The discussion centers on the purpose of using two ball valves in series, known as a "double block and bleed" configuration, with a gate valve for sampling in between. This setup is primarily used to isolate hazardous fluids during maintenance, ensuring safety by allowing venting through the bleed valve while both block valves remain closed. Additionally, it facilitates the calibration of instruments like pressure gauges or transducers by isolating them and applying pressure through the bleed valve. Other applications include providing a common drain or vent and enhancing redundancy for isolation and leakage detection.
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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.