Complex Flare Network Analysis for Oil Refinery Unit Case Study

The Challenge

A large oil refinery with a very complex flare network had become so complex that the tools the refinery was using to evaluate the flows through the flare network could not adequately model the system. The facility had six separate flares, three main headers, and hundreds of relief devices that discharged into the system. As the flare system was modified over the years, multiple cross-connection points were added between the headers in an effort to balance the flow rates through the headers with minimal piping changes. Management no longer had confidence that their model results reflected the actual network performance and therefore, could not be sure the system would perform properly in the event of a global relief scenario at the facility.

Our Approach

Management needed an accurate model and performance evaluation of the flare network during a total refinery power failure scenario. They also wanted a model that could be utilized in the future to evaluate changes or additions to the flare network. The refinery had been using two other well-known commercial flare network software packages to evaluate changes or additions to the flare network, but these programs were not able to properly converge with all the complexities of the network. ioMosaic constructed a model of the flare network using SuperChems™, a component of Process Safety Office®. The backbone of the model included drawing piping isometrics for all the relief devices, headers and flares involved. All relief devices associated with this scenario were also modeled using the valve details provided by the client.  

Once the physical details of the flare network had been entered into the program, the network was divided into sections to assist with the complex calculations. Points where the flows could cross between headers were identified as Nodes. Some of these Nodes had relief devices providing flow into the Node and others were only potential cross-over points between the different headers, but no new flow was added. For each Node where relief devices provide flow into the Node, the relief devices were grouped together. The network ended up with 11 groups of relief devices and 20 network Nodes. The Groups of relief devices ranged from just one relief device up to 19 devices in each group.

A back-pressure curve was generated for each relief device using the specific characteristics of the relief valve and the chemical components flowing through it. Next a back-pressure curve was generated for each Group of relief devices. This provided an initial look at how the relief devices would perform when multiple devices were flowing simultaneously.

Next, all network Nodes were defined with interconnecting piping isometrics and information about where flow could come from and go to for each node. In many instances, flow could go either direction through a cross-over pipe connecting two headers depending on what the pressure was at each of the connecting Nodes. The other software packages ran into trouble without a defined flow path through each of these connection points. However, it wasn’t possible to confirm the flow direction until the pressures were identified.

Once all of the Nodes were defined, SuperChems™ generated the equivalent of back-pressure curves at multiple temperatures and pressures for each Node, called Flow Maps. These Flow Maps were used by SuperChems™ to converge the network solution. The software provided temperatures and pressures for each Node and the flows through the network, including which direction the flow was going through each of the network cross-over connections.

The final step in the analysis was to review each of the groups of relief devices and determine how much flow could pass through each device with the back-pressure at the first network Node as determined above. These relief device flow rates could then be compared to the required flow rates for this particular relief scenario to determine if the relief device and network piping could meet the demand.

Once the model was developed, modifications to the network could be easily examined. If safeguards are present which eliminate the relief load from a particular device, that device can be disabled by a single click. Changes to the type of relief device (conventional, bellows or pilot) can be easily examined. The output from SuperChems™ identifies the pressure drops through each segment of piping, allowing the user to quickly determine which sections of piping should be modified to achieve the greatest performance improvement within the network.

Based on the changes that are made, SuperChems™ can re-evaluate the flows through the network. In some cases, if flows are removed from certain groups, then the pressure drops and flow might reverse direction through one of the cross-over connections. SuperChems™ can identify these changes in flow direction and provide a new solution to the flow network.

The Benefits

This flow network model provided some good insight to the dynamics of the flare system. SuperChems™ was able to highlight some valves that were undersized, regardless of the piping configuration and flows throughout the network. The software identified several relief valves that would not provide any flow because the back-pressures in that section of the flare network would be higher than the inlet pressure for the relief device.  

The model was used to evaluate the effectiveness of various safeguards to reduce the flow rates during this total refinery power failure scenario. The model was also useful in identifying some valves that would benefit from changing from a balanced-bellows style to a pilot device. It also pointed out some bottlenecks in the piping system, which needed to be modified to allow proper flows through all of the relief devices upstream.

In summary, SuperChems™ was able to produce a dynamic model of a complex flare network where all other software packages came up short. Once developed, the model is easily modified to stay current with changes to the system.

Learn More

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