Vessel dimensions are determined by the liquid and gas flowrates and densities during normal maximum operating conditions. This includes potential upset conditions that can jeopardize the downstream equipment.
Liquid Storage Capacity
Vessel length is determined by the requirements for accommodating vessel internals, in addition to the amount of liquid surge volume required to ensure sufficient level control. The design basis must take into account the potential volume of liquid that might “slug” into the vessel during a process upset. This amount is determined by evaluating the upstream process for its potential to entrain liquids in the gas stream in the event of logical operating scenarios, such as a pump or valve failure. This is a judgment requiring careful operations/engineering review.
Surge volume is defined as the liquid capacity below the maximum allowable liquid level. The designer must factor into this vessel dimension any additional capacity necessary to control potential liquid loading. Figure P-1 illustrates surge volume and other design features of a basic vertical knockout drum. For compressor suction knockout service, a minimum recommended fill time of five minutes, based on maximum potential liquid loading, should be provided between the high level alarm (LAH) and the automatic compressor shutdown (LSH). This time period is based on the amount of time necessary for operating personnel to respond to the high level, and should be adjusted to suit the operating circumstances. Discussions with experienced operating personnel is recommended.
Liquid drainage capacity must be addressed in a similar manner. Failure to adequately size drain lines and level control valves for the above conditions can result in uncontrollable high levels and consequential shutdowns. Part of this judgment comes from understanding the location of the vessel and relative availability of operating personnel during upset conditions. A two-inch outlet provision is recommended as a minimum.