 37.5 psi

N07718 Inconel 718

Pressure limited by thread shear strength

Assuming the female thread material is at least as strong as the filling, the shear area of threads at the pitch line (tight) is: Ashear: = π ÷ 2 • (E0 + E1) • Leff ÷ 2 = 0.3in2

Cross-section area on which pressure acts (use pipe OD for conservatism): Ap: = π ÷ 4 • Dp2 = 0.229in2

If the allowable shear stress is 0.6*S, then the allowable pressure limited by the fitting thread strength is: Ptd: = 0.6 • A shear • S3 ÷ Ap = 27722 psi

Thread allowable test pressure at yield: Ptd.test: = 0.6 • Ashear • S3.tst ÷ Ap = 118077 psi

Closure (membrane) calculations: Paragraph 304.4.1

Table 304.1.1 specifies the use of ASME Section VII, Div 1 Paragraph UG-34 for the calculation of the thickness of a flat closure. The 2010 Edition, 2011a Addenda is used for these calculations.

Assuming the head to shell joint is equivalent to Fig UG-34(h): Ch: = 0.33

Part number

Membrane thickness tolerance: Tol: = 0.001 • in

Membrane nominal thickness: tm.noma: = 0.008 • in

Minimum thickness: tma: = tm.noma – Tol – ca = 0.007 in

Membrane maximum allowable pressure: Pma: =  S* Es • tma2 ÷ Ch • Di2 = 51.1 psi

Maximum allowable working pressure: Pa: = min (Ps, Ptd, Pma) = 51.1 psi

Use a maximum allowable working pressure of: MAWPa: = 50 psi at Temp = 300 °F

Membrane allowable test pressure at yield: Pma.tst: = S3.tst • Es • Tma2 ÷ Ch • Di2 = 218 psi

Maximum test pressure: Pa.tst.max:= min (P3.tst.A, Ptd.tst, Pma.tst, 1.5 • Rr3 • Pa) = 80.4 psi

Minimum test pressure: Pa.tst.min:= min (Pa.tst.max, 1.5 • Rr3 • MAWPa) = 78.7 psi

N10276 Hastelloy C276

Pressure limited by thread shear strength

Assuming the female thread material is at least as strong as the filling, the shear area of threads at the pitch line (tight) is: Ashear: = π ÷ 2 • (E0 + E1) • Leff ÷ 2 = 0.3 in2

Cross-section area on which pressure acts (use pipe OD for conservatism): Ap: = π ÷ 4 • Dp2 = 0.229in2

If the allowable shear stress is 0.6*S, then the allowable pressure limited by the fitting thread strength is:  Ptd: = 0.6 • Ashear • S2 ÷ Ap = 19665 psi

Thread allowable test pressure at yield: Ptd.tst: = 0.6 • Ashear • S2.tst ÷ Ap = 32250 psi

Closure (membrane) calculations: Paragraph 304.4.1

Table 304.1.1 specifies the use of ASME Section VII, Div 1 Paragraph UG-34 for the calculation of the thickness of a flat closure. The 2010 Edition, 2011a Addenda is used for these calculations.

Assuming the head to shell joint is equivalent to Fig UG-34(h): Ch: = 0.33

Part number

Membrane thickness tolerance: Tol: = 0.001 • in

Membrane nominal thickness: tm.noma: = 0.008 • in

Minimum thickness: tma: = tm.noma – Tol – ca = 0.007 in

Membrane maximum allowable pressure: Pma: = S2 • Es • tma2 ÷ Ch • Di2 = 38.6 psi

Maximum allowable working pressure: Pa: = min (Ps, Ptd, Pma) = 38.6 psi

Use a maximum allowable working pressure of: MAWPa: = 35 • psi at temp = 300 °F

Membrane allowable test pressure at yield: P ma.tst: = S2.tst • Es • Tma2 ÷ Ch • Di2 = 63 psi

Maximum test pressure: Pa.tst.max:= min (P2.tst.A, Ptd.tst, Pma.tst, 1.5 • Rr2 • Pa) = 57.9 psi

Minimum test pressure: Pa.tst.min:= min (Pa.tst.max, 1.5 • Rr2 • MAWPa) = 52.5 psi

Codes B31.3 versus B40.2

While B31.3 specifically defines the design of fittings and tubing for pressure vessels and boilers, the codes B40.1 and B40.2 are used for analog dial gauges and flange design, respectively. These two codes should not be confused since they serve different roles. Many pressure sensor manufacturers use the B40.2 code for pressure sensors when employing process ports such as 1/4-in. NPT, ½-in. NPT, or F250C. This is a misapplication of the code as it does not take into account diaphragm thickness and metals as required by B31.3. The B40 codes are primarily focused on analog devices and flanges, which does not calculate the minimum diaphragm thickness or limit materials to ensure containment under overload conditions.

The end user is ultimately responsible to use the correct code for pressure sensors and transmitters for boiler controls and pressurized vessels. Failure to comply with code could lead to expensive failures, fines, shutdowns, and unnecessary safety hazards. The end users, particularly in Canada, must check with their pressure sensor suppliers for CRN listings under code B31.3 when specifying pressure sensors in process plants, power plants, boiler controls, and many other critical applications.

Karmjit Sidhu is VP of business development for American Sensor Technologies where he spearheads sales and marketing and also focuses his efforts on the conceptual design of new products.

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