System and method for discharging a fluid

The present invention relates to a system and a method for discharging a fluid from a fluid container in environments where there is a risk of explosions and/or fire (so- called Ex Zone). At plants where fluids are stored which are inflammable and/or explosive, it is necessary to refill the tanks in which the explosive fluid is stored with a gas which mainly contains inert gas, when the fluid is being emptied out. The percentage of inert gas is stipulated in regulations and may, for example, be 95%, which will subsequently be given as a threshold value for the percentage of inert gas in the gas used to refill the tanks which are emptied of explosive fluid.

From the prior art within this field patent applications DE 3426409 Al, EP 369144 A2 and WO 96/34206 Al may be mentioned. In these publications pump systems are disclosed for emptying fluid containers with explosive material. From, these publications, however, it is not known to provide the pump systems with an operating gas generator and an operating gas reservoir with gas pipes to the pump motor.

Rules have been established according to IMO and classification societies confirming that the inert gas should be delivered with 25% overcapacity in relation to the capacity of the discharge pumps. It is also regulated that discharge pumps cannot operate unless the inert gas generator is fully operative. These two technical units are therefore dependent on each other in order to be able to be operated.

A discharge pump is usually mounted in the bottom of the tank and driven by a hydraulic motor which drives the actual pump/impeller. The hydraulic pressure is established via electric motors located in a safe area outside the tank. The discharging operation can thereby be carried out without situations arising where electrical impulses could occur, with the risk of explosions, which could be the case if the impeller/pump is driven directly by, for example, an electric motor.

A hydraulically operated pump of this kind typically consumes approximately 525 KW with a capacity of 1200 m3/h discharge capacity. The IMO requirement then stipulates a nitrogen generator with a capacity of 1200 x 1.25 = 1500 Nm3/h. A typical inert gas generator of this kind (for example a nitrogen generator) consumes approximately the same amount of energy; 520 KW in order to produce this amount of nitrogen.

These 1500 Nm3/h with the prescribed 95% nitrogen are then produced at a typical pressure of 12 bar. With today's solutions this pressure is reduced by the gas being expanded directly in a tank or via a pressure reduction valve before being passed

down into the tanks at approximately atmospheric pressure. The inert gas replaces the volume corresponding to the volume of the load which is pumped out.

In this way a safe atmosphere, which cannot ignite, is maintained in the tank. At the same time this is an energy-demanding process. Since present day systems to a great extent require nitrogen to be produced while the pumps that are pumping explosive fluid out of the tanks are also in operation, a relatively large supply of energy is required when the tanks are emptied. For example, on vessels transporting explosive fluid, the supply of energy is limited, and it is therefore desirable for the process to be made as energy-efficient as possible and for the equipment, particularly the energy-producing equipment, to be utilised in the best and most efficient manner.

Thus it is an object of the invention to provide a method and a system for emptying containers with explosive and inflammable fluid which provides a better energy utilisation than known systems, while making better use of the energy-producing equipment available. This is achieved according to the present invention as it is defined in the independent claims 1 and 7. Further embodiments of the invention are indicated in the associated, dependent claims 2-6 and 8-13.

According to the present invention, therefore, a method is provided for discharging an explosive fluid from a fluid container where a pump is employed for pumping the fluid out of the fluid container, an operating gas reservoir which is provided on the outside of the fluid container and an operating gas generator for production of an operating gas, where according to the method:

- a pump motor is provided in or outside the fluid container for operating the pump, which pump motor is driven by a pressurised operating gas containing a percentage of inert gas which is equal to or higher than a given threshold percentage,

- for operation of the pump motor, the pump motor is supplied with operating gas from the operating gas reservoir or from the operating gas generator, or it is supplied with a mixture from the operating gas reservoir and the operating gas generator, - after having expanded in the pump motor, the operating gas is released into the fluid container in order thereby to maintain a substantially inert atmosphere in the fluid container.

The operating gas is preferably produced from pressurised air, where the air is produced in a booster unit. For this purpose the booster unit comprises a compressor. Furthermore, the booster unit and the operating gas generator are preferably interconnected by means of one or more gas pipes.

In an embodiment of the method, during periods when the pump and the pump motor are not in use, the operating gas generator will produce high-purity operating

gas containing a percentage of inert gas which exceeds the given threshold percentage. The high-purity operating gas is stored in the operating gas reservoir for use when the fluid in the fluid container has to be pumped out. The percentage of inert gas produced and stored in the operating gas generator may be 99% and more. The pressure in the operating gas reservoir may also be increased beyond the 12 bar attained by the operating gas, in order thereby to store more energy for use in pumping the fluid in the fluid container. This can easily be achieved by using a booster pump mounted in front of the operating gas reservoir.

When pumping fluid from the fluid container, high-purity operating gas from the operating gas reservoir may be mixed with operating gas from the operating gas generator containing a percentage of inert gas which is below the given threshold value, with the result that the mixture fed into the pump motor at least contains a percentage of inert gas which is equal to or higher than the threshold value. This is an advantageous way of exploiting the energy-producing equipment since it is substantially less energy-demanding to produce operating gas with a percentage of inert gas well below 95%, such as for example 90%. Total available energy for production of a sufficient amount of operating gas containing a percentage of inert gas which is equal to or above the threshold value (95%) and for operation of pump motors therefore does not need to be so great as would be the case if all the operating gas with a content of inert gas of 95% or more were to be produced at the same time as the fluid motors are fully operative. By means of the present invention, therefore, the energy-producing equipment can deliver a larger amount of operating gas containing a percentage of inert gas which is at least equal to the threshold value (95%) compared to known systems. This means that when fluid in the fluid containers has to be pumped out, a greater part of the available energy can be used to pump out the fluid than is possible today. In other words, the fluid containers will be able to be emptied more quickly. Alternatively, if it is not important to reduce the time spent on emptying the fluid containers, energy- producing equipment can be employed with less capacity than that required for today's known systems.

In a further embodiment the operating gas's pressure is increased before it is admitted to the pump motor. This can be easily done by means of a compressor, which may be placed, for example, before the gas reservoir in order to store more energy or possibly before the operating gas is introduced into the pump motor. A further energy-saving feature of the present invention is to let the fluid pumped out of the fluid container be heated in a second heat exchanger, where the heat employed for the heating process is obtained, by means of a first heat exchanger, from waste heat given off in a booster unit for air. The two heat exchangers are interconnected by means of a pipe system of a type which will be well-known to a person skilled in the art.

A system is also provided for emptying a fluid container wherein an explosive fluid is stored, where the system, in addition to the fluid container, further comprises an operating gas generator for production of an operating gas, an operating gas reservoir for storing pressurised operating gas, at least one pump for pumping out the fluid stored in the fluid container and at least one pump motor for driving the at least one pump. The at least one pump motor is connected to the operating gas generator and the operating gas reservoir via gas pipes, thereby enabling pressurised operating gas to be supplied to the pump motor from the operating gas reservoir, from the operating gas generator or a mixture from both sources, where the operating gas supplied to the pump motor contains a percentage of inert gas which is at least equal to a given threshold percentage, and the system further comprises means for enabling the operating gas which has expanded in the pump motor to be released into the fluid container, thereby maintaining a substantially inert atmosphere in the fluid container. The inert gas which is produced and used in the system will normally be air with a higher percentage of nitrogen than in ordinary air, although the use of other inert gases may also be envisaged. The requirement is for the operating gas to contain a sufficiently large percentage of inert gas to create and maintain an inert atmosphere in the fluid containers in order to avoid the risk of explosion when the fluid containers are emptied. The minimum value for this percentage is stipulated in regulations, where, for example, the operating gas used will be required to contain at least 95% nitrogen if nitrogen is used as inert gas. As mentioned above, we have called this the threshold value for the minimum permitted percentage of nitrogen in the operating gas according to public regulations. In an embodiment of the invention the system further comprises:

- a discharge pipe out through which the fluid in the fluid container is pumped,

- a first heat exchanger which is arranged so that it can exchange waste heat given off by a booster unit for air,

- a second heat exchanger which is arranged so that it can exchange heat with the fluid in the discharge pipe, where the first and the second heat exchanger are connected with fluid lines, thus enabling the heat given off by the booster unit to be used to heat the fluid in the discharge pipe.

In a further embodiment of the invention a return pipe is connected between the discharge pipe and the fluid container, thus enabling heated fluid to be returned to the fluid container for heating remaining fluid in the fluid container.

Thus heat which would otherwise have gone to waste is used for heating the fluid that has to be pumped out and the heated fluid can, if so desired, be used for heating up the remaining fluid in the fluid container before it is pumped out. The system is

provided with means for controlling the fluid. These means may, for example, be different types of valve devices which will be well known to a person skilled in the art.

The operating gas generator for production of operating gas is preferably connected to the booster unit for air by means of a gas pipe, thereby enabling the operating gas generator to be supplied with pressurised air for production of operating gas. The operating gas generator preferably comprises a membrane separator or a PSA unit.

The system preferably further comprises gas pipes, thus enabling operating gas, which is produced by the means for production of operating gas, to be passed to the operating gas reservoir or directly to the fluid pump. It is therefore possible to make use of the time when the fluid is simply stored in the fluid containers to produce a high-purity operating gas with a percentage of inert gas which is above the threshold value, as well as possibly increasing the pressure in the reservoir in order to store additional energy. The percentage of inert gas in the high-purity gas may be 99% and more, and the high-purity operating gas is stored in the operating gas reservoir until emptying of the tanks begins. The operating gas generator can then produce operating gas with a percentage of inert gas which is below the threshold value, for example around 92%. This requires far less energy than producing operating gas with a percentage of inert gas of 95%. The total power requirement for the energy-producing equipment is therefore lower. This is particularly important for vessels. High-purity operating gas is mixed with operating gas with a low percentage of inert gas with the result that the operating gas fed into the pump motor contains a percentage of inert gas which is at least equal to the threshold value. In a further embodiment of the invention the system comprises a compressor which is arranged so as to enable the pressure in the operating gas to be increased before the operating gas is introduced into the pump motor.

Thus in the present invention the pump and the operating gas generator are combined and the operating gas is used directly at its original pressure for driving the discharge pumps. After the operating gas at its pressure (of 12 bar, for example) has expanded over the pump motor, the operating gas is released directly out into the tank. The discharge pump is preferably located in the tank and the nitrogen can thereby simply seep out into the loading tank at the same rate as the discharging operation, either straight out into the fluid in the tank or by the expanded operating gas being conveyed up into the upper part of the fluid container, on top of the surface of the fluid.

As mentioned earlier, the typical purity required is 95% nitrogen. With this degree of purity the membrane's selectivity (recovery) is approximately 50%. Thus 50% of the compressed air which is established upstream of the membrane will remain as

inert gas. In some cases extra energy will be needed to drive the discharge pumps. This is solved by increasing the pressure to a level which gives the desired power by means of a booster compressor. 90-95% of the energy required in order to create the compressed air via the air compressor in the booster unit disappears in heat emission to cooling water and the environment. The fluid in the tanks needs heat in order to sustain a viscosity that enables it to be pumped and in accord with the customer's stipulated requirements. By establishing a secondary circuit on the compressor's cooling water, this heat can be recovered by heating up the load that is pumped out of the fluid containers. The exchange is implemented by a heat exchanger which is established in a circuit driven by the discharge pump.

In order to further increase the efficiency, an operating gas generator is introduced which is filled when the discharge pumps are not in use, for example during sailing if the system is provided on a vessel. The operating gas generator is then placed in a high-purity setting by the flow control valve being adjusted and the amount of produced operating gas reduced, while at the same time the purity is increased to, for example, 99.9%. The tank is then filled with a high-purity operating gas containing 99.9% nitrogen at, for example, 12 bar. The percentage of nitrogen in the high-purity operating gas may, of course, be both higher and lower than 99.9% if so desired. During discharge, therefore, the operating gas generator can produce operating gas with a lower percentage of nitrogen than the prescribed 95% which is mixed in a mixing point with the high-purity operating gas (at, for example, 99.9%) from the tank in order to establish the prescribed percentage of 95% nitrogen in the operating gas before the inlet to the booster compressor and the fluid motor. Depending on how much of the pure nitrogen from the tank is mixed in, the percentage of nitrogen in the operating gas which the generator produces during discharge can be scaled down to, for example, 90%. This increases the recovery by 20% and thereby the potential energy saving by the same 20% for operation of the pump.

On board, for example, a chemical ship today, it is the discharge pumps and the operating gas generator that form the bottleneck in the ship's electrical power balance. The power requirement is greater than what can be established in practice by the on-board generator. It will therefore not be possible to discharge at the rate that is really desired, since the on-board power generation is inadequate.

The present invention reduces the power consumption to a great extent by taking care of the energy established in the nitrogen generator and using it to drive the discharge pumps.

The invention will also give substantial savings on the investment side, since it eliminates a great deal of the generation of hydraulic power and pipelines. At the same time the waste heat from the air booster unit is utilised to keep the load at the

desired temperature. This is currently accomplished by removing heat from the ship's boilers. By looking after the energy, a better solution will also be achieved from the environmental point of view.

Situations may also be envisaged where one chooses to use the operating gas pressure to drive an air motor which in turn drives parts of the hydraulic power package employed in today's existing solutions. This may also be done in cases where one wishes to convert current solutions in order to save energy, or in new plants/boats where one is not quite so concerned with the best solution in terms of energy, but wishes to combine old technology with the present invention. By introducing an operating gas reservoir which stores operating gas with a higher percentage of inert gas than that required when discharging, the object is achieved of being able to run the actual generator at higher capacity by mixing operating gas with a lower percentage of nitrogen than required and high-purity operating gas from the operating gas reservoir. The total energy saving for discharging can be between 30 and 50%.

A non-limiting embodiment of the invention will now be described with reference to the figures, in which

Figure l is a schematic illustration of how a system according to the invention can be built up. Figure 2 illustrates an embodiment where a pump is mounted in a fluid container. Figure 3 illustrates an embodiment with a return pipe for heated fluid.

In figure 1 an example is illustrated of how a system according to the present invention may be realized. Compressed air is established by means of a booster unit 1, comprising a compressor unit 2 and an intake filter 3 where the air is compressed in the compressor unit 2 (for example a screw). The compressor unit may be water- cooled by a first heat exchanger 4. Extra energy may be obtained via this heat circuit by means of a heat exchanger 5 which increases the temperature of the compressed air before it is introduced into an operating gas generator 7 as described in the applicant's Norwegian patent NO 323437. This operating gas generator may be a membrane separator, comprising one or more membranes, or a PSA unit. Due to this temperature increase of the pressurised air by means of the heat recovery, energy is supplied to the operating gas generator 7 and the membrane becomes more permeable, with the result that it will produce more. The liquid in the heat exchanger circuit is controlled by a valve 6 and associated temperature transmitter. Between the membrane's pressurised side and unpressurised side a modulation 12 is established which provides a constant pressure independent of the compressor's fluctuations between unloading and loading set point. This ensures a stable

separation over the membrane. The amount is monitored by means of the flow indicator 8. A valve 9, for example a needle valve, may also be provided as back-up for the flow control valve 10. When the desired purity has been achieved and detected in the oxygen analyser 13, the valve 11 in the product line opens and shuts off the discharge line. The finished operating gas can now be led either to an operating gas reservoir 14 for storage, or it can be led past the operating gas reservoir 14 in a separate pipe, through a flow control valve 15. In periods when the system is not in use for operation, it can be scaled down by adjusting the control valve 10 and high-purity gas at, for example, 99.9% can be stored in the operating gas reservoir 14. In order to increase the percentage of stored energy, the pressure can also be increased further by placing a compressor 23 upstream of the operating gas reservoir 14, i.e. between the operating gas generator 7 and the operating gas reservoir 14. Since high-purity operating gas, i.e. operating gas with a higher percentage of nitrogen than that prescribed for discharging, is being stored here, the efficiency and output of the actual discharging is increased due to the fact that the high-purity operating gas can be mixed with produced operating gas of a lower level of purity than that prescribed. The mixture of high-purity operating gas from the operating gas reservoir 14 and produced operating gas with a lower percentage of inert gas than that prescribed can be mixed in a point 26 where the respective gas pipes are connected. If it is desired to increase the capacity of the discharge pump in relation to the energy and the pressure which are already established in the actual separation process, a compressor 23 may be employed (located either upstream or downstream of the reservoir), whereupon the operating gas with the prescribed percentage of nitrogen of at least 95% is released directly into the pump motor 16. In the pump motor 16 the energy is released and the inert gas expands out into the fluid container 25. The motor power is controlled by a valve 24, for example a choke valve, on the outlet. The pump 17 which is mounted over a suction well 18, now pumps the liquid out through the outlet valve 21. The fluid 109 can be pumped in a return circuit through a return pipe 27 while at the same time a second heat exchanger 5 obtains energy via a secondary circuit, comprising a feeder line 29 for supply of hot water from the booster unit 1 and a return line 30 for returning the water to the booster unit 1. The amount of water flowing in the secondary circuit is regulated, for example by means of a valve 6. In this way the load is kept at a desired temperature and viscosity, for example 65°C, by means of energy which is recovered in the booster unit 1. The temperature is controlled by a temperature transmitter 28 which monitors the temperature of the fluid flowing through the discharge pipe 19 and controls the flow of the secondary heat circuit by means of a control valve 31 in the secondary circuit. When the discharging operation is completed the fluid container 25 is inerted or filled with high-purity operating gas from the operating gas reservoir 14, thereby ensuring the safety of the procedure.

Figure 2 illustrates in more detail how a pump 17 with a pump motor 16 can be arranged. Operating gas with the prescribed percentage of inert gas (preferably nitrogen) is introduced, see arrow A, to the pump motor 101 through a supply pipe 107 and expands over the pump motor. The pump motor 101 drives the pump impeller 102 which pumps the fluid 109 in a suction well 103 out of the fluid container 25 through the discharge pipe 19. After the operating gas has expanded through the pump motor 101, it is passed through the operating gas outlet (105, 20) to the fluid container's upper part where it is released, thereby creating an inert atmosphere 113 in the fluid container. It is also possible to release the operating gas right out into the fluid 109 after the operating gas has expanded in the pump motor 101, thereby enabling the operating gas to climb up to the surface through the fluid 109. In the figure the pump's housing 108 and bearings 104 can also be seen. At the top of the fluid container's upper wall I l i a hatch 1 10 is indicated.

Figure 3 illustrates a pump motor and a pump corresponding to that illustrated in figure 2 and will therefore not be described again in detail. In addition it also shows how the fluid which is pumped out and how the remaining fluid in the fluid container can be heated to the desired temperature in an energy-efficient manner. The fluid which is pumped out through the discharge pipe 19 is heated by means of a second heat exchanger 5. Water which is heated by waste heat from the booster unit 1 for air in a first heat exchanger 4 is passed to the second heat exchanger 5 through a feeder line 29 and returned to the booster unit 1 through a return line 30. A return line 27 is also shown which leads heated fluid back to the fluid container 25 for heating the fluid left in the fluid container. The amount of heated fluid returned to the fluid container 25 can be regulated by means of a control valve 31.

> As explained above, the present invention comprises a number of advantageous solutions for systems and plants for discharging explosive fluid stored in a fluid container. In such a system there are clearly many possible embodiments which have not been described in detail in this description, but which must be considered to be within the scope of the patent protection of the present invention as defined in the independent claims. For example, it will be possible to employ two or more fluid containers even though only one fluid container is shown in the figures. The same applies to operating gas containers for storing high-purity operating gas and other components included in the embodiment of the invention illustrated in the attached figures.