by Usman Khan
Everyone wants to provide a safe atmosphere for workers, facilities and the surrounding environment. The greatest risk in many process facilities comes from fired equipment. Burner management systems (BMSs) are the safety instrumented systems specific to fired equipment. The greatest challenge many asset owners face while evaluating the adequacy of their existing BMS designs comes from the inconsistency of results from one type of fired device to another of the same type (e.g., a heater or boiler) when using a risk assessment technique such as hazard and operability study (HAZOP) or layer of protection analysis (LOPA). Findings and recommendations should be similar for similar installations. The largest contributors of inconsistency are the qualitative nature of the techniques and the strong opinions of the team members. This is even more of a challenge when different teams are used in different risk assessments. However, there are ways to introduce consistency to the studies without turning them into detailed and expensive quantitative risk assessments.
The Environmental Protection Agency has developed simplified protocols for risk management planning to help with this. These protocols utilize equivalent TNT methodologies as contained in the Federal Emergency Management Agency “Handbook of Chemical Analysis Procedures”. Use of this resource can provide the empirical basis needed to drive consistency in the assessment of fired equipment from one asset to another, one facility to another, and one risk assessment team to another.
This technique can be simplified in a seven-step method to yield consistent results for fired equipment. These can be summarized as:
Step 1: Calculate the vapor cloud explosion effect zone of the fired equipment.
Step 2: Calculate the physical explosion and deflagration effect zone.
Step 3: Calculate the pool fire effect zone (for liquid fuels only).
Step 4: Calculate the personnel density in the effect zone and determine extent of impact.
Step 5: Perform a LOPA to determine the frequency for each hazardous event.
Step 6: Determine the required probability of failure on demand (PFD) for each safety instrumented function (SIF).
Step 7: Determine the required safety integrity level (SIL) for each SIF.
Any SIL selection method adopted by a company needs to be easy to use and yield quick results. To make the seven-step method described above easier to utilize, it is recommended that companies develop the following set of tools and procedures:
A spreadsheet application for each type of the most common types of fuels the company utilizes in their fired equipment to calculate each of the three effect zones.
A supporting procedure on calculation of personnel densities.
A spreadsheet application that provides a framework for LOPA for each of the standard SIFs in BMSs.
A supporting procedure to include guidance on how to perform LOPA.
A cost / benefit analysis spreadsheet to support project justification.
Adopting this methodology will allow a company to quickly, efficiently and consistently evaluate their BMSs and make the most cost-effective business decisions. For more details on how to make the right selections, read the paper “Burner Management System Safety Integrity Level Selection.”