Avoiding Hydraulic Shock in Refrigerant Piping
Incorporating slow-acting and multi-stage valves in refrigerant piping can help avoid catastrophic failures due to hydraulic shock. Hydraulic shock is a common and potentially dangerous phenomenon in a range of piping systems. Essentially a sudden local spike in pressure, this shock occurs when a rapid change in liquid flow velocity generates a pressure wave within the liquid media. Loud noises, movements in piping and even the bursting of pipes or line components can result from these pressure spikes.
The magnitude of the pressure wave generated in a hydraulic shock event depends on several factors. The change in pressure is proportional to the fluid density, the change in velocity, and the sonic velocity of fluid in the pipe. As these factors go up, so does the potentially damaging pressure in a hydraulic shock event. Common causes of hydraulic shock include the rapid closure of liquid valves. Known as “sudden liquid deceleration,” this closing action results in a dramatic change in the velocity of the liquid in the pipe in a short time.
Sudden liquid deceleration typically occurs in refrigeration systems the instant a liquid solenoid valve closes. Solenoid valves are fast-acting controls capable of quickly stopping all flow in a line. Bringing the velocity of the refrigerant in the liquid line to zero in this fashion generates a pressure wave in the liquid line upstream of the solenoid valve.
A second common type of hydraulic shock occurs when liquid collected in a refrigerant line is propelled by vapor refrigerant in an intermittent flow regime known as slug flow. This phenomenon commonly is referred to as “vapor-propelled liquid slugs.” The liquid slugs rapidly travel in the line until they encounter a feature, such as a valve, elbow or end cap, that causes a rapid change in velocity and a corresponding dramatic change in pressure, which causes the hydraulic shock event. While the volume of liquid involved in this type of shock is much lower than that in a typical sudden liquid deceleration event, the velocity of the liquid is often much higher. The vapor refrigerant propelling the liquid slugs frequently travels at a velocity an order of magnitude or more higher than typical velocities in liquid lines. The potentially higher velocities can make these slugs particularly destructive.
Vapor-propelled liquid slugs might occur under a number of circumstances in refrigeration systems. One common situation when a slug can be generated is during the opening of a hot-gas solenoid valve at the onset of a defrost cycle. Hot-gas lines often are located in the refrigerated space of a facility. Lower temperatures in the refrigerated space allow refrigerant to condense in these lines during the normal refrigeration cycle. When the hot-gas solenoid opens and releases high-pressure refrigerant vapor to the evaporator, the condensed liquid in this line can be propelled by the hot gas, forming a slug. Another common location for this phenomenon to occur is on the outlet of a closed suction-stop valve where refrigerant often condenses during the defrosting of a hot-gas defrost evaporator. When these valves are re-opened at the end of a defrost cycle, they cause the potential for a vapor-propelled liquid slug to form.
A third type of hydraulic shock event is caused by the rapid condensation of vapor in lines containing both liquid and vapor refrigerant. Known as “condensation-induced hydraulic shock,” this situation occurs when a plug of high-pressure vapor is introduced to subcooled refrigerant liquid. When the high-pressure vapor is trapped in the cold liquid space, rapid condensation occurs. The pressure of the space once occupied by the vapor drops as the refrigerant vapor condenses. The liquid refrigerant then accelerates toward this lower pressure area, collapsing the void. This fast-moving liquid can cause a hydraulic shock event when it encounters the adjacent body of liquid refrigerant.
The circumstances under which condensation-induced hydraulic shock occurs are more difficult to define than those of sudden liquid deceleration and vapor-propelled liquid slugs. There is a potential for this phenomenon to occur when subcooled liquid and high pressure vapor refrigerant are mixed. Some possible situations that could induce this type of hydraulic shock include the pressurization of an accumulator or trapped suction line during defrost, or the pumping of cold liquid refrigerant into a higher-pressure vapor line.
Possible Solution: Slow-Acting Valves
Fortunately, the damaging effects of sudden liquid deceleration, vapor-propelled liquid slugs, and condensation-induced hydraulic shock can be avoided through the careful design and proper application of piping components. System designers can employ a number of methods to minimize or even eliminate the potential for hydraulic shock in refrigeration systems. For example, using valves with a slower closing action can eliminate the potential for sudden liquid deceleration in liquid lines. Commercially available valves such as motor-operated valves or solenoid-type valves with a dashpot design provide a sufficiently slow closing action to avoid the shock caused by a sudden change in the liquid velocity.
Hot gas is the most commonly used method for defrosting low-temperature industrial refrigeration systems today. However, this switching of an evaporator to a condenser and back again creates prime conditions for the potential of vapor-propelled liquid slugs and condensation-induced hydraulic shock events. Low-temperature (less than 0°F [-17°C]) liquid recirculation units are the most susceptible to the hazards of a shock event. The greater pressure and temperature differences in these systems create an environment that makes them more susceptible to shock.
The IIAR 2000 Ammonia Refrigeration Piping Handbook recommends slow-acting or parallel-equalizing valves for the initiation and termination of the defrost cycle. These valves commonly are referred to as the soft-start hot-gas valve and the suction-stop equalizing valve. Smaller solenoid valves piped in parallel usually are used for this application (figure 1).
At the start of the defrost cycle, hot gas should be introduced to the coil slowly, through a smaller soft-start hot-gas valve. As the hot gas enters the cold coil, it will condense. Eventually, as the coil temperature starts to rise, condensation will slow and the pressure in the coil will start to rise. By introducing the hot gas at a slow, controlled rate, the chances of pushing any remaining or newly formed liquid from the coil at a high velocity are minimized. Typically the soft-gas cycle should last about 5 to 10 min, depending on the size of the valve and the hot-gas pressure.1 After the coil pressure has risen to the defrost pressure, the main hot-gas valve can be safely opened to supply full flow for the defrost cycle.
Likewise, at the termination of the defrost cycle, the high-pressure, saturated ammonia contained in the coil needs to be released to the low side in a controlled manner to avoid sending high-pressure liquid and gas through the suction line, where it may encounter elbows or abrupt changes in direction and cause severe damage. Like the hot-gas soft-start valve, a smaller equalizing solenoid typically is used to accomplish this controlled release (figure 1). The smaller solenoid valve, piped in parallel with the suction-stop valve, is opened first to relieve the coil pressure and prevent high-pressure, saturated ammonia from moving at high velocities through the suction line. Once the evaporator pressure has equalized with the suction pressure, the main suction-stop valve can be opened.
Another Possible Solution: Multi-Position Valves
A less expensive way to achieve the desirable effects created by using smaller redundant solenoid valves can be achieved by using one valve capable of providing multiple flow rates. The staged opening and closing action of these valves results in flow control similar to that of redundant valves but with a simpler design (figure 2).
Some of these valves can be used in suction-stop applications where evaporator equalization is desired after defrost. Others are intended for use as hot-gas solenoid valves with a soft-gas feature. Because these dual-position valves incorporate an equalizing feature, they can reduce installation costs by lowering the total number of valves to be installed. In some cases, these valves allow up to 10 welded joints to be eliminated, which reduces installation time and costs while minimizing potential leak paths.
In some of these devices, the valve operation is controlled by the sequencing of two integral pilot solenoids, which, in turn control valve position. The valve can be held in a closed, partially open (approximately 10 percent of full flow) or fully open position. By sequencing the solenoids based on time, users have the flexibility to set each stage to meet their specific needs.
Some multi-position valves also incorporate a fail-safe feature. If a power failure occurs during the defrost cycle, this feature holds the valve in the equalizing position until a safe coil pressure is reached.
Hydraulic shock events in an industrial refrigeration system present designers with unique safety concerns that must be addressed. Uncontrolled hydraulic shock leads to pipes shaking and banging, flanges loosening and, in extreme cases, system rupture. By incorporating slow-acting and multi-stage valves in their refrigerant piping, companies can minimize or prevent catastrophic failures.
Steve Mesner is a product engineer and Joe Nelson is an applications engineer for Parker Hannifin Corp., Refrigerating Specialties Div., Broadview, Ill., a manufacturer and distributor of refrigeration control valves. For more information, call (800) 506-4261 or visit www.parker.com/refspec