Liquid seals are often installed immediately upstream of a flare stack to maintain a positive pressure in the collection system and to provide protection against flashback into the flare header. Figure 1200-39 is a diagram of a typical flare seal drum. Refer to API Recommended Practice 521 for additional description of liquid seals and methods for sizing them. In a ground flare system, a double liquid seal provides a positive means of diverting excess relief
gas into the elevated flare. The maximum volume of relief gas that can go to the ground flare is determined by the water seal depth in the upper seal. Once the relief header pressure exceeds that head, the excess relief gas goes to the elevated flare. Figure 1200-39 shows a double liquid seal.
Some facilities use a dual header design with dedicated high- and low-pressure flares. The high-pressure ground flare is usually self-aspirating and requires no water or steam injection.