Factors Impacting Atmospheric Discharges and Selection of Pressure Protection Disposal Systems Patrick Smith and Abdul Aldeeb MKOPSC 2010 International Symposium College Station, Texas October 26, 2010 Copyright Siemens AG 2010. All rights reserved.
Introduction Direct discharge to atmosphere is a common practice that has long been considered safe, simple, dependable, and economically feasible compared to other overpressure protection mechanisms Discharging highly hazardous chemicals (HHC) to atmosphere remains a major challenge Several recent incidents have shown the need to eliminate HHC disposal to atmosphere
Agenda Current Practices to Design Atmospheric Discharges Factors Influencing Discharges to Atmosphere Discharge Location Decision-Making Approach
Current Practices to Design Atmospheric Discharge Systems The American Petroleum Institute Standard 521 Fifth Edition, 2007 (Addendum, 2008) [API 521] provides design approach for two types of systems: Pressure relief valves that discharge directly to atmosphere 15' RV-1 4 N 6 2' 15' Vessel 1
Current Practices to Design Atmospheric Discharge Systems Disposal through a common vent stack that discharges to the atmosphere
Current Practices to Design Atmospheric Discharge Systems Direct Discharge to Atmosphere API provides two criteria to determine if directing a relief device to atmosphere is advisable Both criteria focus on jet-momentum effects of the discharged vapors High velocities at the discharge of the tail pipe improve the mixing with air and reduce the concentration of the flammable or toxic material below threshold values over a short period of time
Current Practices to Design Atmospheric Discharge Systems Direct Discharge to Atmosphere The first criterion presented in API 521 indicates that the vapor can be diluted below its lower flammability limit due to mixing with air when the Reynolds number, R e, meets the following criteria: R e 1.54 4 10 j Where j is the density of the gas at vent outlet and density of air. is the
Current Practices to Design Atmospheric Discharge Systems Direct Discharge to Atmosphere The second criterion indicates that for exit velocities that exceed 500 ft/s, the discharged flammable hydrocarbon is expected to be efficiently mixed with air and diluted to concentrations below the lower flammability limits at a distance of 120 diameters (based on tail pipe inner diameter) from the end of the tail pipe. Wind Release Plume
Current Practices to Design Atmospheric Discharge Systems Atmospheric Vent Stacks Sections 6.7 and 7.3.4 detail the design requirements associated with atmospheric vent stacks: Recommends modeling of the release to ensure that the flammable vapor concentration does not exceed 10% to 50% of the lower flammable limit Approach presented for individual relief devices is not recommended for atmospheric vent stacks
Factors Influencing Discharges to Atmosphere It is evident from the API guidance that atmospheric discharges of vapors are not necessarily inherently unsafe. For liquid and two-phase discharges, the problem is more challenging. Experiments have shown that at high discharge velocities, the resultant droplets of liquids are extremely small.
Factors Influencing Discharges to Atmosphere In order to address the atmospheric discharge suitability, several factors will be evaluated: 1. Chemical type and concentration 2. Discharge Phase 3. Discharge Location 4. Scenario Likelihood 5. Disposal Options and Cost
Factors Influencing Discharges to Atmosphere - Chemical Type and Concentration Primary hazards are typically associated with the flammability and toxicity of the fluid Corrosive properties and potential formation of explosive mixtures should also be considered. For each property, the maximum acceptable concentration threshold should be determined. Lower Flammability Limit (LFL) Immediately Dangerous to Life or Health (IDLH) Short Term Exposure Limit (STEL) Permissible Exposure Limits (PEL) In many cases, dispersion modeling will be required.
Factors Influencing Discharges to Atmosphere 1. Chemical type and concentration 2. Discharge Phase 3. Discharge Location 4. Scenario Likelihood 5. Disposal Options and Cost
Factors Influencing Discharges to Atmosphere - Discharge Phase Vapor discharges are generally considered safer compared to liquid and two-phase discharges Liquid and two-phase discharges should be evaluated for the additional potential flammable, toxic, corrosive, or explosive hazards
Factors Influencing Discharges to Atmosphere 1. Chemical type and concentration 2. Discharge Phase 3. Discharge Location 4. Scenario Likelihood 5. Disposal Options and Cost
Factors Influencing Discharges to Atmosphere - Discharge Location Discharging to confined spaces or to areas where personnel or source of ignition may be present will impact the design of atmospheric discharges
Factors Influencing Discharges to Atmosphere 1. Chemical type and concentration 2. Discharge Phase 3. Discharge Location 4. Scenario Likelihood 5. Disposal Options and Cost
Factors Influencing Discharges to Atmosphere - Scenario Likelihood Impacted by the operator overpressure guidelines, training, operating procedures, and credits for available layers of protection Atm CWR CWS HLA LT2 LT1 LC1 Stm Cond
Factors Influencing Discharges to Atmosphere 1. Chemical type and concentration 2. Discharge Phase 3. Discharge Location 4. Scenario Likelihood 5. Disposal Options and Cost
Factors Influencing Discharges to Atmosphere - Disposal Options and Cost Once all the previous factors are addressed, a comparison of the various disposal options should be implemented. A decision to accept atmospheric discharge should be impacted the least by the cost factor
Discharge Location Decision Making Approach Process analysis resulted in identifying applicable overpressure scenario. Toxic vapors are expected. NO Liquid or two-phase discharge is possible. YES Toxic, flammable, or corrosive fluid is expected.
Discharge Location Decision Making Approach Toxic, flammable, or corrosive fluid is expected. YES NO Is there a potential for chemical hazard or environmental impact? YES NO Is there an open drain open? NO YES Atmospheric discharge is acceptable. Discharge To closed-system is required.
Discharge Location Decision Making Approach NO Toxic vapors are expected. Flammable vapors are expected. YES YES YES Is concentration <25xIDLH? NO Discharge To closed-system is required. Is the discharge velocity >500 ft/s at device capacity? YES Are all points of interest >120 diameters from the discharge point?* NO NO YES NO Are jet effects sufficient to disperse the material at 25% of device capacity?** NO Does a rigorous dispersion modeling indicate that atmospheric release will be safe? Atmospheric discharge is acceptable. YES NO YES
Conclusions Safe disposal system decision-making approach should consider atmospheric discharge impact and possible alternative discharge locations. Atmospheric pressure relief should only be considered safe when thorough evaluation is completed. Atmospheric pressure relief valves and common atmospheric vent stack design considerations and risk are not exactly the same. Most flare headers have limited capacity and forcing atmospheric relief valves into potentially undersized headers could create more problems than are solved.
Thank you for your attention!