The job spec calls for replacing the existing 3" sch 10 black with 3"
sch 40 galvanized. As I said in a previous post, to do this job
correctly and bring the system up to Code will require more than just
replacing pipe. The mains are essentially flat and the the outlets
are off the side of the main. The mains run out 200-250 ft from the
feed, so you are looking at 5-6" of pitch; now most of the dry
pendents for the second floor will be too short. That wasn't figured
in the spec (prepared by the maintenance staff). Also, there are no
drawings for this place, so it would be a full survey job, which
would take a lot of time and money. Given the economic conditions up
here, someone will probably come in a bid it bottom dollar to just
replace the pipe and say the heck with the pitch, so more than
likely, we won't get it. But, I've got to give my client a quote.
Interestingly, this is the second job that I have been asked to look
at in a couple of weeks with 17 year old +/- 3" sch 10 that has
rotted out. Both are within 5 miles of my house. I believe they were
installed by different contractors.
This job has its own issues, but it did bring up the point about
replacing existing pipe with something different with similar flow
characteristics and the need for calcs.
At 09:00 PM 6/5/2008, you wrote:
Todd, based on the fact that this is a repair, as mentioned, AND the fact
that you're replacing pipe that *should* have been calculated with a
hazen-williams c-factor of 100 (black steel) with piping that has a c-factor
of 120 (galvanized) - I would say you're probably going to be alright - but
better safe than sorry - so why not run calcs?
-B-
On 6/5/08, Todd Williams - FPDC <[EMAIL PROTECTED]> wrote:
>
> To do this job correctly (operative word) will require a lot more than just
> pipe replacement and thus will need to be recalculated. As an example, to
> properly pitch the pipe will require replacement of most of the
dry pendents
> feeding the floor below. The additional 30% increase and a few other things
> have already been mentioned to the contractor. However, the job spec treats
> this as a repair and doesn't really try to solve the problem of water
> trapped in mains (except they want an add for an air dryer in the quote)
>
> My question was more academic in that if you replace one pipe with a
> different type of pipe with a similar loss per foot and make no other
> changes, would it necessarily have to be recalculated. When does repair
> become modification?
>
>
>
> At 12:31 AM 6/5/2008, you wrote:
>
>> If I had high confidence of all the following, I wouldn't see the need for
>> new calcs:
>> - Strictly pipe replacement (no added offsets, elbows, riser
>> nipples, etc)
>> - The original calcs were accurate (done properly and reflect 'as
>> built' conditions)
>> - Water supply hasn't deteriorated.
>>
>> A bit of a side note - if the attic has a roof with slope greater than
>> 2/12,
>> the design area very likely didn't include a 30% increase for the slope.
>> That requirement didn't appear in 13 until 1996. But since the pipe
>> replacement is legitimately a repair, you probably don't need to meet any
>> standard other than what was in effect 17 years ago.
>>
>> Ed Kramer
>> Littleton, CO
>>
>>
>> > I walked through an attic this afternoon where all of the existing
>> > mains need to be replaced. The piping is 3" schedule 10 black steel,
>> > that is developing leaks after 17 years (we found another one on our
>> > tour). The proposal is out there to replace the existing with 3"
>> > schedule 40 galvanized (and provide proper pitch). I did a couple of
>> > quick calculations and at 250 gpm, the friction loss per foot for the
>> > two pipes is very close to the same. (.077 for the sch 10 black v.
>> > .075 for the sch 40 galv). My guess is that the difference in
>> > pressure would be about 0.75 psi. The calc has about 12 psi
>> > remaining. If this was strictly a pipe replacement (no other
>> > modifications), would you recommend new hydraulic calculations?
>> >
>> > Todd G. Williams, PE
>> > Fire Protection Design/Consulting
>> > Stonington, Connecticut
>> > www.fpdc.com
>> > 860.535.2080
>>
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>
> Todd G. Williams, PE
> Fire Protection Design/Consulting
> Stonington, Connecticut
> www.fpdc.com
> 860.535.2080 _______________________________________________
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Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
www.fpdc.com
860.535.2080
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