Problems with low-temperature methanol washing chillers

I think it’s mainly because the opening of the anti-surge valve from the outlet to the high-pressure section results in a pressure entering that section that is higher than normal; as a result, the anti-surge valve on the low-pressure side has to open in order to maintain the proper pressure and flow at the compressor inlet!

This is mainly because, when the compressor starts rotating, the anti-surge valve is fully open, and the pressures at the inlet and outlet of the compressor are almost exactly equal. After the compressor exceeds its zero-crossing speed, the pressure at the outlet of the high-pressure cylinder increases as the speed rises, while the return valve remains in the fully open position. This will cause pressure to build up at the outlet of the low-pressure cylinder, resulting in the surge point in the low-pressure section remaining within the surge region. Only by slowly closing the anti-surge valve in the high-pressure section can the surge point in the low-pressure section gradually move out of the surge zone.

There is some truth to your analysis, but that’s not the main reason for my problem. The pressure ratio at the inlet and outlet of the low-pressure cylinder is not high (much lower than normal), and the inlet flow rate is also high; as a result, the shock point remains within the shock zone, indicating that the unit is in a state of shock. But at this time, the surge point of the high-pressure cylinder is far from the surge zone; the inlet flow rate to the high-pressure cylinder is relatively high (higher than under normal conditions), and the pressure ratio between the inlet and outlet is not very large either. Don’t you find it contradictory? By analyzing the causes of surge, can you explain the situation in the low-pressure cylinder?

A high inlet pressure means that the pressure ratio between inlet and outlet is low, which indicates that surge is less likely to occur. Why is it still in the surge zone? The analysis isn’t very reasonable; let’s continue the discussion.

You are referring to this phenomenon on the DCS during driving time. But this does not mean that the compressor has actually experienced surge. If surge actually occurs, then we won’t be able to operate the compressor. Because this kind of stall happens every time I drive.

Hehe, I’ve just started working with the unit; I follow the principle of \"low inlet pressure, low flow rate, high temperature\"； \"High outlet pressure\" is used to explain the cause of surge! I’m just guessing: it depends on the inlet temperature at the high and low pressure stages. I wonder if, at the moment of startup, the inlet temperature at the low pressure stage was higher than normal!

I think being in the surge zone doesn’t mean surging has occurred; that’s why we can’t close the surge valve in the low-pressure area first!

You are absolutely right; this is indeed the result detected by DCS. There is definitely no surge occurring – with such a high inlet flow rate and such a low pressure ratio, how could surge happen? So I ask you, since the compressor unit isn’t surging, why can’t the surge point emerge from the surge zone? If the surge point cannot be determined, it means that the surge valve cannot move. This question is asking why the DCS mistakenly assumed that the unit was experiencing surge

A guess: Surge is related to flow rate and outlet pressure. When the anti-surge valve on the high-pressure side is opened, the outlet pressure on the low-pressure side increases, making surge more likely to occur. In such cases, it’s not appropriate to rely on the flow rate at the low-pressure side to determine whether the value is at full scale, as this value may be sufficient under normal operating conditions but not necessarily in special situations. Therefore, only by closing the anti-surge valve on the high-pressure side can the low-pressure side be brought out of the surge-prone area

Factors causing compressor base surge (personal opinion): 1. Unit assembly. 2. The drive speed fluctuates greatly. 3. Low inlet flow rate of the compressor. (Maybe I’m ignorant and adding unhelpful information.)

My personal understanding is that the surge point, surge curve, and surge flow rate of the compressor as shown in the DCS are calculated based on the outlet pressure at the current speed. The surge point, surge curve, and surge flow rate in the DCS change as the outlet pressure and speed of the compressor change. It’s possible that incorrect surge points and surge curves are obtained when the outlet pressure and flow rate of the compressor are not normal. (Please give some guidance while I’m learning!)

If the surge valve of the low-pressure cylinder is closed first, it will cause pressure buildup in that cylinder, which in turn will lead to surge in the low-pressure cylinder. This is why the surge zone does not mean the end of surging; this is why the surge valve in the low-pressure area cannot be closed first.

That’s what the answer on floor 12 means

Let me first explain the basis for determining the surge line: At a certain speed, when the pressure ratio remains constant, the control point that ensures the flow rate at the compressor inlet is not lower than the minimum value is known as the surge point. When creating the surge line, pressure ratio and flow rate are used. The surge line is defined as follows: at a certain pressure ratio, surge will not occur as long as the flow rate is above a certain value; this flow rate threshold constitutes the surge line. By adding 7% to 10% to this surge line, a surge prevention line is obtained. So when the signal fed back to the DCS meets the above requirements, the surge point is outside the surge zone. It’s not the case that the surge point cannot be determined when the unit is not operating properly, as some people say. At this point, the DCS serves only as a formula calculator and a display.

Thank you, LZ; I’ve learned something from this. But regarding what LZ said: when the ammonia compressor is started, both the anti-surge valves in the low-pressure and high-pressure sections are fully open. However, after the machine starts up, the surge point in the low-pressure section remains within the surge zone (at this time, the flow rate in the low-pressure section shows the full scale, which is quite high), while the high-pressure section is not in the surge zone. It is only by gradually closing the anti-surge valve in the high-pressure section that the surge point in the low-pressure section can slowly move out of the surge zone. In fact, no surge occurs, and I don’t understand this phenomenon. According to standard safety procedures, when adjusting the pressure after the compressor starts up, one should first close the return valve of the low-pressure cylinder, and then adjust that of the high-pressure cylinder. (Please continue!) This post was last edited by cuixwei on 2009-4-22 01:43

What was said is correct: closing the anti-surge valve of the low-pressure cylinder leads to a decrease in the inlet pressure of that cylinder, which in turn results in a certain drop in the outlet pressure. At the same time, when the surge valve of the low-pressure cylinder is closed, the pressure in the third stage increases, thereby raising the pressure ratio of the high-pressure cylinder. This makes it easier for the high-pressure cylinder to experience surge, which explains the statement you mentioned earlier. However, this is not fixed; flexibility is required at all times. In fact, in many cases, the two surge valves are turned off alternately; there is no strict requirement to turn off one before turning off the other.

The discussion was so intense! Since there’s still no result from trying to bribe God, I’d like to share my own views. My field of expertise is the design and installation of chemical processing equipment; I remember clearly that, while communicating with my mentor, I specifically consulted my professional books on compressors The book states that there is a rotational separation inside the cylinder block (this term is somewhat technical); rotational separation is the cause of surge. Now, I’d like to share my personal opinion on this post – I’m not sure if it’s correct though I hope everyone will point out mistakes and ask questions! As mentioned in relation to bribery, the inlet flow rate to the low-pressure cylinder is very high~ but it still cannot reach the operating range without surge~! During the initial stage of starting up the compressor, it goes through a surge process; during this phase, the anti-surge valve is kept fully open to increase the inlet pressure and flow rate, thereby ensuring successful surge initiation In the low-pressure cylinder, even if the flow rate is high, it doesn’t mean that it is not in the surge zone~! Being in a regional area doesn’t mean that surge has occurred~! If surge occurs, the compressor doesn’t need to be turned on~! During the initial startup and startup surge of the compressor, the opening of the anti-surge valve ensures that the flow rate at the inlet is high enough to prevent the rotor from being pushed out of the cylinder; this is why surging does not occur in that area. If you reduce the size of the anti-surge valve in the low-pressure cylinder, surging will definitely occur. If you think this isn’t accurate, you can give it a try during startup! Hehe? ? :lol :lol I just don’t want to face the consequences. In fact, the low-pressure cylinder ends up in the surge zone because the surge valve of the high-pressure cylinder is open; this results in a high flow rate rather than high pressure, and that’s also a factor. When the flow reaches a certain level, it’s necessary to close the surge valve, and then one has to figure out how to get the low-pressure cylinder out of the surge zone. This is mainly achieved by reducing the flow rate at the inlet of the high-pressure cylinder by closing its surge valve, thereby decreasing the resistance from the low pressure to the high-pressure cylinder and allowing the low-pressure cylinder to exit the surge zone. I’ve explained many reasons, but I’m not sure if I’ve made them clear enough~! I’m just going to explain the real reasons why surge occurs. During normal operation, as mentioned earlier, \"in many cases, the two surge valves are switched off alternately; there’s no strict requirement to turn one off before turning the other off.\" ”

One more thing~! If you have any questions about my views, feel free to post them in the comments~! I try to be as detailed as possible; I’m quite lazy and not very willing to type a lot. I type all this just so that everyone can share what they have~! The goal is to improve together

I think the surge line represents the relationship between rotational speed and flow rate as well as pressure. The pressure difference between the inlet and outlet of the low-pressure section is lower than the normal value, so no surge occurs in the low-pressure section. Surge does not necessarily occur in the surge zone; however, surge always takes place in the surge zone. So why is the low-pressure section in the surge zone? I think the idea that \"each speed corresponds to a performance curve\" means that at that speed, the inlet flow rate is not sufficient to provide a 10% anti-chauffage margin, which is why the system enters the chauffer region!

I personally think that when the high-pressure section is closed, the pressure at the low-pressure end near the high-pressure end increases, which prevents surging from occurring.