Flare Header and Relief System
Occupational Safety and Health
Administration (OSHA) Process Safety
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
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
lines, relief devices, relief
discharge lines, relief disposal
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
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
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
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.
We customize our packages to our customers
requirements. Here is a typical deliverable list:
- A summary of relief rates for each relief device.
- A relief device heat and material balance at designed charge rate. A PFD with flow, temperature, and pressure of main streams will be provided.
- The philosophy for each relief device of why the different scenarios and flow rates were evaluated.
- A simplified PFD sketch showing what equipment is protected by each relief device.
- Detailed calculations for each relief device and relevant assumptions.
- A summary of all scenarios for each relief device identifying the limiting case.
- The maximum allowable Unit feed rate for each feed device based on required and selected relief device orifice sizes.
- A detailed list of new relief devices, specification sheets for all new relief devices, and recommended piping changes.
- 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.