Discussion on the setting of liquid seals per HG20570.17-95

Regarding the liquid seal arrangement specified in clause 2.0.2.10 of the HG20570.17-95 standard, such a design is typically intended to ensure the safety and proper operation of the system. Liquid seals, especially water seals, are primarily used to prevent gas backflow and ensure that gas can only flow through specific paths. The high-pressure steam at the outlet of the jet pump, as you mentioned, can indeed exert pressure on the water seal, but this is taken into account in the design. Setting a water column head of 100 kPa is primarily to ensure a sufficient pressure difference, so that the water seal can effectively prevent gas from flowing back. Even under the action of high-pressure steam, such a water column pressure is sufficient to maintain the water seal stable and prevent water from being forced out. If the inside of the separation tank is under negative pressure, the water column indeed serves to prevent water from being drawn into the separation tank. This provision in the design is intended to balance the pressure in the system, ensuring that the water seal functions effectively under various operating conditions. The questions you raise are very insightful; reasonable doubt and discussion help to better understand and apply these standards. If there are diagrams illustrating the specific settings and your views, I can provide more detailed analysis and suggestions. .

The final system should **operate at the vacuum level required by the vacuum system**

When designing a vacuum system, it is crucial to ensure that the system achieves and maintains the desired level of vacuum. The use of liquid seals, especially those as specified in standard HG20570.17-95, is intended to prevent the backflow of air or other gases during system operation, thereby maintaining a negative pressure or the desired vacuum level inside the system. The function of the liquid seal is not only to prevent gas backflow, but it also absorbs some of the shock waves generated by the ejector pump, thereby reducing pressure fluctuations within the system. Reserving a water column head of 100 kPa is a safety measure to ensure that, even under changing operating conditions such as sudden changes in flow rate or pressure, the water seal can still perform its function, preventing water from being drawn into the separation tank or from being forced out by high-pressure steam. When designing and operating vacuum systems, it is crucial to consider the overall stability of the system. This includes reasonable design parameters, such as the height of the water column, as well as the correct configuration of the various components of the system, to ensure that it can achieve the desired level of vacuum and operate stably. Possible abnormal situations are also taken into account during system design to ensure the safety and stability of the system under all circumstances. Therefore, the installation of the liquid seal and the provision for a water column head are based on a thorough understanding and prediction of the system’s operating conditions, with the aim of ensuring that the system can operate safely and efficiently under various conditions, while simultaneously achieving and maintaining the desired level of vacuum. This design approach reflects both a commitment to system security and an emphasis on maintaining system stability and efficiency. .

The outlet of the jet pump is at high pressure, but the pressure is released once it enters the gas-liquid separation tank. The separation tank has a gas outlet, and there is usually no liquid level inside the tank. Vacuum systems usually have pressure transmitters, and control valves are installed on the steam pipes of steam ejector pumps, or on the process gas pipes feeding into the ejector pumps, as well as on the pipes that return exhaust gases to the vacuum system, in order to regulate the vacuum level of the system. If the control valves are located on the steam side, then when the amount of steam is low, the pressure at the outlet of the ejector pump is close to the pressure in the vacuum system, resulting in a negative pressure. On the other hand, the gas-liquid separation tank has cooling water for heat exchange; the steam coming from the steam ejector is converted into condensed water, reducing its volume and thus creating a negative pressure. A 10-meter water column is considered based on complete vacuum conditions. Similarly, before the jet pump in the vacuum system, there is often a heat exchanger or a separation tank equipped with cooling water or chilled water, and a 10-meter water seal is also required.

The theoretical maximum vacuum level is -0.1 MPa; the liquid seal height is set to more than 10 meters (>0.1 MPa) in order to overcome this vacuum level and allow the condensate to flow smoothly into the storage tank.

How is it achieved, as you mentioned, to prevent water from being forced out by high-pressure steam? A 10-meter water column can flow down by gravity even in the absence of pressure

There is another issue: regarding the gas transfer machinery I’ve been looking at recently, can it really be analyzed using Bernoulli’s equation? Can we simply assume that vacuum creation and pressure increase represent external work done on the system to add mechanical energy, thereby enabling the conversion between kinetic energy and static pressure? In other words, there is always some expansion process that allows the flow to pass from a small diameter to a larger one, with the dynamic pressure being converted into static pressure – which is essentially a process of pressure increase； But if I consider things ideally, based on the ideal gas law, the conclusion is the opposite: according to PV=nRT, as the volume of a compressed gas decreases, its pressure increases; therefore, the process of expanding the diameter seems to be a process of reducing pressure, which contradicts the previous conclusion.

This post was last edited by Sihai Youxian Tian on 2024-3-1 18:44. Thank you for sharing