Process Safety Management: Operating, Safe and Emergency Limits
In this post we continue our discussion to do with safe limits. Previous posts in this series are:
The concept of safe limits can be extended to include operating and emergency limits, as illustrated in the sketch, which shows values for a process variable such as pressure, temperature, level or flow rate.
The innermost range shows the optimum value for this particular parameter. In this case it is 239 to 240. This optimum point may change as target conditions to do with production rates, yields or product quality change.
The operating range represents the upper and lower limits for that variable’s normal value. Supervision is free to move the variable to any point within that range in order to achieve production and quality goals. In the sketch the operating range is 235 to 245.
If operating conditions are allowed to move outside the operating limits, but within the safe limits, then the facility is said to be in trouble, i.e., there are no safety issues to worry about, but the system is operating inefficiently. Troubleshooting efforts to bring the value back into the operating range will save money. Indeed, much of management’s attention will be directed toward trouble shooting because addressing difficulties in this area will often lead to a significant improvement in profitability for relatively little expenditure. Examples of ‘trouble’ include:
Excessive energy consumption;
Product quality problems;
Unusually high use of spare parts; and
Low production rates.
The operating limit values are often quite fuzzy. As the system moves away from optimum operation it will start to exhibit symptoms of unusual operation which will eventually lead into the troubleshooting range.
The next range is defined by the safe limit values. In the case of this example, were the parameter allowed to exceed 275 or go below 210 then the system is in an unsafe condition and action must be taken to bring that value back into the safe range.
The final set of values is the emergency limits. If the process parameter goes beyond one of these limits, then an emergency situation has been created. Immediate action is required; generally the safety instrumentation and safety equipment (such as pressure relief valves) will be activated. In this case the upper emergency limit is 310; there is no lower emergency limit.
