Ultimately, gas delivery pipelines should avoid liquid pockets, while liquid delivery pipelines should avoid air pockets

Air bags and liquid bags are both determined based on the gas or liquid to be transported. It mainly occurs in uneven areas during pipeline installation.

With air bags, for liquids flowing at low speeds, the resistance exerted by the gas inside the air bags may prevent flow; therefore, even when air bags are present, it is necessary to have a venting mechanism

The last edit to this post was made by ylb913 on 2014-12-31 at 16:15. One principle in piping design is to use a layout with gradually increasing or decreasing pressures as much as possible; for example, the process gas lines in our sulfur recovery system must be arranged in this way (for gases, it’s acceptable for the pressure to be higher in the middle and lower at both ends). Such requirements apply even to acidic gas pipelines, but it’s basically impossible to meet them for such pipelines. The solution is to install heat tracing, and we do this 24/7 throughout the year. For liquids, various control valve assemblies, as well as the inlet and outlet of heat exchangers, are led from a higher position to the ground, fitted with valves, and then returned to the higher position (with a vent at the lower point). What I dislike about it is the U-shaped tube formed on the duct; it’s not really appropriate to add a vent in mid-air. ——This is the issue of incomplete shutdown and material removal. For gases, it can be called a liquid bag or a liquid seal. For liquids, it should be called a “bag-shaped” container; calling it a liquid bag is not appropriate. In fact, there are not many cases where gas resistance (gas bags, air cushions) affects liquids; the most typical example in our field is the former gasoline desulfurization unit. The situation is as follows: the normal pressure of gasoline leaving the unit is 1.5 kilograms, while the pressure at the inlet to the unit is around 3 kilograms. If the pressure exceeds 4.5 kilograms, air and alkali can no longer be introduced, and the gasoline may even flow in the reverse direction. The facility consists of three reactors, each nearly 20 meters tall, and there are many gantries in the tank area for loading and unloading. Under normal conditions, I have a flow rate of 40 cubic meters per hour, with an air injection rate of 8 cubic liters per hour – in this case there are no problems. But if the gasoline flow rate drops below 20 cubic meters per hour, while the air injection rate remains at 8 cubic liters per hour, problems arise. At that point, the pressure of gasoline entering the device gradually increases; once the pressure reaches a certain level, the flow rate of gasoline further decreases, until it can no longer enter the device at all, as if the device is blocked. Actually, if you check the pressure at which gasoline is delivered at this point, you will find that pressure has also increased. Our air resistance (air cushion) comes from two sources: one is the air that forms at the top of each reactor, and the other is the air that forms at the upper parts of the gantries. Under normal conditions, when no air is released, the suction force acting downward partially offsets the resistance to the upward movement of gasoline; however, with an air cushion present, the height of each upward-moving liquid column becomes a real obstacle.

I mean, why avoid the formation of air pockets and liquid pockets? Is it due to resistance? Is it to transport gas in order to avoid air bubbles, or to transport liquid in order to avoid air bubbles? Or is it to transport liquid in order to avoid air bubbles as well as liquid bubbles?

Are there air pockets in gas transportation? Are there liquid bags in liquid transportation? Will flow also be difficult due to resistance?

Are you saying that even if a reverse U-shaped pattern isn’t formed, an air cushion might still arise, making it difficult for the liquid to flow?

I thought about it – in liquid transportation as well, it’s necessary to prevent the formation of liquid pockets. If such pockets form, it’s difficult to empty them completely, and in winter they can also cause damage to the pipes. It’s not only in gas transportation that preventing the formation of liquid pockets is important

Whether it’s gas or liquid, it’s necessary to try to avoid this situation, right?

To prevent liquid flow from being blocked by air pockets in liquid transportation, why is it necessary to avoid air pockets in gas transportation? Is it due to friction loss? But it seems that the pressure drop resulting from this should be relatively small, right?

There is also a possibility of liquid accumulation at the lowest points for some gas transfers