Why You Should Avoid Critical Flow In Ammonia Refrigeration Systems
Nationally, the number of significant failures in refrigeration piping systems is estimated to be 15 to 20 per year. Most critical are those that cause major floodbacks to the machinery room and compressors. These failures generally are associated with improper application of suction-line control valves, sequencing or fail-safe positions.
Understanding Critical FlowCritical flow is defined as the rate of flow of a fluid equivalent to the speed of sound in that fluid. Critical flow must be avoided in refrigeration systems because it will cause liquid "cannonballs" that can blow caps off pipes as well as fracture piping. Sudden pressure reductions can induce critical flow from isolated areas of the refrigeration system. This is particularly true in production plants where surge drums and evaporators may create liquid dams. Liquid dams can temporarily cause pressures to build while system pressure rapidly decreases.
Critical flows in refrigeration systems generally are associated with rapid gas expansion. Well-known thermodynamic and gas laws clearly describe rapid gas-pressure reductions and their effects. A reduction of 45.6% at -40°F (-40°C) in ammonia gas pressure will initiate a critical or sonic flow. A basic understanding of molecular theory of gases will help beginning engineers realize that gas molecules move at their average sonic velocities. For ammonia, these are values listed in ASHRAE Handbook 1993, Chapter 17.
Traditionally, gas-suction pipeline velocities of 3,000 to 5,000 ft/min have been the purported norm. In reality, however, pipeline sizing tables show velocities of 13,000 to 15,000 ft/min as acceptable. Rapid reductions in gas pressures by compressors in the machinery room or increases in heat fluxes by evaporators can cause a pressure differential capable of producing critical flow. Critical gas velocities alone may not cause piping system failure, but effects such as wire drawing and valve-component seal abrasion can be expected.
Users are able to produce conditions that can reduce gas pressures by as much as 50% if the refrigerant system is operated at low pressures (less than atmospheric pressure). Therefore, operating procedures for systems functioning at or near vacuum become critical. Care in both design and operation is necessary. Because most systems that use two-phase flow contain evaporators with residual liquid (during normal operation and restart), the combination of high heat flux and rapid pulldown can produce critical flow.
When evaporators are exposed to a rapid pressure reduction, rapid gas expansion can produce slug flow combined with annular and mist slug flow concurrently. Piping and valve components then are exposed to the hydraulic effects of impact, and confined areas potentially can contribute to thermoshock (condensation-induced hydraulic shock) as collapsing gas pockets are formed. The most ardent welding and pipe-fitting practices will not overcome the stresses of high velocity liquid impacts.
Understanding why high velocity liquid impacts occur and designing piping systems that minimize their potential is imperative in automated and large refrigeration systems. Normally, control valves can help prevent sudden liquid releases from evaporators. Utilizing three-dimensional considerations that will accommodate building and refrigeration component geometry is key to designing safe piping systems.
Pipe and Valve FailurePipe and valve failures generally are one of the following three types:
This type of failure occurswhen liquid impacts against pipe or vessel containment walls.
- Thermoshock (Condensation-Induced).
Generally associated with a confined space, in this type of failure, adjacent liquid slugs impact each other and transmit the impact's force to a containment wall or surface.
This type of failure occurs where liquid is confined (generally at low temperatures), then permitted to warm and expand to the point of rupturing containment walls.
When critical flow is permitted to start, valve and pipeline components may fail due to the hydraulic effects of liquid impacting containment walls. Examples of these include air-unit evaporators or shell and tube heat exchangers located lower than pipelines. If an evaporator's stop valves are open on compressor startup or if they inadvertently open during a hot gas defrost, a decrease in gas pressure is permitted, which can lead to pressure reductions of greater than 50%. Rapid change in heat flux also can cause gas evaporation and excite critical flow and liquid slugs. High specific gas/liquid volume ratios necessitate precautions in pressure reductions, whether in evaporators or high heat flux areas such as thermosyphon oil coolers.
Typical design capacity tables (table 1) imply that a 10" pipe at -40°F (-40°C) would have capacity of 887 tons of refrigeration (TR). A line operating at this capacity will have a dry-gas velocity of more than 14,000 ft/min. Systems at this velocity are good candidates for valve and pipe failure. Any unusual occurrence will take the system offline and be followed by a restart, which necessitates higher than normal velocities.
"ASME Code for Pressure Piping, B31.5, 1992 Edition" does not require impact tests for ferrous material used in piping systems designed for temperatures between -20 and -150°F (-29 and 101°C), provided the maximum circumferential or longitudinal tensile stresses do not exceed 40% of the allowable material stress. A53, grade B has a minimum tensile strength of 60,000 psi. Examples using 40% burst pressure with 3 or 4" pipe are nowhere near system standby pressures. Design engineers should be aware of this for the design of suction-line flow velocities. Note that vessel manufacturers are required to provide impact tests for nozzles.
Thermosyphon PipingPiping such as thermosyphon requires a significant understanding of two-phase fluid flow. The primary concerns include life safety, environmental hazards and minimizing production interruptions in storage and processing facilities. Thermosyphon piping requires an in-depth knowledge of high-side flows. Providing equal flow to multiple oil coolers is paramount. A balanced flow is necessary for continual operation of thermosyphon coolers because evaporated refrigerant must return to condenser inlets.
A thermosyphon system will be near discharge pressure at all times, and any sudden variation in discharge pressure will affect oil cooler operation. Sudden pressure reductions will create a significant imbalance in the relative heat flux encountered in the oil coolers. Cooler vessels and pilot receivers accumulate liquid that will respond to such pressure changes. Field observations have shown that sudden reductions of 30 psi (such as that which would occur during defrost of large air units) can cause oil coolers to "geyser" and flash all liquid refrigerant out of the oil cooler.
Liquid supply and gas return lines must be placed on a slope to help maintain flows and to help the system recover quickly after such an interruption. Thermosyphon operation is similar to the risers and downcomers in large power plant boilers that rely on the gravity effects of different fluid densities to sustain convection during operation.
Plant hot-gas supply-line origin or tie-in point also is critical for thermosyphon cooling. A sudden reduction of discharge pressures on the condensers' inlet side can cause reverse flow through the condenser drain systems. Hot gas supplies originating from the receiver maintain proper flow through the condenser and proper supply of liquid to thermosyphon gravity-fed oil coolers.
In some large plants, thermosyphon cooling becomes difficult to operate with gravity systems. A variety of pumped thermosyphon systems have emerged in plants, expanded with different elevations of primary components such as condensers. A thorough understanding of pressure differentials, pipe slopes and velocities is required for proper thermosyphon piping implementation.
Designing and Installing a SystemWhen designing and installing a central refrigeration system:
- Installation and construction engineering drawings must be complete with three-dimensional views and elevations of piping systems.
- The elimination of surplus liquid pools (surge drums or remote accumulators and recirculators) is paramount in automated systems.
- Large automated and computer-operated refrigeration systems require conservative piping practices and a thoroughly designed, properly sloped and untrapped refrigeration pipeline system.
- Understanding the starting sequence for computer-operated systems is key to preventing critical flow. System component (i.e., accumulator and pipe) limitations as a time function of startup of compressors and evaporators must be understood.
- Refrigeration operators must understand and have the skills and manual electrical control options to permit safe and smooth graduated startup.
Understanding thermodynamics and critical flow are an important aspects of designing a refrigeration system. Compresssor and pipe failure can be avoided if these factors are considered when determining pipeline sizes.