What are the requirements for emergency relief and venting systems in chemical process engineering?

1 Sources of overpressure

Overpressure is the condition when the pressure inside a vessel or pipe exceeds its design limit. This can be caused by external fire or explosion, which can heat up the contents and increase their vapor pressure; blockage or malfunction of valves, pumps, or compressors, which can prevent the normal flow of fluids and create pressure buildup; chemical reactions, especially exothermic ones, which can generate heat and gas and accelerate the reaction rate; and thermal expansion, which can occur when the temperature of a liquid or gas changes and causes it to occupy more volume. All of these scenarios can result in overpressure and should be monitored closely to prevent any potential damage.

2 Relief devices

Relief devices are the primary means of preventing overpressure in chemical process engineering and are designed to open automatically when the pressure reaches a predetermined value and release the excess fluid to a safe location. Examples of relief devices include pressure relief valves (PRVs), which are spring-loaded valves that open and close proportionally to the pressure difference, rupture disks, which are thin metal membranes that burst when the pressure exceeds a certain level, safety relief valves (SRVs), which are a combination of PRVs and rupture disks that provide both protection and isolation, and conservation vents, which are weighted or spring-loaded vents that allow the escape of vapors from atmospheric or low-pressure tanks.

3 Venting systems

Venting systems are a key component in chemical process engineering for controlling overpressure. They collect, transport, and dispose of the fluids released by the relief devices and are comprised of various elements. For instance, flare headers are pipes that carry the fluids from the relief devices to the flare stack, while knockout drums are vessels that separate the liquid and vapor phases of the fluids. Additionally, flare stacks are tall structures that burn the flammable fluids and reduce their environmental impact, and seal drums provide a water seal between the flare header and the flare stack to prevent flashback of the flame. Lastly, flare tips are devices that control the shape, size, and efficiency of the flame. Reporting and documenting this system is essential for communicating its status to relevant stakeholders.

4 Design criteria

Designing emergency relief and venting systems in chemical process engineering requires following certain criteria, such as sizing the relief devices based on the worst-case scenario of overpressure and the maximum flow rate of the fluids, and selecting the appropriate type and material of the relief devices based on service conditions and compatibility with the fluids. It is also important to locate the relief devices close to protected equipment, while minimizing inlet and outlet pressure losses. Additionally, sizing venting systems should be based on hydraulic and thermal calculations, as well as flare radiation limits. Moreover, selecting a suitable flare type and configuration should consider flare gas composition and flow rate. Finally, venting systems should be located away from process units and occupied areas, while complying with regulatory and safety requirements.

5 Maintenance and inspection

Maintaining and inspecting emergency relief and venting systems in chemical process engineering is essential for ensuring their dependability and performance. It is recommended to test the relief devices periodically for their set pressure, leakage, and functionality, as well as replacing or repairing any that are damaged, corroded, or fouled. Additionally, it is important to clean and drain the venting systems regularly to avoid plugging, corrosion, or freezing. Furthermore, it is necessary to monitor the flare system operation and emissions and optimize the flare gas recovery and combustion efficiency. Lastly, it is imperative to check the flare system components for any cracks, leaks, or defects and repair or replace them as needed.