In process units, equipment reliability, is dependent upon stable and continuous operation within limits established by the designers. Increases in throughput and severity in feedstock,can cause equipment degradation if operating limits are exceeded. The limits of operating variables (operating envelopes)are defined in terms of controllable critical process variables. Monitoring Operating envelopes is therefore a critical strategy for sustained reliability of Process Units. Guidance can be provided to establish operating limits for process equipment and set up a system of continuous monitoring these limits.