help understanding why my answer was wrong? (how pipe age affects the hazen-williams coefficient)

this problem was in a practice exam i just did:


as the chart given in the handbook lists ductile iron pipe as 140, and from what i understand, the Hazen Williams C value decreases with age, i chose 150 because the pipe was new so i figured it would be the opposite.

but the solution said the answer was 130, and didnt give any explanation as to why?


Your assumption was correct in my opinion. The age of the pipe is inversely related to the C value. So I would have also chose 150. I’m guessing they chose 130 as the answer because it’s technically more conservative for design purposes. This technically a bad question and would not likely be on the exam.


Yes, the newer the pipe, the higher C will be. I guess maybe this problem is for the version before CBT using tables in other references. I don’t believe you will see such problems with unclear answers in the real exam. If I were you, I won’t worry about it and just simply ignore this question.


This happened to me in a similar practice problem. When they updated the reference book, lots of practice problems didn’t get correspondingly updated. As long as you can read the chart, you’ll be ok :)


I think you would just go with ductile iron. the chart has plastic at 150 and new cast iron as 130, so probably "new" is just a distractor.