Selection and design of the condensate main pipe

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Selection and design of the condensate main: As long as the pipes are properly selected, connecting multiple steam traps to a single common recovery main does not pose any problems. In actual connections, certain key points must be taken into account. Elbow joints are better than the common square joints, as they can prevent erosion by high-speed secondary steam and condensate. Some problems may arise when the condensate is discharged into the full recovery pipe. This often happens when the steam main is drained of water. In the layout of steam main traps, to simplify the installation of these traps, the condensate return pipe is often placed parallel to the steam main pipeline. Since main steam traps are required to discharge condensate as soon as it is generated, steam traps that discharge at the saturation temperature or as close to it as possible are usually chosen. When the pressure in the recovery tube is low, the condensate at saturation temperature will generate the most secondary steam. Secondary steam has a larger volume, which will drive the water already present in the pipeline. As the bubbles of secondary steam move through the pipes, they burst rapidly upon contact with cold condensate water or the cold pipe walls, and all of this contributes to the formation of water hammer. The best solution is to avoid such discharge of condensate, and instead return the condensate and secondary steam to the nearest collection point. When this approach doesn’t work, the second option is to use a thermostatic steam trap (such as a pressure-balancing steam trap). To prevent water accumulation in the steam main, it is necessary to use a larger water collection point and install cooling pipes 2–3 meters in front of the steam trap. The volume of the cooling tube allows the condensate water to be cooled to the discharge temperature. Another possibility is to use a float-type hydrostatic valve with continuous discharge characteristics. The stable flow rate of the float-type pressure relief valve is easily absorbed by the piping without causing any problems. When the length of the condensate recovery pipeline exceeds 100 meters, an additional amount of pressure is required with each pumping cycle to help the water in the pipeline overcome inertia and accelerate from a stationary state to full speed. This phenomenon occurs with every cycle of the pump. The flow rate for the recovery pipeline is selected accordingly, with different manufacturers providing relevant data. Watt’s recommendation is to calculate it as the smaller of six times the actual hourly flow rate or 20,000 kg/h. The pressure loss caused by frictional resistance is the value obtained by subtracting the pump’s back pressure from the maximum available head, and then dividing that result by 4. The additional 75% of the available net head needs to be used to overcome the inertia of the condensate water. The selection of pipes should ensure that the flow rate of the condensate water does not lead to any adverse consequences. There is no strict limit on the maximum flow velocity in the pipeline, but experience shows that by keeping the flow velocity such that the pressure drop does not exceed 4.25 mbar/m, problems such as water hammer, noise, and corrosion can be avoided. The momentum of the water flowing in long-distance recovery pipes allows the water to maintain a certain flow rate for a while after a mechanical condensate pump completes one discharge cycle. This causes the condensate water inside the pump to be drawn into the discharge pipe at the end of the drainage process. When the water in the pipe comes to rest, the back pressure behind the condensate pipe will compress and condense the steam bubbles that have been stretched out due to the movement of the water column. The bursting of the bubbles causes contraction, which leads to water flowing back and striking the check valve violently. The water hammer caused by this reverse flow is quite severe. To prevent this from happening, a check valve of the same size as the pipeline is usually installed on the return pipe, with this check valve being about 6 meters away from the condensate pump. In some cases, the recovery pipeline rises not far from the condensate pump; in such situations, a vacuum break valve must be installed at the higher point to ensure that the condensate in the pipeline continues to flow due to momentum.