Analysis of the advantages and disadvantages of adding a back pressure valve at the outlet of diaphragm compressors

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I. Design Background: In the filling process, diaphragm compressors are responsible for pressurizing the purified high-purity hydrogen gas and filling it into the tank. Since the process of filling an empty tank or a tank already containing gas follows a cycle from low pressure to high pressure and back again, when the pressure drops from 19.5 Mpa to 2 Mpa, the exhaust pressure at the first stage of the diaphragm compressor, at 6.5 Mpa, is sufficient to fill the tank; as a result, the second stage of the compressor does not need to do any work. At this point, the diaphragm compressor is no longer in a state of symmetrical balance, and its overall vibration increases significantly. The stress generated over time will inevitably cause problems in some weak points of the equipment, such as cracks or breaks in the bolts that support the middle section of the compressor, the braces used for support, the bolts connecting the various parts of the compressor, and the bolts securing the cylinder body. II. The original design intent was to address the root cause of the unbalanced operation of diaphragm compressors; this issue is mainly caused by too low pressure in the compressor when it is not under load. After communicating with the manufacturers of these compressors, many users decided to install a backpressure valve at the compressor outlet, thereby setting a stable pressure at the inlet – for example, 7 Mpa – to ensure that the pressure at the outlet of the diaphragm compressor remains above 7 Mpa. In this way, the compressor remains in a balanced operating state even when it is operating at low pressure due to no load. III. Working principle of the backpressure valve: The backpressure valve can be considered as a reverse-mounted pressure regulator. By setting a specific outlet pressure, this regulator maintains a stable outlet pressure regardless of how high the inlet pressure is, as long as it is above the set value; the outlet will not allow flow only when the inlet pressure is below the set value. The backpressure valve maintains a stable inlet pressure at all times; no matter how low the outlet pressure is, as long as the inlet pressure is above the set value, the backpressure valve remains in a state of normal ventilation. It only closes and stops ventilation when the inlet pressure falls below the set value. IV. Advantage Analysis 1) The backpressure valve helps maintain a stable operating pressure at the compressor outlet, ensuring that the compressor always operates in a state of symmetrical balance, thereby fundamentally resolving the issue of abnormal equipment vibration that occurs when the unit is switched to low pressure. 2) For devices that need to be started and stopped frequently, a back pressure valve can reduce pressure shocks during startup and shutdown, thereby extending the equipment’s lifespan. 3) For tube train vehicles with particularly low pressure, the backpressure valve can prevent safety hazards caused by excessive hydrogen flow rates and high flow velocities. V. Disadvantage Analysis 1) If a backpressure valve is installed, the exhaust pressure is set at a high level while the compressor is operating; this high exhaust pressure means that the compressor has to overcome the resistance posed by the backpressure valve, forcing it to operate at high power levels and resulting in increased electricity consumption. 2) If the compressor unit is equipped with a follow-up valve, the pressure can increase gradually in line with the vehicle’s pressure. However, if a backpressure valve is installed, once the compressor’s exhaust pressure exceeds the set value, the backpressure valve ceases to function, which goes against the purpose of installing such a valve. VI. Analysis and Summary Through the above brief analysis and comparison, the following selective recommendations can be drawn: 1) It is best not to use a backpressure valve simultaneously with a follow-up valve. Referring to the equipment produced by a large foreign compressor manufacturer, they opted for the use of a backpressure valve without a follow-up valve; as a result, the vibration levels during operation were significantly low. However, the impact of long-term high pressure on the diaphragm must be taken into consideration. 2) For some applications with low flow rates, involving standard gases, in laboratories and research institutions, where it is necessary to maintain the exhaust pressure within a stable or adjustable range and where power consumption is not a concern, diaphragm compressors can be equipped with backpressure valves; however, these applications differ from those involving high-flow hydrogen filling trucks.