Issues with the safety return interface for liquefied petroleum gas

The safety valve returns to the vapor line or the vent line. Also, is the term \"safety return valve\" indeed the correct one?

It is recommended to use a dedicated interface in the storage tank for the safety return valve; if no such dedicated interface is available, then returning the fluid to the liquid phase is an option. Generally, companies opt to return the fluid to the liquid phase in order to save on piping costs, and there is no issue with this from a process perspective. If returning to the gas phase space, that’s fine, but be careful: a dedicated gas pipe must be used separately; it cannot be mixed with the suction gas from the compressor, as it is fatal for the compressor if liquid phase is drawn in. Additionally: The safety return valve is a valve similar to a safety valve; the medium that flows back is a mixture of liquid and gas phases. At the beginning of filling, the pressure inside the cylinder is not high, so the safety return valve does not open. Once the cylinder is full, filling stops when the liquid level reaches its limit. If the amount filled is small or no filling occurs, the hydrocarbon pump will not stop immediately, and excess medium will flow back. Since LPG is easy to vaporize, some gas will definitely flow back through the reflux valve. Thus, there is a difference between installing the safety return valve at the beginning and the end of the filling nozzle: installing it at the beginning mainly allows the liquid phase to flow back, while installing it at the end facilitates exhaust, but the pipeline has to be longer.

I really don’t quite understand the concept of a safety return valve: is it actually a safety valve (used for pressure relief, to protect the pump outlet pipeline and ensure that pressure does not exceed a certain level under shutdown conditions)?； Or a backflow valve (which can be pressure-controlled, or outlet flow-controlled to stabilize the flow or pressure downstream)? Could there be a more detailed description or a flowchart? If the outlet of the safety valve is led back to the side in front of the pump, it still falls under the category of a safety valve; leading it back into the tank (but back pressure must be checked in accordance with API521) poses no problem for the liquid-phase pipeline ； Of course, the simplest and most conventional approach is to return it to the pump inlet; of course, first it’s necessary to confirm that you are indeed referring to a safety valve.

For the return to the liquid phase, many designs have been created

The safety return valve is not directly related to the pump; its main function is to be used in LPG filling stations where bottles are filled manually. Since filling is done manually, there is definitely a time interval between the first bottle filled and the last one, and this interval is quite short. It’s not possible to turn off the pump during this time, especially since hundreds of bottles need to be filled at once. It’s also not advisable to allow the liquid to flow back directly to the pump using a hydrocarbon pump (as if the filler takes a few minutes to rest without turning off the pump, the continuous operation of the pump will cause heating, leading to excessive vaporization of the LPG). That’s why this safety return valve exists – it has a mechanical structure that allows the liquid pumped out by the pump to flow back into the tank during the intervals between bottle fillings, thus ensuring continuous production. It is actually a backflow valve; the addition of the word “safety” is a common term used in the industry, and it is not a safety valve in the traditional sense. Of course, its principle is similar to that of a spring-loaded shut-off safety valve.

Thank you for the clarification; return valves similar to this design are often used in the lubrication stations of large-scale units.