Vents and drains

High points vents and low point drains are a small but important and necessary detail of the piping system in a process plant. Very often they are neglected or missed, even by senior piping designers. Installing vents and drains after pipe has been installed usually result in additional costs which can be very high in the case of alloy piping.
There are two types of drains and vents. Process vents and drains are provided for usage during operations and they are required by process. On the other hand, there are hydro vents and drains which function is to provide proper hydro testing and pipe draining. Purpose of hydro high point (vents) is to expel air during hydro test. Hydro testing required that all air must be expelled and pipe must be completely and absolutely fill with water. After hydro test is done pipe is drained thru low points (drains) by gravity. Typical positions of hydro vents and drains are shown on the figure above. Drains #4 and Vents #5 are installed in low and high points. Drain #2 is required for depressurizing and emptying of the pipework in the case of control valve replacement. It is worth to mention that, theoretically, pipe can be empty by loosening flanges. However, this is not normal practice and can be applied only with customer approval. Control valve internals must be removed during hydro test. Portion of the pipe between the globe valve and the pump can be emptied thru the pump. Note that the pump casing must have installed drain as per API 610. Purpose of Drain #3 is to depressurize and empty portion of the pipe in the case of installing future pipe. Purpose of Vent #6 is to initially expel air during the filling with water, and final air expel will be thru the flange and the spectacle blinds connection. Note that spectacle blinds are used to block pipe allowing hydro testing which is very common in petrochemical industry. It is not common to hydro test vessel and piping together, for the reason of high cost. Spectacle blanks built as per B16.48 are not required to be tested. Spectacle blinds are installed not only for the purpose of hydro test, but also for the safety reason concerning work regulation in a confined space. Purpose of Drain #3 is to allow draining of the vertical portion of pipe. If connection to existing pipe is required, usually we built removable pipe spool 12″ long which will be removed and end of pipe will be blocked by pipe plug during hydro test. Good information about pipe plug can be found here. Please note that internals of check valve must be removed during hydro test. Vent after check valve is required for removing air prior hydro test and drain is required for pipe draining during plant shutdown.
Your customer probably will have standard drawings for vent and drains, and it imperative to follow it. However, there might be case when you work on brownfield project and good starting point for your details can be found at site Piping Info. Note that hydro-test vents and drains are normally 3/4 or 1″ in size and cap at the end valve is required for the safety reasons. Always has collaboration with process department and customer since each plant (customer) has its own preference. Some plants only install temporary valves for use during the hydro testing process. After the test, the valve is removed and a plug is installed. These Plugs are then seal-welded to prevent leaks. Other plants may use some selected hydro-test vents or drains as process start-up vents or drains, so the valves installed in these and are retained. Still other plants require that all hydro-test valves be permanently installed and plugged with no seal-welding. In conclusion, when are you placing hydro vents and drains, always consider how hydro test will be performed, look for customer standard drawing for vents and drains and have good collaboration with mechanical and process department. It is wort to mention that hydro vents and drains are not shown on PID and proper placing them is responsibility of piping designer, whereas process vents and drains must be shown on PID which is responsibility of process departments.