Regarding the calculation temperature for determining the maximum allowable operating pressure
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The last edit to this post was made by Pinocchio on 2012-6-30 at 23:55. Section 3.1.6 of GB150, which defines the Maximum Allowable Working Pressure (MAWP), states, “At the specified corresponding temperature, ...” GB150 provides the following explanation for the Maximum Allowable Working Pressure (MAWP): The allowable stress of the material is required to calculate the MAWP; but what temperature should this allowable stress be based on? “The term “definition” generally refers to the design temperature, or it can also be the lowest designed metal temperature. Assuming that the design temperature of a device is 350°C and the minimum allowable metal temperature is specified at 0°C, the allowable stresses at these two temperatures will differ significantly. So, is it necessary to calculate two MAWPs? Under normal circumstances, the operating pressure is relatively lower at lower operating temperatures, yet the calculated MAWP is relatively high. What is the use of the MAWP at this minimum design metal temperature? In my opinion, MAWP is the maximum allowable operating pressure under design conditions, and it should be calculated based on the allowable stress at the design temperature. Even for low-temperature equipment, calculations are carried out using the allowable stress at 20°C; it seems to have little to do with the lowest design metal temperature. Furthermore, what does the following sentence under “Definition” mean: “When the design documents specify the MAWP, and the pressure-bearing components that control this MAWP...”? Please, fellow sea friends, help clarify this.
This post was last edited by ztr1118 on 2012-7-1 08:53. 1. To calculate the MAWP, the allowable stress of the material is required, and this allowable stress should be that corresponding to the design temperature. For low-temperature equipment, use the allowable stress at 20°C. 2. Regarding “when the design documents specify a MAWP, and the pressure-bearing components that control this MAWP...”, this mainly applies to components such as heads; it is necessary to control the minimum thickness of the head after it has been formed, to ensure that this minimum thickness is greater than the effective thickness.
Regarding the second question, I would like to know the relationship between MAWP and effective thickness. Taking the elliptical head as an example, assume: p=2.0 MPa, t=150 MPa, Di=2000 mm, Φ=1.0, K=1.0, C=1 mm. Then δ = 1.0*2.0*2000 / (2*150*1.0-0.5*2.0) = 13.4 mm. If the deformation thinning is not taken into account, δn=16 mm is used; the head manufacturer uses 18 mm thick steel plates to fabricate the heads. (There is an issue with the value of the allowable stress here.) There is an opening of d=200 mm on the head; in this case, δe=16-1=15 mm, K1=0.9, δ=12.04, A=12.04*200=2408 mm2, and A1=592 mm2. Assume that the final total reinforcement area is 3000 mm2, which meets the reinforcement requirements. Now, how to calculate the MAWP of the head? How is it determined? The GB150 standard version 5-3 does not take into account the issue of reinforcement for openings. Is it necessary to disregard reinforcement for openings when calculating the MAWP, and can the calculation be carried out directly using the effective thickness? Assuming that the actual thickness after head forming is greater than 17 mm, can the additional 1 mm be used in the calculation of MAWP?
In my opinion, if the head has been reinforced by drilling holes using its effective thickness, the MAWP should no longer be calculated based on the head’s original effective thickness. Otherwise, it will result in insufficient reinforcement strength of the actual openings in the head.
Calculations are carried out during the design phase, while the actual thickness of the head after molding is determined during the manufacturing phase. The excess margin definitely cannot be used in the MAWP calculation. Let’s continue the discussion using your example: 1. Assume C1=0.3 and C2=2; these values are convenient for discussion. According to the data, the calculated thickness of the head is 13.4, the design thickness is 15.4, and the nominal thickness is set at 18 (a higher rounded value is used to facilitate consideration of thinning amounts and step changes; in the case of a step change, the recalculation result is assumed to be 13.4). a) The minimum formability thickness is taken as the design thickness of 15.4. The drawing indicates 18 (15.4), and the technical specifications state that the nominal thickness already includes a thinning amount of 2.3 mm (it is up to the head manufacturing factory to determine whether this is sufficient). Enter 18 for the nominal thickness in the calculation sheet, and 4.3 for the corrosion allowance C2, noting that this includes a forming thinning amount of 2.3 mm. The effective thickness at this time is 13.4. Minimum forming thickness = design thickness > effective thickness. b) The minimum forming thickness is calculated based on the thinning rate: 18*0.87-0.3=15.4. ………… 2. Assuming C1=0.2 and C2=0, the calculated thickness of the head based on the data is 13.4; the design thickness is also 13.4, while the nominal thickness is set at 16. a) The minimum forming thickness shall be the design thickness of 13.4. The drawing should indicate 16 (13.4), with the technical specifications stating that the nominal thickness already includes a forming reduction of 2.3 mm (it is up to the head manufacturing factory to determine whether this is sufficient). Enter 16 for the nominal thickness in the calculation sheet, and 2.3 for the corrosion allowance C2, noting that this includes a forming thinning amount of 2.3 mm. The effective thickness at this time is 13.4. Minimum forming thickness = design thickness = effective thickness. b) The minimum forming thickness is calculated based on the thinning rate: 16*0.87–0.3 = 13.7. Reference mark 16 (13.7); the technical requirements state that the nominal thickness already includes a forming thinning amount of 2 mm (it is up to the head manufacturing plant to determine whether this is sufficient). Enter 16 for the nominal thickness in the calculation sheet, and 2.0 for the corrosion allowance C2, noting that this includes a forming thinning amount of 2.0 mm. The effective thickness at this time is 13.7. Minimum forming thickness = Effective thickness > Design thickness. Summary: The minimum forming thickness of the head should be no less than both the effective thickness and the design thickness
As for whether the effective thickness can be used to calculate the MAWP in the case of openings, it depends on the relationship between A-A2-A3-A4 and 0. If the value is greater than 0, it indicates that part of the shell’s thickness is used for reinforcing the openings; in such cases, the minimum required thickness for molding should be greater than the designed thickness. As shown in 2-b above, the effective thickness is greater than the designed thickness, meaning there is sufficient strength available, and the excess portion can be used for reinforcing the openings. In this situation, it is better to use the designed thickness instead of the effective thickness when calculating WAMP. If it is less than 0, it indicates that there is excess reinforcement; in this case, the minimum forming thickness can be equal to the design thickness, and the effective thickness can be used to calculate the MAWP. Personal opinion.
This post was last edited by Pinocchio on 2012-7-1 at 13:59. The analysis above is very thorough; it’s a pity that my rating privileges have been set to zero. I’ll rate it again later – apologies for that. 1. First of all, you believe that when determining the shell thickness in design, the forming thinning amount should be taken into account; I have always held the same view. The amount of thinning is determined according to Appendix J of GB/T 25198-2010, so there should be no issues. Moreover, the actual plate thickness is always slightly greater than the nominal thickness minus the negative deviation; thus, head manufacturers still have some additional margin available for use. 2. Calculation of MAWP under the condition of opening reinforcement: For medium and low pressure vessels, the A1 value usually accounts for a large proportion of the reinforcement area. I think the reinforcement area should not be too large; it is more reasonable if it is greater than A and close to A. A reinforcement area as large as A1 is not necessary; it is neither economical nor advisable, as it may impose excessive constraints on the structure, making deformation coordination more difficult and increasing stress concentration. Therefore, under normal design conditions, the excess area A1 of the housing always serves as an effective reinforcement area. According to your view, the MAWP at this point is based on the design thickness (corrosion allowance and formability reduction should not be taken into account in the calculation of MAWP; do you mean the calculated thickness?) ) to carry out the calculation, and the result of that calculation is the design pressure; does this mean that there is no longer any point in calculating the MAWP?
As you said, using the design thickness in place of the effective thickness to obtain the design pressure is indeed inappropriate! It should be corrected. Under the condition of opening reinforcement, which thickness should be used to calculate the MAWP? Is it necessary to determine the exact thickness required for reinforcement and then find the difference? I'm confused. Is it because of the safety factor that such precision isn’t required for the calculations here? Discussion helps improve things; thanks for the advice