An “Emergency Pressure Control System” has been a requirement in the IFC and IMC for several iterations of these codes, and yet it can be a very confusing requirement as they are not found in the vast majority of mechanical refrigeration systems using Anhydrous Ammonia as the refrigerant. Part of the problem may lie in the fact that the IIAR does not recognize this emergency system as an essential need in the design of an Ammonia refrigeration process (See Appendix I of IIAR-2, 2014). But the gest of this emergency system is to aid emergency responders and refrigeration personnel in mitigating an overpressure condition PRIOR to the operation of relief valve(s). This section of the International Mechanical Code (IMC) establishes a requirement for a fully redundant, automatic, safety control system that ensures that lower pressure zones will be capable of handling additional pressure added by a crossover condition WITHOUT over pressurizing or lifting relief valve(s) in the lower zone. Section 606 of the International Fire Code (IFC) establishes provisions for the automatic crossover valves, overpressure limit set point, manual operation, system design pressure, automatic emergency stop, the operation of an automatic crossover valve and overpressure in low-pressure zones. So let’s break this down…
1105.9 Emergency pressure control system.
Permanently installed refrigeration systems containing more than 6.6 pounds (3 kg) of flammable, toxic or highly toxic refrigerant or ammonia shall be provided with an emergency pressure control system in accordance with Section 606.10 of the International Fire Code.
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606.10 Emergency pressure control system.
Permanently installed refrigeration systems containing more than 6.6 pounds (3 kg) of flammable, toxic or highly toxic refrigerant or ammonia shall be provided with an emergency pressure control system in accordance with Sections 606.10.1 and 606.10.2.
Technological advances in refrigeration system control equipment now make it possible to provide an automatic emergency control system to replace critical functions of the traditional “emergency control box.” The automatic controls required by Sections 606.10.1 through 606.10.2.2 provide a means of mitigating an overpressure condition PRIOR to the operation of RVs and, most likely, prior to the arrival of emergency responders. The automatic valves also eliminate the need for emergency responders to decipher the condition of a system in an attempt to determine whether the operation of manual crossover valves in an emergency control box would be of benefit in mitigating the incident. This section parallels IMC Section 1105.9 to increase the likelihood that designers don’t miss this design requirement in the code. Overall, these provisions add a requirement for a fully redundant safety control system instead of a manual system that has proven itself to be rarely, if ever, utilized by the fire service. This favorably resolves long-standing concerns regarding the potential for harm caused by an untrained person operating valves in an emergency control box. There is no condition where removal of refrigerant from a refrigeration system by the fire service is considered advisable. In contrast, automatic transfer of excess pressure to another zone of the system in conjunction with stopping the pressure source (i.e., the compressors) can safely mitigate an overpressure condition.
606.10.1 Automatic crossover valves.
Each high- and intermediate- pressure zone in a refrigeration system shall be provided with a single automatic valve providing a crossover connection to a lower pressure zone. Automatic crossover valves shall comply with Sections 606.10.1.1 through 606.10.1.3.
The requirement for a single crossover valve between the high and the low sides based on the traditional industry practice of providing a single manual crossover valve in the emergency control box. In the unlikely event that a fire causes an overpressure condition, allowing system zones to interconnect automatically creates a much more significant heat sink to limit pressure buildup while safely containing the NH3. If the fire continues to grow, the RVs will protect the process and the reseating of the RV(s) will automatically limit the release of NH3 to the amount necessary to maintain the system within design limits. In contrast, most emergency responders would not possess the expertise to properly cycle a manual valve in an emergency control box to limit the release of NH3 to the minimum amount necessary for safety.
606.10.1.1 Overpressure limit set point.
Automatic crossover valves shall be arranged to automatically relieve excess system pressure to a lower pressure zone if the pressure in a high- or intermediate-pressure zone rises to within 90 percent of the set point for emergency pressure relief devices.
This section provides a safety buffer between activation of the emergency pressure control system (EPCS) and operation of an RV. Because of variances in operational tolerances among RV(s) and because some RVs may begin to seep at 90% of their rated operating pressure, it is appropriate to have the EPCS shut down a system if system pressure rises to 90% of the relief valve set pressure. This further reduces the potential for any release from a system that has malfunctioned and overpressurized.
606.10.1.2 Manual operation.
Where required by the fire code official, automatic crossover valves shall be capable of manual operation.
This section authorizes the AHJ to require manual control capabilities for the crossover valve. Although this provision is not regarded as necessary by the industry, it is recognized that some local FD’s will prefer to have the manual control capability for backup.
606.10.1.3 System design pressure.
Refrigeration system zones that are connected to a higher pressure zone by an automatic crossover valve shall be designed to safely contain the maximum pressure that can be achieved by interconnection of the two zones.
This provision requires the low side to be capable of handling additional pressure added by a crossover condition without overpressurizing or lifting an RV on the low side. The legacy codes did not address this concern, given the assumption that someone operating a manual bypass valve in the emergency control box would be knowledgeable about system limitations; however, this may or may not be true. Nevertheless, the legacy codes never required the low side of the system to handle the high-side pressure, and as a result, some systems with emergency control boxes present the potential for an emergency responder to overpressurize a system zone by fully opening a manual crossover valve too quickly. The resulting overpressure condition could cause operation of an RV or even a failure in the piping system.
606.10.2 Automatic emergency stop.
An automatic emergency stop feature shall be provided in accordance with Sections 606.10.2.1 and 606.10.2.2.
This establishes the applicability of the requirements for the additional safety feature of an emergency stop feature in refrigeration systems.
606.10.2.1 Operation of an automatic crossover valve.
Operation of an automatic crossover valve shall cause all compressors on the affected system to immediately stop. Dedicated pressure-sensing devices located immediately adjacent to crossover valves shall be permitted as a means for determining the operation of a valve. To ensure that the automatic crossover valve system provides a redundant means of stopping compressors in an overpressure condition, high-pressure cutout sensors associated with compressors shall not be used as a basis for determining the operation of a crossover valve.
The intent of this section is for the automatic crossover system to have a fully redundant means of stopping compressors. Compressors are ordinarily provided with automatic high-pressure cutout controls, but this section requires that these controls not be used to satisfy the requirement. An additional set of controls can be required to serve as a backup means of preventing a severe overpressure condition that could cause operation of an emergency relief vent.
606.10.2.2 Overpressure in low-pressure zone
The lowest pressure zone in a refrigeration system shall be provided with a dedicated means of determining a rise in system pressure to within 90 percent of the set point for emergency pressure relief devices. Activation of the overpressure sensing device shall cause all compressors on the affected system to immediately stop.
The lowest pressure zone of a system cannot be arranged to bleed pressure to another system zone since crossing the lowest pressure zone to a higher pressure zone would most likely result in reverse flow. However, by providing a redundant emergency stop control, which would disengage the compressor, an overpressure condition should be automatically mitigated. Overpressure on a low-pressure zone would most likely result from a defrost line from the high side that is stuck in the open position, and stopping the compressor will disengage the pressure source for the defrost system. Note that compressors will only cut out if an overpressure condition occurs. If the emergency condition involves a leak on the low side, compressors will continue to operate, which is beneficial in pumping down the low side for this type of event.