It would probably help if they had done it 15 years ago. The major
problem, however, is deterioration of the concrete.
I am informed that the owners intend to make a decision today
regarding how they intend to carry out the repair. In gratitude for the
assistance I will at least try to provide the list with a summary of the
final solution.
Regards,
H. Daryl Richardson
----- Original Message -----
From: "Gary L. Hodgson and Assoc." <ghodgson@bellnet.ca>
To: <seaint@seaint.org>
Sent: Friday, May 25, 2007 5:30 AM
Subject: Re: Sulphur Pit Roof Emergency
> Daryl,
> Would stainless steel rebar be a solution to this problem?
> Gary
>
> Jordan Truesdell, PE wrote:
>> Daryl,
>>
>> I had the same reaction as you - steel strongback temporarily,
>> replacement with p/c after knowing you had a 5-7 day window for
>> replacement. 5-7 days is short, so it may depend how flexible the
>> schedule is, and getting everybody in sync.
>>
>> It sounds like the roof design needs a lot of thought. I presume the
>> atmosphere inside is essentially heated sulfuric acid. I'm not qualified
>> to speak to the specifics of the material interactions, but the long term
>> corrosive atmosphere suggests lots of cover, "special" concrete, and
>> either coated steel or corrosion resistant reinforcement material. Is
>> strand even an option, or should might you consider a thicker cap with
>> non-prestressed steel? Sounds like an interesting project.
>> Jordan
>>
>>
>> Daryl Richardson 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
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