Propylene liquid pipeline safety valve
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In an acrylic liquid-phase pipeline where the valves at both ends may be closed, a safety valve needs to be installed to prevent overpressure due to thermal expansion. When the safety valve activates, is there a liquid phase in front of the valve, or is there a vapor phase or a mixture of both phases behind it?
The presence of propylene in the liquid-phase pipeline maintains a constant temperature-pressure relationship; it remains saturated throughout the pipeline. At the moment it is discharged through the safety valve, the temperature stays the same while the pressure decreases, and it will definitely vaporize. Unless the discharge volume is extremely large, there may be some liquid spray, but it will still vaporize immediately. Of course, a phase change enthalpy occurs during vaporization, and the temperature at the discharge outlet drops. When continuous large-scale discharge causes the temperature inside the safety valve’s discharge pipe to fall below its boiling point of around –46°C, liquid may form inside the pipe; however, this is unlikely, as liquids have a relatively low coefficient of expansion. A small amount of liquid discharged by the safety valve is sufficient to relieve pressure, preventing continuous large-scale discharge. Therefore, under normal operating conditions, what is discharged is gas.
The thermal expansion is small and the leakage amount is low; generally, a relief valve with a setting of 20 is selected based on experience. Whether it is in the gas phase or liquid phase depends on where it is emitted When discharged into a pressure source, it is in liquid phase; when discharged into the fuel gas network, it is in gas phase.
For example, if the safety valve of the pump’s outlet pipeline is connected to the pump’s inlet, then the pump inlet serves as the pressure source, while the outlet represents the liquid phase. If it is connected to the flare system, then what is discharged is the gas phase. Is this understanding correct? I’m confused about one thing: when propylene is used as a refrigerant, the compressor pressurizes the gas phase, which is then cooled to turn it into a liquid phase; at this point, it’s in a supercooled state. The fluid then passes through a throttle valve and enters a gas-liquid separation tank, where it exists in a two-phase state under the pressure conditions after the throttle valve. What is the difference between this process and what happens when a safety valve releases pressure?
The back pressure of the safety valve can be selected, while the throttle valve is determined according to process requirements.
Here, it is primarily the pressure increase resulting from thermal expansion that drives the process, and the amount of leakage is very small. If it is discharged into the low-pressure system instantaneously, the bore size must be determined based on the gas phase, which results in a larger safety valve.
The pressure behind both the safety valve and the throttle valve is lower than the pressure before the valve; then why is one discharge in a purely gaseous phase while the other is in a gas-liquid two-phase state?
One has a back pressure of 0.05 MPa, and the other is the pressure under process conditions.
Pressure pipelines for propylene are generally full-pressure pipelines. In other words, the design temperature for liquefied hydrocarbon pipelines might be 50°C, meaning that the pipelines will not explode at 50 degrees Celsius? Furthermore, if the pipeline is overpressured and the safety valve opens slightly, throttling occurs, which inevitably leads to vaporization. Propylene vaporization absorbs heat, and at this point liquid propylene will emerge. If a large amount is ejected, there’s simply no time for it to evaporate. Just like high-pressure, room-temperature propylene, it is discharged to the site through a drain; similar to liquid nitrogen, some of it evaporates, but since there is no heat left for further evaporation, it remains in liquid form in the atmosphere. However, once the safety valve is overpressured, it will definitely activate and then return to its original position; the possibility of it activating again after returning to that position is relatively low. Personally, I think liquid propylene will emerge, and the outlet of the safety valve will frost over.
Under normal circumstances, when liquid propylene causes the safety valve to discharge due to excessive pressure in the pipeline, the pressure in the pipeline downstream of the safety valve is very low, so the propylene exists in a gaseous state. Of course, generally there is a buffer chamber downstream of the safety valve, or a combustion tower is used for combustion in order to reduce the pressure throughout the pipeline.