PSI Explained: Safe Upper and Lower Limits / Consequence of Deviation
Title 8 CCR §5189(d)(2)(D)-(E) and Title 29 CFR §1910.119(d)(2)(i)(D)-(E) require that the information pertaining to the technology of the process include “Safe upper and lower limits for such items as temperatures, pressures, flows or compositions; and, an evaluation of the consequences of deviations, including those affecting the safety and health of employees.” It is worth pointing out that the information that is developed to satisfy this requirement will be used again in the development of Operating Procedures (Title 8 CCR §5189(f)(1)(B) and Title 29 CFR §1910.119(f)(1)(ii)) since it is required that operating procedures address operating limits including consequence of deviation, steps required to correct deviation, and steps required to avoid deviation. Here is an example of how one might document safe upper and lower limits and consequence of deviation for an ammonia refrigeration high pressure receiver:
| Type of Vessel | High Pressure Receiver |
| Normal Operating Pressure (PSIG) | 120 PSIG – 200 PSIG |
| Consequence of Exceeding Normal Operating Pressure | 1. Compressors will shut down on high pressure cutout; 2. Pressure relief valves may lift |
| Consequence of Deviating Below Normal Operating Pressure | 1. Difficulty feeding high pressure liquid to each zone of refrigeration; 2. Difficulty cooling the oil on liquid injection oil cooled screw compressors |
| Steps to Avoid Deviating From Normal Operating Pressure | 1. Operator monitors system pressures daily; 2. System safety devices will shut down equipment before the pressure reaches an unsafe level; 3. Annual mechanical integrity inspection verifies that equipment is in good working condition and that safety devices are properly functioning; 4. System safety devices will alert operator in the event of a high pressure cutout alarm |
| Steps Required to Correct Deviating From Normal Operating Pressure | 1. Check accuracy of gauges; 2. Check setpoint on computer or PLC to make sure that it is set correctly; 3. Turn on/off condensers and compressors to resolve the problem |
| Max. Operating Pressure (PSIG) | 225 PSIG (90% of Relief Valve Setting) |
| Consequence of Exceeding Max. Operating Pressure | 1. Compressors will shut down on high pressure cutout; 2. Pressure relief valves may lift; 3. Vessel may rupture |
| Steps to Avoid Exceeding Max. Operating Pressure | 1. Operator monitors system pressures daily; 2. System safety devices will shut down equipment before the pressure reaches an unsafe level; 3. Annual mechanical integrity inspection verifies that equipment is in good working condition and that safety devices are properly functioning; 4. System safety devices will alert operator in the event of a high pressure cutout alarm |
| Steps Required to Correct Exceeding Max. Operating Pressure | 1. Check accuracy of gauges; 2. Check setpoint on computer or PLC to make sure that it is set correctly; 3. Turn on/off condensers and compressors to resolve the problem; 4. Replace relief valve if it has lifted; 5. Turn zone off if problem can be determined; 6. Perform an incident investigation |
| Normal Operating Temperature (°F) | 70°F – 100°F |
| Consequence of Exceeding Normal Operating Temperature | None (consequence is related to the pressure, not the temperature) |
| Consequence of Deviating Below Normal Operating Temperature | None (consequence is related to the pressure, not the temperature) |
| Steps to Avoid Deviating From Normal Operating Temperature | 1. Operator monitors system daily |
| Steps Required to Correct Deviating From Normal Operating Temperature | 1. Check accuracy of gauges; 2. Check setpoint on computer or PLC to make sure that it is set correctly; 3. Turn on/off condensers and compressors to resolve the problem |
| Max. Operating Temperature (°F) | 108°F (corresponding to 225 PSIG) |
| Consequence of Exceeding Max. Operating Temperature | None (consequence is related to the pressure, not the temperature) |
| Steps to Avoid Exceeding Max. Operating Temperature | 1. Operator monitors system daily |
| Steps Required to Correct Deviating From Max. Operating Temperature | 1. Check accuracy of gauges; 2. Check setpoint on computer or PLC to make sure that it is set correctly; 3. Turn on/off condensers and compressors to resolve the problem; 4. Replace relief valve if it has lifted; 5. Turn zone off if problem can be determined; 6. Perform an incident investigation |
| Fluid | Ammonia |
| Consequence of Using Other Fluids | 1. Water mixed with the ammonia will accelerate internal corrosion; 2. Oil will coat surfaces and decrease heat transfer; 3. Dirt/debris will clog/damage valves and strainers associated with vessel |
| Steps to Avoid Interaction With Other Fluids | 1. Ensure that ammonia supplier supplies the proper quality of ammonia; 2. Ensure that a vacuum is pulled when significant maintenance, repair, or construction work is performed. |
| Steps Required to Correct the System when Interaction with Other Fluids Occurs | 1. Remove foreign fluid completely; 2. Flush/clean the entire affected area |
| Normal Liquid Level (in) | 12″ – 24″ from bottom of vessel |
| Consequence of Exceeding Normal Liquid Level | There is no consequence as long as the vessel is not filled above 85% |
| Consequence of Deviating Below Normal Liquid Level | 1. Difficulty feeding high pressure liquid to each zone of refrigeration; 2. Difficulty cooling the oil on liquid injection oil cooled screw compressors |
| Steps to Avoid Deviating From Normal Liquid Level | 1. Operator monitors the level in the vessel daily |
| Steps Required to Correct Deviating From Normal Liquid Level | 1. Manually shut off the valve supplying the vessel; 2. Continue to drain or feed ammonia from the vessel |
| Normal Fluid Inventory (lb) | 800 lb |
| Maximum Liquid Level | 85% Full |
| Consequence of Exceeding Max. Liquid Level | Hydrostatic vessel failure |
| Steps to Avoid Exceeding Max. Liquid Level | 1. Operator monitors the level in the vessel daily; 2. Vessel level is closely monitored during a pumpdown procedure which will result in temporary increased vessel levels |
| Steps Required to Correct Exceeding Max. Liquid Level | 1. Manually shut off the valve supplying the vessel; 2. Continue to drain or feed ammonia from the vessel; 3. Perform an incident investigation |
| Max. Allowable Pressure (PSIG) | 250 PSIG @ 200°F |
| Consequence of Exceeding Max Allowable Pressure | 1. Compressors will shut down on high pressure cutout; 2. Pressure relief valves may lift; 3. Vessel may rupture |
| Steps to Avoid Exceeding Max. Allowable Pressure | 1. Operator monitors system pressures daily; 2. System safety devices will shut down equipment before the pressure reaches an unsafe level; 3. Annual mechanical integrity inspection verifies that equipment is in good working condition and that safety devices are properly functioning; 4. System safety devices will alert operator in the event of a high pressure cutout alarm |
| Steps Required to Correct Exceeding Max. Allowable Pressure | 1. Check accuracy of gauges; 2. Check setpoint on computer or PLC to make sure that it is set correctly; 3. Turn on/off condensers and compressors to resolve the problem; 4. Replace relief valve if it has lifted; 5. Turn zone off if problem can be determined; 6. Perform an incident investigation |
| Min. Design Metal Temperature (°F) | -20°F @ 250 PSIG |
| Consequence of Exceeding Min. Design Metal Temperature | Vessel failure |
| Steps to Avoid Exceeding Min. Design Metal Temperature | 1. Operator monitors system pressures daily; 2. System safety devices will shut down equipment before the pressure reaches an unsafe level; 3. Annual mechanical integrity inspection verifies that equipment is in good working condition and that safety devices are properly functioning; 4. System safety devices will alert operator in the event of a low pressure cutout alarm |
| Steps Required to Correct Exceeding Min. Design Metal Temperature | 1. Check accuracy of gauges; 2. Check setpoint on computer or PLC to make sure that it is set correctly; 3. Turn on/off condensers and compressors to resolve the problem; 4. Replace relief valve if it has lifted; 5. Turn zone off if problem can be determined; 6. Perform an incident investigation |
very helpful