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Stuffing Box Operation - continued.
Item 1: The sealing material usually needs to be kept moist so that it stays “swollen” and is able to provide a good seal. For this reason, a small amount of leakage is permitted, since it keeps the seal moist. In the figure shown here, external sealing fluid is supplied as auxiliary sealing medium. Through a needle valve, the sealing fluid is allowed to pressurize a point midway in the packing. This sealing fluid does two things. First, it pressurizes the region, thereby stopping the leakage coming along the stem from the valve body. Second, a small amount of the sealing fluid flows upward to leak out along the valve stem. This is a popular method to get a good seal while reducing the leakage to zero.
In most of common applications you DO NOT have external sealant connected to the valve as shown here. In most common application you do not have external sealant at all, you simply allow a little bit of fluid from the valve body to leak along the stem.
Incidentally, this figure also demonstrates the application of a needle valve. In a needle valve the bottom of the stem is provided with a needle like body (rather than a disc.) This allows very fine control of fluid flow. You use these valves in small bore pipes where you want to exercise fine control over the fluid flow.