Two-phase flow pipeline determination
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During a casual conversation, a colleague mentioned that they rarely encounter two-phase flow pipelines in design, which seemed strange to them. This person seems to have had no opportunity to assess the two-phase flow. Unless there is no steam heating (fractionation, vaporization), a two-phase flow will always occur, for the following reasons: 1. The outlet of the reboiler in a fractionation (distillation) tower is 100% a two-phase flow. 2. In a reboiler (evaporator), the steam-heated trap is saturated before it; the back pressure behind the valve is always lower than that in front of the valve. Moreover, since ordinary traps have a slight amount of steam leakage, there is always some steam present. Generally, the outlet of a reboiler is short, so even without performing calculations, there are usually no problems. If the steam pressure is low, the unit is small, and the steam condensate pipeline is short, the condensate enters the tank (at atmospheric pressure) and is then pumped for recovery. But: 1. If there are multiple steam grades in the system and the condensate enters the same recovery system, it is advisable to carry out calculations; otherwise, water hammer effects will prevent determining the cause as \"steam leakage from the check valve,\" and this will not allow the actual problem to be resolved. 2. If the condensate needs to be transported over a long distance under residual pressure, this factor also needs to be taken into account; ultimately, the pipe diameter is determined through calculations. As for the two-phase flow of process materials, it needs to be calculated even more. Otherwise, the problem cannot be solved! --For example, if the material entering a tower via a certain device is in a two-phase flow state and the tower experiences vibration, the issue is addressed by treating it as a two-phase flow problem by moving the control valve assembly to the inlet, but this still does not solve the problem. It’s actually a problem inside the tower.
For two-phase flow pipelines, pipeline erosion also needs to be taken into account
The condensate issue can be ignored if the pipeline is short (leading to the flash tank).
For two-phase flow, it is divided into: gas-liquid two-phase flow, gas-solid two-phase flow, and liquid-solid two-phase flow. The poster is talking about gas-liquid two-phase flow. In gas-liquid two-phase flow, under normal conditions, when the gas phase makes up 6% to 98% of the gas-liquid mixture, it is considered gas-liquid two-phase flow; in such cases, the diameter of the pipeline and the pressure drop must be calculated using the methods applicable to gas-liquid two-phase flow. Gas-liquid two-phase flow is further divided into non-flash and flash types, with different calculation methods for each. Gas-solid two-phase flow, in which gas and solids flow together within a pipe. Mainly in pneumatic conveying. Gas-solid two-phase flow is further divided into dilute phase and dense phase. Its calculation method is also different. For information on gas-liquid two-phase flow, the determination and calculation methods for non-flash and flash types, as well as those for gas-solid two-phase flow in dilute and dense phases, refer to sections 3 Gas-Liquid Two-Phase Flow (Non-Flash Type), 4 Gas-Liquid Two-Phase Flow (Flash Type), and 5 Gas-Solid Two-Phase Flow in HG/T 20570.7-95 Calculation of Pipeline Pressure Drop. For reference.