EP0586294B1 - Device for the distribution of cryogenic fluid to apparatuses using them - Google Patents

Description

The present invention relates to a device for transferring cryogenic fluids in industrial establishments between a storage tank and at least one user device.

Cryogenic fluids are generally transported by truck to their place of use, where they are stored in insulated tanks; from there they are distributed to the user devices by a cryogenic piping or line, under the effect of the pressure which is established in the storage tank by the vaporization of a part of the cryogenic fluid. A balance is established in a few hours between the liquid and gas phases of the tank.

This procedure has the advantage of simplicity, but it has several disadvantages:

When the cryogenic liquid circulates in the piping it undergoes both pressure losses and heat inputs, breaking the balance and causing its vaporization: the circulation is two-phase. The two-phase, for a given mass flow, increases the pressure losses, makes necessary large diameters of line and accessories, and creates irregularities in operation. For nitrogen, for example, lines of more than 50 meters may have a random walk, or a very high cost if they are isolated under vacuum in large diameter.

Another drawback relates to the thermodynamic efficiency of the cryogenic fluid: the usable cold corresponds to the enthalpic variation of the fluid between its initial state, in the reservoir, and its final state, at the outlet of the user device, after vaporization and possible reheating gas produced; it is therefore interesting to find the lowest initial enthalpy, that is to say the lowest equilibrium pressure at the reservoir that can be reached in practice.

A third aspect is not negligible in terms of yield: maintaining the storage pressure means replacing the volume of liquid drawn off by the same volume of gas; however the insulation of industrial tanks is of high quality, and, in the case of nitrogen, if the contents of a tank are used in less than four days, the heat inputs to the tank will be insufficient to maintain the pressure, and 0.5 to 1.5% of the fluid should be vaporized by a "heater".

The above drawbacks, and the paradoxical practice of heating a fluid which is used cold, in order to raise its pressure, are nevertheless accepted because there is no other practically usable means of distributing the cryogenic fluid to the user device.

Some examples are known of raising the reservoir relative to the user station in order to benefit from the pressure of the column of fluid. Known centrifugal cryogenic pumps are ill suited to small flow rates, variations in flow rate, vaporization by loss of hydraulic efficiency in the event of a reduction in flow rate, causing risks of cavitation. Alternative submerged mechanical pumps are known, in installations for filling gas cylinders, pushing the liquid to more than 200 × 10⁵ Pa in a vaporizer; however, these pumps are not suitable for suctioning the liquid at low pressure. The pumping of a cryogenic fluid at low pressure with reciprocating piston, the inlet part of which is immersed in the liquid to be pumped is known from FR-A-2 613 034.

The object of the present invention is to provide a simple and effective cryogenic fluid pumping device, to raise its pressure to a constant and adjustable level, for variable flows, typically between a zero flow and a maximum depending on the size of the user device, this pumping device being advantageously supplemented by one or more reservoirs for accumulating the fluid under pressure.

This device allows a new mode of distribution of cryogenic fluids to user devices; taking the case of nitrogen, the filling on delivery will be at the lowest pressure, which constitutes a control of the quality of the fluid, without regulating the pressure at "unloading", an operation which is often a source of installation irregularities; the storage tank, at low pressure, can be lightened, without a pressure regulation system; the lines will be designed for the monophasic, allowing smaller diameters and / or longer lengths at lower costs. Within the meaning of the present invention, the term “cryogenic fluid” will be understood to mean a liquefied gas such as nitrogen, argon, oxygen, CO₂, etc., the user devices being able to be tunnels, baths, runoffs or spraying liquids, etc ..., but also evaporators, as well as carbon dioxide production devices.

A set of diagrams and drawings makes it possible to describe the device and its operation:

Figures 1 and 2 respectively represent a part of a MOLLIER diagram of nitrogen and CO₂, on which have been plotted the points representative of different distribution conditions of this fluid.

Figure 3 is a partial schematic view of a pumping device according to the invention.

FIG. 4 is a sectional view of the intake valve and of the piston of the pump in FIG. 3.

FIG. 5 is an overall diagram of a distribution of carbon dioxide, according to the invention.

The nitrogen diagram in Figure 1 shows the liquid / vapor equilibrium curve for relative pressures (PR) between zero (atmospheric pressure) and 3 x 10⁵ Pa, along the vertical axis, the enthalpies (H) being carried on the horizontal axis.

It can be considered that liquid nitrogen is normally delivered to its point of use at a pressure of 0.7 x 10⁵ Pa: in fact, nitrogen is less than 0.05 x 10⁵ Pa in the storage facilities of liquefaction. Each pumping increases the enthalpy by 0.7 kcal / kg by loss of efficiency of the centrifugal pumps, which corresponds to an increase of 0.22 x 10⁵ Pa in the pressure of the liquid in this pressure zone. Transport, heat inputs and mechanical effects combined, raises the pressure by 0.07 x 10⁵ Pa for a journey of 100 km. By counting two pumps, and for a transport distance of 200 km, the nitrogen pressure in the tank will therefore be tiny, of 0.63 x 10⁵ Pa. Note that the value rounded to 0.7 x 10⁵ Pa translates the good quality of the fluid.

Nitrogen at 0.7 x 10⁵ Pa is represented in (A1) on the diagram, as well as, in A2, nitrogen in equilibrium at 2 x 10⁵ Pa, very common in the current state of the art; if the point of use is higher than the storage, the pressure will often be 2.6 x 10⁵ Pa, shown in A3).

Points A'2 or A'3 represent nitrogen at the arrival of a user station, the two-phase often representing 3 to 5% by weight, or 150 to 250% by volume. On the other hand, increasing the pressure by pumping from 0.7 to 2.6 x 10⁵ Pa, as shown in A'1, will allow much greater pressure losses and heat inputs, represented by A "1 without formation of vapor.

With regard to the comparisons of the initial enthalpies H1, H2, H3, one must establish the variations, therefore the quantities of cold produced in kcal / kg, for 2 final states: one corresponds to the enthalpy of the gas at - 196 ° and atmospheric pressure, when only latent heat is used, such as immersion, i.e. 18.45 kcal / kg; the other corresponds to freezing tunnels, where the gas can exit at - 50 °, with an enthalpy of 55kcal / kg.

The following table allows the comparison:

Nitrogen at 0.7 x 10⁵ Pa will produce, in the case of immersion, respectively 7.2% and 9.8% more cold than nitrogen at 2 and 2.6 x 10⁵ Pa, and therefore the 6.7% or 9% of the amount of liquid to be saved will be saved.

The recovery of the sensible heat of the gas, in tunnels for example, does not depend on the initial enthalpy, and the savings due to the use of nitrogen at 0.7 x 10⁵ Pa will be between 3.7% and 5%, respectively, with respect to nitrogen at 2 x 10⁵ Pa and 2.6 x 10⁵ Pa.

To these savings, the system according to the invention adds those resulting from the elimination of the pressure maintenance of the tank: even in the event of rapid emptying, the pressure of 0.7 x 10⁵ Pa will drop only by 0.15 x 10⁵ Pa, or 0.55 x 10⁵ Pa in the end, without drawbacks.

On the contrary, the density ratios, in kg / m3 of gas and liquid in equilibrium at 2 and 2.6 x 10⁵ Pa are, respectively: 12.8 / 756.5 = 0.017 and 15.1 / 746 = 0 , 02, which means that 1.7% or 2% of the liquid will have to be vaporized to maintain the pressure at 2 or 2.6 x 10⁵ Pa.

FIG. 2 represents the states of carbon dioxide, plotted on a diagram, the absolute pressures being represented on the vertical axis, and the enthalpies on the horizontal axis; we will quote the analogies and the differences of CO₂ compared to nitrogen.

The major difference is the pressure of the triple point (PT), at 5.18 x 10⁵ Pa. A final state of CO₂ can be the gas leaving a refrigeration tunnel at atmospheric pressure (PA) and -50 ° C ( HT); another state final will be, after production of snow and its sublimation, the "outlet" of gas at -78.5 ° C, represented by HN.

The enthalpy variation, or usable cold, essentially corresponds to latent heat; it is therefore all the more interesting to lower the initial enthalpy:

The usual initial state is the delivery of liquid CO₂ at -20 ° C and 20 x 10⁵ Pa, represented in D1), which can be stored completely in the manner of liquid nitrogen, in vacuum-insulated tanks; generally less efficient storages are used, the pressure of which is controlled at 20 × 10⁵ Pa by a refrigeration unit; in the latter case, centrifugal pumps are known for looping the liquid CO C to keep the line cold, and not for the purpose of obtaining subcooling of the liquid by increasing pressure. The gas formed by the losses in efficiency of the pumps and the heat inputs, returns at the end of the loop to the tank, where it is condensed by the refrigeration unit.

The most frequent is the use of a single line, and in this case, the CO₂ arriving at the use is represented by D'1, which allows normal operation; The interest of the reduction in the enthalpy and the initial pressure, motivated distributions of liquid CO₂ at 8 x 10⁵ Pa and -45 ° C, represented by D2: on arrival at use, represented by D '2, the proximity of the pressure of the triple point requires special precautions. We can say that even under 10 or 11 x 10⁵ Pa the risks of plugging by solid CO₂ (plugging) exist.

The pumping device which is the subject of the invention makes it possible to use CO₂ brought in the vicinity of the triple point, either by cooling, or by vaporization and reduction of the pressure, as represented by D3: the pressure can be increased to the point representative of D'3, so that in use (D "3) the risk of solid formation is reduced. The pump admits the circulation of fluid loaded with particles, and if the low pressure tank is loaded on its inlet , we could even use a mixture of liquid and solid CO₂ at the triple point.

FIG. 3 schematically illustrates the pumping device according to the invention: an isolated storage tank 1 contains the cryogenic fluid at the lowest possible pressure: for nitrogen, this tank is storage, receiving liquid nitrogen at 0.7 x 10⁵ Pa; for CO₂, it is a low pressure tank, generally separate from the delivery tank.

A line 2 is the liquid outlet from the reservoir, which feeds by gravity the pumping device via a stop valve 3; it is desirable to oversize the latter 3 ', and to reduce the horizontal distance during pumping. The outlet of tank 1 must be 0.5 to 1 meter higher than the inlet of the pumping device.

The pumping device comprises at its lower part a small reservoir 4 used to supply liquid devoid of gas to the inlet valve 6 of the pump; for this it is provided with a vent tube 5 connected by an insulated tube to the gas phase (upper part) of the reservoir 1; the tube 5 is also used for cooling the pumping device in the case of the use of CO₂. It can alternatively be replaced by a gas eliminator.

The pump is of the piston and valve type, vertical. A tube 7 fixed by flanges to the reservoir 4 carries the cylinder 8 in which the piston 9, itself carrying a discharge valve, is actuated by a rod 10 passing through a set 11 and 11 'of seals, between which gas is injected under pressure from the discharge, through a heating tube 12. Only the lower part of the pump is insulated, as shown by the dashed line 13: the liquid leaves the tube 7 is made by a nozzle 14, located just above above the cylinder 8 and the tube and pump fixing flange, so that the upper part of the tube 7 contains a volume of slightly conductive gas, and that the seals 11 and 11 ′ remain at room temperature.

The piston control rod 10 is connected to a motor 15 giving an adjustable reciprocating movement, a particularly advantageous embodiment using a motor with pneumatic or hydraulic cylinders. In fact, an important characteristic of the movement of the piston is that it must be slow during the ascent, which corresponds to the suction or admission phase of the pumping cycle, compared to the descent movement, which corresponds to the setting. in fluid pressure and in its discharge, and which can be as fast as it is mechanically possible.

The pressure of the working fluid actuating the engine 15 is regulated by a pressure reducer 17: the ratio of the active sections of the motor cylinder and the pump piston and the pressure setting of the pressure reducer define the maximum pressure to which the cryogenic fluid will be brought. A possible adjusting member 18 conditions the flow of working fluid, and therefore the overall duration of an up / down cycle. This duration must be compatible with the operation of the motor fluid distributor 19, actuated by a double timer, or by a timer and a limit switch 21. An adjustable unidirectional flow reducer 20 makes it possible to adjust the slow ascent rate (admission), the discharge being free or regulated by the member 18. Another limit switch 21 'can allow the control of the piston ascent rate.

The operation of the pump is volumetric, in free liquid: the detector 21 can be used for counting and determining the instantaneous flow rates, means, or cumulative quantities of cryogenic fluid supplied to an item of equipment.

Two distribution systems can raise the pressure of counted and regulated quantities of two different liquefied gases, such as nitrogen and oxygen, so that after mixing in the liquid phase, the pressure is greater than the bubble pressure; liquid phase metering can also be used before vaporization and mixing in the gaseous state.

FIG. 4 shows the details of the pump and in particular of the piston and of the intake valve: the body of the valve 6 is fixed on the cylinder 8; the valve itself is a flat disc with rounded edges, the lifting of which is limited by a rod 25; it rests on a seat 24, and is guided by the tie rods 26 for assembling the valve bottom 27. The disc is preferably made of high density polyethylene or PTFE.

The piston is constituted by a tubular body 9 fixed on the control rod 10, and provided with windows 22 for passage of the pumped fluid; a hollow screw 28 fixed on the piston of the annular seals 30 with upward-facing lips, made of polyethylene, PTFE, or leather, by tightening spacers 29. A ball 31 forms a sealing valve by resting on the screw 28; the rod 10 abuts the lifting of the ball.

In the low position, the piston touches the intake valve 23 in order to reduce the "dead volume" of fluid between the piston and the valve: in fact, upon lifting, the acceleration of the valve is several times the gravity, by pressure by below, from the liquid column, and by vacuum from above, due to the upward movement of the piston; we limit this effect and the vaporization, reducing the volume of liquid concerned, by bringing the piston and valve as close as possible.

The liquid arrives from the storage tank through line 2 and completely fills the pump tank 4 with evacuation of any gas through the vent 5; the liquid descends through the space between the body of the valve 6 and the reservoir 4.

When the piston is raised, the valve 23 is lifted by pressure difference between the lower and upper faces: the valve must be as light as possible, and in the case of a seat diameter of 80 mm., It will be set to rest on a spider carried by an annular piece 27; the liquid must strictly follow the upward movement of the piston, since any vaporization must be avoided. The pressure of the fluid on the discharge side, and the pressure of the gas contained in the tube 7, which are identical, are exerted on the upper face of the piston and on the ball 31.

On descent, the intake valve 23 closes, the liquid contained between piston and valve sees its pressure rise above the discharge pressure, so that the liquid passes through the screw 28 and the ball valve 31; the pressures are little different, and the force on the rod 10 is low.

• . 0,1 % d'azote à pression atmosphérique, formant 17,5 % de gaz
• . 0,07 % d'azote à 0,7 x 10⁵ Pa relatif, formant 7,1 % de gaz,
• . 0,019 % de CO₂ à 6,8 x 10⁵ Pa abs., formant 1,2 % de gaz.

The difficulty of cryogenic pumping by reciprocating pump lies in the need to avoid any vaporization: any depression immediately vaporizes liquid by "flash"effect; on the contrary, compression does not immediately recondense the gas formed, because there is a transfer of heat between gas and liquid: compression must therefore compress the gas before raising the pressure of the liquid, and for a critical volume of gas, the process gets worse until gas compression-expansion, without liquid flow. For information, the vaporization of fluids at equilibrium, in% by weight, and the correspondence in volume, for examples of nitrogen and CO₂, and for a vacuum of 1000 Pa, are:

• . 0.1% nitrogen at atmospheric pressure, forming 17.5% gas
• . 0.07% nitrogen at 0.7 x 10⁵ relative Pa, forming 7.1% gas,
• . 0.019% CO₂ at 6.8 x 10⁵ Pa abs., Forming 1.2% gas.

For example, for a pump having an internal diameter of 45.3 mm, a stroke of 100 mm, a valve with a diameter of 54 mm covering a 40 mm seat and a 5 mm valve lift, the following observations were made:

There is no reduction in the volume of nitrogen pumped per cycle up to a rate of 165 cycles per minute, if the height of the liquid nitrogen above the valve is equal to or greater than 250 mm. nitrogen, or 2000 Pa. The flow rate is then 1400 liters / hour.

The piston rise speed is 0.55 m / s: this is also the nitrogen rise speed. The narrowest flow section is in the vicinity of the valve seat; this is also the highest speed of nitrogen, around 1.30 m / s, corresponding to a dynamic pressure of 700 Pa. The overall pressure drop of the valve is less than 2000 Pa, the coefficient of pressure drop of the valve is therefore less than 2000/700 = 2.86 value due to the particularities of the configuration described in FIG. 4: rounding of the profiles, and seat size.

The observations carried out made it possible to establish characteristic relationships for the pumping of cryogenic fluids at equilibrium by alternative pumps:

The ascent of the piston is the flow phase through the inlet valve; the piston ascent rate must typically have a maximum value of 0.5 m / s for the rise of the liquid to follow the rise of the piston. For a given pump, the rate of rise of the piston conditions the flow rate, and it is desirable that, in a cycle, the descent phase is as brief as mechanically possible, since there is no thermodynamic drawback, and the flow will be optimum in chained cycles.

For optimized cycles, the flow rate is proportional to the active surface of the piston, ie the square of its diameter.

Depending on the geometry of the valve and the rounding in the fluid direction change sections, a pressure drop factor of less than 2.86 has been found; multiplied by the dynamic pressure of the highest speed of the fluid in the valve, it indicates its overall pressure drop. To keep the same pressure drop, the same speed must be kept in the smallest section of the valve, namely in the vicinity of the seat, the valve lift being always in practice limited to 5mm; keeping the same fluid speed requires that the diameter of the valve seat be proportional to the square of the piston diameter. If a smaller valve is used, the piston ascent rate must be reduced: the diameter of the piston will be oversized, causing greater pressures of the working fluid.

The transition times from rise to fall, and vice versa, of the piston are between 25 and 50 ms, and the average speed of the piston is little different from the instantaneous speed during a movement of the cycle. Therefore, the stroke length is not an essential factor of flow, but frequency, with a mechanical interest.

The pump described above, with piston diameter 45 mm, valve seat 40 mm, stroke 100 mm, can be industrially used at a rate typically of around 125 to 130 cycles / min, corresponding to a flow rate of 1100 to 1200 liters / hour of cryogenic fluid, suitable for many applications.

A double flow pump, i.e. 2200 to 2400 liters / hour, would have a piston diameter multiplied by √2, or approximately 65 mm, a valve seat diameter proportional to the flow rate, ie 80 mm, and an operating rate of about 125 to 130 cycles / min. if the stroke is 100 mm, and about 85 cycles per minute, if the stroke is 150 mm.

These examples show that the alternative pump is suitable for an installation whose instantaneous flow rate, which must be less than the maximum flow rate of the pump, will not be very high or different from the average flow rate of use. We know cases of large instantaneous flows, for small average flows. In CO₂ in particular, we have to inject a few kg of snow in a few seconds, corresponding to 2000 or 3000 kg / h of instantaneous flow of liquid, for an average flow of a few hundred kg / h; on the other hand, a pressure drop near the triple point causes catastrophic blockages.

The present device provides a solution by the use of accumulators under pressure, either at the point of use, or online.

In the CO₂ distribution installation according to FIG. 5, we recognize the low pressure tank 1 supplying by line 2 the tank 4 of the pumping device shown diagrammatically by its tube 7. The tank 1 is maintained by a pressure switch P2 at a pressure 6 x 10⁵ Pa, for example, the CO l'équilibre balance being at -53 ° C

The reservoir 1 receives the CO₂ from a delivery reservoir 32, at a pressure P1 of the order of 20 × 10⁵ Pa, via a line 33; a float 34 controls a filling means 35 to keep a constant level; the liquid at 20 × 10⁵ Pa is introduced into the tank by means of a phase separator 36. A demister 37, placed in the tank 1, is located at the intake of a single-stage compressor 38 associated with a gas-gas exchanger 39. The compressor 38 raises the pressure of the CO₂ gas in the tank 1 from 6 to 22 × 10⁵ Pa, to reliquefy it by the condenser 40, connected at 41 and 42 to a refrigeration circuit (not shown). Alternatively, the tank pressure can be lowered by refrigeration to -53 ° C.

The discharge line 14 of the pumping device ensures a flow of liquid CO₂ at -53 ° C, at a pressure of 10 x 10⁵ Pa, for example, (therefore sub-cooled relative to the equilibrium temperature at 10 x 10⁵ Pa, which is - 40 °), until the use represented by a solenoid valve 48. A gas trap 53, for example with float 54, ensures that the line is kept cold, even if it is not there is no traffic. The gas at 10 x 10⁵ Pa, leaving the trap 53, is brought through a pipe 55, to the gas phase of the tank 1, or to the reliquefaction. This arrangement advantageously replaces the technique of keeping cold by loop of liquid with circulation pump.

To obtain instantaneous flow rates greater than the nominal flow rate of the pumping device, an isolated accumulator tank 43 allows operation if the average flow rate is lower than the nominal flow rate of the pumping device: the latter fills the accumulator 43 up to the level defined by a detector 44; as long as the level is not reached, a valve 45 in a line connecting the upper parts of the tanks 1 and 43, is open, and the gas repelled by the liquid in the tank 43 joins the tank 1, passing through a spillway 46 set to maintain an upstream pressure of 8 x 10⁵ Pa for example; this spillway can also be replaced by simple rolling.

When the accumulator tank 43 is full, without use, the pumping device 7 maintains the pressure at zero flow, by the adjustment established on the pressure of its working fluid.

When the operating solenoid valve 48 is open, a solenoid valve 49 for pressurizing the accumulator tank 43 disposed in a line connecting the upper parts of the tanks 43 and 32 is also opened; the pressure of the pressurization gas coming from the storage tank 32 is adjusted by the pressure reducer 50 which ensures, for example, a downstream pressure of 11 x 10, Pa, so that the fluid reaching use 48 comes first from the accumulator 43; on the contrary, it will be possible to adjust the downstream pressure of the regulator 50 to a value lower than the pumping pressure, for example 9.5 x 10⁵ Pa, so that the flow rate of the pumping device is used as a priority, and the accumulator 43 only as a backup.

To avoid false operations, a level control 47 can be used in the accumulator tank 43 stopping the supply of fluid supplied for use 48 in the event of excessive draining of the accumulator, the size of the latter having to be adapted to Requirement ; one can also use a pressure switch P3 to check that the pressure in the accumulator 43 is always at a certain level above that of the triple point.

This accumulator system can optionally be adapted to fluids such as liquid nitrogen; in this case, the evacuation of the gas from the accumulator takes place in the atmosphere, and the pressurization can be carried out by increasing the size of the vaporizer 12 of FIG. 3, and by connecting it to the solenoid valve 49. We can place a non-return valve on the discharge piping to avoid a return to the pumping device, and above all, to create an additional pressure drop and favor the pressurization of the accumulator 43.

For the record, the usual safety and purging devices have been shown, respectively a valve 56 and a valve or solenoid valve 57; a drain valve for the pump reservoir 4 must also be provided when the installation is stopped.

Claims (7)

1. Device for distributing a cryogenic fluid to the means for using it, this fluid being stored at low pressure in a storage reservoir (1) supplying a pumping device with a piston (9) having a reciprocating movement, the inlet part (6) of which is immersed in the liquid to be pumped, characterized in that the pumping device comprises operating means ensuring a slow upward movement of the piston, corresponding to aspiration of the fluid, preferably less than 0.5 m/s, and a rapid downwards movement corresponding to delivery under pressure.
2. Device according to claim 1, characterized in that movement of the piston (9) is obtained by the action of at least one jack.
3. Device according to claim 1 or 2, characterized in that the pumping device comprises an inlet valve (23) and in that the piston (9) in the lowest position of its stroke, comes into contact with the valve (23).
4. Device according to claim 3, characterized in that the pumping device is connected in a liquid tight manner to a reservoir (4) surrounding the inlet valve (23), and comprising a vent (5) for evacuating gas.
5. Device according to one of claims 1 to 4, characterized in that it comprises a means for counting the cycles of the pumping device, so as to obtain an evaluation of the volume flow rates of the fluid used in an installation.
6. Device according to one of claims 1 to 5, characterized in that it comprises at least one insulated reservoir (43) connected to the delivery of the pumping device, this reservoir being filled by evacuating gas through a valve (45), and being emptied during the periods of use, with pressurization by means of a regulated inflow of gas above the accumulated liquid, controlled by a valve (49).
7. Distribution device according to one of claims I to 6, characterized in that the fluid is carbon dioxide, pressurized by the pumping device, and is conveyed through an insulated pipe (14) kept full of liquid by at least one gas trap (53), the gas leaving from these traps being led again (55) to the storage reservoir (1).

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