In process industries, pressure can be an explosive force. When pressure increases beyond the capacity of a vessel, pipeline, or process system, the outcome can range from simple equipment damage and production losses to fire, explosion, and loss of life. Design of pressure relief and flare systems is a means of controlling the consequences of a process failure.
The engineering principles can be straightforward. The challenge is to connect them properly!
Begin the design process with questions related to the scenario - not the valve
Blocked outlet, control valve failure, external fire, utility loss, runaway reaction, thermal expansion, and gas blow-by from a higher-pressure system are some of the causes of overpressure in a system. Each of these scenarios will create different physical behavior and have different requirements for relieving capacity.
Therefore, it is an engineering mistake to select a pressure safety valve (PSV) before knowing what scenarios are credible.
The engineer must determine the protected equipment's design pressure and identify the credible causes of overpressure. He/she must also determine the governing relieving case and calculate the required relief load. The largest load is not always the most probable, and various scenarios may govern various items.
Standards such as API 520 and API 521 contain sizing methods for selecting and applying pressure-relieving devices and disposal systems. These standards' true value can only be achieved if the designer thinks carefully about the process.
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A pressure safety valve is a means of protecting equipment by relieving the pressure in the system when the set pressure is reached. However, opening the valve is only part of the solution. Just as important is what happens to the fluid that is relieved.
For hazardous, flammable, or toxic process streams, simple atmospheric discharge may not be an acceptable means of disposal. The relief stream may need to be directed to a flare system, where combustible gases are safely burned at designed operating conditions.
Notice that the PSV protects the equipment and the flare system provides a safe means of disposing of the relieved material and its associated hazards.
Therefore, the flare system design and its layout must accommodate the credible relief loads without creating unacceptable backpressure that would limit the relieving devices' capacity. This design must consider the composition and phase of the relieved fluids, piping pressure losses, liquid carryover, flare header hydraulics, knockout drum, radiation, noise, emissions, and potential for flame instability.
Thus, a flare system that successfully relieves pressure but creates an unacceptable fire, toxic exposure, or overpressure hazard elsewhere has not met the safety goal.
A crucial design consideration is the determination of the amount of material that will have to be disposed of simultaneously
Summing up the maximum rated capacities of all the PSVs can lead to overdesigning the flare system. Equally important, assuming that only one valve will ever discharge is probably too optimistic. The designer must identify credible common-cause events and determine which relief devices could be in service at the same time.
Suppose a total power failure, loss of cooling water, or a significant process upset can affect multiple units simultaneously. Or perhaps simultaneous fire exposure creates different simultaneous relief requirements. This flare load scenario establishes the governing flare size.
Good engineering must understand credible events and their interactions to provide adequate protection against the resulting demands.
Following is a suggested design workflow:
Document the assumptions, including built-up and superimposed backpressure, which may limit the relieving device's capacity and affect its stability, depending upon the type of valve used.
Flare hydraulics must be determined relative to the limits for each type of relieving device. Flare knockout drum sizing must be based upon credible liquid loadings, droplet size, and adequate residence/disengagement time. Radiation must be determined for credible operating and emergency conditions, including personnel and equipment layout. Dispersion and noise studies, as applicable, can confirm that the disposal system will be acceptable.
Simulation software can speed up the calculations and identify problem areas, but assumptions are still required.
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The pressure relief and flare system design must be based on sound assumptions and rigorous calculations to determine the requirements and limitations of each component.
There are three connected questions every good pressure relief and flare system design must answer:
These questions require the design input of multiple disciplines, and assumptions must be adequately documented and independently verified.
Design simplifications should make the process clearer, not less safe. A smaller flare header may save money but can result in unacceptable backpressure and capacity limitations. A different type of relief valve may reduce the purchase cost, but it may not have the required relieving capacity for the governing scenario. Even more important, technically correct design calculations are not always a guarantee of an inherently safe design if the assumptions are invalid.
In summary, the design of a pressure relief and flare system is made easier if the various components are viewed as a total system with one goal - to protect people and equipment.
The flare tip, knockout drum, header, relief valve, and piping all play a part. Each must function as intended to protect people and equipment.
The relief device, discharge piping, flare header, knockout drum, and flare tip form a pressure relief and flare system in which each component is necessary for the system to function as a whole. The designer must ensure that each part of the system is adequate for the intended application.
Ultimately, the success of pressure relief and flare system design should be based on an understanding that the purpose of pressure relief is not to prevent a vessel from exploding but to limit the consequences when a process goes beyond its intended operating limits.
Engineering reference points: API 520 (sizing, selection and installation of pressure-relieving devices), API 521 (pressure-relieving and depressuring systems), and applicable ASME Boiler and Pressure Vessel Code requirements. Actual designs must use the applicable editions, jurisdictional requirements, and project-specific engineering criteria.
Click Here to Download Readymade Engineering/Process Equipment Design, Continuous Improvement, Quality, Production, ISO 9001, ISO 14001, ISO 22000, ISO 45001, FSSC 22000, HACCP, Food Safety, Integrated Management Systems (IMS), Lean Six Sigma, Project, Maintenance and Compliance Management etc. Kits.