drain and vent

This has come up before. Here is my response the first time the question was asked.

High Point Vents and Low point Drains
The question :
“High point vents and low point drains, what they are? Why do we have them? These are both very good questions. So let’s us take a look at some answers to these questions.

My answer:
First you might ask if there are there other types of vents. Yes, there are other types of vents. And by association there are also other types of drains besides the Low Point Drain. There are the Process vents and there are the Process drains. The process vent and process drain is provided for multiple usages during the proper active operation for which the plant was designed.
Every plant that is built with piping systems must insure that the piping systems are properly fabricated, installed and safe. Some of these systems may have very hot or very cold or hazardous or toxic chemicals in them. These types of commodities can kill, maim or seriously injure the operators working in one of these plants or the people living around these plants.
In order to insure that a piping system is secure, the system must be tested and properly prepared prior to start-up. The method used for this testing is to fill the system with water and then apply pressure. This is called “Hydro-testing.” Hydro-testing can be done with other types of fluids but water is the normal choice. The water, under pressure will seek and find weak points in the piping system and then squirt out. Water under pressure is not as dangerous as a normal commodity such as high pressure steam or sulfuric acid. Some systems only need to be tested for tightness. Others must be tested then because the water is not compatible with the process, it must be flushed with a solvent or other cleaning agent to remove the water. For this Hydro-test to be done and be affective the system must be absolutely and completely full of water (or other fluid).
In order to completely fill a piping system you must be able to get all the air out of the pipes. The air will naturally accumulate in the high points of the system. Thus we install “high point vents” to be able to get rid of the air. Once the test is completed we must be able to get rid of or drain out all the water. The water naturally gravitates down to the low points of the system. So we install “low point drains.”
Hydro-test vents and drains are normally ¾” or 1” in size. Most of these vents and drains are only used when the plant is first built or when a system is modified and must be retested. These vents and drains do not always require valves. Some plants only install temporary valves for use during the testing process. After the test the valve is removed and a plug is installed. Other plants may use some selected hydro-test vents or drains as process start-up vents or drains so the valves installed in these are retained. Still other plants require that all hydro-test valves be permanently installed and plugged.
In conclusion, high point vents and low point drains are a small but important and necessary detail of the piping system in a process plant.