Water hammer – the hidden bomb in heat exchange systems

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Water hammer – the hidden bomb in heat exchange systems. Water hammer is a common phenomenon in steam systems; it not only damages pipes, valves, and equipment, leading to downtime and disruptions in production, but can also cause safety accidents in severe cases. Recently, some of the enterprises visited by Fred were in the chemical, pharmaceutical, petrochemical, and electronics industries, and all of them faced problems with water hammer in their steam systems; the most severe cases of water hammer occurred in the heat exchange systems. Today, Fred would like to share some insights with you on the water hammer issue in heat exchange systems. Water hammer in heat exchange systems occurs in three locations: the system’s steam inlet, inside the heat exchange equipment, and at the equipment’s outlet/condensate recovery pipe. Water hammer at the steam inlet of the heat exchange system occurs because the steam supplied usually contains water (except for superheated steam), and due to heat loss and condensation during pipeline transportation, the steam supply pipes of the heat exchange system often contain condensed water. When, at a certain moment, the steam speed increases due to a sudden rise in load, the condensate water in the pipeline creates waves. Driven by the rapid flow of steam, these waves gradually build up in intensity. Once they reach the top of the pipeline and block it, they form a temporary water seal that prevents steam from passing through. This results in an increased pressure difference between the front and back sections of the pipeline, and the steam is propelled forward at high speed, like a bullet leaving a gun barrel. Moreover, during high-speed surging, the amount of condensate increases progressively, and the speed also rises; eventually, when it strikes the valves, elbows, or the metal walls of the equipment, water hammer occurs. When the system is restarted after being shut down, due to the large amount of condensate in the pipes and rapid changes in steam velocity, water hammer tends to be more severe. In steam supply pipes, when the amount of condensate is high, especially when the level of condensate approaches 80% of the pipe diameter (which is more likely to happen in pipes with a small diameter), water hammer can occur even if the load remains stable, if there are bends in the pipe or control valves installed in it. Water hammer in heat exchange equipment: Water hammer that occurs inside heat exchange equipment is often caused by water accumulation within it. Water accumulation in the equipment caused by flow loss issues. Flow loss occurs when the total back pressure behind the check valve is greater than or equal to the pressure in the steam space within the heat exchange equipment, which prevents the condensate from being removed and leads to water accumulation. There are mainly two possibilities for loss of flow in heat exchange equipment: 1) Loss of flow is caused by an excessive heat exchange area. When the heat exchange area of a heat exchange device is too large, its heat exchange capacity increases, and the steam pressure required for the heat exchanger decreases. This results in too low a steam pressure inside the heat exchanger, leading to loss of flow and an inability to discharge condensate properly through the drain valve ； 2) Flow loss is caused by an increase in the secondary side fluid inlet temperature, a decrease in flow rate, or a decrease in the outlet set temperature. All three of these situations can lead to a reduction in the load on the heat exchange system, prompting the control valve to close in order to lower the steam pressure. This results in a low pressure difference before and after the drain valve at the equipment outlet, causing loss of flow and water accumulation inside the heat exchange equipment. The issue of incorrect selection of steam traps leads to water accumulation in heat exchange equipment. Choosing a steam trap that is too small, or using a steam trap with a too low drainage temperature (such as thermostatic types), can also result in condensate water inside the heat exchange equipment not being removed in time and thus accumulating there. Under what circumstances can internal water accumulation cause water hammer inside the equipment? Practice has shown that when the temperature of the condensate at the bottom of the equipment is equal to the steam temperature, no steam will be condensed by the condensate, nor will water hammer occur ； When the temperature of the condensate is much lower than that of the steam, the steam condenses instantly upon contact with the cold condensate, resulting in only a few steam bubbles and almost no water hammer ； When the temperature difference between the condensate water and the steam is 20–30°C, the steam is condensed more slowly by the condensate water, and the contact time between the steam and the condensate water is longer; as a result, numerous bubbles form. When the steam reaches its condensation point, it condenses rapidly, causing these bubbles to burst, with their volume decreasing by several hundred to thousands of times in an instant. A vacuum zone is created immediately, and the condensate water is drawn into this vacuum space at high speed, which leads to water hammer. Water hammer in the equipment outlet/condensate recovery pipe: Water hammer at the outlet occurs due to a loss of flow in the equipment. When there is a loss of flow in the equipment, the condensate water cannot be drained, resulting in water accumulation inside. This leads to a decrease in the temperature on the secondary side; as a result, the opening of the steam control valve increases, causing the steam pressure to rise suddenly. This pushes the condensate water within the equipment against downstream valves and pipes, creating water hammer. Water hammer becomes more severe, especially when there are right-angle connections in the condensate pipelines. Secondary steam causes water hammer. Due to the reduced pressure, the condensate at the outlet of the steam trap will flash, so a mixture of liquid and vapor often exists in the condensate recovery pipeline. Sometimes, bubbles generated by secondary steam can exist below the surface of the condensate water (for example, when the outlet pipeline of the trap is connected to the lower end of the main condensate pipeline). When the temperature of the condensate recovery pipeline is lower than that of the secondary steam at the trap’s outlet, the condensate water in the recovery pipeline cools these bubbles, causing them to burst. The bursting of these bubbles creates hollow areas in the condensate water, and the condensate water surrounding these areas flows toward them, resulting in water hammer. Furthermore, in the water-vapor two-phase system, the velocity of the steam is 10 times that of the water. Similar to what happens in steam supply pipes, if the wave of condensate water rises to the top of the pipe and blocks it, a temporary water seal is formed before and after this wave of condensate water. This water seal prevents steam from passing through, resulting in a drop in pressure behind the seal. The pressure difference between the two sides drives the water seal to rush forward at high speed, thereby causing water hammer. Water hammer in heat exchange systems is like a hidden bomb that endangers the safety of equipment, facilities, and personnel; it is imperative to eliminate this threat! (This article was first published on the WeChat official account “Fured Steam Energy Saving”)