Project Synopsis

Perform sizing calculations of depressurization orifices and pressure safety relief valves on a mol-sieve based hydrocarbon dehydration and desulfurization skids.

Project Summary


Process Engineering Associates, LLC (PROCESS) was contracted by the client, a company that specializes in the design and fabrication of compressed air, gas, and liquid dehydration and purification systems, to size several pressure relief valves (PRVs) as well as to size flow orifices for emergency depressurization of several full liquid-hydrocarbon vessels.  The vessels were configured on several individual skids and used to dehydrate and desulfurize liquid hydrocarbons in molecular sieve towers. 

The skids consisted of a typical two-vessel molecular sieve arrangement with one vessel absorbing and the other regenerating.  Regeneration of the media is through hot, pressurized nitrogen.  Nitrogen is supplied from an on-site liquid storage tank and is brought to the vapor state in a shell and tube vaporization unit.  Heated nitrogen is provided by a shell and tube heat exchanger using medium pressure steam as the heat source.  The skids were also equipped with vaporizers (for liquid hydrocarbons), blowers, and knock-out drums.

The first component of this effort was to size various orifice plates in hydrocarbon service per API 521 for emergency depressurization.  The orifice plates are located on the outlet piping on each of the vessel towers (one absorbing and one regenerating).  During an emergency, the active absorbing vessel must be depressurized of product (propane, 1-butene, etc.).  The vessel discharge lines are equipped with an on-off depressurization valve that is closely followed by a flow orifice.  The flow orifice was sized in accordance with API 521 and allows for the depressurization of the vessel within 15 minutes.  A total of four (4) flow orifices were sized; one for each two-vessel skid.

The second, and more comprehensive purpose of this work was to size ten (10) PRVs located on the skids in various services.  Each tower vessel is equipped with a PRV.  There are also PRVs on the process-side of the shell and tube heat exchangers (vaporizers), as well as on some of the regeneration equipment.  Depending on the hydrocarbon serviced by the skid, the PRV relief fluid phase can be in the vapor, liquid, or supercritical condition.  For the most part, “external pool fire” was the controlling contingency that dominated valve sizing.

Results from all the valves were summarized into complete and comprehensive reports, and datasheets for each valve were generated.  PROCESS' work product was submitted to the client and PROCESS personnel participated in phone conferences with the client and the client’s client.  PROCESS performed these calculations for the client due to the client being unfamiliar with the methodology required to accommodate liquid-full vessels containing hydrocarbons.


Industry Type

  • Upstream Hydrocarbon Processing

Utilized Skills

  • Relief systems design

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