Agenda Item 620-295
Title: Compression Ring Leak Testing
Paragraph Q.8.5.4 & R.8.4.4
Date: Nov 19, 2008 (As turned in for S09 SCLB)
Revision: 0
Handled By:Doug Miller
ChicagoBridge and Iron Company
14105 S. Route 59
Plainfield, IL 60544-8984
Telephone: 815-439-6522
Fax: 815-439-6560
Email:
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Purpose: Provide alternate leak testing method for compression ring welds when rotation of the weld joint is restrained.
Source: Doug Miller Aug 20, 2008 email
Impact: Allows welds, which are difficult to reach during the pneumatic test, to be inspected earlier when scaffolding is in place.
Discussion:
The Problem: Paragraphs Q.8.5.4 and R.8.4.4 are within a sequence of steps for Pneumatic Pressure testing and contain a default requirement of solution film testing (SFT) of the joints above the water during the tanks’ pneumatic pressure test. Alternate vacuum box checking (normally during construction) is permitted with some limitations. However, all compression ring welds have been previously off-limits for the substitution since the last sentence of this paragraph states
“The solution-film inspection shall still be made, above the water level, on all welds around openings, all piping joints, and the compression ring welds, including the attachment to the roof and shell.”
SFT of the compression ring joints during pneumatic testing requires close-in viewing of the welds. This is especially difficult for the outside shell-to-bar weld. Scaffolding is commonly used which requires removal following the completed tank pressure test. Removal of scaffolding shell connections requires clean up with possible weld repairs after test. Other methods of access can also be used but with lesser safety.
The Solution: This proposal would allow leak detection using vacuum box testing prior to the pneumatic test in lieu of the SFT during pneumatic test for certain compression bar welds. These particular welds can be defended as having very low chance of having a leak path open up during tank pressurization as discussed in rationale below.
The Rationale: I think the reason for the past requirement of SFT of the compression ring welds under test pressure is due to high strains on fillet welds in some of the common details used at this tank discontinuity as illustrated in Figure 5-6. With the roof pressurized, the compression ring will try to rotate as well as pull inward as the discontinuity tries to round out. Common details that apply components with significantly different thickness cause the rotation to concentrate at these junctions. Weld details such as a one sided roof plate to compression ring weld offer little resistance to rotation and also subject the weld to high strains. Such high strains could open up a leak path that was formerly tight during any pre-test leak checks thus supporting the need to make a leak check with the roof pressurized
But other commonly applied compression ring details are more efficient, are less susceptible to rotation at the joint and thus have lower strain levels at the weld. One example of this is a double-lap roof plate to compression ring weld (1 in sketch). Also, details with a thickened upper shell ring (like f and f-1) cause peak rotation to shift from top (2 in sketch) to bottom of the cylindrical ring. The base of such a thickened ring is at a location with a butt weld (3 in sketch). Full fusion butt welds are much less prone to having strain-induced leak paths open.
Meaning of Visual Inspection in this Context Also in this agenda item the “visual inspection” clause has been removed. This is because it was very misleading in that it seemed to refer to visual inspection of the welded joint. Actual visual examination of welds (as per 7.15.5) during pneumatic testing (whether for the compression bar or for anywhere else on a tank shell) would require complete scaffolding of the outer tank. I don’t think that has ever been done. All that has been done is a visual check of the structure (similar to description in 7.18.3.4) during pneumatic pressure to look for signs of distress. That is done regardless of how leak checking is accomplished.
Proposed Changes:
Q.8.5.4
Above the water level all welded joints, all welds around openings, and all piping joints against which the pneumatic pressure is acting shall be checked with a solution film. A prior vacuum box checkvisual inspection may be substituted for the solution-film inspection if the welded joint has previously been checked with a vacuum box. The solution-film inspection shall still be made, above the waterlevel, on all welds around openings, all piping joints, and the compression ring welds, including the attachment to the roof and shellexcept as listed below:
- two sided roof to compression ring welds
- shell to compression ring welds applying a thickened upper shell ring detail similar to figure 5-6 details f or f-1 The thickened upper shell ring shall be greater than ½ of the conical compression ring thickness and greater than two times the adjacent shell ring thickness.
- Full fusion butt welded connections
R.8.4.4
Above the water level all welded joints, all welds around openings, and all piping joints shall be checked with a solution film. A prior vacuum box checkvisual inspection may be substituted for the solution-film inspection of the welded joints if they have been previously checked with a vacuum box. Above the water level, the solution-film inspection shall be made, of all welds around openings, all piping joints, and the compression ring welds, including the attachment welds to the roof and shellexcept as listed below:
- two sided roof to compression ring welds
- shell to compression ring welds applying a thickened upper shell ring detail similar to figure 5-6 details f or f-1 The thickened upper shell ring shall be greater than ½ of the conical compression ring thickness and greater than two times the adjacent shell ring thickness.
- Full fusion butt welded connections
620-295page 1 of 3
620-295page 1 of 3