The purpose of the Sulphur Seal Leg is to contain process vapours to the condensers and prevent emitting H2S & SO2 to the environment, allowing free draining of liquid sulphur at the same time. The sulphur seal leg works much like the seal beneath your sink/toilet (except in reverse). It prevents obnoxious odours from the drain/sewer system from entering your house.
GPSA recommends that a Seal Leg be designed to withstand the air blower maximum discharge pressure plus an additional 6" to 12" safety factor for transients, with a minimum Seal Leg depth being 13 ft.
Typically, the process side of SRUs are designed for a pressure of 15 psig. Using the density of liquid sulphur at process tmeperature, this is equivalent to a seal leg depth of ~11.5 ft.
To determine the pressure rating of your seal leg, look at the mechanical drawings to get the depth. Convert the depth to psi using the density of liquid sulphur at process temperature.
Hope this helps, Brian