Turbine-Driven Centrifugal Compressor Operation
Turbine-Driven Centrifugal Compressor Operation
(OP)
I have experienced something that I consider strange in regular centrifugal compressor operation. I would be grateful if someone of forum members could clarify these issues:
Turbine-driven refrigerant compressor (isobutane) in alkylation unit is suffering from frequent surging when operating in automatic mode. Automatic operating mode means the following:
- antisurge flow controller is in automatic mode
- suction pressure controller is in automatic mode (suction vessel PC connected to high pressure steam servo assembly, which regulates the RPM of compressor by manipulating steam flow into the turbine; steam is condensed under vacuum)
- suction temperature is always constant, meaning that composition of the compressed gas is also unchanged
At minimum alkylation unit capacity, compressor operates at 95% of maximum RPM, developing polytropic head 80% of design value (?). Lowering the RPM pushes the machine into surge region and raises the suction pressure, so the operators found that it is better to run the compressor with almost maximum RPM in manual mode, in order to have relatively smooth operation of the plant. This somewhat causes suction pressure to vary with time, but with no significant consequences.
What surprised me the most is the following:
1) With this parameters I described, antisurge FCV is open 52%. Polytropic head is 80% of design value, as I said.
2) Lowering the RPM from 7000 to 6800 RPM does not affect suction and discharge pressure (?), but it causes antisurge valve to open further, up to 56%! Moreover, machine goes into surge cycles.
3) Switching from manual to automatic mode of RPM control (via suction PC), makes incredible changes in compressor operation: relatively smooth operation is turned into surging cycles, so the automatic operation is completely abandoned.
My questions are:
1) If actual gas composition, suction pressure and temperature are as designed, why cannot we achieve design polytropic head? Is it possible that there is so little process gas (compared to spillback stream), that antisurge flow (52% valve open) pushes the compressor so much right off the curve, developing less head? Is it possible that machine is mechanically damaged, causing lower polytropic head at 95% of design RPM?
2) Why antisurge valve continues to open further when RPM is reduced, if suction and discharge pressures are unchanged? Isn't it contradictory, practically impossible? Less RPM should require smaller recycle stream (if being far enough from the surge point) in order to achieve the same head - that is what I (thought) I knew about centrifugal compressors.
Can you please throw some light on this.
Thanks in advance.
Turbine-driven refrigerant compressor (isobutane) in alkylation unit is suffering from frequent surging when operating in automatic mode. Automatic operating mode means the following:
- antisurge flow controller is in automatic mode
- suction pressure controller is in automatic mode (suction vessel PC connected to high pressure steam servo assembly, which regulates the RPM of compressor by manipulating steam flow into the turbine; steam is condensed under vacuum)
- suction temperature is always constant, meaning that composition of the compressed gas is also unchanged
At minimum alkylation unit capacity, compressor operates at 95% of maximum RPM, developing polytropic head 80% of design value (?). Lowering the RPM pushes the machine into surge region and raises the suction pressure, so the operators found that it is better to run the compressor with almost maximum RPM in manual mode, in order to have relatively smooth operation of the plant. This somewhat causes suction pressure to vary with time, but with no significant consequences.
What surprised me the most is the following:
1) With this parameters I described, antisurge FCV is open 52%. Polytropic head is 80% of design value, as I said.
2) Lowering the RPM from 7000 to 6800 RPM does not affect suction and discharge pressure (?), but it causes antisurge valve to open further, up to 56%! Moreover, machine goes into surge cycles.
3) Switching from manual to automatic mode of RPM control (via suction PC), makes incredible changes in compressor operation: relatively smooth operation is turned into surging cycles, so the automatic operation is completely abandoned.
My questions are:
1) If actual gas composition, suction pressure and temperature are as designed, why cannot we achieve design polytropic head? Is it possible that there is so little process gas (compared to spillback stream), that antisurge flow (52% valve open) pushes the compressor so much right off the curve, developing less head? Is it possible that machine is mechanically damaged, causing lower polytropic head at 95% of design RPM?
2) Why antisurge valve continues to open further when RPM is reduced, if suction and discharge pressures are unchanged? Isn't it contradictory, practically impossible? Less RPM should require smaller recycle stream (if being far enough from the surge point) in order to achieve the same head - that is what I (thought) I knew about centrifugal compressors.
Can you please throw some light on this.
Thanks in advance.





RE: Turbine-Driven Centrifugal Compressor Operation
To analyze the second problem of percieved lower than design head, start with a work balance on the turbine side and compare wih compressor side. Verify that what you calculate to be the compressor flow is consistent with the horsepower input and discharge (T,P) conditions.
best wishes,
sshep
RE: Turbine-Driven Centrifugal Compressor Operation
Antisurge flow controller is a "characterized antisurge system", as you said. It calculates minimum suction flow at actual RPM (with 10% safety margin) and acts on the spillback valve. It is mostly operated in "manual with backup" mode, because automatic operation pushes the compressor in surge cycles (even with RPM control on manual).
Concerning your second question, what can I conclude from heat/work balance of the compressor assembly? It simply operates far below his working curve conditions at given RPM.
Regards
RE: Turbine-Driven Centrifugal Compressor Operation
As this is a clean system (regrigerant), the system can be easily analyzed. I think you can solve this mystery.
best wishes,
sshep
RE: Turbine-Driven Centrifugal Compressor Operation
I will check the force balance very soon. But still (maybe because of my long explanation), there is one thing that is definitely not in consistency with common sense. Let me describe it again, I 've checked today in the plant.
If there is really so little process gas and 52% antisurge valve opening is required to prevent surging (which means that compressor operates at only 10% higher suction flow - 10% away from surge point, at maximum RPM), why is there so dramatically reduced compressor discharge pressure, if suction pressure and temperature are at design conditions? It should be 6.46barg, and in real operation it is only 5.12barg! Compressed gas composition is as designed, and it has been checked in the laboratory (GC analysis). Refrigerant compressor is delivering much lower polytropic head than it should do, and consequently much lower discharge pressure (measured just at the compressor discharge outlet). Since process conditions, in my opinion, are not the cause of abnormal compressor operation, I was wondering if someone may have a clue what is going on. Also, when running the compressor in manual mode, the whole system operates smoothly; as soon as we switch to automatic suction pressure control (PC connected to turbine motive steam valve), the compressor starts surging. At all times, steam is condensed under vacuum and there are no significant changes of steam condensate outlet parameters.
Thanks again,
RE: Turbine-Driven Centrifugal Compressor Operation
steam rate,
steam inlet and condensing pressure,
isobutane flow,
compressor inlet temp and pressure,
compressor outlet temp and pressure
best wishes,
sshep
RE: Turbine-Driven Centrifugal Compressor Operation
T (suction) = -6C
P (discharge) = 4.600barg
T (discharge) = 54C
Steam rate = 5050 kg/h
Steam inlet pressure = 43.1barg
Steam inlet temperature = 378C
Steam outlet pressure = 0.17bar abs (-0.83barg)
Steam outlet temperature = 58C
Refrigerant flow = 12690 m3/h
Compressor RPM = 6760
Gas (weight) composition:
6.5% C3
91.4% i-C4
2.1% n-C4
Maximum rated compressor RPM is 7346, corresponding to discharge pressure of 6.460barg - at suction volumetric flow 10% higher than surge point flow.
RE: Turbine-Driven Centrifugal Compressor Operation
RE: Turbine-Driven Centrifugal Compressor Operation
I'm a little confused about the compressor discharge pressure. You say it should be 6.4 barg but it is only 5.12 barg. Are you trying to maintain a specific discharge pressure (say with a backpressure valve)? If not, the the discharge pressure will be suction pressure plus the sum of the system resistance to flow at the actual flow rate. For the pressure to be 20% low then either you've got your system resistance or your mass flow rate wrong.
David
RE: Turbine-Driven Centrifugal Compressor Operation
Here is additional information: recycle (spillback) stream enters the compressor suction trap without previously being cooled, but after mixing with sub-cooled liquid refrigerant (from liquid refrigerant accumulator drum) in special vapor-liquid mixing nozzle. The bottom line: suction temperature is not changing by time, as well as suction pressure - with compressor operating in manual mode (fixed RPM). So generally, as you can conclude, it is very much stable process.
There is a sketch of refrigeration section available at: http://www
Note that all downstream control valves maintain resistance to flow practically unchanged: LIC10 is in manual mode, PDIC (vapor bypass) is on automatic mode. Everytime when LIC10 is closed by only 0.2% of valve controller output, compressor starts surging. In my opinion, for LIC10 to operate in the automatic mode, automatic operation of compressor suction pressure controller needs to be provided; otherwise, small changes in LIC10 valve % opening affects compressor load (required polytropic head).
As you can see, there is no backpressure PC valve at the compressor discharge section. Downstream pressure is "fixed" by condenser outlet temperature and pressure drop of compressor downstream equipment. I did not notice any significant changes in this particular pressure over time (PR10 on the sketch). For emergency purposes, or in the case of propane accumulation in the system, there is HCV connected to flare system (the pump for liquid C3/C4 stream transport to depropanizer is currently out of service).
My question regarding compressor operation is: if compressor operates at maximum RPM on manual mode, and if inlet flow is equal to surge point flow + 10% safety margin, will it develop the same polytropic head at the compressor discharge nozzle, regardless of changing downstream resistance? Or not? And why?
My question regarding instruments tuning and antisurge control: what causes unstable operation when switching from manual to automatic mode of suction pressure control, if process is stable while operating in manual mode?
sshep: concerning the steam expansion/isobutane compression work balance, I didn't manage to come up with any conclusion. Calculations show much higher compression work compared to steam expansion work, for given (measured) flows. Do you have any further suggestions?
RE: Turbine-Driven Centrifugal Compressor Operation
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RE: Turbine-Driven Centrifugal Compressor Operation
The arrangement of air coolers and water coolers has minimized your cooling water and MAXIMIZED your horsepower on the compressor, that usually isn't the most efficient way.
Because you can't give the exact composition, the alkylate's mole wt and its head and actual pressure can cause surging along with the action of LIC10 and PDIC. Most people take the flash spill back as you have drawn it for a new plant under maximum loading. If the unit is way under loaded, then you need some cooling because the hot minimum flow also changes the temperature, which means the density changes, which means the discharge pressure drops and you will cycle up into surge too. So, some of the minimum flow needs to come from the REF ACC tank as a liquid. The amount from the the REF ACC is controlled by a TIC which allows injection of liquid C4 with the hot gas from the spillback valve.
RE: Turbine-Driven Centrifugal Compressor Operation
Can you make a hand-drawing of that modified scheme and post it somewhere, for example at http://www.imagehosting.com ?
I do not understand exactly what do you mean, by only reading your post.
What happens with compressor discharge pressure (measured at the compressor discharge nozzle) when you close LIC10 a little bit (1-2%, for example)? Does it develop the same polytropic head?
P.S. You have refrigerant gas composition in one of my previous messages. It is very close to design composition.
Thanks,
RE: Turbine-Driven Centrifugal Compressor Operation
Compressor Polytropic Effi= 0.725
Compressor Power= 925kW
Turbine Power= 1000kW (assumed 75% isotropic effi)
best to make a check of my calcs yourself.
This doesn't seem to be an unreasonable balance or compressor efficiency. I am curious about what the design effi was because the statement that the "polytropic head is only 80% of design" is not so clear to me as efficiency. Also due to compression ratio, if the actual suction pressure is slightly below what your instrument shows, the discharge pressure will appear to be much lower for the same power. If the suction pressure is really lower by even 0.15 bar, it would explain your lower discharge pressure. You should verify this pressure with a gage as close to the suction as possible.
The lower discharge pressure does not seem like a serious operational problem. As was pointed out above, the design looks like it would waste energy because you compress to higher pressure than would be needed to condense against air. It also seems like a lot of possible interactions among controllers- including some process considerations due to the open loop nature of the system. It doesn't seem surprising to me that that there could be instability controlling the suction pressure by speed for reasons unrelated to the compressor efficiency observations.
With regard to the surging, can you clarify how you calculate the compressor inlet flow is measured (or calculated), and the flow inputs to/from the surge controller?
best wishes,
sshep
RE: Turbine-Driven Centrifugal Compressor Operation
Suction pressure is actually somewhat higher than designed (0.105barg VS 0.08barg). This should move the compressor away from surge conditions.
Surge control: total discharge flow is measured and then recalculated for suction conditions (p, T). This flow is compared to the minimum suction flow (at given RPM) plus 10% safety margin, which acts as the software controller output on antisurge valve. There is an option to run antisurge (spillback) valve in 3 modes of operation:
- automatic
- manual with backup (you can raise spillback flow above minimum calculated flow, but not below)
- manual (it is possible to open/close spillback valve in the full range 0-100%)
You said: "I am curious about what the design efficiency was because the statement that the "polytropic head is only 80% of design" is not so clear to me".
According to compressor ACFM-Hp chart, at maximum RPM compressor should develop 6.46barg discharge pressure (also given as tabular data), instead of 5.1 or 5.3bar - which is actually the case (remember, we have even slightly higher suction pressure than designed). Gas composition is almost exactly the same as designed, so I really cannot find the process-side reason for so much smaller discharge pressure at maximum RPM of the compressor.
When analyzing compressor problems in the past few days, actually I became so confused with the facts (and measurements), that I have suspicious about - at least I think so - fundamental principles of centrifugal compressor operation.
I am in love with this Alkylation unit, that is for sure!
RE: Turbine-Driven Centrifugal Compressor Operation
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RE: Turbine-Driven Centrifugal Compressor Operation
There is nothing at this point to suggest that any plant data is bad, on the otherhand there is no reason to shutdown early to check the compressor for damage.
The surging is a seperate issue. Can you get speed on control by using extra spillback (manual with backup)?
========
This is my own view of the operation if it helps. NOTE: I am speculating about your system as most of my refrigeration experience is with self regulating variable area condensing (as Decasto's looks to be). There is nothing in your sketch to indicate a liquid level in the finfan.
Speed control is normal in a variable area condensing case as it regulates the system so that the compressor is handling only the flow needed for the refrigeration load and the refrigeration temp is also controlled (vaporizing pressure). Your system runs at constant speed so suction ressure floats. My speculation is that the condensing pressure is set by the finfan air flow (and temp) and exchanger area (assumes area fixed=no liquid level in finfan). Consider the the fixed position finfan control valve pipe as extra pipe resistance since the position is fixed. There is a pressure (around 5barg) at which the required flow will be completely condensed over the available cooler area- i.e. if pressure were lower not all would condense and pressure would rise, if discharge pressure were higher then all would condense and the pressure would drop. The system in your constant speed case becomes self regulating through small (but important) suction pressure changes against the refrigerent condenser. If I am correct about the condensing pressure at a given refrigerent load being fixed by the finfan area and air flow, it may not ever be possible to also control the suction pressure. Such a case seems over specified since refrigerant flow (process heat load), inlet (speed controller) and outlet pressure (finfan area) are all set independently.
The net result of this to efficiency is that I think you should be conceptually wondering why the suction pressure is higher for the constant speed case rather than asking yourself why the discharge pressure is low. This was my confusion about what the "only 80% of polytropic head" really met. I know that you believe the suction pressure is low, but this would be one of the measurements that I would field check as it wouldn't need to be that much off to make everything ok. In addition my estimate of dew pt temp for your vapor mix at 0.105 barg was about 5C higher than your data, suggesting maybe you really do have a lower suction pressure (on the otherhand it may mean nothing but that my vapor pressure properties are a bit off).
Thanks for the sketches and all. I am sure that someone can give a better explaination about the speed vs pressure profile than I gave. If I am wrong then I would like to know what you think sets the compressor discharge pressure in your case. Anyway don't give up on your compressor theory, just try and put it into the context of the process (refrigeration loads and condensing conditions).
best wishes,
sshep
RE: Turbine-Driven Centrifugal Compressor Operation
Could you please post a larger picture of that modified flow scheme? I'm having problems in figuring out certain numbers and streams. Also, about new TIC - where does it take the temperature signal from? I apologize in advance, but something is missing in my analysis of this conception you proposed.
Wouldn't be better to install a PC valve instead of HIC valve in depropanizer feed drum (refrigerant blowdown drum) and fix the condensing pressure of compressor discharge stream? This service (refrigerant blowdown to De-C3 in FCC Gas plant) is not functioning because of depropanizer charge pump minimum flow problems and choking of the depropanizer with additional feed, which essentially does not contain C3 when blowdown is continuous. Without fixed discharge pressure, pressure rise in the refrigerant blowdown drum during day is causing compressor suction pressure to rise equally - because compressor is operating on manual, remember? Propane accumulation is very slow but consistent, so I think some sort of purging is necessary - through PC valve instead of HCV... With small losses of i-C4 to the flare system. Could you please explain your concept in more details?
sshep:
Yes, we are using manual with backup type of antisurge control. I do agree with everything you have written in your last post.
About discharge pressure what do I think? Here is my concept:
- Compressor is always on manual (fixed RPM)
- Antisurge valve is in "manual with backup" mode, which means that surge conditions should always be avoided, if system is well-tuned
- Equipment downstream pressure (refrigerant blowdown - depropanizer charge drum pressure) is set by condenser outlet temperature and C3 content in compressor gas. Without purging, C3 content gradually rises regardless of condenser outlet temperature. This means that downstream pressure also swings (rises) by time, because it is impossible to hold C3 in liquid
- Since compressor operates with fixed RPM, suction pressure rise is the consequence of downstream pressure rise
- Having the same suction temperature and higher suction pressure, ACFM drops (I am very suspicious about fast-acting of antisurge controller) and, probably, moves the compressor toward surge point. This is something that happens always in AUTOMATIC mode of suction pressure control, but not in manual mode
- I think without downstream pressure control (PCV) it is impossible to run the plant in regular way, since propane accumulation (even in 7-day cycle) is a fact.
RE: Turbine-Driven Centrifugal Compressor Operation
In existing equipment configuration, if compressor discharge pressure floats (depending on system resistance and backpressure from the blowdown drum), having suction pressure controller in the automatic mode will continuously speed-up the compressor untill it trips off. Because of C3 accumulation and non-functioning of the liquid purge to De-C3 tower, I think it would be very difficult to maintain constant suction pressure without PC valve to flare system. Do you agree with that?
Regards,
RE: Turbine-Driven Centrifugal Compressor Operation
The TIC watches the temperature in the REF suction drum. You need to set the tempearture about 5 to 10 C above the dew point temp of the feed stream. I'd never seen this before until I was troubleshooting a 20,000 HP propane refrigeration system with 2 9,000 HP gas turbine single shaft 6 stage centrifugal compressors with a 2000 HP steam helper/starter turbine. They had a CCC antisurge system in manual mode, We called CCC in for training and returned the system to automatic and all the operational problems went away.
RE: Turbine-Driven Centrifugal Compressor Operation
What is the purpose of TIC controller when all liquid inventory from refrigerant accumulator ends up in the compressor suction drum, in both ways? LIC09 on refrigerant accumulator drum is controlling liquid refrigerant flowrate to the suction vessel. From simple heat balance it is easy to see that is not important how much refrigerant flows through LV and how much through TV, when both streams enter compression suction drum together with spillback gas flow. This means that all condensed refrigerant must end in the compressor suction drum. So I cannot grasp your concept about TIC.
I agree with the proposed PFD modifications in terms of achieving maximum condensing duty by removing LIC10 (LV10) and PDIC valve, and placing liquid refrigerant sub-cooler downstream of the fin-fan condensers. This would maximize refrigerant heat removal at compressor discharge and lower the required pressure for total condensation.
Forget about feed to depropanizer at the moment - I am almost 100% sure it will not function for the next few months, maybe even longer. When reading my last two previous posts, what do you think about C3 accumulation and necessity to have PC valve on the refrigerant blowdown drum? I can't see how can we rid of C3 and provide stable downstream pressure for smooth compressor operation without maintaining constant pressure in the last drum (refrigerant blowdown drum, not refrigerant accumulator drum)?
Onwards,
RE: Turbine-Driven Centrifugal Compressor Operation
RE: Turbine-Driven Centrifugal Compressor Operation
In alkylation reactor, C3/C4 olefins react with isobutane to make a high octane gasoline. The process is catalysed by a strong acid like sulphuric acid. The process requires low temperature (0-5 deg.C) and high Ic4/olefin ratio (6-10).
The process is characterized as exothermal, need high recycle of iC4 and need high mixing energy.
Ic4 is a reactant and also a good refrigerant. Therefore an opened refrigeration system like the system in this thread is normal used. The refrigerant compressor is designed to remove reaction heat plus heat added from mechanical equipment (circulation pump, mixing impeller) and keep reaction zone at the required temperature. Recycle iC4 comes mainly from the De-iC4. A part of iC4 is separated from the reactor effluent (in compressor suction drum) before it is sent to the De-iC4. The reaction zone is kept cold by: heat exchange with cold effluent (which is flashed at compressor suction pressure, recycle of cold iC4 refrigerant.
Before going to the compressor, some words about the propane content in the process are needed. Propane comes with the feeds. A small amount can also be made by a side reaction in the reactor (craking). All propane shall be removed from the system to avoid accumulation. It can be done by a purge stream from where it is most concentrate, either at the top of the de-iC4 or at the discharge of refrigeration compressor. In the system shown in the sketch, the discharge gas is partially condensed in the fin-fan, the remaining gas will have higher concentration of C3 and is condensed at lower temperature in the CW cooler. The flow rate of purge stream to De-C3 shall be set high enough to reduce C3 content to a required level (set by the process designer). It will depend on how much C3 entered with the feeds. Higher level of C3 will increase pressure in the refrigeration system (both suction and discharge) to keep the same operating temperature. If the C3 amount is low, the purge can be a gas purge to flare where the loss of iC4 is insignificant.
Back to the compressor, as said above the compressor shall remove the heat from a low temperature (heat source) and reject the heat to ambient (heat sink) at higher temperature. The temperature difference between heat source and heat sink will decide the head of the compressor (compression ratio). The amount of heat to be removed will decide the flow rate of refrigerant to be condensed in the fin-fan.
The relation between composition, temperature and pressure in the suction vessel and the refrigerant accumulator is that the pressure is equal to vapour pressure of the given liquid at the actual temperature. The propane content in liquid plays important roll for the vapour pressure, even at the small change. Given that the propane level is controlled in the allowable range. The temperature & pressure of the suction and discharge vessel is almost fixed.
The compressor operating point (head, flow rate) will be the intersection of the heat curve at given RPM and the system curve (pressure-flow rate relation) at the discharge of the compressor. The head at compressor discharge does depend on the down stream pressure and resistance . The system back pressure is the pressure at refrigerant accumulator plus DP of fin-fan. Since the DP is constant by the action of PDIC. The discharge pressure of compressor shall be constant at all flow rates. The system curve is a horizontal line. However, the compressor head curve will change with the rotation speed. The fan law says that compressor head is proportional with the speed or the flow rate through compressor is proportional with the speed given that other factors are unchanged.
The compressor controller loop (suction pressure – steam flow – speed – flow rate) will work in a certain range, which seems quite narrow. This is a feed back system, if flow rate is less than required, heat removal is low, suction temperature will increase and the suction pressure will increase, which opens for more steam. The input for the controller is suction pressure. It should be in theory equivalent to the temperature of the refrigerant in suction vessel, which is a more direct target of the (temperature of reaction).
If the turn-down causes the refrigerant flow to condensed goes below the surge limit, the surge valve shall be opened to reject the exceed flow. The surge flow does not remove any heat from the system as intention of system designer.
Before answer the questions it should be clear that if the propane level is not in control as it seems to be, it might have effect on the control of the whole system.
- Why can not achieve the design polytropic head ? I assumed the head here means discharge pressure. The head (in meter) is depend on gas properties like MW, Cp/Cv, etc. As mention above, the operating point is an intersection of system curve and compressor head curve. If the back pressure is low, the discharge pressure is also low. It should be constant provide that the down stream equipment is in their function range, check if the PDIC is not total opened or total closed and check the pressure of refrig accumulator.
- Why surge control valve open further when RPM is reduced in manual mode? I have no idea. The more opening does not need a indicator for more flow, if the discharge pressure is not constant.
- PCV at De-C3 feed drum: may be a good ide if the propane import is low such as the propane feed pump is not in function the most of time. I does not recommend to change the layout of CW cooler, unless the feed will be free of C3 in future, in this case the propane feed drum and the pump can also be removed. The current layout can be used to increase the concentration of C3 in purge gas. A new pipe is needed to send the condensate from De-C3 feed drum back to the suction drum by for example a flow control.
Hope it may help to give some light on the problem.
RE: Turbine-Driven Centrifugal Compressor Operation
I would not agree that compressor head is dependant on gas properties, as you wrote. It is a function of compressor mechanics and inlet volumetric flow. Only discharge pressure depends on gas properties, but not head developed by the compressor.
Still cannot imagine alky unit design (or any other unit) without compressor discharge pressure control. In spite of having relatively clean service (>90% i-C4 in compressor gas), propane accumulation and downstream pressure buildup will, eventually, cause overspeeding and tripping-off the compressor if suction pressure control is on automatic mode. If nothing else, having fin-fans as discharge condensers will result in different heat duties during one single day, not to speak about months or seasons. In other words, if refrigerant blowdown system is not functioning properly as it is the case now (no liquid pumping to De-C3, because of inadequate pump and De-C3 operational problems with blowdown stream), PC valve on blowdown drum looks like the most simple solution, resulting in minor losses of i-C4 into flare system.
I think it is the first thing we must resolve before continuing to work on compressor control problems.
Thanks,
RE: Turbine-Driven Centrifugal Compressor Operation
I mean that the discharge pressure in this system is controlled indirectly via the condenser outlet temperature. What happen when both outlet temperature of fin fan and CW cooler goes down, e.g by 5 deg.C ? The pressure at refrigerant accumulator or back pressure will goes down. It causes refrigerant flow to the fin-fan to increase. This again causes temperature of the suction drum to go down, which makes suction pressure to go down. The controller will reduce steam to turbine until a new operating point is reach (lower discharge pressure with approximate the same refrigerant flow to the fin-fan).
I believe that the instability when running automatic mode is caused by the control loop “suction pressure – steam flow – speed – flow rate”, based on the fact that when the speed is on manual, instability is gone. The feed back of the loop is break in this case. The system still works since surge flow is a mean to fix how much heat is removed from the system, even it is not a optimal operating point.The control system might be not well tuned or operate outside the range it is tuned for. It must be very sensitive if the compressor head curve in the relevant range is flat (a small change in pressures cause a large change in flow). In that case, reduce proportional factor of the controller may help (The bias will increase).
Regards
Nghia
RE: Turbine-Driven Centrifugal Compressor Operation
A vent to flare for controlling C3s will work if you don't have depropanizer feed option. A flow control drag off the refrig acumulator will be effective. This seems to be what the controls indicate now except that the draw is condensed. Any chance you can go to fuel gas insteadof flare?
This has been an interesting thread, most especially the controls. It challenges my mind to understand how many independent variables are available across the whole system. Whenever a system with so much is going on is unstable, it can be hard to figure out if the problem is tuning or a fundamental instability (including overspecification). Can you cover again how we are running the LIC/LV10 and the PDIC?
best wishes,
sshep
RE: Turbine-Driven Centrifugal Compressor Operation
LIC10 is always on manual (50-55% open), while PDIC is on automatic mode with 0.4bar set point.
In theory, when purge stream flow to De-C3 is increased (through FCV at pump discharge) LIC/LV10 closes for a few % and pushes more refrigerant gas through PDIC. As a result, level in refrigerant blowdown drum increases to compensate increased purge stream to De-C3.
In practice, I cannot see the reason for not opening LIC10 100% when refrigerant purge stream to De-C3 is not functioning.
When I asked day shift supervisor why LIC10 is not always 100% open, he answered that "system always needs positive differential pressure across the condenser in order to maintain refrigerant flow in the right direction" - which I cannot understand and do not agree with (for example, FCC wet gas compressor does not have hot vapor bypass on discharge condensers). It is also interesting that, according to PDIC measurement, PDIC valve is almost always completely closed in order to maintain set differential pressure of 0.4bar - which indicates that ~100% refrigerant gas flow is going through discharge condensers. In such circumstances, keeping LIC10 partially closed does not have sense at all.
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