PSV calculation for storage tanks

Context:
A chemical company had several above-ground tanks for storing xylene. For this installation, the sizing of the pressure relief systems (PSVs) had to be re-evaluated in light of current operating conditions and applicable international standards. Demonstrable compliance with API 520/521, the PED directive and the requirements of the Belgian Code on Well-being at Work was essential.

Approach:
Prohead Engineering carried out the calculations and analyses using established international methods:

• Determination of the design and operating conditions of the tanks, including the liquid properties of xylene (vapour pressure, boiling point, heat capacity).
• Scenario selection for possible overpressure conditions:
  • external fire load,
  • inert gas blanketing in- and outflow,
  • thermal expansion of the liquid,
  • operational errors (filling too fast).
• Calculations of the required relieving capacity of the PSVs, including the F-factor for xylene, to take account of the specific heat transfer coefficients.
• Drawing up capacity curves for the various tanks to visually substantiate the relationship between F-factor, heat load and vent capacity.
• Validation of the existing vent sizing and comparison with the theoretically required flow diameters.

Results and conclusions:
The calculations showed that:

• The existing PSVs were largely correctly sized, but for one tank the vent diameter had to be adjusted to fully cover the required capacity in the fire scenario.
• The fire load was the governing scenario, considerably more decisive than thermal expansion or inert gas overpressure.
• Using the xylene-specific F-factor was crucial to arrive at realistic and safe sizing.

Added value:
Thanks to the thorough methodology and detailed reporting, the client now has demonstrably compliant and technically robust PSV sizing. The reports with calculation curves also form a valuable reference for future inspections, audits and expansions.