Flare Design Basis & Analysis

We will provide you with a PSM compliant flare system design that is not based on overly conservative assumptions.

Smith & Burgess has the experienced staff to solve relief systems related problems. We can provide relief systems documentation as required by OSHA either from scratch or update existing documentation. Have other contractors told you that your flare loads are above your existing systems’ capacities? We have reviewed multiple flare installations and can typically find 20% to 40% reductions in the flare sizing design basis.

Where Does the Flare Design Basis Come From?
Our experience has shown that typical flare design basis comes from the relief systems documentation. General engineering work flows only spend enough time to ensure that the individual relief valve is adequately sized and installed. These loads are then combined to form a set of flare system loads. While there is nothing wrong with this work flow, it tends to overpredict the flare loads for the design cases (typically a power failure or partial power failure scenario). Smith & Burgess has found the following engineering assumptions prevalent in flare system designs that can lead to overprediction of required relief rates:

  • Using Normal Flow Rates – Whether the flow is coming from a pump or from compressor. Typically, as the pressure in the system rises, flows reduce due to the limitations of the equipment. Other times, recycle compressors would be shut down during the power failure case being instigated, but the analysis relieved the entire rate and did not account for the reduction due to the loss of the compressor.
  • Using Normal Exchanger Duties – For Fractionators, Columns or Towers that are reboiled with steam; the exchanger may not be capable of adding the normal duty to the system during a relief upset. In some cases, systems were designed such that the temperature of the steam is lower than the bubble point temperature of the system fluid at relief pressure. Analyzing exchanger “pinch-out” due to a reduction in the LMTD driving force can have a large impact on the relief rate if not done.
  • General Power Failure – In some cases, the flare system design is based on a general or generic power failure. These cases take the worst combination of electrical equipment losses and add them up to create a power failure (often called a total power failure). However, when these cases are dissected, there is no actual electrical failure that can cause the listed results. These analyses tend to assume that some electrical equipment continues to operate while others do not. With this assumption basis, the final design is based on assumptions and not the actual electrical system of the plant.

These are the major items that tend to result in overpredicting the flare system design case. Each system is unique and no list can comprehensively cover all design considerations. Call our experts to find out more ways we can help with all of your relief systems documentation needs.

Bottom Line
Smith & Burgess has on staff experts that are fully trained in relief systems design and documentation methodology. Our knowledge of relief systems translates into accurate flare system designs that do not result in overly conservative basis. We have in the past lowered flare system design rates by over 50% (while 20% to 40% is more common). We will generate fewer concerns which will save money and will provide documentation that is compliant with the PSM standard.