Considerations regarding the relief operation of safety valves.
Thread Content
This post was last edited by ice2008 on 2022-12-20 at 16:14. 1. If the safety valve has discharge under two operating conditions, for example, in Condition 1, it discharges in gaseous form； Condition 2 is liquid phase release ； How should this be taken into account when selecting a safety valve? 2. For example, in the case of a pressure vessel with the inlet valve of the liquid pipeline open and the outlet valve closed, is the discharge volume of the safety valve equal to the amount of liquid entering, that is, the liquid flow rate? (An answer already exists; API 520 specifies that it is a liquid-phase discharge condition.) 3. If a safety valve operates in both a liquid-phase discharge mode (with the outlet valve closed) and a gas-phase discharge mode (in case of a fire), then how should the fluid discharged by this safety valve be handled – directly to the flare or into a liquid storage tank? – I noticed that in previous projects, the liquid went straight to the flare tower; there was no separate liquid separation tank before it reached the flare tower. I don’t understand why, in the case of a leak where the liquid is released, it would end up in the fire suppression pipelines – it seems quite unreasonable.
I have never seen a situation like the one you described, but I can give a similar example. The water entering the boiler’s drum is in liquid form, while the safety valve discharges steam under normal conditions. However, liquids are generally not allowed to flow out through the safety valve, as liquids are incompressible fluids. In a safety valve designed to release gas, if the flow rate of discharge is not fast enough to keep up with the inflow rate, it can lead to overpressure in the container. This is generally how it’s done: a safety valve is installed at the top to discharge gas and regulate pressure, while a level gauge is placed on the steam drum to control the liquid level. Once the level exceeds the warning threshold, the emergency discharge pipe at the bottom of the steam drum is activated to lower the liquid level. The diameter of the safety valve and that of the emergency discharge pipe are calculated separately based on the requirements for exhaust/water discharge; generally, the diameter of the emergency discharge pipe is larger than that of the safety valve’s exhaust port.
This post was last edited by Suspected law-abiding citizen on 2022-12-20 at 15:16. I don’t know why, but the post was posted twice; I couldn’t delete it, so I’m editing it again.
I also believe that the safety valve directs flow to the flare. In such a situation, if the outlet valve of the container closes accidentally, causing overpressure in the equipment, the pump located upstream of the container will definitely give an alert, and the issue should be resolved fairly quickly； Secondly, a liquid separation tank is installed before entering the torch ； And if an additional liquid separation tank is added, it’s not impossible, but the back pressure at the outlet of the container’s safety valve must be taken into full consideration. Furthermore, in accordance with existing standards, the design pressure of the container at the pump outlet must take into account the possibility of the outlet valve of the container closing accidentally
1. Depending on the medium being discharged, they are classified into relief valves, safety valves, and safety relief valves. The activation process of safety valves varies depending on whether gas or liquid is involved; refer to API520 for details. As a result, the discharge coefficients used in calculations also differ. Therefore, for question 1, it is necessary to calculate the discharge volume for each different scenario, and then use the manuals provided by the supplier to select appropriate discharge coefficients for the selection and calculation of safety valves. 2 If the outlet valve is closed, the volume of the gas phase is first compressed; therefore, it is the gas phase that is released. However, the capacity for release is calculated based on the volumetric flow rate of the pump. Another scenario is where the outlet valve is not closed, but the diameter of the outlet pipe is significantly smaller than that of the inlet pipe, resulting in a situation of liquid overpressure; in this case, it is the amount of liquid that is released. 3 This topic still needs to be discussed in the context of specific scenarios; it’s not uncommon to see designs for torches that include a catch pot.