Safe upper and lower limits are intended to relate to the equipment itself. For example, a process circuit will have multiple pieces of equipment within the circuit with varying design maximum pressure ratings. Whereas each piece of equipment will have different pressure rating, the safe upper limit for the circuit should be something less than the maximum working pressure of the lowest rated equipment or component in the circuit. For example, if “Vessel A” has a maximum rating of 250 psi, its safe upper limit is something less than 250 psi. Likewise If “Vessel B” has a maximum rating of 275 psi, its safe upper limit is something less than 275 psi. However, if “Vessel A” and “Vessel B” and in fluid communication with each other and are part of a common circuit, the effective safe upper limit of equipment with that circuit may be controlled by the vessel/component with the lowest rating. Sources of safe operating limits may be manufactures designs (e.g., information in a U-1 form for a pressure vessel), or codes or specifications.
Often operators will confuse the concept of operability limits with safe upper and lower limits. An operator of the circuit discussed above may never intend to operate above 175 psi and “establish” a safe operating limit of 180 psi. That operator may reason, since my equipment is rated well above my operations limit, it is extra safe. Although the logic is sound, the operator’s action does not match OSHA’s and EPA’s intent. Safe upper and lower limits are a component of process safety information.