Q_Goest said:What you're describing sounds like a "double block and bleed". Two valves are put in series on a piping run (block valves) with another valve off a T between them that runs to a vent (bleed valve).
One purpose is to isolate hazardous fluids so that maintenance can be performed safely. The procedure would have both block valves closed and the bleed valve open to vent during any maintenance. That way, if the one block valve leaks back toward where maintenance is being performed, it will go out the vent and you have a second block valve protecting you from this leakage.
I've also seen them being installed where the T has something in it such as a pressure gage or transducer that needs periodic calibration. The two block valves are closed to isolate the component and the bleed valve is used to apply pressure for calibrating the instrument.
Those are the two main reasons I've heard of but there could be others.
The purpose of putting 2 ball valves together is to create a more secure and reliable system for controlling the flow of a liquid or gas. By using two ball valves, you can ensure that there are no leaks or disruptions in the flow of the substance.
When 2 ball valves are connected, they work in tandem to control the flow of a substance. One valve is used as the main shut-off valve, while the other is used as a backup or isolation valve. When the main valve is closed, the backup valve can be opened to allow for maintenance or repairs without disrupting the flow.
Using 2 ball valves provides added security and reliability compared to using just one valve. If one valve fails or needs to be shut off for maintenance, the other valve can still control the flow. This can also prevent potential hazards or accidents from occurring.
Compared to other types of valves, 2 ball valves are easier to maintain and have a longer lifespan. They also offer more precise control over the flow of substances and are less likely to leak. However, they may be more expensive and take up more space than other types of valves.
2 ball valves are commonly used in industrial settings, such as oil and gas pipelines, to control the flow of substances. They are also used in plumbing systems, water treatment plants, and other applications where precise control and reliability are important.