We normally do not calculate MAWP for S&T HE since it is difficult to calculate MAWP for tubesheet. We design HE for Design Pressure. Is there any paper/ write-up which state that for S&T HE, MAWP shall not be calculated?

If client insist to calculate MAWP for S&T HE, how to do it?

Sunil Agrawal
Tecnimont ICB, Mumbai
Ph: +91 9820658733
E-mail: s.agrawal@ticb.com

I AM NOT SURE WHETHER THIS WOULD BE HELPFUL TO U .....................

MAXIMUM ALLOWABLE WORKING PRESSURE IS THE PRESSURE CALCULATED BASED ON THE ACTUAL THICKNESS USED FOR FABRICATION OF PRESSURE VESSELS.

e.g.- IF THICKNESS DERIVED FROM CALCULATION IS 17 MM, WE USE 18 MM PLATE (AS 17 MM THICK PLATE IS NOT GENERALLY ROLLED & 18 MM IS THE NEXT AVAILABLE SIZE IN THE MARKET).
THEN WE HAVE TO DO THE REVERSE CALCULATION & FIND OUT THE MAAXIMUM ALLOWABLE PRESURE FOR WHICH THE 18 MM IS SUITABLE.

THUS, MAWP IS GREATER THAN DESIGN PRESSURE.

Calculation of MAWP is an interesting problem. For a formed head (say, hemispherical or ellipsoidal, etc) ASME Code provides only a single design formula for required thickness based on internal pressure. Thus to determine MAWP it is necessary only to solve one formula...calculate pressure based on the thickness.

A cylinder is almost as simple but in this case there are two ASME design formulas relating thickness and pressure: one for circumferential stress and one for longitudinal stress; typically circumferential stress will govern but it is quite possible that longitudinal stress may govern. Both formulas must be checked for MAWP and the smaller such value will govern the MAWP of the cylinder.

Transitions (cones) present a much more complex situation. ASME provides design formulas for circumferential stress and longitudinal stress at both small and large ends of the cone, as well there are the requirements for reinforcing area given in Appendix 1-5. Thus there are 6 design formulas relating thickness and pressure, any one of which can govern the MAWP of the transition.

Nozzles become even more complex. There are many ASME Code requirements applicable to nozzles, any one of which may govern the nozzle's MAWP (eg: shell thickness, nozzle neck thickness, area per UG-37, area per Appendix 1-7(a), stress calculation per Appendix 1-7(b), stress analysis for external loads on nozzles as per WRC-107, analysis of the nozzle neck for longitudinal stresses due to external loads on nozzle, etc, etc).

Determining the MAWP of the tubesheet under UHX requirements is even much more complex. Conceptually it is clear as to how to proceed, this is given in UHX itself by the step-by-step procedure, the last of which is that the tubesheet thickness may be reconsidered if too small for the given pressure, or too large (uneconomical). But because of the many many calculations that must be performed, and the fact that there is no linear system of equations that may be readily solved, an iterative numerical solution method must be used to solve for the "magic" pressure (MAWP) which results with all Code requirements not exceeded, but at least one such requirement is "at" its corresponding permissible Code value (the point being that if no such requirement is at the limit then there is room for an increase in pressure). This iterative solution might be obtained by trial and error on a hand calculator with good guesses, or by a roomful of highly trained monkeys with calculators, or by specially design numerical algorithms that could be used by hand with calculator or programmed into a computer program.

There is no ASME Code requirement that the chamber (or tubesheet, etc) MAWP be calculated; the test pressure per UG-99(b) is based on the MAWP but per the Code may be based on design pressure if the MAWP is not calculated. However, the vessel owner may wish the MAWP to be calculated, which is their prerogative.