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Selection and determination of the pipes before and after the steam trap: An inappropriate selection of the pipes surrounding the steam trap can affect the proper drainage of condensate as well as the proper functioning of the trap. The pipeline before the steam trap refers mainly to the drainage system from the equipment to the steam trap. The condensate must flow from the drainage outlet of the steam-using equipment to the drain valve. The pressure inside the equipment and the steam trap is the same; therefore, the flow will be driven by gravity, which means the steam trap must be located below the equipment. The piping layout from the device’s drainage point to the steam trap should have a certain slope. Watt Energy-saving steam engineers recommend to customers a flow rate that is based on maintaining the lowest possible pressure drop. The diameter of the steam trap does not necessarily have to be the same as the diameter of the drain pipe connection. Because the equipment must be designed to handle different working pressures and flow rates. When selecting the pipe size, the amount of condensate that can be discharged is not necessarily the drainage volume when the equipment is operating at full capacity. When the equipment starts up, the amount of condensate water can be as much as twice the operating load. At the same time, there is also air in this section of the piping system. Typically, the calculated displacement is actually based on twice the steam load and 1.4 mbar of frictional resistance per meter. Based on experience, Watt Energy-saving Steam Engineers recommend the following operating load values for various pipe diameters: 15 mm – 100 kg/h, 25 mm – 500 kg/h, 50 mm – 3000 kg/h, 65 mm – 6000 kg/h, and 100 mm – 20,000 kg/h. For example, an operating load of 2000 kg/h corresponds to a pipe diameter of 50 mm. The pipeline downstream of the steam trap, that is, the steam trap discharge line, refers to the section at the outlet of the steam trap; in this pipeline there is not only condensed water, air, and other non-condensable gases, but also secondary steam generated as a result of pressure reduction. Whenever possible, the piping should be connected as steeply downward as possible to the following equipment: • Secondary steam recovery tank • Water storage tank for the condensate pump • Boiler feed water tank or deaerator. Considering various practical conditions: at startup, the condensate is relatively cold, and there is little or no secondary steam. But at the same time, the amount of condensate water is the highest, and it also includes some air. The diameter of the pipe must be at least the same as the diameter of the steam trap inlet. After running for a period of time, the condensate volume stabilizes at the operating load. But as the temperature of the condensate rises to the steam temperature, secondary steam is also generated. Calculations carried out by Watt Energy Saving Steam Engineers show that the proportion of secondary steam is as follows: pressure before the trap = 4 barg; pressure of the secondary steam (pressure in the condensate pipeline) = 0 barg (released directly into the atmosphere). The percentage of secondary steam is 10%. When selecting the drain pipe for the trap, the amount of secondary steam is the most important parameter. The selection of the condensate pipe downstream of the steam trap is based on the diameter chosen for the secondary steam. Considering costs, the condensate main is always made as small as possible. Usually, the amount of water to be treated is taken into consideration during design, but in reality, what is discharged from the steam trap is always a mixture of condensate and secondary steam. Not everyone is aware that the volume of secondary steam can be as high as 400 times that of the condensed water. This indicates that the diameter of the condensate recovery pipeline should be determined based on the steam volume rather than the condensate volume. Looking at the previous example again: the pressure at the pressure relief valve is 4 barg, while the pressure in the recovery pipe is at atmospheric pressure. The amount of secondary steam is about 10%. The condensate load is 1000 kg/h, which means that in the discharge pipeline, each kilogram of discharged fluid consists of 0.1 kg of steam and 0.9 kg of water. Among them, secondary steam accounts for 99.44% of the total volume, while condensate accounts for 0.56% of the volume. In other words, the secondary steam occupies almost the entire volume of the pipeline. (Usually, since the discharge pressure is slightly higher than atmospheric pressure, the amount of secondary steam released is also slightly less.) The condensate will flow at the bottom of the pipe at a speed lower than that of the secondary steam. Its mass flow rate, plus the mass flow rate of steam, constitutes the total mass flow rate of the fluid passing through the steam trap. Due to the condensate water at the bottom of the pipes, the steam will inevitably be moist; therefore, the flow velocity of the secondary steam must be limited to 15 m/s, otherwise water hammer and erosion will occur in the curved sections. From a practical perspective, there are other factors to consider when selecting pipes. Temperature control. If the device features temperature control, the pressure will drop by nearly half when the control valve is partially open. This phenomenon occurs when the equipment is operating at full load and the valve is selected based on the critical pressure drop. But sometimes the valve does not need to be selected based on the critical pressure drop, in which case the pressure drop will be much less than half. For example, when the pressure downstream of the control valve decreases, the amount of secondary steam generated is reduced. Thermostatic steam traps. These types of steam traps [such as pressure-balanced steam traps or bimetallic steam traps] are designed to drain water at temperatures below the saturation temperature. For different types of steam traps, the temperature difference can range from 10 to 50ºC. For example, the liquid chamber of a pressure-balanced steam trap is designed to operate at temperatures 13ºC below the saturation temperature. This will reduce the amount of secondary steam generated. Below is an example for selecting the main condensate recovery pipe: The saturation temperature at 7 barg is 170.5ºC; saturation temperature minus 13ºC equals 170.5 – 13 = 157.5ºC. According to the steam table, the pressure corresponding to 157.5ºC is 5.00 barg. Therefore, the upstream pressure should be set at 5 barg, rather than 7 barg. This will reduce the diameter of the condensate pipe. Starting load. The starting condition refers to the situation where the amount of condensate is at its maximum when the equipment is cold. There are great differences in the methods used to calculate the starting load, and manufacturers often simply apply a coefficient (2 or 3) based on experience. For equipment that is not used frequently, it is appropriate to select it using the starting load. Sometimes, for a particular factory, not all the equipment is in use at the same time, so these changes must be taken into account. For example, in a laundry room, hot water needs to be added at startup for washing clothes. Then a dryer is used, followed by a steam iron. The use of a flash tank in equipment and boiler room deaerators results in higher pressure in the condensate main lines; it is therefore necessary to ensure that this increase in pressure does not affect the performance of the steam traps. In their daily work, Watt Energy-saving steam engineers have found that the feasibility of connecting several check valves operating at different pressures to a single common recovery main is often questioned. This is because it is believed that high-pressure condensate may interfere with the discharge of low-pressure condensate. However, one fact is overlooked: high or low pressure exists only within each steam trap; at the outlet of the steam trap, the pressure is that of the common recovery pipe, perhaps plus the back pressure generated by the movement of secondary steam. If the pipe size is chosen to be too small, it is certainly possible for the back pressure to rise excessively, thereby restricting or even hindering the operation of the steam trap.