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Check ASME Vessel and Exchanger Stresses Before Testing

The ASME Code requires that all vessels undergo hydro (or pneumatic) testing following fabrication and before Code stamping. As this test provides mechanical stress relief it may be thought of as the final step in the fabrication process. The hydrotest is an effective way to screen for design, material and fabrication deficiencies as well as to prove the structural integrity of the equipment.

However, this raises the question - what constitutes a hydrotest "design deficiency"? Although ASME Section VIII-1 does not prescribe a test stress limit, vessels that undergo "objectionable distortion" during testing may be rejected by your AI. The COMPRESS Hydrotest Feature assists you by:

• Calculating component test stresses and warning if they are excessive (objectionable distortion check).
• Checking for formed head buckling.
• Checking that the vessel supports can carry the weight of the test water.
• Warning if the test water temperature is too cold (brittle fracture check).
• Determining the test pressure to apply (as read on the gauge at the top of the vessel).

How do I view the Hydrotest Report?

The Hydrotest Report is available to all COMPRESS users. To prepare your report:

1. Design your vessel or exchanger.
2. Select your test basis in Set Mode Options > Defaults > Testing.
3. Press F3 to generate the Code calculations. Click "Hydrostatic Test" on the left to view the report.
4. Check for overstressed components and revise your design as necessary. To speed your review, COMPRESS also includes any overstressed components in the Deficiency Report.

Download Sample Hydrostatic Test Report

Criteria for Limiting Hydrotest Stress in COMPRESS

Because ASME VIII-1 does not provide guidelines for determining allowable stress values for the test condition, COMPRESS uses the following method by default. It is taken from the 2004 Edition of ASME VIII-2, paragraph AD-151.1. If needed, you can define a different percentage of yield allowed at test in the Set Mode Options > Defaults > Testing area.

(a) P_m ≤ 0.9 * S_y
(b) P_m + P_b ≤ 1.35 * S_y for P_m ≤ 2 / 3 * S_y
(c) P_m + P_b ≤ 2.2 * S_y − 1.5 * P_m for 2 / 3 * S_y < P_m ≤ 0.8 * S_y

In the equations above,  P_m is the primary membrane stress and P_b is the primary bending stress. The yield stress S_y is taken as the smallest of the shell, reinforcing pad or nozzle neck S_y values.

The discontinuity stress state at a nozzle includes both primary membrane and bending components. As equation (a) governs for shells and either equation (b) or (c) governs for nozzles, the test allowable may be different for nozzles and shells even when they are made from the same material.

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