Steam recovery from secondary flashing of low-pressure steam condensate
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Operating condition description: The low-pressure steam pressure is 0.45 Mpa. The pipeline for collecting low-pressure steam condensate has a diameter of DN200; the system operates at atmospheric pressure with emissions to the atmosphere at the top. Low-pressure steam is primarily used for the heating systems in the workshop as well as in two small towers (with very little being used in the towers). There are nearly 600 drain traps with a diameter of DN20, and the pressure measured using a gauge on the main pipe for low-pressure steam condensate is around 70 Kpa. There is a hot water pump at the bottom of the low-pressure steam condensate tank, which is used for heat circulation purposes such as heating the tank; any excess water is cooled by a cooler before being sent to the desalinated water system. Cold water is also added to the low-pressure steam condensate tank, but this is done using the hot water returned from the hot water pump, rather than through spraying from the top. The inlet for the condensate main pipe is located at the top of the tank, which causes the condensate to flash vaporize upon entering the tank, resulting in a large amount of water being carried along with it. Technical modifications are now necessary to achieve the following objectives: 1. Since the low-pressure steam condensate main pipe is connected to numerous traps, and some of these traps have excessively long pipelines, the back pressure of the trap main pipe must not increase as a result of the modifications, as this could lead to poor drainage of water from the traps. 2. Since it is not possible to aggregate the quantities, everyone can estimate the amount of steam condensate; valuable and informative calculation methods are hoped to be provided. 3. If it is necessary to add heat exchangers or similar equipment, I hope everyone will not be stingy – could you provide the calculation process? Bounty post; hope the moderator will pin it.
The condensate pipe pressure is 70 kPa, with available headroom. Install a U-turn at the discharge outlet ； For DN200 pipes, an non-condensable gas discharge point should be installed at the highest point. The key is to control the steam and air so that they don’t mix; by allowing exhaust through a single main pipe, the situation you described will occur
I’m not sure if you are planning to carry out modifications for closed-loop recovery of condensate; if so, it will place higher demands on the performance of the steam traps. Traditional check valves have a significant impact on back pressure. Therefore, it is recommended to use venturi-type steam traps, as these types of steam traps are not affected by pressure and can still function properly even under a backpressure of over 95%. Moreover, it doesn’t need to be replaced for 10 years, offering stable and reliable performance.
Original poster, how did the renovation turn out? Could you share it?
It has been modified, and the result is excellent. The approach involves extending the outlet pipe to allow for natural cooling; then a plate heat exchanger is installed at the end of the pipe, with its outlet connected to a silencer. This setup works very well – no gas escapes at all. . .
With the pipeline being extended and additional plate exchangers added, the back pressure has increased significantly. Has this affected the operation of the back pressure valve?
A few suggestions: First, eliminate the steam traps and replace them with condensate tanks; combine the condensate pipes by considering factors such as different floors and lengths of the heating pipelines. The outlet of the condensate tanks should be controlled via liquid level interlock, in order to minimize losses due to steam leakage from the steam traps. Adding a plate heat exchanger to the outlet pipeline, whether using water cooling or air cooling, results in unnecessary additional equipment investment and energy consumption. There are 600 DN20 steam traps, and the total amount of steam is not large. I usually use one of two methods: either discharge the steam directly into the cooling tower, where the secondary steam is cooled by the return water, with some of this secondary steam condensing to serve as water for replenishing the cooling tower; or extend the pipes to a higher altitude, as shown in the diagram, by adding a custom-made air-cooled return pipe at the end, in order to minimize the amount of secondary steam that is discharged directly. Finally, if necessary, this steam can be used for the nearby hot water system.