GB2026859A - Method and apparatus for storing a volatile liquid - Google Patents

Abstract

In order to reduce evaporative loss of petrol (or other volatile liquid) from a closed storage tank 2, the tank 2 is provided with a gas e.g. nitrogen supply pipeline 20 and a pressure relief valve 44. The pipeline 20 has in it a series of three pressure reducing (or demand) valves 22, 24 and 26 which reduce the supply pressure of the nitrogen in stages to, say, 18 inches water gauge. The pressure relief valve is set to open at a pressure above the reduced supply pressure, and typically, at a pressure of, say, 20 inches water gauge. <IMAGE>

Description

SPECIFICATION Storage method and apparatus This invention relates to a method of and apparatus for storage of a volatile liquid, particularly a volatile organic liquid.

To store in bulk at ambient temperature a volatile organic liquid such as petrol, a large closed tank having a pressure/vacuum flap valve is customarily used. The flap valve is adapted to open when the vapour pressure in the ullage space of the vessel exceeds the atmospheric pressure by a chosen amount and is thus able to relieve the excess vapour pressure. Such an excess vapour pressure in the ullage space may, for example, be created when filling the tank or may arise as a result of an increase in the ambient temperature. The flap valve is also adapted to open in the opposite direction when the pressure in the ullage space falls below atmospheric so as to allow ambient air to be drawn into the ullage space to restore the pressure.Such a belowatmospheric pressure may, for example, be created when some organic liquid is discharged from the tank or as a result of the fall in the ambient temperature.

We have found that there is a very considerable loss of the organic liquid to the atmosphere as a result of repeated drawing-in of air and repeated venting of air-vapour mixture to the atmosphere. We have also found that, surprisingly, such loss of organic liquid can be reduced by relatively simple means.

According to the present invention, there is provided apparatus for storing a volatile liquid in a closed vessel at ambient temperature, which apparatus includes a gas supply pipeline communicating with the interior of the vessel; a pressure regulating (or demand) valve set to open when the pressure in the ullage space of the vessel is below a chosen minimum, the chosen minimum pressure being discretely above atmospheric pressure, and a separate pressure relief valve set to open when the pressure in the ullage space of the vessel is above a chosen maximum.

The invention also provides a method of storing a volatile liquid in a closed vessel at ambient temperature, in which method a gas is supplied to the ullage space of the vessel through a pressure regulating (or demand) valve when the pressure in the ullage space falls below a chosen minimum, the chosen minimum pressure being discretely above atmospheric pressure, and vapour/gas mixture is vented from the ullage space through a separate pressure relief valve when the pressure in the ullage space of the vessel is above a chosen maximum.

It is possible to use air as the aforesaid gas.

However, this is undesirable from the point of view of safety. Preferably, the gas is one which does not react chemically with the volatile liquid or with any impurities, such as water, in the liquid. A reactive gas would be undesirable as it would cause loss of product, further formation of impurities and possibly corrosion of the tank.

Typically, nitrogen will be the preferred gas as it does not support combustion and does not react with most organic liquids. Nitrogen may also be supplied conveniently at an above-atmospheric pressure. Typically, the source of nitrogen may be a container in which the nitrogen is stored in liquid state. Vaporisation of the liquid raises the necessary supply pressure. Alternatively, the nitrogen may be supplied under pressure from cylinders.

The method and apparatus according to the invention are particularly suited for use in the storage of the volatile organic liquid. In particular, the method and apparatus according to the invention may be used in the storage of petrol in bulk containers. The invention is not, however, restricted to the storage of petrol (gasoline). It may also be used, for example, in the storage of organic liquid such as acetone, methanol, ethanol, xylene, benzene and methyl ketone, to give a few examples of the very many volatile organic substances than can be stored in accordance with the invention.

An existing storage tank or vessel may readiiy be converted so as to be able to operate in the method according to the invention. As previously mentioned, such a conventional vessel will typically have a pressure/vacuum flap valve, In addition, it will typically have a manhole to allow access to be gained to the interior of the vessel. Typically, such a vessel or tank may be converted by sealing off the outlet in which the flap valve is situated, and leading the gas supply pipeline through a cover for the manhole. In addition, a pipe having the pressure relief valve disposed therein may be fitted to the cover of the manhole.

The maximum pressure may be chosen to be just below the maximum that the vessel or tank can withstand. Typically, for a conventional bulk storage tank this pressure will be 20 inches water gauge.

The chosen minimum pressure is preferably at least 10 inches water gauge above atmospheric pressure. Typically, it is about 2 inches water gauge below the chosen maximum pressure. Thus, if the chosen maximum pressure is 20 inches water gauge, the chosen minimum pressure is preferably 18 inches water gauge. We have found that the higherthe chosen minimum pressure the greater is the suppression of vaporisation of the volatile liquid during normal storage. Thus, if the temperature rises or if the tank receives a further batch of the liquid being stored, and the pressure rises, most of the fluid vented from the ullage space through the pressure relief valve will be the chosen gas. The method and apparatus according to the invention are particularly suited for storing liquids in bulk storage tanks.By the term "bulk storage tank" as used herein, is meant a tank which has a capacity of over 100tonnes.

Typically, the bulk storage tank may have a capacity of over 1000tonnes.

The gas supply pipeline may typically have more than one pressure reducing (or demand) valve connected in series. This enables the pressure to be reduced in steps from the supply pressure. Typically, the gas supply line will also be fitted with a non-return valve downstream of the final (most downstream) pressure regulating (or demand) valve.

The or each pressure regulating (or demand) valve and the pressure-relief valve may be of conventional design with appropriately spring-biased or springloaded valve members to enable the valves to close or open, as the case may be, at the required pressures.

The method and apparatus according to the invention will now be further described by way of example with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram illustrating a conventional tankforthe bulk storage of petrol which has been adapted to perform the method according to the invention; and Figure 2 is a schematic diagram illustrating a modification of the apparatus shown in Figure 1.

Referring to Figure 1 of the drawings, a bulk storage tank 2 has a sloping roof 4 which has a central outlet 6 at is apex. Originally, the outlet 6 contained a flap valve (not shown). However, as shown in the drawing, the outlet is now sealed off by a closure 8.

The tank is shown containing a volume 10 of petrol.

In the sloping roof 4 of the tank 2 there is a manhole 12 fitted with a cover 14. There is also a port 16 in the sloping roof, which port 16 may be opened to allow a dipstick to be inserted into the interior of the tank therethrough so as to measure the volume of liquid into the tank.

A gas supply pipeline 20 has one end inserted in the manhole cover 14 so as to communicate with the ullage space 18 of the tank 2. The other end of the pipeline 20 communicates with a source (not shown) of nitrogen under pressure. In the pipeline 20 are disposed three pressure regulating (or demand) valves 22, 24 and 26. These valves are in series with one another. Downstream of the valve 26 is a non-return valve 28. A by-pass pipeline 30 communicates at one end with a part of the pipeline 20 upstream of the valve 22 and at its other end with a part of the pipeline 20 downstream of the non-return valve 28. In the pipeline 20 are disposed a pressure reducing valve 32 and a non-return valve 34.Downstream of the union between the pipeline 20 and the downstream end (ie. the end communicating with the pipeline 20 downstream of the non-return valve 28) of the pipeline 30 is a pipe 36 having disposed in zit a pressure relief valve 38.

Immediately downstream of each pressure regulating (or demand) valve is a pressure gauge 40. In addition, a pressure gauge 40 is upstream of the first (almost upstream) pressure regulating valve, and another such gauge 40 is upstream of the union of the upstream end of the pipeline 30 with the pipeline 20. Also communicating with the ullage space 18 through the manhole cover 14 is a pipeline 42 having a pressure relief valve 44 disposed therein.

Typically, the valves may be set as follows: if the supply pressure of gaseous nitrogen to the pipeline 20 is 150 psig, the valve 22 may be set so as to open when the pressure on its downstream side is less than 50 psig. The valve 24 may be set to open when the pressure on its downstream side is less than 3 psig. The valve 26 may be set to open when the pressure in its downstream side is 18 inches water gauge or less. This pressure will, or course, be equal to the pressure in the ullage space 16 of the tank 2.

The pressure relief valve 44 may be set to open at a pressure of 20 inches water gauge. The minimum pressure in the ullage space 16 will be maintained at or very close to 18 inches water gauge. If the pressure in the ullage space 16 rises to 20 inches water gauge the pressure relief valve 44 will open and some of the gas-vapour mixture in the ullage space will be maintained between 18 and 20 inches water gauge. If the tank cannot withstand a pressure as high as 20 inches water gauge the valves may be arranged to help the pressure in the ullage space between lower valves.

The pressure relief valve 38 is provided as a precautionary measure to ensure that there is no risk of a dangerous pressure being built up in the ullage space 18 in the event of the pressure relief valve 44 failing.

The non-return valve 28 is provided to prevent nitrogen-petrol vapour mixture passing back through the pipeline 20 to that region thereof where the valve 22,24 and 26 are located.

The by-pass pipeline 30 is provided as a precautionary measure in the event of either of the valves 24 and 26 failing. The valve 32 is set to open should a pressure below atmospheric occur on its downstream side. The non-return valve 34 prevents nitrogen-petrol vapour mixture from reaching the valve 32.

The pressure gauges 40 may be observed from time to time in order to check the valve 22,24 and 26 are functioning properly.

Typically, in order to prepare the tank 2 for operation in accordance with the method of the invention, the closure 8 will be fitted to the outlet 6 and the pipelines 20,30 and 42 with their associated valves and gauges installed. The connections to the source of nitrogen are made, and nitrogen is caused to flow into the ullage space 18 of the vessel each time pressure therein falls below 18 inches water gauge and nitrogen-petrol vapour will be vented each time the pressure in the ullage space reaches 20 inches water gauge.

Instead of including the valve 32 in the pipeline 30, a series of pressure reducing or regulating valves identical to the series of valves 22, 24 and 26 may be included with a pressure gauge 40 downstream of each valve. Such an arrangement is shown in Figure 2, the additional valves being indicated by the references 52, 54 and 56. The sets of valves 22, 24 and 26, and 52,54 and 56 can be of chosen dimensions such that in the event of failure of one of the valves in one set, the valves in the other can cope with the total demand for nitrogen, while during normal separation each set will pass half the demand at any one time. It is alternatively possible that one train could be set to operate in a slightly lower pressure range (eg. 16 to 18 inches water gauge) than the other. We have found that a tankforthe type shown in the drawing, before having the closure 8 fitted, and the pipelines, valves and gauges provided, lost 8000 gallons of petrol in a month owing to evaporation. The tank had a capacity of 1400 tonnes. The tank was then converted for operation in accordance with the invention, the chosen maximum pressure being 20 inches water gauge and the chosen minimum pressure being 18 inches water gauge. It was found that in a period of a month operation in accordance with the invention reduced the evaporative loss of petrol to 3000 gallons.

Claims (17)

1. Apparatus for storing a volatile liquid in a closed vessel at ambient temperature, which apparatus includes a gas supply pipeline communicating with the interior of the vessel; a pressure regulating (or demand) valve set to open when the pressure in the ullage space of the vessel is below a chosen minimum, the chosen minimum pressure being discretely above atmospheric pressure, and a separate pressure relief valve set to open when the pressure in the ullage space of the vessel is above a chosen maximum.

2. Apparatus as claimed in claim 1, in which the vessel is a bulk storage tank.

3. Apparatus as claimed in claim 2, in which the tank has a capacity of over 1000 tonnes.

4. Apparatus as claimed in any of the preceding claims, in which the gas supply pipeline has more than one pressure reducing (or demand) valve connected in series.

5. Apparatus as claimed in claim 4, in which the gas supply pipeline has in it a non-return valve downstream of all the pressure regulating (or demand) valves.

6. Apparatus for storing a volatile liquid substantially as herein described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings.

7. A method of storing a volatile liquid in a closed vessel at ambient temperature, in which method a gas is supplied to the ullage space of the vessel through a pressure regulating (or demand) valve when the pressure in the ullage space falls below a chosen minimum, the chosen minimum pressure being discretely above atmospheric pressure, and vapour/gas mixture is vented from the ullage space through a separate pressure relief valve when the pressure in the ullage space of the vessel is above a chosen maximum.

8. A method as claimed in claim 7, in which the gas is one which does not react chemically with the volatile liquid or with any impurities, such as water, in the liquid.

9. A method as claimed in claim 8, in which the gas is nitrogen.

10. A method as claimed in any of claims 7 to 9, in which the volatile liquid is organic.

11. A method as claimed in claim 10, in which the organic liquid is petrol.

12. A method as claimed in claim 10, in which the organic liquid is acetone, methanol, ethanol, xylene, benzene, or methyl kentone.

13. A method as claimed in any of claims 7 to 12, in which the chosen minimum pressure is at least 10 inches water gauge above atmospheric pressure.

14. A method as claimed in claim 13, in which the chosen minimum pressure is about 2 inches water gauge below the chosen maximum pressure.

15. A method as claimed in any of claims 7 to 14, in which the vessel is a bulk storage tank.

16. A method as claimed in claim 15, in which the tank has a capacity of over 1000 tons.

17. A method of storing a voltile liquid, substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.