Analyzing Overpressure Scenarios

When isolating equipment that contains liquid anhydrous ammonia, the potential exists for it to become over pressurized due to thermal expansion effects. The overpressure from liquid expansion could then result in the rupture and loss of ammonia from the equipment. And that rupture and loss, if large enough, could potentially have a serious or catastrophic consequence.

Liquid and vapor anhydrous ammonia expand and contract with changes in pressure and temperature. For example, if 0°F liquid anhydrous ammonia is in a partially filled, closed container and it absorbs enough heat to increase to 68°F, the volume of the liquid will increase by about 10 percent. If the same container is 90 percent full at 0°F, it will become 99 percent full when it reaches 68°F. At the same time, the pressure in the container will increase from 16 pounds per square inch gauge (psig) to 110 psig.

Thermal expansion results in the physical change of size of the ammonia molecules due to higher thermal energy of the object. Heat (thermal energy) when absorbed into the ammonia causes increased atomic motion. This change in the kinetic energy of the atoms and molecules causes it to increase in volume. It is a physical characteristic of matter as opposed to a chemical characteristic. The mass of the ammonia remains the same, while its volume increases and its density decreases.

Equipment and piping sub-sections that can be isolated and can trap liquid ammonia should be protected against overpressure due to thermal expansion:

  1. automatically during normal operation;
  2. automatically during shutdown by any means, including an alarm or a power failure;
  3. during planned standby or seasonal conditions (e.g. situations when the valves in the ammonia lines to/from evaporative condensers are closed during cold weather conditions);
  4. and, in situations with an equipment or component fault.

Protection can be accomplished in these situations by either installing a hydrostatic relief device or check valve that relieves to another part of the closed-circuit system or an expansion compensation device.

The manual isolation of equipment and piping sub-sections, for any purpose, should be undertaken by trained technician(s) taking all necessary precautions to protect against overpressure due to thermal expansion of trapped liquid ammonia.

If trapping of liquid with subsequent thermal expansion can occur only during maintenance – i.e. when trained technician(s) are performing maintenance tasks, either engineering controls or administrative controls should be used to relieve or prevent the overpressure.

Where Lockout/Tagout is required for the energy control, the procedure and training should be in compliance with OSHA 29 CFR 1910.147, Control of Hazardous Energy.

Refer to Standard ANSI/IIAR 2-2008 (With Addendum B), Section 11.4, Equipment and Piping Hydrostatic Overpressure Protection for control requirements to protect against overpressure due to thermal expansion.

The refrigeration management plan of the ammonia system, which is subject to OSHA’s Process Safety Management (PSM) and the EPA’s Risk Management Plan (RMP) if it contains at or over the threshold quantity of 10,000 lbs., or at a minimum, the General Duty Clause, should include a process hazard analysis (PHA) that addresses the hazards of the process, which includes overpressure protection.

The analysis will include consequences of failures and human factors that must be addressed. Analyzing overpressure scenarios when isolating equipment or components to assure that methods for its prevention are implemented is crucial. It is critical that a current and accurate operating and maintenance procedure, on which the qualified operator(s) and technician(s) are trained, includes the appropriate steps to safely evacuate the liquid and isolate the equipment during maintenance to prevent overpressure.

An IIAR Member Alert was recently released to address this matter and it informed members that IIAR had received clarification from OSHA on an ongoing question surrounding OSHA’s requirement for hydrostatic relief for various pieces of equipment, especially evaporative condensers. OSHA’s position on the issue was originally that hydrostatic relief is required when equipment containing liquid can be isolated. That position was stated in an interpretation letter commonly known as “the Palmer letter.” However, a few years ago, the agency unofficially noted that administrative controls, i.e. trained operators/technicians are acceptable when isolating equipment as outlined in Standard IIAR-2. Nevertheless, OSHA did not officially revise its position and the interpretation remained on the agency’s website.

The inconsistency created confusion about OSHA’s requirement. Therefore, IIAR pursued OSHA to encourage the agency to make an official decision on this issue. As a result, IIAR successfully convinced OSHA to remove its requirement for hydrostatic relief (e.g. during manual isolation for maintenance), provided administrative controls are used for the isolation, as outlined in Standard IIAR-2.