US3739453A - Refloating sunken floating roofs - Google Patents

Abstract

When the floating roof in a covered floating roof tank has sunk and it is desired to refloat the same, the tank is first ''''safed'''' by lowering the product level to the top of the rim plate on the roof, introducing water into the tank below the roof (or above, if necessary), pumping the product from the top of the water, then pumping in gas oil to absorb any product, and stripping this off. After safing of the tank, the roof drain plug is removed and the water completely removed from the tank. Then, after reinstalling the drain plug, gas oil is pumped in to raise the roof to a suitable level for inspection thereof. If there is no visible damage and no leaks, the tank may be returned to service after cleaning the roof.

Description

REFLOATING SUNKEN FLOATING ROOFS [75] Inventor: Joseph C. Vankoski, Wilmington,

Del.

[73] Assignee: Sun Oil Company, Philadelphia, Pa. [22] Filed: Sept. 7, 1971 [21] Appl. No.: 178,259

[52] U.S. Cl 29/401, 210/242, 2lO/DIG. 21 [51] Int. Cl 1323p 7/00 [58] Field of Search 29/401, 429, 469;

210/242, DIG. 21, 1, 198

[56] References Cited UNITED STATES PATENTS 3,508,652 4/1970 Woolley 2lO/DlG. 21 3,635,819 1/1972 Kaiser 2l0/DIGi 21 3,662,892 5/1972 Sorensen.. 210/242 3,198,731 8/1965 DeLew..... 2l0/DlG. 21 R24,997 6/1961 Hanna 29/429 'sEAT ZCC LOWER PRODUCT LEVEL TO TOP OF ROOF RIM PLATE INTRODUCE H; O

BE LOW ROOF MEASURE VAPO R CONCENTRATION ADJUST H;O LEVEL TO 1 June 19, 1973 Primary Examiner-Charles W. Lanham Assistant ExaminerD. C. Reiley, lll Attorney-George L. Church, Donald R. Johnson Wilmer E. McCorquodale Jr. et a].

[57] ABSTRACT When the floating roof in a covered floating roof tank has sunk and it is desired to refloat the same, the tank is first safed by lowering the product level to the top of the rim plate on the roof, introducing water into the tank below the roof (or above, if necessary), pumping the product from the top of the water, then pumping in gas oil to absorb any product, and stripping this off. After safing of the tank, the roof drain plug is removed and the water completely removed from the tank. Then, after reinstalling the drain plug, gas oil is pumped in to raise the roof to a suitable level for inspection thereof. If there is no visible damage and no leaks, the tank may be returned to service after cleaning the roof.

4 Claims, 2 Drawing Figures HOT TAP NOZZLE INTO SHELL mrnouucs H2O ABOVE ROOF 1F ROOF FLOATS ON HZO-PRODUCT INTERFACE IF Q3 PUMP IN (5A5 OIL AND STRIP OFF REMEASU RE VAPO R N TOP OF ROOF mm PLATE co CENTRATION REMOVE ROOF 0mm PLUG am) INSTALL sv uou LINES AS macs SSARY DRAIN ALL H3O FROM TANK REINSTALL RooF MAIN PLUG PUMP IN GAS OIL, PIN ROOF SUPPORTS AT HIGH POSITION mspcc'r ROOF FOR 1F LEAKS ziga i gcz A A 0AM cc, LE KS QR DAMAQE FOR REPAIRS N M CLEAN ROG AND EY'URN TANK T'O SERVICE PATENIEDJIIII I 9 I973 same urz LowER PRODUCT LEVEL TO TOP OF RooF RIM PLATE I INTRODUCE H2O PUMP FRoDUcT FRoM TOP OF I-I,o

PU P IN GAS OIL AND STRIP OFF OPEN FREE VENTS AND MANHOLES HOT TAP NOZZ LE INTO SHELL BELOW ROOF I INTRODUCE H2O ABOVE RooF IF RooF FLOATS ON I HZO-PRODUCT INTERFACE I MEASURE vAPoR Q 3 PUMP N GAS OIL. CONCENTRATION AND STRIP DFF IF 0.3 6 I REMEAsURE VAPOR ADJUST H1O LEVEL To CONCENTRATION TOP oF RooF RIM PLATE REMOVE ROOF DRAIN PLUG AND INSTALL SYPHON LINES AS NECE SSARY DRAIN ALL H2O FRDM TANK REINSTALL ROOF DRAIN PLUG INSPECT ROOF FOR IF LEAKS FROM SERVICE DAMAGE. LEAKS OR DAMAGE FOR E A RS IF NO AKS CLEAN ROOF AND RETURN TANK TO SERVICE 1 REFLOATING SUNKEN FLOATING ROOFS This invention relates to floating roof tanks of the type commonly used for the storage of hydrocarbon products, and more particularly to a procedure for refloating floating roofs which have sunk. The invention is particularly applicable to covered floating roof tanks" (which are tanks having a fixed cone roof and an internal floating roof).

Covered floating roof tanks have certain advantages over open type floating roof tanks, so that the former have come into increased use during the last several years. Covered floating roof tanks (known to those skilled in the art as covered floaters"), however, have a drawback in that the floating roof in a covered floater is more prone to sinking in the product than is the floating roof in its open type counterpart. There are several causes for this.

In the first place, one leak in a floating pan, not compartmented, will cause complete inundation of the roof; the time required to sink the roof is a function of the size of the leak. The leak can go undetected for a long period of time, since the internal floating roof is hidden from the sight of personnel who are involved in the normal operations of the tank (e.g., gauger, sample taker, etc.).

Overfilling of a covered floating roof tank is another possible cause of damage to the floating roof, and consequent sinking. The floating roof will jam against the cone roof structure and liquid will overflow onto the roof, or the roof will be ruptured due to a large pressure buildup under the roof. Small overflow holes are provided in the shell of a covered floating roof tank as a standard design, but these holes are not intended to relieve the flow rate which would be required to prevent damaging the floating roof; they merely serve as a high level alarm to a person who happens to be in the area.

Violation of the industry practice of keeping the filling stream at a rate of 3 feet per second or less when the floating roof is not buoyant (that is, when the roof is resting on its supports, so that a vapor space exists under the roof) is suspected as a cause of roof sinking. When this violation has occurred, product has been detected on the floating roof in the vicinity of the fill line.

Further, filling the tank with a material having a vapor pressure or temperature significantly higher than the product in the tank has also been postulated as a cause for the sinking of floating roofs, through vapor evolution or high turbulence due to severe thermal gradients. In this case, the propensity of the vapor to escape may cause product to boil past the peripheral seal and onto the roof, or it may lift and suddenly drop the roof, so that product overflows the rim.

In a covered floating roof tank, the fact that the roof has sunk can go undetected for a long time, since the internal roof is hidden from the sight of personnel who would (in the case of open type floating roof tanks) normally detect a malfunction prior to sinking. Only after discrepancies in gauging occur is the roof found to be sunk.

In an emergency, a covered floater can be operated with a sunken roof. In order to operate it safely, it is necessary (immediately upon discovering that the floating roof has sunk) to seal or block off the tank free vent facilities (e.g., the shell and cone roof free vents). The efficiency of the air circulation venting facilities of a covered floater provides a hazardous situation when the roof is submerged. An explosive condition is likely to occur as the vapor concentration varies with the air flow through the tank free vents. It becomes desirable to ensure that the vapor space is too rich to explode, especially with expectations of thunderstorm (lightning) activity.

To ensure high vapor concentration, the shell and cone roof free vents are sealed (as previously mentioned), and all-weather type conservation-vent valves, properly sized for tank volume and pumping rates, are installed in the cone roof.

The tank should not be emptied, since the weight of the liquid trapped on the (sunken) roof may crush the roof, thus requiring large repair expenditures.

It is obviously desirable that the tank be returned to v proper operating condition (which involves refloating of the floating roof) as soon as possible after tlie discovery of the roofs sunken condition. The present invention is concerned with a procedure for the refloating of a sunken floating roof.

An object of this invention is to provide a novel procedure for refloating a sunken floating roof, in a covered floating roof tank.

Another object is to provide a novel method for free ing a covered floating roof tank (in which the floating roof has sunk) of hydrocarbon vapor.

The objects of this invention are accomplished, briefly, in the following manner: By means of a procedure involving, among other things, the introduction of water into a covered floating roof tank (wherein the floating roof has sunk), the tank is freed of hydrocarbon vapor, so that a workman may enter the tank to remove a drain plug in the roof. After the water is all drained from the tank, the drain plug is reinstalled, the roof is cleaned, and (provided the floating roof is undamaged) the tank is returned to service, by pumping product to be stored into the tank.

A detailed description of the invention follows, taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a vertical sectional view of half of a covered floating roof tank, with a sunken floating roof therein, illustrating the conditions existing therein at a certain stage in the refloating procedure of the invention; and

FIG. 2 is a block diagram (in flow chart form) of a reflating procedure according to the invention.

Referring first to FIG. 1, which illustrates a covered floating roof tank T, an upstanding; cylindrical wall 1 constitutes the shell of the tank whose longitudinal (vertical) center line is indicated at 2. Around its lower edge, shell 1 is sealed to the imperforate bottom wall 3 of tank T, and to the upper edge of shell 1 there is sealed a fixed cone roof 4. At the center of roof 4, there is provided a conventional mushroom type free vent 5.

Several circulation vents (free vents) such as 6 are provided in the shell 1, near the top thereof, and several overflow slots such as 7 are also provided in shell 1, near the top thereof but below vents 6. The usual tank suction/fill line 8 extends through shell 1, near the bottom thereof. Also, one or more manholes (not shown) may be provided in the shell I, for entry of workmen into the tank.

A floating roof R is telescopically disposed within tank T. The roof R is generally of pan shape, having an upstanding plate 9 at its radially outer edge which defines a rim. A somewhat conventional peripheral seal 10 is secured to the extreme outer edge of roof R, just beyond rim plate 9, this seal engaging the inner face of shell 1 and the outer face of rim plate 9, to provide a seal between roof R and tank T as the roof moves up and down within the tank. A ladder L, whose upper end is secured to the roof 4 adjacent a ladder hatch l1 and whose lower end is secured to the tank bottom wall 3, passes through an opening 12 in roof R, suitable sealing means (not shown) being provided in this opening to prevent passage of vapors therethrough. The ladder L and hatch 11 provide for entry into the tank, for maintenance and/or repair purposes.

Preferably, the roof R is compartmented into a central deck area 13 and a plurality of radially outer compartments, one of which is indicated at 14. One of the compartment-forming bulkheads is indicated at 15. A threaded plug 16, which threads into a suitable aperture in the central deck area 13, serves as a roof drain plug.

The roof R is adapted to be supported above the bottom wall 3 of the tank T by means of a plurality of legs or supports 17 (one of which is illustrated), when the floating roof is in its lowered position. The legs 17 telescopically fit in sleeves 18 which are rigidly affixed as by welding to the roof R. The axial position of the supports 17 relative to the sleeves 18 may be adjusted by means of bolts (not shown). The sleeves 18, by engagement with the tank bottom wall 3, provide an operating level or low level support for the roof R. For the maintenance level or high level support of the roof R, the leg 17 is secured (by means of the aforementioned bolt) to the sleeve 18, through a hole 19 in sleeve 18 and a hole 20 in leg 17. During normal operation of the tank, a bolt through hole 20, only, prevents leg 17 from falling downwardly, out of sleeve 18.

An open-ended well 26, some ten inches in diameter,

' is sealed at its lower end into a suitable hole provided in the central deck area 13 of the roof R, the upper end of this well being in the same horizontal plane as the top of the roof rim plate 9. During normal operation of the tank T (i.e., when the roof R is floating), the upper end of well 26 is covered and sealed by means of a cover plate 27 provided on its lower face with a sealing gasket 28.

A sleeve 18' is welded near its upper end to cover plate 27, sleeve 18' being thus sealed through a suitable central hole in the cover plate and passing downwardly through well 26. Sleeve 18 has a length such that, when roofR is resting on its supports 17 in the low level position, as illustrated in FIG. 1, the lower end of sleeve 18' engages the tank bottom 3 as illustrated and lifts the cover plate 27 approximately 6 inches above the upper end of well 26; thus, in this roof position the well 26 is open at both ends and provides free communication between the upper and lower sides of roof R.

An inner leg 17 is telescopable within sleeve 18'. Leg 17 carries a removable pin 29 which is adapted to be inserted through a pair of aligned holes 30 in sleeve 18 for the high level support position of roof R; thus, in the high level position of the roof, the lower end of leg 17 engages the tank bottom 3 and maintains (by way of items 29 and 30) the cover plate 27 spaced above the upper end of well 26. Therefore, in the high level position of the roof also, the well 26 is open at both ends and provides free communication betwee the upper and lower sides of roof R.

The petroleum-derived liquid hydrocarbon products with which the invention is concerned, and which may be generally stored in tanks, are all lighter than water and are immiscible with water, or substantially so.

Refer now to FIG. 2. As previously mentioned, once the fact of sinking of the floating roof R has been detected (it is illustrated in a sunken position in FIG. 1), the shell free vents 6 and 7, and the cone free vent 5, should be immediately sealed, and one or more properly sized conservation-vent valves (not shown) should be installed on the cone roof 4. These steps are necessary to ensure high vapor concentration during the initial steps of refloating the roof, as well 'as being necessary if it is desired to operate the tank with a sunken roof.

The returning of the tank T to proper operating condition (i.e., the returning of the roofR to floating condition) requires safmg of the tank (which is to say, freeing it of hydrocarbon vapor) so that a workman may enter the tank from the top (by way of hatch 11 and ladder L) to remove the drain plug 16. Attempts should be made to see if the roof R is resting on its supports 17 (at low level or operating level) by checking the readout of gauging equipment that is attached to the roof and/or hand gauging to the roof pan. Experience has proven that the roof R was always in fact on its supports, and not jammed at some other level.

The first step is to lower the level of the stored hydrocarbon product to the top of the roof rim plate 9, by drawing the product down through the tank suction line 8. The product flows mainly through the (now uncovered) roof well 26, to line 8, although some product also finds its way past the seals around the roof support legs 17, and around the seal in opening 12.

Next, water is pumped into the tank below the roof R, by way of the suction/fill line 8. This introduction of water should be done in a cautious manner, since water and light hydrocarbons can generate enough static electricity to produce a spark. The water should be pumped into the tankto a level above the roof rim plate 9. During this step of the procedure, this situation may be as illustrated in FIG. 1. In FIG. 1, the roof R is resting on its low level supports, the water level (hydrocarbon-water interface) is momentarily at 23, and the product level is momentarily at 24.

A nozzle 21, with a valve 22 connected thereto, is then installed on the shell 1 by hot tapping, at a point above the roof R. This hot tapping (welding) of the nozzle to the tank shell must be done with the water level above the nozzle.

if when pumping water into the tank as above, the roof R begins to float on the water with the product above, it is necessary to introduce water above and onto the roof R, by means of a hose over the tank top, or through the hot tapped shell connection 21 (if the latter has been installed). The water should be discharged below the product surface, to prevent the generation of static charges caused by freefall or splashing.

Next, the product is pumped from the top of the water. Depending on the tank size and the available quantities of water (which govern the amount of water previously introduced into the tank, below, or above, the roof R), this pumping is done either by means of a hose that is lowered through the tankcone roof 4 (in this case, the product must be near the top of the tank), or by means of nozzle 21.

After the product is stripped from the surface of the water, a quantity (such as a 6 inch layer) of so-called gas oil (a rather heavy liquid derived from crude petroleum) is pumped into the tank T, to absorb any product that may still exist on the water.

Following this, the gas oil is stripped off (e.g., in the same manner as the product was previously stripped from the surface of the water), and then the tank shell vents 6 and 7, the cone roof free vent 5, and the manholes such as 11 are opened. At this time, a measure ment for vapor concentration is taken, using for example a conventional J. W. combustible indicator. If this reading is not satisfactory (that is, if the reading is greater than 0.3), gas oil is again pumped in and stripped off, as many times as necessary, until a satisfactory reading is obtained. When the reading is satisfactory, the safing operation is considered completed.

At this point in the procedure, the water level is adjusted (by means of suction line 8 and the open well 26) to the top of the roof rim plate 9.

A workman can now enter onto the floating roof R (by way of ladder L) to preliminarily inspect the roof and remove the roof drain plug 16. If the roof is compartmented as illustrated, the workman also will install syphon lines or hoses such as 25 (FIG. 1), from the compartments 14 to the central deck area 13, so that the compartments will drain along with the central area. The water is now completely removed or drained from the tank T, by way of suction line 8.

When this has been done, the floating roof R will be visible, and both this roof and tank T will be essentially free of contained product and water. Then, the plug 16 is reinstalled, and gas oil (or furnace oil) is pumped into the tank, to raise the roofR to a level such that the support legs 17 may be pinned (at bores 19 and 20) for the high level position. The roof is inspected for leaks during this period, and if severe leaks are detected, the roof R must be lowered back to its low level support position. If there is no visible damage and/or no leaks, and if the seal 10 is tight, the tank T may be returned to service after a thorough cleaning of the floating roof R, with frequent inspection for roof leaks. The return to service involves the lowering of the roof to its operating level or low level support position (support by means of sleeves 18), and the pumping of liquid prod uct into the tank at a slow rate (approximately 3 ft. per second) until the roof R becomes buoyant, or floats.

Roof leaks, damaged (torn) seals, or obvious roof buckling require removing the tank from service for cleaning, complete inspection, and necessary repairs, following which the tank may be returned to service as previously described.

The invention claimed is:

1. In a procedure for reconditioning a sunken pantype floating roof in a storage tank for liquid hydrocarbon products, the steps of lowering the product level in the tank to the top of the roof pan rim, introducing water into the tank, at a location below the sunken roof, to float the remaining product on top of the water, withdrawing the product from the top of the water, introducing a scavenging liquid into the tank to absorb the remaining product, and stripping off the scavenging liquid with its contained hydrocarbon product.

2. Procedure set forth in claim 1, including the additional step of adjusting the water level to the top of the roof pan rim.

3. Procedure set forth in claim 1, including the additional step of draining all water from the tank.

4. Procedure set forth in claim 3, including the additional step of introducing product into said tank to refloat the floating roof.

Claims (4)

1. In a procedure for reconditioning a sunken pantype floating roof in a storage tank for liquid hydrocarbon products, the steps of lowering the product level in the tank to the top of the roof pan rim, introducing water into the tank, at a location below the sunken roof, to float the remaining product on top of the water, withdrawing the product from the top of the water, introducing a scavenging liquid into the tank to absorb the remaining product, and stripping off the scavenging liquid with its contained hydrocarbon product.

2. Procedure set forth in claim 1, including the additional step of adjusting the water level to the top of the roof pan rim.

3. Procedure set forth in claim 1, including the additional step of draining all water from the tank.

4. Procedure set forth in claim 3, including the additional step of introducing product into said tank to refloat the floating roof.

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