For most large facilities, the process of managing the lifecycle of relief and flare systems data is complex and fraught with challenges and risks. Based on our experience executing many large scale projects for refineries, chemical, and petrochemical facilities, we know these to be the key reliability requirements for relief and flare systems information.
The quality of relief systems evaluations is heavily influenced by many technical and human elements. The qualification of engineers performing the analysis should be verified. While drafting of isometrics and data entry can be adequately performed by a nonrelief system expert, scenario identification, modeling of complex dynamics, and the assessment of corrective actions will require deep expertise in process safety and relief systems.
The quality of the end result of the process simulation or relief systems assessment will also depend on the quality of thermodynamic, transport, chemical reactivity, and kinetic data available for the calculations. Vapor-liquid equilibrium data in particular are very important, especially for reaction systems.
The evaluation process should use field verified equipment ratings, relief piping isometrics, and flare network data for existing systems. For new design, the initial evaluation should be revalidated after the construction drawings are issued and also audited for compliance after construction is complete and prior to startup. This is necessary as isometrics can be changed during contruction without considerations of pressure loss, elevation changes, and other factors that can impact relief device stability and structural dynamics and integrity.
Scenario identification and development is best performed by experienced process safety and relief systems engineers. Once scenarios are developed, a formal scenario review with plant operations should be conducted in order to validate the scenarios. Operating personnel in facilities can provide a wealth of knowledge regarding scenarios that have occurred in the past or more importantly near misses as well as deep insight into the control systems.
Up to date and truly representative material and energy balance data are very important, especially for relief systems scenarios that are dependent on the flow capacity of a plant or a particular unit within the plant. Note that many older plants in the United States have been debottlenecked and increases in capacities have pushed operating conditions much closer to the operating design limits.
Consideration for reaction forces and vibration risk developed during relief is often poorly addressed in many systems we at ioMosaic have reviewed. In some unique cases, poorly supported and/or designed relief piping can exhibit resonance with other systems components and fail catastrophically.
There are many unique systems that require special expertise such as high pressure ethylene systems, where the pressure can be as high as 1,500 bars during normal operations and pressure relief can lead to very large dynamic reaction forces as well as cold cryogenic temperatures downstream of the flow restricting devices. In general, it is very difficult to support structures that are exposed to more than 25,000 lbf of thrust during relief.
| There are many unique systems that require special expertise such as high pressure ethylene systems, where the pressure can be as high as 1,500 bars during normal operations and pressure relief can lead to very large dynamic reaction forces as well as cold cryogenic temperatures downstream of the flow restricting devices. |
Reaction and multiphase flow systems also require special expertise. Unlike non-reaction systems, where small uncertainties in PVT data and/or other design variables such as fire flux can be tolerated, reaction systems are very unforgiving. Reactions rates will typically double every 10 degrees. An erroneous estimate of the actual temperature at the relief device set point pressure can yield much faster reaction rates and ultimately fail due to inadequate relief. The vast majority of single-phase flow, two-phase flow estimates are more uncertain and the reliability of designs involving two-phase flow are best validated by an expert.
Depressuring systems represent another area where special expertise is required and where poor designs have led to accidents. For depressuring systems, we have to worry about both cold and hot temperatures during depressurization. Cold temperatures reached in vessel or downstream (especially if dew point is reached) can reach the embrittlement of typical carbon steel. High superficial vapor velocities during depressuring can cause liquid carryover and two-phase flow, excessive noise, and vibration risk.
Condensing gas/two-phase flow downstream can lead to more liquid accumulation and cold temperatures. Condensation can lead to the formation of liquids at their associated dew point and can cause localized stress concentrations and embrittlement. Many offshore depressuring systems are suspect to hydrate formation as small amounts of water are present in the vessel hydrocarbon contents. Hydrate formation can cause plugging.
Even under fire exposure, large temperature differences at the vapor/liquid wall interface have been observed. This is where most failures are observed to initiate for two-phase vessels engulfed in a pool fire. A vessel under fire exposure that is rapidly depressurized can still be exposed to cryogenic temperatures because of the poor free convection heat transfer characteristics from the vessel wall to the vapor space during the first few minutes, as the walls have not reached a sufficiently high temperature for radiation heat transfer to become dominant between the inner metal wall surface and the vapor/liquid contents. Localized flame jet heating on vessel walls (dry) and thermal heating from ignited PRV discharge are scenarios where protection by a relief device may not be possible or practical.
ioMosaic Software Tools
Our Process Safety Office® SuperChems™ software is an advanced tool for pressure-relief design, consequence analysis, and thermal hazard assessment. It's the ideal tool for simple and complex flow dynamics, pressure relief systems and vent containment design, chemical reactivity management, quantitative risk analysis (QRA), and consequence analysis.
We Can Help
Our experts have experience in all areas of design, from reactivity testing for design basis determination to calculations for Z-axis deflection from dynamic loads. When you need to audit or revalidate your relief systems to comply with internal company standards and global industry standards (such as CCPS, ACC, API Standard, OSHA’s PSM Standard, and the EPA’s RMP Rule) call us at 1.844.ioMosaic or send us a note. We'd love to help.