Field of the invention: A combined vessel protection and monitoring system for tanks storing gas at constant pressure is described. In particular a facility and method is described.

Background:

It is very common for gases of various sorts to be stored in tanks - usually called “cylinders”. These cylinders are normally relatively stiff compared with the internal gas. That is to say, if a small increment is made in the mass of stored gas, the proportionate increase in the volume of the cylinder is typically negligible compared with the increase in volume that would occur in the gas if that gas was allowed to expand back to its original pressure whilst being retained at the same temperature. In such cases, the pressure of the stored gas rises as the mass of stored gas increases. Conversely, the pressure of the stored gas falls as mass of stored gas reduces again. These cylinders are approximately “isochoric” - i.e. constant volume.

There are several disadvantages to isochoric containments (cylinders) for gas. One obvious disadvantage is that the machinery used to compress and expand the air must deal with a range of different storage pressures and it is difficult to design machines that have optimal, or near-optimal, performance over a range of different storage pressures. A second disadvantage is it will usually be impractical to withdraw more than about 50% of the maximum charge of the gas from the store. The state of stress of the walls of the containment changes is affected by the internal pressure and large oscillations in this state of stress can cause material damage in the walls. A third disadvantage relates to heat transfer. If storage pressure falls as gas is withdrawn during a discharging process, the temperature of the stored air also tends to fall and heat leaks into the stored air through the cylinder walls. The opposite happens during charging where the stored air tends to rise in temperature and heat leaks back out. The net effect of this heat transfer over a complete charge-discharge cycle is a loss of exergy and this loss of exergy translates itself into a loss of turnaround efficiency in the energy storage process. Isobaric (constant pressure) storage of the gas resolves all of the above problems. The compression/expansion machinery can operate with the same pressures on inlet and exhaust at all states of charge and discharge. The state of stress of the containment walls stays (almost) constant and the temperature of the stored gas is not inclined to change.

The attractions of isobaric storage are widely understood and there are several ways in which this can be achieved. One commonly-proposed method is to use hydraulic compensation. Hydraulic compensation involves pumping an incompressible liquid such as water into the containment to displace gas as gas is being withdrawn and conversely allowing the liquid to be driven back out of the containment as gas is being re-introduced into the containment. We will describe this liquid as the hydraulic compensation liquid (HCL). A pump drives the HCL into the cylinder whilst pressurised gas is being discharged and clearly this pump absorbs mechanical power but the amount of power used by the pump is far lower (typically 5-10 times lower) than the work done by the gas being exhausted. When the system is charging again, liquid is driven out again and most of the work done to pump the liquid into the containment can be recovered as the liquid flows from the high-pressure containment into a holding tank at or near to ambient-pressure. For obvious reasons of cost, the most popular candidate for the HCL is water-based.

Most implementations of hydraulic compensation do not attempt to include an impermeable barrier between the liquid and the stored gas. It is possible to introduce a bladder to ensure that the HCL does not come into direct contact with either the inner surface of the cylinder or with the stored gas. The present invention relates mainly to arrangements where there is no bladder and where a water-based HCL is used.

Most pressure vessels have metal inner surfaces and are prone to corrosion difficulties associated with the presence of water at the inner surface together with some free oxygen. The presence of oxygen in the water is most serious in the context where the gas being stored is pressurised air but even in the case of other gases, oxygen may diffuse into the water-based HCL whilst that is held in a non-pressurised vessel and that oxygen is carried into the pressure vessels with the HCL as the gas charge is depleted. Summary of this invention.

According to a first aspect of the present invention there is provided a facility for storing gas at constant pressure, said facility comprising: one or more metal tanks each tank comprising: a lower port fluidly couplable to liquid pipework, through which a hydraulic compensation liquid can enter and leave; and an upper port fluidly couplable to gas pipework through which stored gas can enter and leave, wherein each tank comprises an internal coating that separates the wall of the metal tank from the enclosed fluids electrically and chemically and wherein each tank is electrically isolated from the liquid and gas pipework.

In an embodiment, the facility may further comprise an electrical power source for providing an electrical potential difference having a non-zero mean (DC) value between the liquid pipework and the tank wall such that corrosion effects that might arise through chemical interaction between the hydraulic compensation liquid and the tank wall is inhibited.

The facility may also comprise a measurement facility in place for measuring the mean (DC) current flowing. The electrical power source may be further configured to provide a small alternating (AC) value in addition to its mean (DC) value. The or a measurement facility may be provided for tracking electrical current and monitors both the mean (DC) and alternating (AC) values of electrical current.

According to a second aspect, there is provided a method for protecting and monitoring the state of health of one or more metal pressure vessels in use as constant-pressure gas stores, said method comprising the steps of: coating on the inside wall of the metal pressure vessels, said coating providing both chemical and electrical isolation between a hydrostatic compensation liquid and a wall of the vessel, as well as electrical isolation between the wall and pipework at both top and bottom ends of the vessels such that a potential difference can be sustained between the wall and the pipework containing the hydraulic compensation liquid without large electrical currents flowing through a continuous metal path. The method may further comprise the steps of imposing an electrical potential difference with a non-zero mean value tending to oppose the occurrence of chemical corrosion at inner surfaces of the tank walls even where small breaches exist in the internal coating. The method may further comprise the step of imposing an alternating (AC) component of electrical potential difference between the main tank wall and the pipework containing the hydraulic compensation liquid in addition to any mean value (DC) of electrical potential difference, such that the measurement of the AC current provides information about the state of continuity of the internal lining of the tank.

In an aspect there is provided a facility for storing gas at constant pressure comprises one or more metal tanks each having a lower port through which a hydraulic compensation liquid can enter and leave and an upper port through which the stored gas can enter and leave, with each tank having an internal coating that separates the metal tank wall from the enclosed fluids both electrically and chemically and with each tank also having electrical isolation from the pipework on both the gas and liquid sides.

An electrical potential difference having a non-zero mean (DC) value is maintained between the pipework containing the hydraulic compensation liquid and the tank wall such that corrosion effects that might arise through chemical interaction between the hydraulic compensation liquid and the tank wall is inhibited and with a measurement facility in place for the mean (DC) current flowing.

The electrical potential difference between the pipework containing the hydraulic compensation liquid and the tank wall has a small alternating (AC) value in addition to its mean (DC) value and where the measurement facility in place for tracking electrical current can monitor both the mean (DC) and alternating (AC) values of electrical current.

In another aspect a method for protecting and monitoring the state of health of one or more metal pressure vessels in use as constant-pressure gas stores involves having a coating on the inside wall of the metal tanks providing both chemical and electrical isolation between the hydrostatic compensation liquid and the tank wall as well as electrical isolation between the tank wall and the pipework at both top and bottom ends of the tanks such that a potential difference can be sustained between the tank wall and the pipework containing the hydraulic compensation liquid without large electrical currents flowing through a continuous metal path.

The method may involve imposing an electrical potential difference with a non-zero mean value tending to oppose the occurrence of chemical corrosion at the inner surfaces of the tank walls even where small breaches exist in the internal coating.

The method may involve imposing an alternating (AC) component of electrical potential difference between the main tank wall and the pipework containing the hydraulic compensation liquid in addition to any mean value (DC) of electrical potential difference such that the measurement of the AC current can provide information about the state of continuity of the internal lining of the tank.

The present invention is a combination of three core ideas: (i) applying an electrically- insulating protective coat to the inner surface of the pressurised tanks, (ii) insulating the metalwork of the pressurised tanks electrically from the connecting pipework and (iii) adjusting the electrical potential of the pressurised tanks relative to the connecting pipework such that even if/where the coating does not provide complete separation between the HCL and the inner wall of the cylinder, the chemical corrosion action is suppressed by normal “cathodic protection”.

This invention is applicable to all systems where a gas is intended to be stored within one or more tanks and where the pressure of the stored gas is required to remain fairly constant. A common instance of this requirement occurs in compressed air energy storage systems where the pressurised air is stored in tanks. There are also applications in the storage of other gases such as methane, propane and hydrogen.

In all cases, the integrity of the tanks is a critical concern and the present invention provides both protection against internal corrosion and a means of detecting the vulnerability to corrosion.

Description of the Figures

Figure 1 is the only figure provided. This shows one cylinder (1 ) for containing the pressurised gas (5) together with its electrically-insulating inner coating (2). The electrically-insulating inner coating (2) separates the pressurised gas (5) and the HCL liquid (6) from the metal cylinder (1 ) and this separation is both chemical and electrical. The cylinder (1 ) is connected to external pipework (4) via electrically-insulating arrangements (3) such that the electrical potential of the cylinder metalwork (1 ) can be maintained at a level different from that of the connecting pipework (4) without significant electrical current passing through an all-metal path between the two. The electrical potential difference between the cylinder metal wall (1 ) and the external pipework (4) is maintained by an electrical power source (7) whose rated power could be very small.

Embodiment.

Figure 1 shows an embodiment of the present invention as described above. The hydraulic compensation liquid (HCL) (4) rests in the bottom part of the cylinder beneath the stored pressurised gas (3). The metal wall of the cylinder (1 ) is coated internally with an electrically insulating coating (2) which separates the fluids (5), (6) within the cylinder from its metal wall (1 ). This separation is both chemical and electrical.

If the internal coating (2) could be relied-upon to begin and remain continuous and intact for the lifetime of the cylinder, then the electrical aspects of this invention would have no relevance. These electrical aspects are motivated by the realisation that in most practical contexts, it is necessary to cater for the eventuality that the coating might not offer complete protection on its own. Even a very localised damage site on the vessel might initiate a crack that could potentially propagate quickly.

The HCL (6) would normally have additives in it to reduce the propensity for corrosion anyway but these might not provide complete protection against the possibility of a breach of the internal tank coating (2).

To complete the protection, an electrical connection is formed between the lower part of the connecting pipework (4) that contains the HCL (6) and the metal wall of the cylinder (1 ) and a small voltage source (7) is connected between the two. The reason for the electrical isolation between the metal cylinder wall (1 ) and the connecting pipework (4) is so that current does not simply pass through an all-metal path. Such a path would either consume a lot of electrical power or limit the achievable potential difference to very low levels.

In normal operating conditions, the small voltage source (7) would be set to maintain a constant potential difference that would prohibit corrosion even in the presence of faults in the internal coating layer (2) and in the event that additives in the HCL (6) were not fully effective on their own in suppressing corrosion. The amount of current that flows would be an indication of the extent to which the potential difference was actively providing protection. The current would be expected to increase over the lifetime of the system.

It is anticipated that in addition to using a constant potential difference (DC), some alternating current (AC) components of potential difference could also be used to provide further information on the extent of electrical (and hence also chemical) contact between the HCL (6) and the metalwork of the cylinder (1 ). In some cases an additional AC voltage source similar to (7) could be used to apply relatively high- frequency AC at locations elsewhere on the metal cylinder wall (1 ) so that any damage indicated could be localised a little more.