Interlock between electrolyzer and hydrogen compressor

(1) The automatic control valve for electrolytic venting releases gas according to the set pressure, thereby maintaining the operating pressure of the electrolyzer; if adjustment is not possible, manual intervention is used for adjustment. (2) If both electrolyzers trip, it will depend on whether the automatic control valve in the output area can adjust fast enough! It’s unclear whether the hydrogen processing units will be affected later; sometimes it’s better to shut down the system to identify the cause, as it also impacts the chlorine processing units. (3) Shutting down the hydrogen unit is relatively simple; if it does not interfere with maintenance work, the unit can be vented directly there. Resume hydrogen operation after the fault is resolved. (4) Why not simply purchase two hydrogen compressors capable of meeting the production demand of 50,000 tons of caustic soda, with one in operation and one as a backup? This would ensure both the stable operation of the electrolyzers and consistent production. Production must be stable and safe; leadership attitudes need to change. The loss resulting from a single stoppage could possibly exceed the cost of a hydrogen compressor.

If a cell voltage difference interlock is set in place, the hydrogen compressor and the electrolyzer will generally also shut down as part of this interlock mechanism, unless the interlock value is high enough to provide sufficient time for manual intervention – which is practically not feasible. If one electrolyzer shuts down, it’s usually still possible to bring it back online through the control circuits, depending on the operator’s skill; however, if things proceed slowly, all of them will shut down as well. Therefore, I suggest not considering the issue of fault interlocks, as most people won’t be able to handle them, unless the interlock values are increased. There’s simply no time to release pressure through a water seal; it’s impossible to explain this here. The regulation of the entire system is closely related to the system pressure and the interlock values. Don’t try to protect a particular section of the system – it’s essential to trigger the shutdown when it’s necessary. If pressure is not managed properly, the membrane that was supposed to last three years will need to be replaced after just two years as a result of repeated stress. The cost in such cases becomes very high, making it not worth it.

This post was last edited by nanren2 on 2017-11-11 at 14:39. A reply was already given in the QQ group today; here I will explain in more detail to the original poster: (1) If one of the two hydrogen compressors stops suddenly, the pressure of the electrolyzed hydrogen is bound to rise – how is this controlled? All the hydrogen goes to the dehydrogenation compressor without being exhausted; at this point, the system pressure is maintained by the backflow in the hydrogen system or by the backflow within the dehydrogenation compressor itself! In the past, the electrolyzers would stop when the hydrogen press stopped, but that’s no longer the case! There are two methods. One involves a water seal before the hydrogen inlet to the scrubber tower (although there are also those placed after the scrubber tower). The pressure at which this water seal breaks is slightly higher than the normal operating pressure. When the hydrogen compressor stops working, hydrogen escapes through the water seal as a safety mechanism for pressure relief. As the pressure rises, the vent valve also gradually opens to regulate the pressure (its setting is higher than the normal hydrogen pressure but lower than the pressure at which the water seal breaks, in automatic mode). Eventually, as the vent valve opens more, the pressure drops and the water seal is restored, with the vent valve then regulating the pressure in the hydrogen system! Another approach is to use a gas tank; this method is more stable, as it provides a buffer between the electrolyzer and the hydrogen compressor. There is no need for a interlock between them, but when the liquid level in the gas tank drops too low, an interlock stops the hydrogen compressor to prevent the tank from collapsing (2) When three electrolyzers are operating simultaneously and one or two of them stop suddenly, the pressure in the hydrogen main pipe will inevitably drop significantly. How is this controlled? At this time, control during normal operation is generally achieved through the backflow from the hydrogen compressor itself, as the diameter of the backflow orifice in the compressor is small, resulting in minimal fluctuations in pressure! I’m not sure what the configuration of the equipment used by the original poster is like; generally, each hydrogen compressor has its own backflow valve, and there is also a large backflow control valve in the system. In this case, the return valve of one hydrogen compressor is set to the pressure controlled by the automatic stabilization system, while the set value for the return valve of the other hydrogen compressor is slightly lower than the operating pressure, so that it remains fully closed in automatic mode during normal operation. The overall system return setting is also set a little lower, and again it remains fully closed in automatic mode. For example, the operating pressure of the hydrogen system is controlled by setting the return valve of one of the hydrogen compressors at 22, so that the system pressure remains around 22 at all times. Therefore, the set value for the return valve of the other hydrogen compressor can be kept at 20, while the system’s maximum return flow rate can be set at 18. In the event of an anomaly, such as one of the electrolyzers suddenly stopping, the hydrogen pressure will drop rapidly; at that point, the other hydrogen compressor and the system’s maximum return flow rate will activate in sequence to compensate for this situation! It provides a sequential protective effect. (3) In the subsequent hydrogen usage process, if there is a malfunction, the hydrogen pressure will inevitably increase; how is this controlled? In such cases, the hydrogen control valve on the outlet distribution panel of the hydrogen compressor usually opens. In other words, even when the hydrogen vent valve is closed, it should be in automatic mode with a pressure setting that is about 5 KPa higher than the normal operating pressure; this way, the vent valve can open promptly when the pressure rises abnormally! At this time, due to fluctuations in the system, the pressure in front of the hydrogen compressor also rises. Generally, there is a hydrogen vent valve before the hydrogen inlet to the scrubber; its set pressure is slightly higher than the operating pressure of the system, and it remains closed in automatic mode (as mentioned in point 1). If the pressure rises, this valve will gradually open as well. If it still cannot be adjusted, the water seal will break and pressure will be released; as the vent valve opens further, the water seal will return to its normal state. The entire system is still working fine! In my opinion, the stability of a system isn’t maintained by having people keep a close eye on it constantly; when an anomaly occurs, it only lasts a few seconds – who can react that quickly! What’s the problem with not acting in time? Why wasn’t it adjusted promptly? It wasn’t detected in time! Those are the ideas of the leaders; what era are we in now? Cars are already driverless; if that’s the case, the spacecraft used for lunar missions would exhaust the people on Earth and the astronauts as well! Therefore, when there are problems with the system, we need to find the root cause! Just like with safety, intrinsically safe is the most reliable option! The above are merely personal opinions!

Firstly, when one hydrogen compressor stops operating, it is mainly addressed by breaking the water seal in front of the scrubber and making adjustments to the electrolysis air vent valve manually; if the DCS gives clear alarm signals in this case, handling it should not be difficult. Secondly, as for the situation where one or two electrolyzers stop operating, your company’s previous methods of handling this remain largely the same, with the addition of adjustments related to the backflow from the hydrogen compressors. In my opinion, if it’s possible to adjust these properly, do so; otherwise, trigger a sequential shutdown. If only one unit stops operating, there should be no major problems. Thirdly, this is simple: in cases of hydrogen-related issues, design the water seal based on the hydrogen pressure, set up high/low pressure interlocks for venting, and make manual adjustments according to pressure changes. If possible, it’s best to add gas tanks for buffering, which will ensure 100% reliability for points 1 and 3; point 2 can still be handled in the same way as before

Perfectly solves the issue of recovering heat from the catalytic oxidation of hydrogen without direct combustion