In this Issue:
1. Project Spotlights
- New High Purity Propylene Distillation Process Design
- Refinery Relief Valves and Flare System Capacity Evaluations
2. Technical Discussion – Ventilating Your Processing Area
3. Safety Pause: The Ins and Outs of Breather Valves
4. Company Announcements
New High Purity Propylene Distillation Process Design
PROCESS was contacted by a producer of aerosol propellants, fuels and refrigerants to provide design and engineering services for a distillation train to product high-purity (99.995%) propylene from polymer grade propylene. PROCESS needed to rate the existing towers for the new service and if those were inadequate, design new towers and equipment. The high-purity requirement and close boiling components presented significant challenges. Would the existing equipment be adequate without capacity limitations? Keep reading
Refinery Relief Valves and Flare System Capacity Evaluations
An independent petroleum refinery contacted PROCESS to execute a refinery relief valve and flare system capacity evaluation. The client had over 300 relief valves and needed appropriate engineering documentation for sizing. But, before the valves could be calculated, PROCESS needed to use simulation software to develop mass balances for six refinery unit operations. PROCESS would go on to use its proprietary in-house relief valve capacity computer program to evaluate each valve for all credible relief scenarios. But, that wasn’t enough! The project required development of a unique program to more accurately define the fire case. Conventional fire calculations provided results that would have required a considerable change to accommodate a flow rate that in reality would never be attained. For more details, keep reading here.
Ventilating Your Processing Area
A safe working environment requires the evaluation and careful consideration of both general exhaust ventilation requirements and localized capture and control requirements for a chemical processing area or building. A combination of general area exhaust systems, point source capture and control systems, and emergency release capture and control systems are required to ensure that hazards are minimized. A systematic approach can be used to determine potential requirements for exhaust ventilation in your processing area or building. The approach includes review of applicable standards such as the International Building Code (IBC), International Fire Code (IFC), and International Mechanical Code (IMC) as well as the National Fire Protection Association (NFPA) standards.
The systematic approach would involve the following:
- Documenting the hazards of all chemicals handled in the area
- Determining for each chemical the maximum quantity stored and/or used
- Evaluating general exhaust requirements that may apply to the processing area
- Determining if more stringent general exhaust requirements may apply to specific hazardous materials
- Determining if localized point source capture/control requirements may apply for highly hazardous chemicals
- Determine if there are any special requirements (e.g. compressed gases, emergency release, spill) that may apply.
Understanding the physical and health hazards of the chemicals you handle in the area is paramount to completing a good technical review of ventilation requirements. Therefore, material safety data sheets (MSDS’s), NFPA 704, and other sources must be used to define such things as the corrosivity, flammability (e.g. flash point and lower explosive limit LEL), and toxicity (median lethal dose (LD50) and median lethal concentration (LC50)) of the chemicals. In addition, understanding (both qualitatively and quantitatively) whether or not a chemical can be present in the area as a vapor, gas, fume, mist or dust during any part of the operation is also important. The basis for the hazards and any assumed concentration should be well documented.
Specific, more stringent requirements will apply to areas where hazardous chemicals are stored and used in amounts that exceed the Maximum Allowable Quantity (MAQ) per control area (as defined in both Chapter 3 of the IBC and Chapter 50 of the IFC). For example, the MAQ/control area for the storage of a corrosive liquid is 500 gallons. If a corrosive liquid is stored above this quantity, more stringent ventilation requirements may apply. The MAQ/control area for each hazardous chemical needs to be carefully defined.
General Exhaust Requirements
Regardless of the quantity of a hazardous chemical handled in the area, the codes require that general exhaust systems be provided, maintained and operated to make sure any fumes/mists/vapors/dusts that may present a physical and/or health hazard are discharged outdoors with no chance of re-entering through the building ventilation system. Some examples of general exhaust requirements provided in Chapters 4 and 5 of the IMC include the following:
- If natural ventilation is used, ensure a minimum of 4 percent of the floor area is openable to the outdoors.
- Provide adequate makeup air and maintain a neutral or negative air pressure throughout the area
- Locate inlets to the exhaust systems at areas of heaviest contamination.
Other physical design requirements are provided in these Chapters.
General Exhaust Requirements for Hazardous Materials
Additional requirements exist in areas where hazardous materials are stored or dispensed and used in amounts greater than the MAQ per control area. Using the same example above, if a corrosive liquid is stored, used or dispensed in a quantity greater than 500 gallons, the mechanical exhaust system for the area will also have to meet additional requirements including:
- Design capacity for 1 cfm/ft2 of floor area over storage or use area.
- Operate continuously.
- Equip with a manual shutoff switch, labeled and located outside the room adjacent to the access door.
- If the vapor density is greater than air, the exhaust vents should be located no more than 12 inches off the floor (for chemicals lighter than air, exhaust from a point within 12 inches of the highest point of the room).
- Design to provide air movement across all portions of the floor (no dead spaces) and allow no recirculation of exhausted air back into the room.
Localized Exhaust Requirements
Ventilation may need to be expanded to include localized point source capture and exhaust if more hazardous conditions can potentially exist, some of which include the following:
- A hazardous chemical with an NFPA health hazard rating of 3 or 4 is used in amounts exceeding the MAQ per control area.
- A “corrosive” material is dispensed and/or used in amounts exceeding the MAQ per control area
- A highly-toxic or toxic liquid is dispensed and/or used in amounts exceeding the MAQ per control area
As an example, if a Chlorine solution (corrosive, NFPA 4 rating) is pumped to a tank that has an open vent, a localized point source capture and exhaust system may need to be designed around that vent.
Hazardous Materials-Specific Requirements
Once general exhaust and localized exhaust requirements are defined, additional requirements should be identified for specific hazardous materials conditions; for instance, the potential for a spill or accidental release of a highly toxic chemical. It is important to define the potential worst-case spill or accidental release scenarios and to estimate the concentration of harmful fumes that could be generated and emitted. The mechanical exhaust system may need to be equipped with a scrubber system to process these vapors (if the concentration is potentially harmful). There are other requirements for specific hazardous materials, such as those for storage or use of highly-toxic and toxic compressed gases, or flammable and combustible liquids, which would be considered, as relevant. These are well-defined in the standards referenced.
A good ventilation review requires a thorough understanding of the chemicals in the area, how they are stored and used, and their potential hazards. With that information, a systematic technical review can be implemented to summarize the ventilation requirements for your processing area.
Safety Pause: The Ins and Outs of Breather Valves
API 520 clearly defines the hydraulic loss requirements on both the inlet and outlet piping for a conventional spring loaded relief valve. These often familiar rules are referred to as the 3% or the 10% rule, meaning that losses up to 3% of the set pressure on the inlet pipe are acceptable and losses up to 10% of the device set pressure on the outlet piping are acceptable. Don’t we like that as engineers? There is a rule, an equation and a number that is right or wrong. It’s Clear as Day.
But, what about the tanks that fall under API-2000 – those tanks rated to less than 15 psig that are outfitted with conservation (Breather) vents and manways? Enter, the mud….
Under API-2000 “Venting Atmospheric and Low-Pressure Storage Tanks”, there is not a set guideline for hydraulic loss. Rather, it’s a more iterative process that makes sure the venting system can flow the required relief rate. Per API-2000 Seventh Edition, section 3.6:
“….When designing inlet or outlet pipework for a pressure/vacuum relief valve, consider the influence of the following on the valve set pressure, the set vacuum and on the flow rate:
- Flow resistance of pipes, bends, and installed equipment;
- Possible back-pressure or vacuum within the system
To further muddy the waters, most vent vendors report vent capacity as if the valve were directly mounted onto an equivalent size nozzle with no additional piping or fittings. How then do you determine if the valve is adequate since the reported capacity isn’t reflective of your system? For breather vents, one must think of the relief system venting the required relief rate and staying at or below the maximum allowed pressure. The iteration works by adjusting set points or piping configurations until you find an acceptable system.
As an example for a pressure conservation vent:
- Using the valve vendor sizing data, determine the pressure drop across the valve for your required flow and set point
- Calculate the hydraulic losses for the valve inlet and outlet piping
- Determine if there is a superimposed backpressure on the vent (i.e. discharges into a header, flare, scrubber, knock out drum, etc)
- Determine the maximum allowable pressure for the protected vessel
- If the calculated tank pressure is below the maximum allowable, then the venting system is adequate.
As you can see, the Tank Pressure in the above example is 16.007 inWC which exceeds the maximum allowable tank pressure of 15 inWC. This is not an adequate venting system. Now, you must consider which variables you can adjust such as the valve set point or making piping modifications. By adjusting the valve set point down to 10.5 inWC, the tank pressure is now at 14.512 and below the maximum allowable and therefore acceptable.
Now that you have your breather vent all figured out, don’t forget to check and make sure its set point is above your blanketing valve operating band and that it also fully vents before your manway starts to crack. That’s a lesson for another day.
PROCESS ENGINEERING ASSOCIATES, LLC is pleased to publicly offer a 3 1/2 day Process Hazard Analysis (PHA) Leader Training Course.
The purpose of the 3½ -day training course is to assist personnel at chemical plants, petrochemical plants, petroleum refineries, and manufacturing plants in becoming proficient in leading and documenting process hazard analyses (PHAs) by becoming familiar with various qualitative hazard review techniques and industry best practices for conducting and documenting PHAs.
Who should attend: Managers and engineers responsible for conducting PHAs at chemical plants, petrochemical plants, petroleum refineries, and manufacturing plants.
Detailed instruction of the following hazard review methodologies will be included in the course:
- Hazard and Operability (HAZOP)
- Failure Modes Effects Analysis (FMEA)
An introduction to Layer of Protection Analysis (LOPA) will also be included.
Courses to be offered in Houston, TX; Portland OR; Tulsa, OK; Philadelphia, PA; Denver, CO and New Orleans, LA.
Check our website in April (www.processengr.com) for more detailed course information and registration.
PROCESS offers a full range of Process Design and Process Safety Services to clients around the globe. Our services include conceptual process design; simulation and modeling feasibility studies; FEL-0,1,2, and 3 (Schedule-A process design packages); front-end-engineering design (FEED); debottlenecking; process optimization; on-site operations support; process safety training; safety program development; on-site process safety auditing, performance of hazard assessments; and other services designed to meet the needs of our clients.
Process Engineering Associates, LLC
700 South Illinois Ave Suite A-202
Oak Ridge, TN 37830
Call Us: (865) 220-8722