Introduction

With the development of cryogenic technology, cryogenic liquid products have been playing an important role in many fields such as national economy, national defense and scientific research. The application of cryogenic liquid is based on the effective and safe storage and transportation of cryogenic liquid products, and the pipeline transmission of cryogenic liquid runs through the whole process of storage and transportation. Therefore, it is very important to ensure the safety and efficiency of cryogenic liquid pipeline transmission. For the transmission of cryogenic liquids, it is necessary to replace the gas in the pipeline before transmission, otherwise it may cause operational failure. The precooling process is an inevitable link in the process of cryogenic liquid product transportation. This process will bring strong pressure shock and other negative effects to the pipeline. In addition, the geyser phenomenon in the vertical pipeline and the unstable phenomenon of system operation, such as blind branch pipe filling, filling after interval drainage and filling of air chamber after valve opening, will bring different degrees of adverse effects on the equipment and pipeline. In view of this, this paper makes some in-depth analysis on the above problems, and hopes to find out the solution through the analysis.

Displacement of gas in line before transmission

With the development of cryogenic technology, cryogenic liquid products have been playing an important role in many fields such as national economy, national defense and scientific research. The application of cryogenic liquid is based on the effective and safe storage and transportation of cryogenic liquid products, and the pipeline transmission of cryogenic liquid runs through the whole process of storage and transportation. Therefore, it is very important to ensure the safety and efficiency of cryogenic liquid pipeline transmission. For the transmission of cryogenic liquids, it is necessary to replace the gas in the pipeline before transmission, otherwise it may cause operational failure. The precooling process is an inevitable link in the process of cryogenic liquid product transportation. This process will bring strong pressure shock and other negative effects to the pipeline. In addition, the geyser phenomenon in the vertical pipeline and the unstable phenomenon of system operation, such as blind branch pipe filling, filling after interval drainage and filling of air chamber after valve opening, will bring different degrees of adverse effects on the equipment and pipeline. In view of this, this paper makes some in-depth analysis on the above problems, and hopes to find out the solution through the analysis.

The precooling process of the pipeline

In the whole process of cryogenic liquid pipeline transmission, before establishing a stable transmission state, there will be a pre-cooling and hot piping system and receiving equipment process, that is, the pre-cooling process. In this process, the pipeline and receiving equipment to withstand considerable shrinkage stress and impact pressure, so it should be controlled.

Let’s start with an analysis of the process.

The whole precooling process starts with a violent vaporization process, and then appears two-phase flow. Finally, single-phase flow appears after the system is completely cooled. At the beginning of the precooling process, the wall temperature obviously exceeds the saturation temperature of the cryogenic liquid, and even exceeds the upper limit temperature of the cryogenic liquid — the ultimate overheating temperature. Due to heat transfer, the liquid near the tube wall is heated and instantaneously vaporized to form vapor film, which completely surrounds the tube wall, that is, film boiling occurs. After that, with the precooling process, the temperature of the tube wall gradually drops below the limit superheat temperature, and then favorable conditions for transition boiling and bubble boiling are formed. Large pressure fluctuations occur during this process. When the precooling is carried out to a certain stage, the heat capacity of the pipeline and the heat invasion of the environment will not heat the cryogenic liquid to the saturation temperature, and the state of single-phase flow will appear.

In the process of intense vaporization, dramatic flow and pressure fluctuations will be generated. In the whole process of pressure fluctuations, the maximum pressure formed for the first time after the cryogenic liquid directly enters the hot pipe is the maximum amplitude in the whole process of pressure fluctuation, and the pressure wave can verify the pressure capacity of the system. Therefore, only the first pressure wave is generally studied.

After the valve is opened, the cryogenic liquid quickly enters the pipeline under the action of pressure difference, and the vapor film generated by vaporization separates the liquid from the pipe wall, forming a concentric axial flow. Because the resistance coefficient of the vapor is very small, so the flow rate of the cryogenic liquid is very large, with the forward progress, the temperature of the liquid due to heat absorption and gradually rise, accordingly, pipeline pressure increases, filling speed slows down. If the pipe is long enough, the liquid temperature must reach saturation at some point, at which point the liquid stops advancing. The heat from the pipe wall into the cryogenic liquid is all used for evaporation, at this time the evaporation speed is greatly increased, the pressure in the pipeline is also increased, may reach 1. 5 ~ 2 times of the inlet pressure. Under the action of pressure difference, part of the liquid will be driven back to the cryogenic liquid storage tank, resulting in the speed of vapor generation becomes smaller, and because part of the vapor generated from the pipe outlet discharge, pipe pressure drop, after a period of time, the pipeline will re-establish the liquid into the pressure difference conditions, the phenomenon will appear again, so repeated. However, in the following process, because there is a certain pressure and part of the liquid in the pipe, the pressure increase caused by the new liquid is small, so the pressure peak will be smaller than the first peak.

In the whole process of precooling, the system not only has to bear a large pressure wave impact, but also has to bear a large shrinkage stress due to cold. The combined action of the two may cause structural damage to the system, so necessary measures should be taken to control it.

Since the precooling flow rate directly affects the precooling process and the size of cold shrinkage stress, the precooling process can be controlled by controlling the precooling flow rate. The reasonable selection principle of the precooling flow rate is to shorten the precooling time by using a larger precooling flow rate on the premise of ensuring that the pressure fluctuation and cold shrinkage stress do not exceed the allowable range of equipment and pipelines. If the pre-cooling flow rate is too small, the pipeline insulation performance is not good for the pipeline, it may never reach the cooling state.

In the process of precooling, due to the occurrence of two-phase flow, it is impossible to measure the real flow rate with the common flowmeter, so it can not be used to guide the control of precooling flow rate. But we can indirectly judge the size of the flow by monitoring the back pressure of the receiving vessel. Under certain conditions, the relationship between the back pressure of the receiving vessel and the pre-cooling flow can be determined by analytical method. When the precooling process progresses to the single-phase flow state, the actual flow measured by the flowmeter can be used to guide the control of the precooling flow. This method is often used to control the filling of cryogenic liquid propellant for rocket.

The change of the back pressure of the receiving vessel corresponds to the precooling process as follows, which can be used to qualitatively judge the precooling stage: when the exhaust capacity of the receiving vessel is constant, the back pressure will increase rapidly due to the violent vaporization of the cryogenic liquid at first, and then gradually fall back with the decrease of the temperature of the receiving vessel and pipeline. At this time, the precooling capacity increases. 

Tuned to the next article for other questions！

HL Cryogenic Equipment

HL Cryogenic Equipment which was founded in 1992 is a brand affiliated to HL Cryogenic Equipment Company Cryogenic Equipment Co.,Ltd. HL Cryogenic Equipment is committed to the design and manufacture of the High Vacuum Insulated Cryogenic Piping System and related Support Equipment to meet the various needs of customers. The Vacuum Insulated Pipe and Flexible Hose are constructed in a high vacuum and multi-layer multi-screen special insulated materials, and passes through a series of extremely strict technical treatments and high vacuum treatment, which is used for transferring of liquid oxygen, liquid nitrogen, liquid argon, liquid hydrogen, liquid helium, liquefied ethylene gas LEG and liquefied nature gas LNG.

The product series of Vacuum Jacketed Pipe, Vacuum Jacketed Hose, Vacuum Jacketed Valve, and Phase Separator in HL Cryogenic Equipment Company, which passed through a series of extremely strict technical treatments, are used for transferring of liquid oxygen, liquid nitrogen, liquid argon, liquid hydrogen, liquid helium, LEG and LNG, and these products are serviced for cryogenic equipment (e.g. cryogenic tanks, dewars and coldboxes etc.) in industries of air separation, gases, aviation, electronics, superconductor, chips, automation assembly, food & beverage, pharmacy, hospital, biobank, rubber, new material manufacturing chemical engineering, iron & steel, and scientific research etc.