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Flare Header and Relief System Analysis

The U.S. Occupational Safety and Health Administration (OSHA) Process Safety

Management (PSM) Standard 29 CFR 1910.119 requires that a Management of Change

(MOC) be generated when the design rate in a process unit is increased. The MOC

documentation should include a review/analysis of the affected parts of the relief system

due to the increased design rate. The relief system review/analysis includes affected relief

device inlet lines, relief devices, relief discharge lines, relief disposal equipment, and

the flare system to determine if revisions to the relief system are needed. A thorough steady state review and analysis of these systems is an important but time consuming task. Ascent Engineering's group of experienced engineers can perform the thorough analysis and create the detailed documentation required to comply to all process safety standards.

In addition to offering steady state analysis, Ascent Engineering can, at the customers request, create a dynamic simulation of the process. Dynamic simulation provides accurate calculations of relieving rates. Relieving rates calculated by dynamic simulation are often lower than relieving rates calculated by steady-state methods and dynamic simulation provides a real time understanding of the relief process. In some cases, accurate Dynamic simulation can indicate significantly smaller relief loads than conventional methods and can save the refiner hundreds of thousands of dollars in capital costs. The savings are particularly significant in cases where the flare header is limiting, here the cost savings could be in the millions of dollars.


Relief Valve Studies


Ascent evaluates all PRD devices against all relevant cases including fire, thermal relief, blocked equipment, power failure, loss of instrument air, loss of cooling water and all other relevant utility failures. We simulate scenarios to insure accurate relief valve sizing. Depending on the case and customers requirements, Dynamic simulation and detailed flare modeling can also be performed.


All of our engineers have had HAZOP and PHA training, and are familiar with OSHA requirements and API standards regarding PRD devices.


Our Typical Scope of Work for a Relief Valve Study


1. Evaluating all existing relief devices based on API RP 520 and 521 for the specified Design Basis. Ascent can aid the customer in developing a Design Basis if needed, and we can simulate expansion cases and determine if all existing relief valves meet the Unit's maximum capabilities. 


2. Performing rigorous hydraulic calculations to evaluate the relief devices' inlet and outlet lines, including evaluation of whether the existing inlet and outlet lines meet API 520/521's 3% rule and backpressure limitations. Where existing lines do not meet API 520/521, piping modifications are recommended,. In some cases results which are over 3%, but less than 5% will be further evaluated prior to recommending modifications.


3. Where flares are involved, the relief valve discharge piping to the flare is evaluated by verifying the built-up back pressure at the relief drum using an existing model if available. Ascent can develop this model if necessary.


Typical Deliverables


We customize our packages to our customers requirements. Here is a typical deliverable list:


  1. A summary of relief rates for each relief device.

  2. A relief device heat and material balance at designed charge rate. A PFD with flow, temperature, and pressure of main streams will be provided.

  3. The philosophy for each relief device of why the different scenarios and flow rates were evaluated.

  4. A simplified PFD sketch showing what equipment is protected by each relief device.

  5. Detailed calculations for each relief device and relevant assumptions.

  6. A summary of all scenarios for each relief device identifying the limiting case.

  7. The maximum allowable Unit feed rate for each feed device based on required and selected relief device orifice sizes.

  8. A detailed list of new relief devices, specification sheets for all new relief devices, and recommended piping changes.

  9. The relief study results will be delivered in a binder with individual tabs for each relief device. A CD will also be provided to allow easy electronic archival of all documentation.