The new hydrogen compressor features one-stage feedback to the same stage and two-stage feedback to the corresponding stage. What is the purpose of this?

1. When the compression ratio is too high (the ratio of the exhaust pressure to the intake pressure in the cylinder), it can cause the compressor valves to overheat and reduce their lifespan; by adjusting the valve opening degrees to one open and one closed, or two open and two closed, the compression ratio can be reduced, thereby bringing the valve temperatures back to normal levels. But at this point, the compressor does more useless work, resulting in a decrease in efficiency ； 2. When the air volume regulated by the unloader cannot meet the process requirements, a one-to-one or two-to-two configuration can also be used to regulate the air volume. These are just my humble opinions; I hope experts can provide corrections

Here are my humble opinions: Currently, there are generally two types of control systems for new hydrogen compressors: 1. One two-way control valve is used, with pressure control in a single circuit; this approach is simple to implement, but the compression ratio is greatly affected by external factors. 2. In the first and second cases, control is more complex; generally, a stepped recursive control system is used to effectively control the compression ratio of the new hydrogen compressor, but achieving automatic control is rather difficult.

The control method for the new hydrogen compressor has clear advantages: it allows for the adjustment of the compression ratio at each stage, thereby providing better protection for the compressor. The downside is that it is complex to operate in practice, and operators are prone to making mistakes or even causing accidents.

It is one of the methods for controlling flow rate and compression ratio.

The control methods for reciprocating machines include, first, the step-by-step feedback method, which was proposed by the inventor; this method allows for adjustment of the pressure ratio at each stage of the unit, which is beneficial for its operation. The second method is three-feedback-one, wherein flow is directed directly from the outlet of the unit back to the inlet, with the control system regulating the amount of hydrogen added. The pressure ratio cannot be adjusted in this method, but it is relatively simpler to operate compared to the step-by-step feedback method. Both of these methods are currently in use.

The third floor covers the topic in more depth. Generally, there are compressors with a 2-in-2 or 1-in-1 configuration, and their control scheme employs a split-range + selective logic control approach. The data measurement points are the pressures at the inlet and outlet of the compressor’s first and second stages, with the aim of keeping it operating within normal ranges. But indeed, this method of adjustment is very difficult to implement; at least domestic instruments find it hard to do so. But even without automatic control, under manual control, the pressure at the inlet and outlet of the compressor can be adjusted effectively using a two-to-two or one-to-one configuration, and the primary and secondary stages can be adjusted separately, which is very convenient. Additionally, hydrogen can be used in many parts of refining plants. When the secondary hydrogen pressure is high, the primary medium-pressure hydrogen is easier to use. Therefore, in cases where some of the grade-1 hydrogen is diverted for use elsewhere, the two-stage-two and one-stage-one processes become even more practical.

The compression ratios at the two stages can be adjusted to provide better protection for the compressor. The actual operation is complex, and operators are prone to making mistakes, which can even lead to accidents.

It’s just two different ways of regulation. By comparing one reverse-one with two reverse-two and two reverse-one, it is possible to adjust the compression ratio; however, such step-by-step progression makes automatic control difficult to achieve, whereas automatic control is easy to realize with two reverse-one.

I would like to ask: Which control method is better for controlling the compressor?

First, thank you for your answer. So why can 2-back-to-2 and 2-back-to-1 achieve better control over the compression ratio?

Those used to adjust the compression ratio are not that complex in medium and low pressure applications; generally, they only have an outlet and an inlet, while high pressure applications may require more sophisticated designs

It mainly serves to balance the thrust of the compressor piston rod and extend the operating life of the compressor. Only the recursive control method of one cycle then one or two cycles is adopted; the two-cycle-one approach tends to generate heat, which is not favorable for the operating conditions of the gas valve. In general, recursive control is used whenever it is possible.

It is primarily used to control the pressure of the reaction system through the partial return of fresh hydrogen. For two-stage fresh hydrogen compressors, there are two methods to achieve this: one is what the moderator referred to as \"three returns to two\" and \"two returns to one\", which requires two control valves (four for two units); the load on the inter-compressor coolers is higher in such cases； Another option is three returns and one; two units can share one control valve, but an inter-stage cooler needs to be added.

For new hydrogen compressors with three-stage compression, the pressure control can be achieved using the following two methods: 1. Direct return of the third-stage outlet to the first-stage inlet (three-to-one return)； 2. A step-by-step return control scheme in which the third-level outlet returns to the third-level inlet, the second-level outlet returns to the second-level inlet, and the first-level outlet returns to the first-level inlet (due to the relatively complex process and numerous control points, new refineries generally prefer to use Scheme 1). There are two purposes for this approach: 1. The need to control the pressure in the high-pressure system. When the system pressure is high, flow returns from the third-stage outlet to the first-stage inlet; as the flow at the third-stage outlet decreases, the amount of new hydrogen injected into the reaction system also decreases, the pressure in the high-pressure separator drops, and thus the system pressure falls ； Conversely, when the system pressure decreases, the return valve closes partially, the flow rate at the third outlet increases, the amount of hydrogen injected into the system rises, and the system pressure increases. 2. The need for power balance of the compressor. Reciprocating compressors with multi-stage compression generally adopt a horizontal, symmetrically balanced design; an even number of columns is chosen for reasons related to dynamic balance. The optimal design involves equal masses for the reciprocating parts in each column, with hydrogen returning in each stage, so that the compression ratio of each stage is equal to or close to the designed value. This helps to reduce the influence of inertial forces associated with reciprocating and rotating motions, thereby ensuring the stable operation of the machine over extended periods of time.

Facilitates adjustment of the compression ratio, prevents overheating, and extends the compressor’s lifespan

Today I just learned about the control method for the second-stage hydrogen compressor. In simple terms, it involves staged control plus low-pressure selection. I would like to ask an expert in instrumentation: when the pressure is high, is it such that the system pressure is maintained first, then the pressure of the second stage, and finally the pressure of the first stage? In other words, when the pressure reaches a certain level, the safety valve at the outlet of the first stage activates to protect the compressor. What about when the pressure is low? Is it the opposite in that case? First, maintain the pressure of stage 1, then that of stage 2, and finally the system pressure. In other words, if low-pressure alarms are set for all of these, shouldn’t the alarm go off first for the low pressure in the main system pipeline? But for ordinary compressors, a low alarm is set at the inlet, while a high alarm is set at the outlet?