Which two valves do the sampling valves consist of

Atmospheric pressure sampling can be a valve.  High pressure sampling generally requires two valves, and both valves must be high pressure valves!  The liquid gas sampler can also be installed straight, tilted to a certain point of view may be to avoid sampling personnel contact with the medium, considering safety aspects.  

The receiving position of gas sampling is on the horizontal pipeline. When the pipe size is DN≤80mm, it is connected from the top of the pipeline.  When the size of the pipe is DN≥100mm, it can be connected from the top or side of the pipe. On the straight pipe, when the gas moves from bottom to top, the sampling and receiving should be connected from the Angle of 45° up the straight pipe.  When the gas moves from top to bottom, the sample is received from a straight pipe pen directly out.  

The quick loading sampling valve is generally composed of two valves. Compared with the guideline valve, the gas is sampled in a straight pipe. The sampling port is generally inclined 45 degrees upward when the device is installed, because the gas is going up, so as to better obtain the desired sample.  Upright devices can also be used, but the sampling is inadequate and errors may be large.  

General medium sampling pipeline is a single globe valve or directly is the sampling valve;  But for the toxic, harmful media, it's going to be double valve.