How to determine whether a valve is leaking inside or outside？

During the use of the valve,we often encounter a variety of failures, the leakage of the valve is one of the most common failures, and the leakage of the valve is generally divided into internal leakage and external leakage. So, what is the difference between the two?
 


The internal leakage of the valve, generally refers to when the valve is completely closed, there is still medium flowing out, leakage or dripping from the flow diameter outlet of the valve.Generally speaking,the valve is not tightly closed and the sealing is not good.This fault is usually caused by erosion and wear of the medium or impurities in the medium.Even if the valve stem is screwed in place, the medium can still flow through because there is still a gap between the valve core and the valve body.
 
The external leakage of the valve generally refers to the leakage of medium in the external sealing element of the valve packing after the valve is completely closed or opened.That is, the sealing between the valve and flange or thread is not tight，resulting in the outflow of the medium connecting the sealing surface,or the medium leakage caused by the poor sealing between the valve stem and the gland,or external leakage caused by sand holes on the valve body blank,and because the valve body is worn by media erosion resulting in external leakage.