Regulating valve piping fluid bag

For any type of pipeline, it is generally attempted to avoid the formation of air pockets or liquid pockets, unless there are specific process requirements, such as the use of liquid seals. The liquid bag of the control valve is sometimes necessary for ease of maintenance and operation; this is also why drain pipes are installed before and after the control valve. I’m not sure if this helped you:D

For certain special media used in some devices, it is not allowed to have liquid pockets even at the control valves. For most conventional media, the control valves are connected to the pipeline system from above and back to it again; this is done to facilitate operation. In cases where condensate might form and create a liquid seal that affects operation, then heat tracing must be added.

Well, your answer is quite detailed. There is a low-point drain in front of the control valve, but I would like to ask further: what exactly are the liquid bag and the air bag, and where in the control valve are they located? Please advise. Thank you

This post was last edited by ylb913 on 2016-12-14 06:10. 1. A liquid bag refers to gases. During the transportation of gas, either due to foam being carried out from the outlet of previous equipment or as a result of temperature drops during transport, a large amount of condensed oil or water forms in the U-shaped sections of the gas transmission pipes, thereby creating a liquid seal. ——A bag is just a shape of a pipe; whether a liquid seal will form depends on the medium and operating conditions. There is also the issue of whether a liquid seal will form, which is concerning. (1) This is not a problem for media under high pressure; it generally does not draw much attention, nor does it have any significant impact on the normal operation of the device. We found such problems due to sampling. This is actually also related to the flow rate of the gas; when the gas flow rate is low, sampling at the U-tube was intended to yield a gas sample, but upon opening the sampling valve, a large amount of liquid came out (later we changed the sampling point to the outlet of the subsequent tank). For such U-tubes, when the flow rate is high, this isn’t a problem, as an equilibrium is established between the amount of condensate and the amount that is carried away by the gas (in fact, at high flow rates, there is basically no issue with liquid remaining in the U-tube, which also includes the situation where the temperature drops when the flow rate is high). ——A typical example of this is when we use a catalytic dry gas desulfurization tower to treat hydrogenated light gas. When the catalytic unit is shut down, the gas flow rate is too low, which leads to excessive carryover of condensed oil. However, the pressure of the light gas is high; aside from affecting gas sampling, this has no significant impact on the operation of the unit. (2) For systems with lower pressure, let’s take an example: it involves the long-distance transportation of acidic gas from the top of the desulfurization amine liquid regeneration tower. The pressure of this regenerated acidic gas is less than 0.1 MPa. Once a liquid seal forms, the acidic gas arriving at the downstream receiving system will be delivered in bursts, resulting in significant fluctuations. This prevents proper air supply for the sulfur recovery process; moreover, when enough liquid accumulates, the acidic gas from the regeneration system cannot be discharged properly. Our approach to this is to maintain steam heating throughout the year, or to use the system that is turned on earliest and turned off latest among all the steam heating systems in the plant. 2. The air pocket refers to pipelines used for transporting liquids; at the highest points of such pipelines, a gas phase forms (this also includes the gas already present in the pipelines at the time the system is first put into operation). This is the issue associated with the “inverted U” shape of the pipelines. For example, system ducts have many \"gantry\" structures, all of which are in a \"reverse U\" shape. If gas is released at each high point, that gas volume will keep accumulating over time. What is air rent under such operating conditions? ——Starting with siphoning: in the absence of gas evolution, the liquid rises first and then drops, which is a result of the siphon effect. The pressure drop associated with this flow is very small; in fact, the pulling force from behind can be greater than the pushing force from ahead. (Our plant’s refined oil storage area is located at a lower level, and for the examples I’ll mention later, this storage area exerts a pulling force on our equipment.) But once there is gas (a gas cushion) at each high point, the siphoning effect disappears completely; the suction force from behind is lost, and as the liquid rises, it has to overcome the pressure caused by the height of the liquid column in front of each gate. ——A typical example of this is the old gasoline desulfurization unit (with the improvement in gasoline quality, this unit is being dismantled these days). Since desulfurization requires air to be injected into the gasoline reactor, as the flow rate of gasoline decreases while the air injection rate remains constant, air lockages occur. These air lockages lead to an even further reduction in the flow rate of gasoline entering the unit; in severe cases, the high level of air lockages can prevent gasoline from entering the unit at all. ——In reality, the pressure of our air is only around 0.45 MPa. If the pressure of gasoline entering the system exceeds this value, the gasoline will flow back into the industrial air system, which is very dangerous. Our approach to solving such problems is to remove one or two reactors. We have three gasoline desulfurization reactors, each about 10 meters tall, connected in series (with low inlet and high outlet for gasoline). Gas bubbles first appear at the top of the reactors; by removing one or two of them, the pressure resistance caused by the rising liquid column is significantly reduced. At the same time, the amount of air injected is decreased to prevent air from forming at the top of the reactors. As the pressure of the gasoline entering the system decreases, the gasoline reactors can be brought online one by one. 3. The low-point drain of the control valve is activated only when it is necessary to vent air after the unit is taken out for maintenance or during purging when the plant is shut down; it has no relation to normal operation. 4. There are situations where a liquid seal is required, but this is generally specified clearly in the design drawings. The U-shaped or inverted U-shaped shapes that naturally form in the piping, as I mentioned earlier, are things that aren’t given much consideration during the design process; the solutions to these issues have also been outlined earlier. Regarding the issue with air pockets at the time of initial operation, it was common to install vents at high points for exhaust. Later on, we introduced material of the same medium but at higher pressure to push out the gas before normal use; however, using vents at high points proved to be too troublesome.

Thank you for your answer. I’m slowly understanding and absorbing it by relating it to real-life situations

This post was last edited by ylb913 on 2016-12-14 at 12:21. You might as well just say what medium (or device)’s absorption you want me to explain. Absorption is generally a physical process (dissolution), but it is also possible for processes that involve chemical reactions to be referred to as absorption as well; for example, the towers used to remove hydrogen sulfide from gases and liquefied gases using an aqueous solution of ethanolamine are called absorption towers. Also in the context of absorption, there is a process known as extraction; for example, using alkaline solutions to remove thiol compounds from liquefied gas. Additionally, there are some industries called extraction. Also, absorption generally involves using a liquid to absorb a gas; in both extraction and distillation, the phases are liquids. ——Maybe I made a mistake and clicked on something; at least I can’t see the reply posts from the 7th floor.