Allowable Stress limit in hydrotest

Allowable stress for materials are listed in the ASME B&PV code section II "Materials" part D "Properties". There are many tables that list allowable stress at temperatures and whether or not the material is permitted for use in vessels stamped under under section III, VIII-1, VIII-2 and/or XII.

http://www.est.asme.org/knovel2/asme/default.jsp

The allowable stress at design pressure and design temperature is generaly but not allways either 1/3 the yield strength or 1/5 the ultimate strength whichever is lower.

For the safety of all involved it is best to get a code stamp shop to design, fabricate and test your perssure vessels.

A good reference is PRESSURE VESSEL HANDBOOK, by Eugene F. Megyesy, published by Pressure Vessel Publishing, Inc. P.O. Box 35365 Tulsa, Oklahoma 74153 USA.

There is also computer software that will guide you through the design process. It is called Compress provided by Codeware Inc., 3100 S. Gessner Rd., Suite 610 Houston, TX 77063. The codeware web site is:

http://www.codeware.com/

What is the allowable stress which we generally take during hydrotesting of pressure vessels and the basis for that?Is there any value set by code(ASME SEC VIII-Div-1) on the same?

I'd like to differentiate between the Allowable Tensile Strength of material, S used in design equation, and the Stress Limits of stresses arise due to internal pressure, hydrostatic test, and or any other loading, say Sn.

S can be located from ASME BPVC, Section II, Part D, where Sn is required to be calculated due to hydrostatic pressure, internal pressure, stresses at midspan and at supports due to weight of testing fluid and weight of vessel itself, wind load, earthquake ……, etc.

So a stress analysis like ZIK analysis is required to be carried out at various loading cases to include :

A. Longitudinal Bending Stresses S1: at saddles, midspan, and due to internal pressure for horizontal vessel as example. Max(S_{1at saddle}, S_{1at midspan}) + S_{int. press} must be < SE.

B. Tangential Shear stresses : in shell or head must be < 0.8 S

C. Circumferential Stresses: at horn of saddle must be < 1.5 S & at bottom of shell must be < 0.5 S_yield.

For ZICK analysis :

COADE Discussion Forums: Zick Analysis and Saddle Calculation

Not exceeding 80% of yield strength at hyro conditions is my understanding of acceptable practice.

I do not believe the code limits vessel stress during hydrotest. Can any one confirm this?

As per my experience, requirements of Shop hydrostatic test pressure is described below.

1. Shop HT pressure = 1.3 * MAP (New and Cold condition)
2. MAP used of HT pressure calculation is allowable pressure of the strongest category A weld.
3. Caused stress of all components is below 90% of its min. yield stress
4. MAP is determined nozzle flange pressure rating or the calculated pressure.

While Hydrostatic test pressure is determined with the following procedure.

Calculate

1. MAP*1.3 (As per code)
2. MAP for Nozzle flange pressure rating
3. Check caused stress of all parts in Compress Calculation (As per spec)
4. i.,e calculate Stress Ratios = caused stress/material stress value at design temperature

Material stress is already by default 0.9*yield stress of material in Compress software

If stress ratio < 0.9 Then increase test pressure so that stress ratio approaches to 1

If stress ratio > 0.9 Then decrease test pressure so that stress ratio is less than equal to 1

Please note that this requirement is specifically for my Project. Shop test pressure by the strongest Category A weld is more higher than typical establishment of hydrostatic test pressure by ASME Code 1.3 times MAP.

And note that the body flanges may be caused some problems of the leakage under this more severe test pressure in potential.

ASME specify the HT pressure of minimum. (HT pressure = 1.3*MAWP ) And these is a heat exchanger specification to decide HT pressure. Job specification gives higher value than ASME. But we have to obey the specification as per client requirement.

I have made this draft in short time so if anyone still have confusion then do tell me.

As it depends upon the historical condition of the vessel in question, the answer can be found in the domain of the Engineer/Surveyor for the company that is supplying burst/collapse indemnity insurance cover on it as a result of the historical condition records it generates.