Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Sulphur Pit Roof Emergency

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Hello Daryl,

I work in the precast concrete industry. I have done
repairs/replacement of roof slabs for two refineries
in Southern California. here is how I had designed the
earlier two roof slabs. I have a very good idea of how
these panels look like after they have deteriorated.

I would suggest that they be deisned using prestressed
panels and also applying a coat of epoxy paint on all
sides of the panel except the top side. This epoxy
paint is specially made to resist acids and is used in
refineries.

Al of the manufacturing and epoxy ccoating could be
done at a precast facility and then shipped to site
for replacement.

Meanwhile your temporary fix sounds reasonable but if
the current situation of the panels could survive a
few more days then opt for a quick delivery of new
precast panels and replace the old ones.

If you need further information please let me know and
I will try to help.

Arshad Vali, S.E.
Fontana, California
 
--- Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca> wrote:

> Fellow engineers,
> 
>         I have a project involving a sulphur pit
> with a roof which is on the verge of collapse.  I
> would like to have some help in the form of
> brainstorming to find suitable ways and means of
> carrying out an emergency repair.  Any ideas, no
> matter how "wild and wooly" would be welcome.
> 
>         The particulars of the subject facility are
> the following.
> 
>         A sulphur pit is a reinforced concrete
> structure resembling a swimming pool with some type
> of roof structure which may be steel, aluminum, or
> concrete in nature.  The purpose is to serve as a
> holding tank for liquid sulphur (melting temperature
> about 243 degrees F.) during the process of removing
> sulphur (an undesirable impurity but saleable by
> product) from natural gas.  These pits seem to
> suffer from severe cracking due to the high
> temperature gradient through the walls and from
> extreme corrosion of the concrete due to the nature
> of the service (sulphur vapor + water vapor + oxygen
> in the space above the liquid level).  The life of a
> sulphur pit is commonly less than 10 years between
> major restorations.  Petrochemical processing
> facilities like to operate for four to five years
> between major shut down periods for scheduled
> maintenance.
> 
>         The subject sulphur pit is rectangular, 33
> feet by 60 feet, of unknown and unimportant depth
> (probable guess about 12 or 16 feet) with steam
> pipes (for keeping the sulphur molten) a foot or so
> above the floor.  The wall thickness is probably
> about 12 inches.  The top is abut 30 inches above
> grade.
> 
>         The roof structure is precast concrete
> panels 12 inches thick by 6 feet wide spanning the
> 33' direction.  There are four voids about 14 inches
> wide by 7 inches deep filled with some type of
> particle insulation.  The main reinforcing is post
> tensioned grouted duct with about 1.5 inch clearance
> to the bottom; there seem to be 8 strands.  The
> cement type is Type CSA 10 (ASTM Type 1), 35 MPa
> (about 5,000 psi) mix design, with 4.5%air.  The
> panels are seated on a Styrofoam strip and calked
> with a Tremco material.  The roof has been in
> service for 15 years!!  All of this information
> comes from the drawings; the "as built" situation
> may be somewhat different.  The drawings do NOT bear
> the stamp of a professional engineer!
> 
>         The apparent condition of the roof is as
> follows.
> 1.) There is a visible "sag" in the panels (perhaps
> as much as 2 inches).  I have not yet checked the
> curvature due to the thermal gradient through the
> roof slab.
> 2.) I was informed that there was originally a
> camber of 3 inches (which I find doubtful but I
> guess it's possible).
> 3.) One panel (the third from one end) has a
> noticeable sag of at least 1 inch more than the
> panels adjacent to it.  I suspect that at least some
> of the strands in this panel have failed; but how
> many??  This panel also shows signs of a horizontal
> shear failure at one end; there is a horizontal
> split about mid height across the full width of the
> panel and the top portion cantilevers out over the
> bottom portion about half an inch.
> 4.) I am recently informed that fires in sulphur
> pits are a very frequent occurrence.  Apparently
> these fires do not cause ay significant problem from
> an operating perspective; the operators can put them
> out very quickly once they are discovered.
> 5.) It is a standard procedure in ALL facilities
> (not just this one) that no one is ever permitted to
> walk on the roof of an operating sulphur pit.
> 6.) This plant appear to be able to take this pit
> out of service for not more that 5 to 7 days without
> shutting the facility down.  No one will tell me the
> cost of such a facility shut down but my guess is
> about $200,000 to $300,000 per day.  The next
> scheduled major maintenance shut down is in about
> two years.
> 
> Possible repair procedures already considered.
> 
> 1.) As a short term repair while the plant is "hot"
> (kept in service) span beams (say W16x36 at 6 feet
> spacing spanning in the 33' direction) a foot or so
> above the existing roof, build a working platform on
> top of these, suspend the existing precast roof from
> these beams using a mechanism similar to a drywall
> anchor that could be dropped through a hole cored
> through the precast voids.  Once the mechanism was
> activated it would resemble an inverted T supporting
> all or most of the ribs in the precast.  Such
> support could be achieved at the third points (or
> even the quarter points) of ALL of the precast
> panels forming the roof.  I formulated this scheme
> before I was aware that a shut down of 5 to 7 days
> was possible; I now prefer the next alternative,
> never the less, the owner wants to give this scheme
> a more formal consideration.  The risk of failure
> with this scheme seems to me to be fairly small (but
> not zero) with a good and careful contractor;
> however, the consequences of failure are,
> admittedly, very high and could involve serious
> injury or worse.
> 2.) Design and install a new precast roof.  It would
> seem possible to have panels precast in advance and
> installed within the 5 to 7 day window.  The risk of
> failure would seem to be no more than the risks
> normally associated with construction and the
> consequences of failure would seem to be only
> financial, related to not meeting schedule.  One
> decided advantage would be the opportunity of
> inspecting the upper portion of the pit walls and to
> better plan the work to be done in the next shut
> down in two years.
> 
>         As I said above, any other ideas or
> thoughts, no matter how "wild and wooly" would be
> gratefully received and would be treated
> respectfully.
> 
> Thank you for anything you might submit.
> 
> Regards,
> 
> H. Daryl Richardson



       
____________________________________________________________________________________You snooze, you lose. Get messages ASAP with AutoCheck
in the all-new Yahoo! Mail Beta.
http://advision.webevents.yahoo.com/mailbeta/newmail_html.html

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********