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Re: Sulphur Pit Roof Emergency

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Daryl,

I had a similar situation last year however the pit I looked at had a cast-in-place concrete roof.  I saw pictures of a previous repair of the roof and what they did was to put stainless steel scaffolding inside the pit.  The bottom roof concrete was nearly all gone and the bottom layer of rebar exposed.

You can bet the roof concrete inside your pit is really bad and the post tensioning bar exposed and/or corroded away.  Your backer beam approach sounds good but since the concrete panels are suspect I would use several attachments along the beam.  I would also suggest through bolts with very large plates both of which will likely need to be A317 stainless steel (check with your Piping Material specialist if available).

Replacing the precast panels with new ones might be your best bet if they can be ready and at site for the turn around.  As a minimum I would use Type V cement with 7 to 10 percent silica fume and provide 3 inches of concrete cover and significant supplemental non-prestressed rebar.  Definitely a situation for belts and suspenders.

Thomas Hunt, S.E.
Fluor



Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca>
05/23/2007 06:57 PM
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Sulphur Pit Roof Emergency





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
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