System for draining a liquid storage tank

The present invention relates to a system which makes it possible to remove undesirable liquids, generally an aqueous phase, which precipitate to the bottom of tanks for storing liquid products. Use is made of a drainage device (30) which comprises a concave body (31), a seat (26), and an interface float (8) which has a plug portion (25). This interface float (8) is designed to sink into the product and to float on the aqueous phase. As the level of the aqueous phase drops, the plug portion (25) settles sealingly on the seat (26), halting the drainage operation since all the accumulated aqueous phase has already been drained off. The drainage system may be automatic or manual. In the former case, use is made of a sensor (12) for detecting the presence of an aqueous phase and commanding the process of opening a shut-off valve (11) and allowing drainage of the accumulated aqueous phase. In the manual case, an operator periodically checks for the presence of an aqueous phase inside the storage tank (1), through a port (28) for inspecting the level. If such a phase were to accumulate, he opens a manual shut-off valve (29) so as to allow all the accumulated aqueous phase to drain away.

FIELD OF THE INVENTION 
The present invention relates to equipment for draining precipitated 
liquids, especially water, which collect at the bottom of a tank for 
storing liquids. The equipment of the present invention makes it possible 
to drain off the precipitated liquids which accumulate at the bottom of 
such a storage tank. 
PRIOR ART 
Storage tanks are often used in the petroleum industry and are essential to 
the functioning of an operational unit. They may, for example, be for 
storing crude oil, intermediate products or final products. 
When the product is being stored in the tank, it is normal for there to be 
undesirable liquids, generally an aqueous phase which is usually heavier 
than the product being stored, deposited at the bottom of the tanks. 
Delay in draining the aqueous phase out from the tank gives rise to a 
number of problems. There may be a loss in product storage capacity, due 
to the formation of a layer of sludge. There may be corrosion of the 
bottom caused by the accumulation of the aqueous phase. There may be an 
interruption of production operations in order to allow the storage tank 
to be cleaned or even the replacement of the damaged bottom of the tank. 
It may be necessary to re-paint the metal plates making up the bottom of 
the tank in order to protect them against corrosion. Also, there may be a 
need to install sacrificial anodes in the internal part of the bottom of 
the tank in order to provide protection against corrosion. 
It is worth pointing out that, before the bottom of the storage tanks can 
be painted, it is necessary to remove the corrosion with a sand jet or 
water jet, and then to apply several coats of special protective paint, 
which requires production to be interrupted for a number of days until the 
tank can be used once again. Such a painting operation is fairly costly. 
The present invention proposes the adoption of a system, for draining a 
tank for storing liquids, which may be automatic or manual and prevents 
the accumulation of an aqueous phase at the bottom of the tank. 
SUMMARY OF THE INVENTION 
A first aspect of the present invention provides a system for draining a 
liquid storage tank, which is intended to promote removal, from inside the 
storage tank, of denser phase which is deposited in the bottom of the tank 
below a less dense phase which is not to be drained, characterized in that 
it is operated automatically and comprises the following components: 
a) a drainage device to be fitted at the lowest level of said liquid 
storage tank, said drainage device comprising a concave body which ends in 
a seat, an interface float which sinks into the less dense phase and 
floats on the denser phase and thus prevents the formation of a vortex 
when the denser phase is being drained off, means actuated by the float 
for plugging the seat, and stop means which limits the travel of the 
interface float inside the drainage device but allows the passage of 
liquids; 
b) a drainage line connecting the seat of the drainage device to a drainage 
box which is connected to a drain line; 
c) a shut-off valve fitted in the drainage line and having an actuator 
which is intended to control the flow of liquid which passes to the 
drainage box; 
d) a pressure-equalization line connecting the concave body of the drainage 
device to the drainage box; 
e) a pressure-equalization valve fitted in the pressure-equalization line, 
said pressure-equalization valve having an actuator and functioning to 
promote the equalization of the pressures below and above the interface 
float after the plugging means has settled on the seat of the drainage 
device; and 
f) a sensor to be fitted on the side wall of the storage tank slightly 
above the level of the drainage device, and functioning to detect the 
presence of the denser phase and to emit a command signal to the actuator 
to open the shut-off valve for a predetermined period of time which makes 
it possible to drain off to the drainage box the denser phase which has 
accumulated in the drainage line; wherein, after a predetermined period of 
time during which the shut-off valve has remained closed, the sensor emits 
a command signal to the actuator to open the pressure-equalization valve 
for a predetermined period of time, which causes an equalization of the 
pressures to which the upper and lower parts of the interface float are 
subjected, enabling said interface float to float again on the denser 
phase, thereby initiating a new drainage flow. 
A second aspect of the invention provides a system for draining a liquid 
storage tank which is intended to promote removal, from inside the storage 
tank, of a denser phase which is deposited in the bottom of the tank below 
a less dense phase, characterized in that it is operated manually and 
comprises the following components: 
a) a drainage device to be fitted at the lowest level of said liquid 
storage tank, said drainage device comprising a concave body which ends in 
a seat, an interface float which sinks into the less dense phase and 
floats on the denser phase and thus prevents the formation of a vortex 
when denser phase is being drained off, means actuated by said interface 
float for plugging the seat, and stop means which limits the travel of the 
interface float inside the drainage device but allows the passage of 
liquids; 
b) a drainage line connecting the seat of the drainage device to a drainage 
box which is connected to a drain line; 
c) a first manual shut-off valve fitted in the drainage line and intended 
to control the flow of liquid which passes to the drainage box; 
d) a pressure-equalization line connecting the concave body of the drainage 
device to the drainage box; 
e) a second manual shut-off valve fitted in the pressure-equalization line, 
and operative to promote the equalization of the pressures below and above 
the interface float after the plugging means has settled on the seat of 
the drainage device; and 
f) a level gauge, for inspecting the liquid level in the tank, intended to 
be fitted in the bottom of the side wall of the storage tank slightly 
above the level of the drainage device, and operative to enable an 
operator to detect the presence of aqueous phase in the bottom of the 
storage tank, whereupon the operator can open the first manual shut-off 
valve and keep it open until the end of the drainage operation, close it 
at that end of the drainage operation, and subsequently open the second 
manual shut-off valve and keep it open for a predetermined period of time 
to equalize the pressures to which the top and bottom of the interface 
float are subjected, thereby enabling the said interface float to float 
again on the denser phase, thereby initiating a new drainage flow. 
This interface float is designed to sink into the product and to float on 
top of the denser phase underneath the product. As the level of the denser 
phase drops, the plug portion of the interface float settles onto the 
seat, stopping the drainage operation since all the accumulated denser 
phase has now been drained off. 
The drainage system may thus be manual or automatic. In the manual version, 
an operator periodically checks for the presence of an aqueous (denser) 
phase inside the storage tank via a port for inspecting the level. If such 
a phase were to accumulate, he opens a manual shut-off valve so as to 
allow all the accumulated aqueous (denser) phase to drain away. 
In the automatic version, use is made of a sensor for detecting the 
presence of an aqueous (denser) phase and commanding the process of 
opening a shut-off valve and allowing drainage of the accumulated denser 
phase. A sensor may be fitted in the drainage line to detect an undesired 
passage of product (less dense phase) and to command closing of the 
shut-off valve which controls drainage in response to detection of product 
passing the sensor.

DESCRIPTION OF THE INVENTION 
FIG. 1 shows a storage tank 1 equipped with an automatic system according 
to the present invention, for draining tanks for storing liquids. The tank 
1 has a side wall 6, a foundation ring 5, a floating roof 2, and a bottom 
3 having its centre lower than the edges and also a ramp 4 which makes 
drainage more efficient. Any product, generally a hydrocarbon, can be 
stored inside the storage tank 1 and the fact that the roof 2 floats on 
the product prevents any inflammable gaseous phase from occurring above 
the liquid surface. 
The system for draining liquid storage tanks may be used in any type of 
storage liquid tank, irrespective of the shape of the tank bottom; the 
type of tank mentioned above is chosen only to facilitate the description 
of the invention. 
A drainage device 30 is fitted at the lowest part of the ramp 4, as close 
as possible to the foundation ring 5 of the storage tank 1; this is the 
lowest level at which there can be a deposit of liquid. 
The drainage device 30, which may be seen better in FIG. 4, comprises a 
concave body 31 which may, for example, be frustoconical or polyhedral. 
Inside the concave body 31 there is an interface float 8 which has a plug 
portion 25 at the bottom. A guide 9 is fitted over the concave body 31 and 
its function is to serve as stop means to limit the upward travel of the 
interface float 8. The guide 9 may be constructed so as to allow the 
liquid to pass without difficulty to the inside of the drainage device 30. 
At the bottom of the concave body 31 is a seat 26 which can be sealingly 
engaged by the plug portion 25 of the interface float 8. A drainage line 
10 and a pressure-equalization line 23 are connected to the seat 26 of the 
drainage device 30. FIG. 1 shows that the drainage line 10 and the 
pressure-equalization line 23 extend beyond the foundation ring 5 of the 
storage tank 1 and end in the drainage box 13. A normally closed shut-off 
valve 11 having an actuator 11A is fitted in the drainage line 10, and a 
normally closed pressure-equalization valve 24, having an actuator 24A, is 
fitted in the pressure-equalization line 23. 
The bottom of, the drainage box 13 has an outlet to the discharge line 22 
through which the drained aqueous phase will drain off. It is expedient 
for the drainage box 13 to be as high as, or higher than, the useful 
height of the side walls of the containment basin 50 of the storage tank 1 
so as to ensure that there will be no undesirable communication between 
the containment basin and the discharge line 22. A drain line 20, having a 
shut-off valve 21, is fitted in the bottom of the drainage box 13. This 
line 20 makes it possible for any drainage of the containment basin wall 
to be carried out via the drainage box 13. To do this, it is sufficient to 
open the shut-off valve 21. The containment basin 50, which is not part of 
the invention, is the area limited by depressions or walls (dykes) on the 
ground intended to contain the fluids of the tank in case of a leakage. In 
the drawings only a small part of it is shown. 
A sensor 12 is fitted in the bottom of the side 6 of the storage tank 1 
slightly above the level of the drainage device 30. Its function is to 
detect the presence of an aqueous phase and to emit a command signal to 
the actuator 24A of the pressure-equalization valve 24 and to the actuator 
11A of the shut-off valve 11, as will be better understood below. 
Operation of the system for draining liquid storage tanks, according to the 
present invention, is fairly simple. The storage tank 1 normally receives 
the product which it is desired to store. If there should be any 
precipitation of aqueous phase, this tends to deposit in the bottom of the 
storage tank 1 because it is denser than the product. 
The interface float 8 is designed to float on the aqueous phase and to sink 
into the product thereabove. The drainage device 30 is located in the 
lower part of the storage tank 1, which results in the precipitated 
aqueous phase accumulating around it. When the interface float 8 floats on 
aqueous phase, the passage to the drainage line 10 is unobstructed, which 
makes it possible for the precipitated aqueous phase to pass through the 
line 10. This drained aqueous phase will be accumulating inside the 
drainage line 10 because the shut-off valve 11 is closed. 
As time passes, the level of the precipitated aqueous phase will rise until 
it reaches the sensor 12 which, on detecting the presence of an aqueous 
phase, immediately emits a signal to the actuator 11A to command the 
opening of the shut-off valve 11 for a preestablished period of time 
sufficient to allow all the aqueous phase to drain off to the inside of 
the drainage line 10. The presence of the interface float 8 prevents the 
formation of a vortex inside the drainage device 30, which could wrongly 
give rise to drainage of product even though there might still be 
remaining aqueous phase to be drained off. 
As the level of the aqueous phase inside the drainage device 30 drops, the 
interface float 8 will also drop, and its plug portion 25 approaches the 
seat 26. When all the aqueous phase has been drained off, the plug portion 
25 will settle on the seat 26, closing the passage and preventing the 
outflow of product via the drainage line 10, thereby completing the 
drainage process. The pre-established period during for the shut-off valve 
11 to remain open will then end, and the valve again closes. 
As soon as more of the aqueous phase accumulates around the drainage device 
30 the interface float 8 would tend to float again, due to its buoyancy, 
but meanwhile, owing to the action of the static head of product acting on 
the top of the interface float 8, such floating does not occur 
immediately. 
To avoid this problem, it is necessary to equalize the pressures above and 
below the interface float 8. In order that such a pressure equalization 
may occur, after the predetermined period of time during which the 
shut-off valve 11 is closed the sensor 12 emits a signal to the actuator 
24A to command opening of the pressure-equalization valve 24 for a 
predetermined period of time which is sufficient for pressure equalization 
to occur, after which the pressure-equalization valve 24 closes once 
again. At this point, the interface float 8 again floats on the aqueous 
phase, initiating a new drainage cycle. 
FIG. 2 shows a variation of the system, according to the present invention, 
for draining tanks for storing liquids. With the aim of providing a safer 
drainage operation, a sensor 17 is fitted in the drainage line 10. Its 
function is to detect an undesired passage of product through the drainage 
line 10, which would signify an operational failure of the drainage device 
30. In this case, the sensor 17 would emit a command signal to the 
actuator 11A, which would immediately close the shut-off valve 11, thereby 
interrupting an undesired flow of product to the drainage box 13. The 
sensor 17 may also send a signal to a remote panel installed in a control 
room, for example, where some type of alarm would alert the operator in 
charge to the problem so that the operator can immediately take the 
necessary measures to rectify it. 
Various types of sensor may be used to function as the sensor 12. It is 
possible, for example, to use a liquid level switch, a conductivity sensor 
or a capacitive sensor. 
Similarly, use may also be made of various types of sensor for the sensor 
17. Purely by way of example, use may be made of a conductivity sensor or 
a capacitive sensor. 
It is unnecessary to state that the types of sensor mentioned in the above 
two paragraphs are merely illustrative examples and may not in any way be 
regarded as limiting the present invention. 
The drainage system for tanks for storing liquids may also be operated 
manually. To this end, as shown in FIG. 3, a liquid level gauge 28 for 
inspecting the level may be fitted instead of the sensor 12, a manual 
shut-off valve 29 instead of the actuated shutoff valve 11, and a manual 
shut-off valve 27 instead of the pressure-equalization valve 24. 
The system would then be operated by an operator who would periodically 
check for the presence of an aqueous phase in the level indicator. If such 
a phase were to accumulate, it would signify the need to drain off the 
aqueous phase that had accumulated in the bottom of the tank. To this end, 
it would suffice to open the manual shut-off valve 29, which would make it 
possible for all the aqueous phase to drain off to the drainage box 13. 
The drainage device 30 would operate exactly as described above, enabling 
all the accumulated aqueous phase to drain off. 
When the operator detects the fact that there is no more aqueous phase to 
drain off to the drainage box 13, he would next close the manual shut-off 
valve 29 and would then open the manual shut-off valve 27 for a 
preestablished period of time, with the aim of equalizing the pressures 
above and below the interface float 8, enabling the float to float again 
on the accumulated aqueous phase, thereby initiating a new drainage cycle. 
It is preferable that the plug 25 is formed on the interface float 8, but 
it is also possible that the float actuates an external plug, and the 
aforesaid description should not be taken as a limitation of the 
invention.