WO2002095283A1

WO2002095283A1 - Liquefied gas tank with heat exchange device

Info

Publication number: WO2002095283A1
Application number: PCT/EP2002/003448
Authority: WO
Inventors: Vera Fogliato
Original assignee: Selen S.R.L.
Priority date: 2001-03-30
Filing date: 2002-03-27
Publication date: 2002-11-28
Also published as: ITTO20010300A0; ITTO20010300A1

Abstract

A tank for liquefied gas, in particular LPG, includes a body (2) having a heat exchange device (25) arranged within the body (2) near the bottom (29) of the tank (1). The heat exchange device (25) includes a tubular casing (24) for containing a service fluid and auxiliary piping (32, 34) with a heat transfer fluid circulating therein for warming the gas contained in the tank and arranged inside the tubular casing. The tubular casing (24) includes at least one pair of side-by-side branches (26, 28) each with an aperture (20a, 22a) for accessing the inside of the casing and joined to each other at one end, and the auxiliary piping includes at least one pair of branches (32, 34), for delivery and return respectively and joined together at one end, each branch (32, 34) being arranged inside a corresponding branch (26, 28) of the tubular casing.

Description

LIQUEFIED GAS TANK WITH HEAT EXCHANGE DEVICE

The present invention relates to a tank for liquefied gas, in particular for LPG.

Tanks for liquefied gas are generally used, for example, to store inflammable gas for industrial use and include a structure made of electrically welded sheet steel elements defining a chamber for containing pressurized liquefied gas. Accessory equipment is usually associated with the tank, for example for refilling the tank with gas, for supplying gas to utilities, and safety equipment.

In particular, after the tank has been refilled with gas, around 80% of its storage volume contains gas in liquid phase while the remaining 20% is filled with gas in vapour phase. ^•

Utilities are always supplied with gas in its vapour phase which can be drawn directly from the tank.

However, when gas is drawn off, the pressure within the tank falls which inevitably causes part of the remaining liquid- phase gas to pass into vapour phase, thereby expanding and causing the contents of the tank to cool. This cooling can cause the gas to freeze, a physical process which, if repeated over a long period, can cause cracks to form in the welding, a problem which is made all the more dangerous for not being immediately apparent since gas tanks are obliged by law to be buried underground.

This phenomenon is even more critical in industrial applications, where the flow of gas from the tank per unit time unit may vary considerably in dependence on the requirements of the utilities, and is made all the more serious by the fact that the tanks are buried and therefore not warmed by the sun.

For the reasons explained above, tanks for liquefied gas are normally fitted with a forced evaporation device, warmed by hot water supplied from a boiler at a temperature of around

70÷80²C, or by electrical resistances. Safety regulations require such conventional evaporation devices to be securely insulated from the gas contained in the tank.

The liquid phase of the gas drawn from the tank is delivered to the evaporation device, generally arranged separate from the tank, and the gas is evaporated in it.

However this location of the evaporation device separate from the tank makes the whole installation more complex and more expensive.

In order to overcome these disadvantages, Italian patent N. 1 300 768 provides a tank which includes a heat exchange device inside the body, near the bottom of the tank, this heat exchange device comprising an elongate tubular casing with a heat exchange fluid circulating therein for warming the gas in the tank.

The evaporation device described in the aforesaid patent includes a pair of pipe branches for the circulation of the heat-exchange fluid arranged alongside each other, connected by means of an elbow joint, inside a tubular casing containing a non-inflammable diathermic auxiliary fluid. This equipment does not allow a sufficient exchange of heat and is shaped so as to occupy the bottom of the tank, making access difficult if it is necessary to insert a probe, or during cleaning and maintenance operations. In addition, the structure of the tubular casing makes it impossible to perform in situ draining operations at the inside of said casing. Furthermore, various problems could occur such as, for example, the heat exchange fluid freezing because of a stopping of the boiler, leading to occlusions in the pipes containing the heat exchange fluid, especially at the elbow joint portion. It is difficult to remove these occlusions by conventional means owing to the configuration of the pipes inside the tubular casing.

The object of the present invention is to provide a tank for liquefied gas, in particular LPG, which, while fully complying with safety regulations, includes gas-warming equipment designed to overcome the problems listed above and also provides a relatively inexpensive tubular casing.

This object is achieved according to the invention by providing a tank which includes a body having a heat exchange device arranged therein, near the bottom of the tank, this heat exchange device including a tubular casing for containing a service fluid and auxiliary piping, arranged within the tubular casing, for the circulation of a heat- transfer fluid for warming the gas contained in the tank, characterised in that the tubular casing includes at least one pair of side-by-side branches, joined at one end and both having respective, adjacent access openings outside the tank, and in that the auxiliary piping includes at least a pair of branches, for delivery and return flow respectively, joined together at one end and each arranged inside a corresponding branch of the tubular casing.

Thanks to this arrangement, the tank of the invention enables twice the heat exchanging power to be obtained, thereby reducing the consumption of energy required to warm the gas . In addition, it is possible to locate all the gas control instruments on the bottom of the tank as well, and to gain access to it for cleaning or maintenance operations. Furthermore, the tank of the invention makes it possible to carry out any draining operations on the service fluid in the tubular casing in situ, without needing to destroy and dig up the tank and take it to a suitable maintenance site. Also, since the two branches of the auxiliary piping are not constrained within the same portion of tubular casing, it is possible to join them, for example by a curved connector portion, making it easier to remove any occlusions with conventional equipment known to a person skilled in the art.

An additional object of the invention is to provide an apparatus which includes the aforesaid tank.

Further characteristics and advantages of the present invention will become apparent from reading the detailed description which follows, with reference to the appended drawings which are provided purely by way of non-limitative example, and in which:

Figure 1 is a partially sectioned side elevation view of a tank of the invention;

Figure 2 is a front elevation view of the tank of Figure 1, sectioned along the line II-II, and

Figure 3 is a plan view of the tank of Figure 1, sectioned along the line III-III. With reference to the drawings, a tank for liquefied gas under pressure, in particular for PG or propane, is indicated 1. The body 2 of the tank 1 is constructed, in a manner known per se, of sheet steel, electrically welded so as to form a cylindrical central body, with a pair of substantially hemispherical caps 4 fixed to either end thereof .

The body 2 has an upper portion 6, intended to be positioned at the top when the tank 1 is installed, extending along the axis of the cylindrical section of the body 2 and having associated fittings known per se. These fittings include, for example, a main inspection aperture 10, fitted with appropriate devices such as a pressure gauge 10a and a thermometer 10b, for measuring the pressure and temperature of the gas, a duct 12 housing a ball cock device indicating the level of the liquid phase of the gas, a pipe union 14 for drawing off the vapour phase of the gas, a filling duct 17 and a device for measuring the density 18 of the liquid phase of the gas. Valve assemblies with the appropriate safety devices demanded by law are associated with the various fittings. These devices extend upwardly from the upper portion 6 of the tank 1 in such a way that they project from the ground when the tank 1 is buried.

A pair of adjacent, outwardly projecting bodies 20, 22 are fixed to the top 6 of the body 2 so as to form the outside ends of a tubular casing 24 which is inside the body 2 and is made of electrically-welded steel sheet analogously to the body 2, so as to make it strong and able effectively to insulate its internal cavity from the gas contained in the tank 1. The casing 24 houses a heat exchange device of the tank 1, generally indicated by the numeral reference 25 and provided to maintain the liquid phase of the gas held in the tank 1 at a substantially constant predetermined temperature.

The casing 24 includes a pair of side-by-side branches 26, 28 having respective substantially rectilinear first and second portion 26a, 28a which extend near the bottom 29 of the body 2 on opposite sides with regard to a vertical plane containing the general axis of the body 2, for example along a direction parallel to the said axis. The branches 26, 28 are connected to the projecting bodies 20 and 22 respectively and thereby to first and second access apertures 20a and 22a, located in the ends of these projecting bodies, while the other ends of the two bodies are joined together. Under normal operating conditions, appropriate lids close the apertures 20a, 22a. The casing 24 acts as a jacket, containing the auxiliary piping for the circulation of a heat-transfer fluid, usually water and glycol, as will be seen better below.

The auxiliary piping preferably includes a pair of delivery and return branches 32 and 34, arranged within the branches 26 and 28 respectively of the tubular casing 24.

Each of the branches 32, 34 includes an end portion 32a, 34a leading into and out of the body 2 respectively and protruding from the corresponding projecting body 20, 22 for connection with a separate apparatus for heating the heat transfer fluid.

A service fluid of a diathermic, non-inflammable type is interposed between the casing 24 and the auxiliary piping in order to improve heat transmission from the branches 32, 34 to the casing 24 and thence to the gas contained in the body 2. The service fluid can be constituted, for example, by a glycol-based liquid, preferably a mixture of water and glycol. Under normal operating conditions this service fluid is static.

It will be clear to an expert in the art that the whole casing is both simple and relatively inexpensive to manufacture and makes it easier to drain the service fluid in situ through the apertures 20a, 22a. This draining operation is carried out during maintenance operations according to known methods, taking advantage of the tubular configuration of the casing 24.

In order to improve heat exchange between the heat-transfer fluid circulating in the auxiliary piping and the gas contained in the body 2, it is convenient if the branches 32, 34 and/or the portions 26a, 28a of the casing 24 have fins (not shown) .

In an installation, which is not shown, which utilizes gas stored in the tank, the intake and outlet portions 32a, 34a of the auxiliary piping of the device 25 can be connected to a boiler, arranged separate from the tank 1 for supplying the device 25 with hot water at a temperature of around 50÷70²C. The boiler can be conveniently fuelled by gas drawn from the tank 1. A control assembly can be arranged next to the boiler which includes, in per se known manner, a compressor, a safety control unit and any other auxiliary equipment required. In order to comply with safety regulations, the boiler and the assembly must be located at a particular distance from the tank 1.

Claims

1. A tank for liquefied gas, in particular LPG, includes a body (2) with an associated heat exchange device (25) arranged inside the body (2) near the bottom (29) of the tank (1) , the heat exchange device (25) including: a tubular casing (24) for containing a service fluid, and an auxiliary piping (32, 34) with a heat transfer fluid circulating therein for warming the gas contained in the tank, arranged inside the tubular casing, characterised in that the tubular casing (24) includes at least one pair of side-by-side branches (26, 28) each having an aperture (20a, 22a) for accessing the inside of the casing and being joined together at one end, and in that the auxiliary piping includes at least one pair of branches (32, 34) for delivery and return respectively and joined together at one end, each branch (32, 34) being arranged inside a corresponding branch (26, 28) of the tubular casing.

2. A tank according to Claim 1, characterised in that the body (2) has a generally cylindrical shape and in that the side-by-side branches (26, 28) of the tubular casing extend either side of a vertical plane containing the general axis of the body (2) .

3. A tank according to Claim 1 or Claim 2, characterised in that the tubular casing (24) has at least one pair of bodies (20, 22) projecting from the body (2) of the tank, the said projecting bodies being connected respectively to the said branches (26, 28) and each having an aperture (20a, 22a) for accessing the inside of the casing (24) .

4. A tank according to any preceding Claim, characterised in that at least the outside surface of at least a bottom portion (26a, 28a) of the branches (26, 28) of the tubular casing (24) has fins.

5. A tank according to any preceding Claim, characterised in that the auxiliary piping (32, 34) has external fins.

6. An apparatus for distributing gas from a tank for liquefied gas, in particular LPG, characterised in that it includes a tank (1) according to any Claim from 1 to 5.