RU2761148C1 - Method for transporting cryogenic liquids - Google Patents

Abstract

FIELD: cryogenic liquids transporting.

SUBSTANCE: invention relates to a method for transporting cryogenic liquids, especially liquefied gases, and can be used in the design and construction of pipelines transporting cryogenic liquids. The method for transporting cryogenic liquids includes transporting LNG through pipelines and cooling the pipeline before transportation. The pipeline is made of ultra-high molecular weight polyethylene, covered on top with several layers of insulation, including at least one thermal insulation layer, then at least two vapor barrier layers and a cover layer. The operating pressure in the pipeline must be higher than the vapor pressure of the cryogenic liquid. In motion, the temperature of the cryogenic liquid is allowed to rise to a temperature that does not exceed the saturation temperature of the cryogenic liquid at a given operating pressure. The pipeline is cooled with an inert gas, which is used as nitrogen, in a gaseous state with a liquefaction temperature lower than that of a cryogenic liquid.

EFFECT: reduction of hydraulic resistance during fluid movement.

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Description

The invention relates to a method for transporting cryogenic liquids, in particular liquefied gases, and can be used in the design and construction of pipelines transporting cryogenic liquids.

A known method of transportation of liquefied gases (Patent SU No. 903650, publ. 02/07/1982), containing a double-walled heat-insulated pipeline, the annular space of which is divided into sealed sections, they are supplied with intermediate gas at a pressure equal to the average pressure of the transported medium in this section of the pipeline. The pipeline contains a laminated insulation containing an elastic adsorbent, the space between which is evacuated.

The main disadvantages of this method are that during the initial filling of the pipeline, uneven compression of its sections occurs, which leads to the occurrence of significant stresses caused by the temperature difference between the cryogenic liquid and the pipeline, as well as due to the difficulty of maintaining vacuum along the entire length of the pipeline, there are restrictions on the length and the diameter of such pipelines. In addition, there is a gradual loss of the absorbing properties of the adsorbent.

A known method of preparation for transportation of a mixture of liquefied hydrocarbons through main pipelines in a cooled state (Patent RU No. 2584628, publ. 05/20/2016), including the purification of natural gas, its multistage cooling to a temperature from -30 to -50 ° C with the addition of cooled to a temperature from -20 to -50 ° C condensate in an amount of 3 to 10 wt. %. The resulting hydrocarbon mixture is cooled to a temperature of -40 to -50 ° C at a pressure of 10 to 12 MPa to a single-phase liquid state and transported through the main pipeline.

The main disadvantage of this method is that for such a significant decrease in the temperature of pumping liquefied natural gas, condensate containing heavy hydrocarbon fractions (pentane, hexane) is additionally introduced, which significantly worsens the quality of liquefied natural gas supplied to the consumer.

A known method of pumping LNG through main pipelines (Main pipelines of cooled and liquefied natural gas. Executive editor AE Polozov / NP Akulshina, VA Andrianov, VI Zorkaltsev and other URO RAS, Komi Science Center. SYKTYVKAR, 1988, p. 158), which includes pumping stations, cooling stations for the pumped product and linear points for controlling its temperature. Pumping is carried out through pipelines with a diameter of 1220 to 1420 mm, at a pressure of 5.5 MPa and a temperature range of 153K - 173K.

The main disadvantage of the described method is the need to cool a large mass of metal, which requires a large volume of cooled gas and subsequently liquefied natural gas supplied from the head liquefaction plant. The total time of the cool-down process for a pipeline with a diameter of 1420 mm and a length of 100 km is more than a day. In addition, the specially designed economical alloy steel 10KhGNMAYU, offered for construction, is in short supply, at the moment there is no industrial production in the world.

The known method of transportation (Patent WO2012127261A1, publ. 09/27/2012) liquefied gases, especially LNG, at a temperature below the saturation temperature. In the proposed method, the liquefied gas is cooled before loading into the tanks of the carrier vessel (for example, a ship, a car, a railway carriage) to a temperature below the saturation temperature at the design pressure in the tanks during transportation. During transportation, the flow of heat from the environment is absorbed due to the free increase in the temperature of the liquefied gas from a supercooled liquid to a saturated liquid state.

This method is inapplicable for the conditions of pipeline transport of cryogenic liquids, since the pipeline is an extended structure, then transportation with a free rise in temperature at atmospheric pressure (0.103 MPa) will inevitably lead to boiling of LNG.

There is a known method for transporting cryogenic liquids and a pipeline for its implementation (Patent RU No. 2156400, publ. 09/20/2000), including creating a reserve of the length of the pipeline, cooling the inner pipe of the pipeline to the temperature of pumping the product, pumping the transported product and regulating the temperature and pressure of the pipeline. The pipeline for transportation of cryogenic liquids consists of two concentrically installed pipes, fixed supports and technical means for regulating temperature and pressure. The outer and inner pipes of the pipeline are installed with the ability to move relative to each other, the outer pipe is equipped with two nozzles, and at least two supports of each linear section of the pipeline are sealed chambers with a length equal to the longitudinal displacement of this section at the operating temperature, while the chambers are connected to the outer pipeline pipe using telescopic expansion joints.

The disadvantage of this method is that during chilling, liquefied gas is fed into the annular space and, despite the use of a telescopic expansion joint system, the inner pipe will experience strong compressive loads, since the liquefied gas will sharply cool the line. Thus, it is not possible to avoid the formation of a two-phase flow and boiling of the liquid, which will also lead to increased loads on the pipe wall material, which can lead to an emergency. In addition, the complexity of creating and maintaining a vacuum along the entire length of the pipeline imposes restrictions on the length and diameter of such pipelines, and there is a gradual loss of the adsorbent's absorbing properties.

A known method of transportation of liquefied natural gas (Patent JPH05141599A, publ. 06/08/1993), taken as a prototype, in which LNG is transported through pipelines and cooled by liquid air obtained using the cold energy of liquefied natural gas in a separate process, before or after transportation (just before downloading). LNG stored in the tank is pumped in and pumped to the consumer via a double-walled pipeline, the space between which is filled with perlite and evacuated. The pressure rise in response to the temperature rise during transport is controlled to such an extent as to keep the LNG in a liquefied state.

The disadvantage of this method is the mixing of the flow of LNG and liquefied air, which degrades the quality of LNG and can lead to the formation of an explosive mixture. As noted above, the use of vacuum insulation imposes restrictions on the length and diameter of such pipelines, and there is also a gradual loss of the absorbing properties of the adsorbent.

The technical result is a decrease in hydraulic resistance during fluid movement by using pipes made of ultra-high molecular weight polyethylene (UHMWPE).

The technical result is achieved by the fact that the pipeline is made of ultra-high molecular weight polyethylene, the top is covered with several layers of insulation, including at least one heat-insulating layer, then at least two vapor barrier layers and a cover layer, the working pressure in the pipeline must be higher than the vapor pressure of the cryogenic liquid, while in motion, the temperature of the cryogenic liquid is allowed to rise to a temperature that does not exceed the saturation temperature of the cryogenic liquid at a given operating pressure, and the pipeline is cooled with an inert gas, which is nitrogen, in a gaseous state with a liquefaction temperature lower than that of a cryogenic liquid.

The method is illustrated by the following figures:

fig. 1 is a schematic diagram of LNG pumping through pipelines;

fig. 2 - cross-section of the pipeline;

fig. 3 is a graph of the pressure drop in a pipeline made of ultra-high molecular weight polyethylene (UHMWPE) and similar steel;

fig. 4 is a graph of the temperature distribution of LNG in a pipeline made of ultra-high molecular weight polyethylene and similar steel, where:

1 - liquefaction plant;

2 - transfer pumping station;

3 - additional cooling station;

4 - regasification unit;

5 - low-temperature storage;

6 - consumer;

7 - gas carrier;

8 - UHMWPE pipeline;

9 - heat-insulating layer;

10 - vapor barrier layer;

11 - cover layer.

The method is carried out as follows. The cryogenic liquid is fed for filling from the liquefaction plant 1 (Fig. 1), into a pipeline pre-cooled with liquid nitrogen. When a pipeline is filled with a cryogenic liquid, it irregularly fills its cross-section and accumulates in the lower part of the pipe, because of this, uneven stresses arise along the cross-section of the pipeline. The lower part of the pipe is subjected to strong compressive loads. In order to reduce such loads, the pipeline is pre-cooled with an inert cooled gas, whose liquefaction temperature is lower than the temperature of a cryogenic liquid, for example, nitrogen. The cooled gas is evenly distributed throughout the entire volume of the pipeline provided to it. Thus, during filling, a decrease in temperature stresses along the axial section of the pipeline is achieved, which makes it possible to reduce the likelihood of a two-phase flow and associated unfavorable gas-hydrodynamic effects, such as pressure pulsations, hydraulic shocks, as well as geyser effects during subsequent filling with a cryogenic liquid. To prevent mixing of LNG and nitrogen, a gastight piston separator made of cryogenic temperature resistant material can be used. The source of cooled nitrogen can be gas from a liquefaction plant, used in refrigeration cycles during liquefaction, or mobile units with liquefied nitrogen, which evaporates in the evaporator and cooled down is fed for filling through mobile compressor units.

Liquefied natural gas is pumped through a low-temperature insulated pipeline by a pumping station 2 (Fig. 1). For transportation, a pipeline made of UHMWPE 8 (Fig. 2) is used, made of ultra-high molecular weight polyethylene (UHMWPE). UHMWPE proposed as a pipeline material has significantly lower pipeline wall surface roughness and thermal conductivity coefficient in comparison with widely used nickel-containing steel. UHMWPE is capable of operating at temperatures down to -269 ° C while maintaining high impact toughness. These are the main characteristics that provide an increase in the length of the pipeline, which is also achieved by reducing the number of pumping stations for maintaining pressure and stations for additional liquid cooling.

The insulating coating consists of at least one heat-insulating layer 9 (Fig. 2), which is made, for example, of polyurethane foam, expanded pearlite sand, varieties of mineral wool or basalt fiber, approved for use at the temperature of cryogenic liquid transportation.

On top of the thermal insulation layer 9, at least two vapor barrier layers 10 are mounted, which can be made, for example, from a polyethylene film or foil.

Then the cover layer 11 is installed, made, for example, of composite materials or metal sheets.

The transportation parameters are determined by the formulas below, describing the process of movement of a cryogenic liquid, in particular liquefied natural gas, through a pipeline. The pressure loss in the considered section of the pipeline is determined from the Darcy-Weisbach expression:

where λ is the coefficient of hydraulic resistance;

x is the length of the pipeline section;

d _int - inner diameter,

v is the flow rate,

ρ is the density.

In formula (1), the effect of local resistances is taken into account by increasing the pressure loss by 1.5%.

Pressure change at any point in the flow:

p _i - pressure at the beginning of the pipeline section, Δp _i - pressure drop in the considered section of the pipeline

When moving through the section under consideration, liquefied natural gas changes its temperature due to an increase in heat content, which occurs due to heat input from the environment, for example, soil, as well as due to the performance of work when moving a cryogenic liquid with a mass flow rate G with overcoming the resistance idx. The change in temperature due to a change in the flow rate is neglected due to its insignificant value, since it is assumed that a stationary steady flow takes place in the pipeline. In the region of cryogenic temperatures, the phenomenon of inversion of the Joule-Thomson coefficient takes place, i.e. a change in its sign, and in this case it will contribute not to the cooling of the liquefied gas, but to its heating. Thus, the heat balance equation taking into account the Joule-Thomson effect takes the form:

G is the mass flow rate of the cryogenic liquid;

c _p - coefficient of heat capacity of LNG;

K is the total heat transfer coefficient;

d _int - internal diameter of the pipeline;

T is the LNG temperature at the pipeline section x;

T ₀ - soil temperature at the depth of the pipeline; g - acceleration of gravity,

i - hydraulic slope, p _i , p _{i + 1} - pressures at the beginning and at the end of the pipeline section, respectively;

x is the length of the pipeline section, D _h is the Joule-Thomson coefficient.

An expression that allows you to determine the temperature at any point in the flow:

Heat flows from the ground to the insulated pipeline, then from the insulation coating to the pipe wall and from the pipe wall to the pumped stream. This process is mathematically expressed by the total heat transfer coefficient:

λ _p , λ _ins are the coefficients of thermal conductivity of the pipe wall and the pipeline insulating material, respectively; d _ext is the outer diameter of the pipeline; d _ins - diameter of the pipeline with an insulating coating; α ₁ - internal heat transfer coefficient characterizing the movement of heat from the inner surface of the pipeline to LNG:

λ _LNG - coefficient of thermal conductivity of LNG;

Nu is the Nusselt number.

α ₂ - external heat transfer coefficient characterizing the movement of heat from the ground to the outer surface of the insulating coating:

λ _gr - coefficient of soil thermal conductivity;

h ₀ - equivalent depth of the pipeline.

If the initial pressure and diameter of the pipeline are unknown, then a hydraulic calculation is performed to determine them, assuming that the flow regime is stationary at a constant temperature selected from the phase diagram for a liquid of a given composition. They are used to examine the area to the left, which lies above the curve of the beginning of evaporation. Based on the need to maintain the flow in a single-phase state, the calculation to determine the initial pressure in the pipeline is performed along the upper boundary of the selected temperature range, since in this case there is the greatest probability of a two-phase flow formation. To determine the pressure at the beginning of a pipeline section, it is assumed that the pressure at the end of the section is equal to the saturation pressure of the LNG. Thus, the pressure at the beginning is the sum of the pressure loss along the length of the pipe (formula 1) and the pressure at the end of the pipeline section.

When a cryogenic liquid moves through a pipeline, its temperature can be increased to an intermediate temperature, but not exceeding the saturation temperature at a given pressure. The working pressure of the pumped cryogenic liquid must be higher than the saturated vapor pressure at a given temperature.

After filling the line with cryogenic liquid and reaching operating parameters, depending on the required transportation distance, the selected pressure and the temperature interval of pumping, intermediate pressure maintenance and cooling stations can be installed along the pipeline route. It should be noted that the installation of intermediate stations may be required in cases where the transportation distance is 50-100 km. However, so far, the transportation of LNG over such distances is almost never used. The calculation provides the necessary mode of transportation over a distance of about 20 km.

At the final section of the pipeline, depending on the needs of the end user, an additional cooling station 3 (Fig. 1) is installed, or LNG is fed directly to the regasification unit 4. At the additional cooling station, the cryogenic liquid is cooled to a temperature below the normal boiling point, in cases where the cryogenic liquid stored in a low-temperature storage 5 at atmospheric pressure, goes directly to a consumer 6 or for further loading into a gas carrier 7.

As an example, confirming the efficiency of the method, the calculation of the LNG pipeline was performed with the composition (in% vol.): СН ₄ - 99.8%; N ₂ - 0.13%; C ₂ H ₆ - 0.07%, according to the above formulas 1-7 for a pipeline made of ultra-high molecular weight polyethylene (UHMWPE) and AISI 321 steel, an analogue of 12X18H10T. The inlet pressure and temperature were 2 MPa and -160 ° C, respectively. As a heat-insulating layer, polyurethane foam is used, the cover layer is made of rolled fiberglass and two layers of vapor barrier material made of polyethylene heat-shrinkable film. The initial characteristics of the pipeline are presented in the form of table 1.

Table 1 - Initial data

Parameter The magnitude UHMWPE AISI 321 Outside diameter of the pipeline, d _ext , mm 350 351 Internal diameter of the pipeline, d _int , mm 286 325 Outside diameter of a pipeline with an insulating coating, d _ins , mm 410 411 Length of the pipeline, L, m 20,000 Roughness of the pipeline wall, mm 0.00022 0.2 Mass flow, G, t / day 5000 Thermal conductivity coefficient of the pipe material, λ _pipe, W / (m K) 0.3 eleven Coefficient of thermal conductivity of the insulating coating, λ _ins, W / (m K) 0.03 Coefficient of thermal conductivity of soil, λ _gr, W / (m K) 1.9 Equivalent laying depth, h _0, m 2

FIG. 3 shows a graph of the pressure drop in an underground LNG pipeline made of steel (dashed line) and UHMWPE (solid line) depending on its length, obtained from the results of the calculation performed. The pressure loss in the steel pipeline is much higher than that of the UHMWPE, which is obviously explained by the higher roughness values of the steel. As you can see from the graph, the pressure drop is almost linear.

FIG. 4 shows the change in the LNG temperature for a pipeline made of UHMWPE and AISI 321 steel, built according to the calculation results.It shows that the temperature rise rate in a UHMWPE pipeline is lower than in a similar steel, which is primarily associated with a significantly lower thermal conductivity polymer material 0.3 W / (m K) in comparison with 11 W / (m · K) for a steel pipeline.

However, the most significant is the difference in pressure losses during movement, which is confirmed by the basic formulas of hydraulics and different roughness of pipes, as well as the ability of polymers to self-lubricate. Based on this, it can be concluded that it is possible to transport cryogenic liquids over long distances without the construction of additional pumping stations and additional cooling stations.

Claims (1)

A method of transporting cryogenic liquids, including the transportation of LNG through pipelines and cooling the pipeline before transportation, characterized in that the pipeline is made of ultra-high molecular weight polyethylene, the top is covered with several layers of insulation, including at least one heat-insulating layer, then at least two vapor barrier layers and a cover layer, working the pressure in the pipeline must be higher than the vapor pressure of the cryogenic liquid, while in motion the temperature of the cryogenic liquid is allowed to rise to a temperature that does not exceed the saturation temperature of the cryogenic liquid at a given operating pressure, and the pipeline is cooled with an inert gas, which is used as nitrogen, in gaseous state with a liquefaction temperature lower than that of a cryogenic liquid.

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