An unstable process in transmission

In the process of cryogenic liquid pipeline transmission, the special properties and process operation of cryogenic liquid will cause a series of unstable processes different from that of normal temperature fluid in the transition state before the establishment of stable state. The unstable process also brings great dynamic impact to the equipment, which may cause structural damage. For example, the liquid oxygen filling system of the Saturn V transport rocket in the United States once caused the rupture of the infusion line due to the impact of the unstable process when the valve was opened. In addition, the unstable process caused the damage of other auxiliary equipment (such as valves, bellows, etc.) is more common. The unstable process in the process of cryogenic liquid pipeline transmission mainly includes the filling of blind branch pipe, the filling after intermittent discharge of liquid in the drain pipe and the unstable process when opening the valve which has formed the air chamber in the front. What these unstable processes have in common is that their essence is the filling of the vapor cavity by cryogenic liquid, which leads to intense heat and mass transfer at the two-phase interface, resulting in sharp fluctuations of system parameters. Since the filling process after intermittent discharge of liquid from the drain pipe is similar to the unstable process when opening the valve that has formed the air chamber in the front, the following only analyzes the unstable process when the blind branch pipe is filled and when the open valve is opened. 

The Unstable Process of Filling Blind Branch Tubes

For the consideration of system safety and control, in addition to the main conveying pipe, some auxiliary branch pipes should be equipped in the pipeline system. In addition, safety valve, discharge valve and other valves in the system will introduce corresponding branch pipes. When these branches are not working, blind branches are formed for the piping system. The thermal invasion of the pipeline by the surrounding environment will inevitably lead to the existence of vapor cavities in the blind tube (in some cases, vapor cavities are specially used to reduce the heat invasion of the cryogenic liquid from the outside world “). In the transition state, the pressure in the pipeline will rise because of valve adjustment and other reasons. Under the action of pressure difference, the liquid will fill the vapor chamber. If in the filling process of the gas chamber, the steam generated by the vaporization of the cryogenic liquid due to heat is not enough to reverse drive the liquid, the liquid will always fill the gas chamber. Finally, after filling the air cavity, a rapid braking condition is formed at the blind tube seal, which leads to a sharp pressure near the seal

The filling process of the blind tube is divided into three stages. In the first stage, the liquid is driven to reach the maximum filling speed under the action of pressure difference until the pressure is balanced. In the second stage, due to inertia, the liquid continues to fill forward. At this time, the reverse pressure difference (the pressure in the gas chamber increases with the filling process) will slow down the fluid. The third stage is the rapid braking stage, in which the pressure impact is the largest.

Reducing the filling speed and reducing the size of the air cavity can be used to eliminate or limit the dynamic load generated during the filling of the blind branch pipe. For the long pipeline system, the source of the liquid flow can be adjusted smoothly in advance to reduce the velocity of the flow, and the valve closed for a long time.

In terms of structure, we can use different guiding parts to enhance the liquid circulation in the blind branch pipe, reduce the size of the air cavity, introduce local resistance at the entrance of the blind branch pipe or increase the diameter of the blind branch pipe to reduce the filling speed. In addition, the length and installation position of the braille pipe will have an impact on the secondary water shock, so attention should be paid to the design and layout. The reason why increasing the pipe diameter will reduce the dynamic load can be qualitatively explained as follows: for the blind branch pipe filling, the branch pipe flow is limited by the main pipe flow, which can be assumed to be a fixed value during qualitative analysis. Increasing the branch pipe diameter is equivalent to increasing the cross-sectional area, which is equivalent to reducing the filling speed, thus leading to the reduction of load.

The Unstable Process of Valve Opening

When the valve is closed, heat intrusion from the environment, especially through the thermal bridge, quickly leads to the formation of an air chamber in front of the valve. After the valve is opened, the steam and liquid begin to move, because the gas flow rate is much higher than the liquid flow rate, the steam in the valve is not fully opened soon after evacuation, resulting in a rapid drop in pressure, liquid is driven forward under the action of pressure difference, when the liquid close to not fully opened the valve, it will form braking conditions, At this time, water percussion will occur, producing a strong dynamic load.

The most effective way to eliminate or reduce the dynamic load generated by the unstable process of valve opening is to reduce the working pressure in the transition state, so as to reduce the speed of filling the gas chamber. In addition, the use of highly controllable valves, changing the direction of the pipe section and introducing small diameter special bypass pipeline (to reduce the size of the gas chamber) will have an effect on reducing the dynamic load. In particular, it should be noted that different from the dynamic load reduction when the blind branch pipe is filled by increasing the blind branch pipe diameter, for the unstable process when the valve is opened, increasing the main pipe diameter is equivalent to reducing the uniform pipe resistance, which will increase the flow rate of the filled air chamber, thus increasing the water strike value.

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.