Hello everybody:
Regarding the construction of steel penstocks for hydropower plants, I have some questions to ask to the members of this Forum:
Penstock in hydropower plant B has a thickness t = 22 mm; for plant C have a thickness t = 25 mm; for plant D, t = 9,5 mm and for plant E, t = 12 mm to t = 25,4 mm. The diameters are between 2,00 m and 3,50 m.
Hydropower plants A, B, C and D are in commercial operation; plant E is under construction.
The material for these penstocks is steel S355JR that is equivalent to steel A572 Gr 60 (P1, Gr2). According to Table UCS-57 of ASME Code Section VIII, Division 1 – 2010, the thickness above which full radiography examination of butt joints is mandatory is 1¼ (32 mm).
I wonder why so different specifications for non-destructive testing were delivered for penstocks of these hydro plants; particularly, the radiographic test that was specified to be performed to the full length of circumferential and longitudinal joints; as well on T junctions an bi and trifurcations.
Am I misunderstanding the code? Am I applying it out of order?
Few days ago, I was talking with a Certified Welding Inspector regarding to the radiographic inspection on a trifurcation and he told me that those inspections were (sometimes) very difficult to perform, so instead of that, it was a common alternative practice to achieve Ultrasonic Test on bi and trifurcations.
Thanks in advance for your comments.
Suppose that the wall thickness of a pressure vessel designed to ASME VIII Div 1 is 3/4 of an inch = 19,2 mm. Suppose that the diameter is 3.000 mm and the straight height is 4.500 mm. According to the Code, full Rx is not mandatory and in fact only spot Rx was performed. During the hydrostatic test, a leak developed allowing a few drops of water to spill. The vessel was emptied, the weld repaired and that was all. Everybody is happy.
Now take into account the penstock of a hydro power plant. The penstock interconnects the bottom of the dam to the hydro turbine inlet. There's no stop valve on the bottom on the dam, where the penstock begins. The only valve, usually a butterfly one, is located at the hydro turbine inlet. There's no practical way to perform a hydrostatic test in the penstock. So, when everything is ready, water is admitted into it. Following your comment, no full Rx was performed. A leak develops, allowing a tiny little jet of water to spill. How are you going to repair the weld, if there's no valve upstream the leak?
That's the reason the Project engineers specified full Rx test on every penstock weld. As a matter of fact, a hydro power plant penstock doesn't fall entirely under ASME VIII.
Giovanni S. Crisi
Sao Paulo - Brazil
Hello everybody:
Thanks Giovanni for your prompt response. As a manner of explanation, I can tell you this:
Hydropower plants B and C do not have a shutoff valve at the Penstock Intake, plant D has a shutoff butterfly valve at the Penstock Intake and, plant E will have installed a shutoff valve in that point.
Additionally to the Non-Destructive Test of Weldings, for all these hydropower plants it was performed a Hydrostatic Test of the penstock as a whole. After this hydrostatic test, and only after it, when no leaks were detected, the backfilling task was achieved.
I would appreciate if you can tell me what code (s) in addition to ASME Section VIII, Div 1 can be applied for testing penstocks.
Giovanni, do you have any comment regarding to the application of the Ultrasonic Test in lieu of the Radiographic Test? especially in wye branches.
I have prepared a table in an Excel worksheet where are described the types of inspections requested for the weldings of these penstocks but, in this Forum I could not find the proper way to upload that archive. Can you or someone else give me a hand on this subject?
Thanks for all. Obrigado e saudações.
As far as I know, hydroelectric power plants penstocks do not fall under any specific standard.
They're not pressure vessels because there's water flowing into them, so they don't fall under ASME VIII. Neither they're piping, because of their dimensions: a large diameter and a short length, so they don't fall under ASME B.31.
In my opinion, the specification for design, calculation, construction and testing of penstocks should be issued by the Project (or Consulting) Engineers that have designed the plant.
That's what happened with the hydro power plant you're working at: the Project engineers have specified full radiograph of all welds. The reason I've told you: it's better to spend some time and money and make sure the penstock won't leak than having to repair leaks after water has been admitted.
As a matter of fact, making radiographic tests on angle welds is difficult and sometimes impossible, and in this case the RX may be replaced by an US test.
The American Water Works Association has issued excelent standards for welded steel fittings and branches. Take a look at their site: www.awwa.org
Good luck
Giovanni S. Crisi
Just to add some information, there is an ASCE Manual out (ASCE No. 79) specifically for Steel Penstocks. Section 13 is on testing.
kyle
You're right, thanks to you I've learned that steel penstocks are ruled by ASCE 79 std.
ASCE is the American Society of Civil Engineers. I wouldn't have never imagined that steel penstocks falled under a civil engineers' standard.
Giovanni S. Crisi
Forum
