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Problem in crack gas compressor start up

Problem in crack gas compressor start up

Problem in crack gas compressor start up

(OP)

Hi everyone
During start up of an ethylene plant, discharge temperature of the crack gas compressor (fifth stage) was drastically increased, meanwhile very high amount noises like fast opening and closing of a valve was observed. Moreover the compressor faced with insufficient flow. In this case, opening of the antisurge valve was 100%.It should be noted that there is no FE on antisurge loop Of CGC stage 1 and no piping pockets in the suction. However, a pumping trap is installed in upstream of suction line CGC stage 1. Antisurge valve with a difuser is specified to be a low noise
Other trends could be listed as below:
1- Outlet temperature of crack gas from quench tower was increased about 4-5 deg.C.
2-Level of discharge and suction drums of the crack gas compressor (stage 4) were increased during starup.
3-Discharge temperature of 5th stage of the CG compressor was increased to 130 C.
4- Fluctuation of flow was observed in all five stages of the compressor.
5-Alongside the flow fluctuation some small fluctuinations of pressure drop was observed in caustic tower.
6- No vibration in CG compressor was detected by DCS.

The following activities to remedy have been done:
* Antisurge valve was opened and fully inspected . No failure was detected.
* All suction and discharge drums, check valves, orifices, cooling water exchangers and caustic tower in the line were inspected and no chocking or unusual thing was seen. 
* About 1000kg mixed yellow oily water was observed in cooling water exchanger of 5th stage of the crack gas compressor. Antisurge valve is branched from outlet of this exchanger. The water was completely drained. 
*Stage 5 of the compressor was inspected by a borescope camera and no failure was detected.

After these remediations and start the plant for the next three times, the noise, insufficient flow and increased temperature were again repeated.
I was wondering if anyone could propose a solution? 

RE: Problem in crack gas compressor start up

As far as I remember, 130 deg C is borderline as for polymerization temperature of cracked gas.
Is the train equipped with some washing system (e.g. Naphta injection nozzles)?
I could speculate the 4th stage may be causing the problem to the stages forward.
Other question?
Which stage anti-surge valve is opening ?
If there is no FE on anti-surge loop, how does the control system controls the valve?

Is this steam turbine driven as I suspect? is the gas composition sampled and molecular weight within specs. ?


Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
thank you rotw
when the compressor was started and passed the min. governor speed to reach the normal speed, the discharge pressure and temperature of stage 5 increased while discharge flow did not rise and began to fluctuate. Reaching to about 130 C at discharge, the speed was reduced and the train was stopped.
A wash oil injection system is provided in the CGC but it is normally used in normal operation and not in start up.
Anti surge valve of stage five is fully opened and other antisurge valve located between stage one and four is about 60% opening.
The first and second antisurge valves are controlled by discharge temperature and pressure of stage four and five respectively. However FEs are installed on discharge line of CGC both stages four and five.
The turbine as you mentioned is steam driven.
Normally during each start up the gas composition is not sampled and the outlet temperature of furnace is the only criteria to be in spec, however it is analysed via the lab and the results are in range including gas composition and MW.

RE: Problem in crack gas compressor start up

First impression, is that problem on stage 5 is probably an induced problem and some stage(s) upstream stage 5 is at fault. But it's hard to say, the least we can say is that it is not easy to diagnose machinery issues let alone such a complicated beast. It gets even more complicated if there dynamic effects on top (start-up).

Some speculations:
- Stage 1 to 4 have a common anti-surge valve, this means the 'weakest' stage from 1 to 4 is going to trigger recycle for the complete arrangement 1-4. Having a combined anti-surge valve decreases capital costs but squeezes operating range. Cracked gas compressors have typically modest operating range depending on the gas heaviness. So, when the stage at fault has recovered from surge it has done so at the expense of other stages operating off design and with possible efficiency drop because each stage is not equipped with a dedicated valve.

- If anti-surge valve opens, there may be a (dynamic) effect of drop of forward flow to the successive stage (five) which has triggered the 2nd anti-surge valve to fully open, this has positioned the operating point of that stage onto the left of the speed curve (i.e. control line flow) which corresponds to higher delivery pressure and higher discharge temperature.

Is this a start-up for new unit? If so, maybe you could check if the design is referenced especially in terms of anti-surge valve arrangement? Is the process open art or licensed and check if the anti-surge valve and control system manufacturer vendors have also references. If the set up is referenced for this capacity or equivalent, fine, if not you could start to wonder what has been done.

- Is there some sort of boiler feed water injection to the impellers in order reduce the discharge temperature (cooling) in order to prevent polymerization temperature?


Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
Thank you so much rotw
The train has been started for many times and this problem has not occurred up to now.
BFW is injected to impellers in stages 1-4 and wash oil is injected intermitently. Recently wash oil and BFW was injected in stage 5 (before the last stoppage).It seems the impellers are not coated.
It should be noted that a caustic tower is located between stage 4 and 5 so the discharge of stage 4 is directed to the tower and the CG from overhead of the tower is sent to suction of stage 5.
One more thing, a pressure valve (PV) is located in discharge of stage 5 to flare and when the valve is limited the discharge temperature and noise is drastically increased.

RE: Problem in crack gas compressor start up

If there is failure of after cooler of stage 5 then temperature of gas going to anti-surge valve will increase. There might also be obstruction of flow passage on heat exchanger (fouled or altered after BFW injection and naphta oil washing?). The hot gas goes back to suction of stage 5 and while it is mixed with main feed flow it has drastically increased temperature and pressure at the suction and incidentally at discharge of stage 5. This has triggered the anti-surge valve of this stage to fully open. Possibly this has also induced effect on stage 1 to 4 which started also to recirculate facing higher system resistance from stage 5.

Highly speculative in any case.

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
The after cooler of stage 5 is NEN type exchanger which CG passes through tubes and cw in shell. The exchanger was inspected and no plugging was observed in tube side. When wash oil was used during one of unsuccessful start up, about 1 ton wash oil and polymer was observed in the outlet and tube side of the aftercooler. The mixture was completely drained and the train was started. The noise, heightened temp. and insufficient flow were repeated.
What about the plugging of the stage 5 of CGC ?

RE: Problem in crack gas compressor start up

You have a parallel post on this topic - we can continue with this one to keep correspondence together. E-Tips policy discourages double posting on any one topic.
High discharge temp on stage 5 tells me maybe you've got low pressure on stage 5 suction. Is stage 5S pressure within normal range? - Any blockage on the demister in the caustic tower?
Am trying to understand compressor antisurge scheme:
One ASV on Stage 4D returns to stage 1S and is operated by either stage 4D high temp or high pressure
One ASV on stage 5D returns to stage 5S (downstream of caustic tower and upstream of stage 5 suction drum, I presume) and is operated by also by high temp or high pressure on stage 5D.
Stage 4D and 5D flow elements have no input to antisurge control.

Presume there is no input from turbine speed to the antisurge algorithms, so the surge line is most likely assumed conservatively to be the one at max permissible operating speed.
Is there a capacity control recycle valve ? What happens when you have low pressure on stage 1S, especially during startup when cracked gas flow is low? A capacity recycle valve would then recycle on stage 1S low pressure also.
Another reason may be foaming in the caustic tower due to some kind of contamination of the recirculating caustic solution. This may be the reason for high level in stage 5S drum.

RE: Problem in crack gas compressor start up

(OP)
Dear rotw
Trends of discharge and suction temperature of stage 5 show that when the discharge temperature increases, the suction temperature decreases or almost constant. So, the failure of after cooler stage 5 is debunked.

RE: Problem in crack gas compressor start up

What about the plugging of the stage 5 of CGC ?

Did you look at historian / data trends to understand the behavior vs. time?

Another question is, why has the oil washing and BFW injection been applied to stages 1 to 4th only and not to stage 5th?
What was OEM/operator instruction in this regard and have the prescriptions (if any) been followed in the past?

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
georgeverghese
Thank you for your hint.
At discharge of both stages 4 and 5 the flow is cooled through cooling water after coolers to about 45 C. So, the flow in antisurge lines are cooled enough and have lower pressure than discharge line. FEs are installed on discharge of stage 4 and 5, however there is no FE on antisurge line. The opening of first and second antisurge valve is adjusted based on the suction temperature and pressure of stage 1 and 5, respectively.
I supposed there is no capacity recycle valve in the line.  
When the discharge pressure stage 5 is increased, the discharge temperature and suction pressure of stage 5 are rised. The suction pressure of stage 1 is fluctuate, first increased next reduced then rised.
In case of low pressure on 1S the antisurge valve will open.
The level in suction drum stage 5 was remained constant and the demister of caustic tower was inspected, it was clean.

RE: Problem in crack gas compressor start up

(OP)
rotw
The available trends show that there is no such significant deviation in performance of CGC.
Injecting wash oil and BFW in stage 5 may increase the level of discharge drum stage which may increase the possibility of liquid carryover and send much more liquid to the dryer. Recently discharge temperature of stage 5 has been rised approximately 4-5 C however the common discharge temperature was deviated from design value about 10 C. So it was decided to start the injection of BFW and wash oil in stage 5. The vendor was also recommended to inject BFW in all stages of CGC.

RE: Problem in crack gas compressor start up

Has the operator taken the decision/responsibility to deviate from vendor recommendation that BFW injection and naphta oil injection shall be applied to all stages? I suggest to maybe check this point; also because you did mention the impellers do not have special coating.

Normally after washing stage #5, the performance should go back to normal; surge line for instance should match the parameters of the configuration of origin in the controller (assuming none has manipulated the anti-surge controller configuration).

However if the impellers have experienced irreversible alteration (as an induced effect to extensive fouling), means a degradation that would probably not 'disappear' upon washing, then the process stage map could shift (degradation case) and incidentally the anti-surge controller would no more be configured adequately. So, boroscopic inspections indicates no failure, Ok, but did they get to the impeller surface? I am myself ignorant on boroscopic investigation, so just wondering.

I also would like to say that 4-5 deg C deviation is not negligible (all other things being equal).

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

Again, your antisurge scheme is not clear. You now say
"In case of low pressure on 1S the antisurge valve will open."
Previously you said the ASV on stage 4 and 5 only works on high pressure or high temp of stage 4D or 5D respectively.
I think we need to have a complete picture on this control scheme and also a complete PFS or sketch.
The plant I worked on shows a large low point piping pocket in the feed to the intermediate caustic tower, which is steam traced and insulated, presumably to vaporise condensate that would have collected upon CGC shutdown /cooldown. Do you have such an arrangement in your plant, and is the condensate cleared out of this low point in the caustic feedline before CGC startup? - liquid slugging in this line may be the cause of these pressure fluctuations?

RE: Problem in crack gas compressor start up

(OP)
rotw
It should be added to previous note that when the discharge temperature of stage 5 was increased they decided to inject BFW as per vendor recommendation.
By the way, the CGC stages 4 and 5 are opened and no fouling or degradation are seen. So the plugging of CGC stage 4 and 5 are rejected.
As far as I know, no body has manipulated the anti-surge controller configuration from the previous normal operation up to now.

RE: Problem in crack gas compressor start up

(OP)
georgeverghese
A schematic of compression stages is attached.
The ASV controller uses suction temperature and pressure, discharge flow, temperature and pressure to adjust the valve opening. During start-up before reaching to minimum speed the ASV is fully opened. Above the minimum speed if the compressor was not in the surge,in case of low pressure on 1S the antisurge valve will open.
During the previous start up I checked the trends and I found that in stage 5:
Increasing the CGC speed >>> Increasing Discharge Pressure & Temperature >>> Increasing Suction Pressure & Temperature >>> Increasing the level of suction drums stage 1-4 and discharge drum stage 4 (no change in level suction drum stage 5) >>> DP of caustic tower was fluctuated (increased and decreased)
It worth noting that the level of discharge drum stage 4 (inlet to caustic tower) was as high as 98% during the first startup but in the next trials the level was controlled.
There is no low point from the discharge drum stage 4 to the caustic tower and the feed to the tower is preheated in a steam jacket via LP steam.

RE: Problem in crack gas compressor start up

There are some details missing on this sketch, but we can guess where/ what it is:
a) Speed control of turbine is presumed to be taken from a PIC on stage 1S
b) Stage 5D line to flare PIC is presumably upstream of propane chiller
c) Stage 5 antisurge line is shown to be taken from downstream of stage 5D cooler / upstream of stage 5D drum - but it should be from downstream of stage 5D drum.
The sketch does not show the inputs to the antisurge loops, so we will go by your description so far.

The plant drawings I have here also have this pumping trap on stage 1S and there is no CGC stage 1S drum. It also shows this LP steam jacket heating on caustic tower feed to be located on a piping low point. These drawings I have are for Assaluyeh naphtha cracker based ethylene plant.

Am running out of guesses, but a few more will be:
1)Any instability seen in the line to flare PCV on stage 5D?
2)Compressor casing drains all working well ? Where are these routed to?
3)Heat tracing on piping downstream of ASV working okay
4)Is there an input from steam turbine speed transmitter to stage 4 and 5 antisurge loops, and is there any irregularity in these feed signals from the turbine ST?
5)You said stage 4 antisurge loop also recycles gas to stage 1 in case of low pressure on stage 1S, which may happen when turbine speed is already decreased to min speed. Does the same concept apply to antisurge loop on stage 5 i.e. does it recycle gas to stage 5S on detection of low pressure on stage 5S?

RE: Problem in crack gas compressor start up

(OP)
About the missed details:
a) Speed of turbine is controlled from a PIC on stage 1S.
b) Stage 5D line to flare PIC is upstream of stage 5D after cooler.
c) Stage 5 antisurge line is exactly from downstream of stage 5D after cooler / upstream of stage 5D drum and then go to inlet of suction drum stage 5.

This plant is ethane cracker based ethylene plant and the feed line to caustic tower has no piping low point. A sketch of isometric drawing from discharge drum stage 4 to caustic tower is attached.

About the questions:
1) The 14" line from discharge stage 5 to flare has no PCV and the flow is controlled via a hand gate valve. However, no instability could be seen in the gate valve, the trends from DCS show the instability and fluctuation of discharge flow of stage 5, discharge P stage 5, suction P stage 5, DP caustic tower. Moreover, field operators reported frequent closing and opening of the check valve at the discharge of caustic tower during the start-up.
2) Compressor casing drains are routed to ditch( ground) and all working well.
3) If you mean steam jacket by heat tracing on piping downstream of ASV, yes it is working well. In other case if you mean heat tracing in discharge stage 5, there is no heat tracing in this stage.
4) No, there is no input signal from steam turbine to stage 4 and 5 antisurge loops. The input signals to both antisurge controllers are suction T&P and discharge P & DP (or flow).
5) In case of low pressure on stage 1S (less than 0.2 bar) the stage 4 antisurge loop recycle gas to suction to prevent ESD activation but the same concept does NOT apply to the second antisurge loop i.e. stage 5 antisurge. However, the turbine speed did not decrease to min speed during the start-ups.

"The surge line in this compressor is defined by X-axis : sqrt (DP * Pdischarge)/Psuction and Y-sxis: Pdischarge/Psuction
The anti-surge control uses the signals of the differential pressure transducer in the discharge line of the compressor , the suction pressure
transducer and the discharge pressure transducer. The anti-surge control valve is opened pneumatically, which means that it is necessary to convert the
electrical controller output into a pneumatic signal. This function takes place in the current to a pneumatic (I/P) converter and acts directly to the pneumatic
positioner mounted at the control valve. The solenoid valve mounted at the anti surge valve has to ensure a quick opening of the anti surge valve.Quick opening occurs if a potential free contact from the compressor PLC to the solenoid valves fails for any reasons. This contact is normally closed as the compressor has reach minimum speed."


RE: Problem in crack gas compressor start up

Typically, train is started with stage 1 and 2 in re-circulation. Turbine ramps up. Probably there is idle speed sequence for turbine as well.
Turbine reaches minimum operating speed. Discharge Check valves should be open. Anti-surge valves gradually close. Turbine speed is ramps up gradually. At normal operating speed, anti-surge valves are normally closed.

It seems at this point, discharge pressure and temperature of stage #5 continued to increase, flow reduced. Anti-surge valve no.2 went fully open. Anti-surge valve no.1 opened at 60%. Which one occurred first is not clear.

This is just guess. So, could you post a brief / more correct summary of the starting sequence of CGC if it is minimum effort; I cannot say that it will really help solve the issue. It would just be interesting to investigate the start up sequence in details and maybe get a hint of which point exactly things went wrong.


Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
Here is a summarized starting sequence of CGC:
1-Before starting check that anti-surge valves are fully opened.
2- When turbine reaches minimum speed, DCS operator adjust opening of anti-surge valves based on the deviation from surge line.
3- Before closing anti-surge valves, open gate valve on discharge line stage 5 to flare to control the discharge pressure.
* At this point, discharge P & T stage 5 increased and flow fluctuated. Anti-surge valve 2 remained fully open while anti-surge valve 1 closed to 60%.
4- Ramp up the speed turbine to normal condition, close the anti-surge valves and close gate valve on discharge line stage 5 to flare.

RE: Problem in crack gas compressor start up

* At this point, discharge P & T stage 5 increased and flow fluctuated. Anti-surge valve 2 remained fully open while anti-surge valve 1 closed to 60%.

After flow fluctuations were noticed (are these fluctuations usual or not?), I understand operator continued turbine ramp up until normal operating speed is reached (questionable?). Is this correct?
Then compressor at nominal speed starts feed forward flow to downstream system and anti-surge valve was gradually and finally closed by operator with anti-surge control system active. Is this correct?

So what did happen next?
Pressure continued to build up, flow at suction reduced and temperature increased at stage #5; this initiated trip (ESD or else?).
Was the trip on discharge temperature high, and was it discharge temperature of stage#5 or of another stage?

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
In this case the flow fluctuations are unusual. When the flow was fluctuating, the discharge temp stage 5 was increasing drastically so the operator shut down the CGC. It did not reach to normal speed.
Your second point is correct for normal start up to reach normal operation.
Exactly as you mentioned, pressure build up & flow reduction & increasing temp stage 5. However there is ESD on high temp, DCS operator tripped the compressor. The main reason to trip CGC is high temp stage 5.

RE: Problem in crack gas compressor start up

Some other thoughts:

- To me it looks like the gate valve to flare is there to help the compressor relief the pressure (similar to a blow off) and force the compressor to start along the choking line / far from surge. An anti-surge valve fully open that is sized too big can choke the compressor which is not recommended all though it is good for dynamic response anticipation; so the gate valve to flare looks like a way to regulate / fine tune the path of the compressor during ramp up so it goes along the choking line without putting the compressor into deep choke neither. Maybe the capacity of that gate valve was no more sufficient.

- Consequence of surge is not only vibations and noise but a surge cycle could lead to a drastic increase of temperature at the suction. So I do not know if this can be correlated to your statement in the OP: "fast opening and closing of a valve was observed"

- It is also needed to know the design of the anti-surge control system. Do you have control line that moves dynamically? Do you have safety line that would put the valve fully open if a certain threshold of the deviation is crossed. Is there a surge counter?

I maybe on the wrong track, but just as brainstorming exercise might be good to have a broad look at things.

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

Re your responses,
c)I find it strange that your recycle line take off to ASV on stage 5 is taken from aftercooler discharge and not from downstream of stage 5D drum. So there would be lots of liquid in this recycle line, causing slugging and pressure fluctuations etc, especially when you are injecting BFW or wash oil in to stage 5 casing - is this ASV recycle line a free draining line? Standard practice is to install this take off from downstream of stage 5D drum, so you will then have only gas being recycled.
3)Yes I was asking about heat tracing - okay.
Your response to (1) about the chattering on the check valve on caustic tower exit line brings up another suspect cause: Maybe the cracked gas feed to CGC is too low in mol wt to enable getting to the final discharge pressure, and this is most apparent in stage 5 - the compressor has pushed it self up to max speed on startup and is then too close to the surge line. Hence the stage 5 ASV at 100% open. This also matches the high temp on stage 5D - low mol wt gas causes high temp. Compressor is close to or on the surge line and pressure fluctuations occur on stage 5 and stage 4 ( and then reflected back all the way to stage 1). Have you checked the mol wt of this cracked gas? If this is found to be true, then maybe there is something wrong with the exit gas from the furnace. Is it exiting at the correct temp range or is there too much H2?





RE: Problem in crack gas compressor start up

(OP)
rotw
I believe the capacity of gate valve is so enough based on the process data sheet and previous runs.
The suction temp stage 5 did not increased so much. Field operators reported "fast opening and closing of a valve" which it was apparently the check valve after caustic tower and before suction drum stage 5.
There is a control line, a safety line and a surge line in the anti-surge control system. The control line is specified as being at a distance of 5 % of the volume flow along a performance curve represential constant speed. When this line is reached, the controller opens the limit recycle valve. The safety line which is located between the surge line and the control line and when is reached, the control system is overridden by the valve jump.
Can you explain more by surge counter.

RE: Problem in crack gas compressor start up

(OP)
georgeverghese
Its an important point that you mentioned but the temperature of stage 5D drum is 15C. The anti surge line goes to suction drum stage 5 and the liquid may remove in this drum. However, there is a drain in low point of after cooler discharge stage 5.
The mol wt of the CG and H2 conc. in feed during the previous start-up are 20.05 and 32.53 mol%, respectively. The recommended values by vendor are 22.14 (MW) and 31.9 mol% (H2). Moreover, the temperature of the exit gas from the furnaces is in the normal range.

RE: Problem in crack gas compressor start up

For surge counter, you may want to view some description on page 33 of 35 of the following article:
https://oaktrust.library.tamu.edu/bitstream/handle...

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

Agreed, 15degC is too cold for this recycle valve feed on stage 5. In standard practice, a new hot discharge drum would be required on stage 5D (upstream of PR kettle chiller). You can connect / relocate the stage 5 recycle takeoff to be just downstream of this hot 5D drum.
Mol wt and H2 content for cracked gas feed in the previous startup appears to be within spec range. Last few suspect causes would be :
a) There is some defect in the operation of the stage 5 ASV
b) There is some recent modification or adjustment made to stage 5 antisurge algorithm and /or antisurge controller P/I/D settings in DCS
c) Some deterioration in compressor polytropic efficiency on stage 5 and 4 - but you have checked impellers / stator diffuser vanes and all are clean.

RE: Problem in crack gas compressor start up

(OP)
a) ASVs were sent to shop. No failure or plugging for both valves has been seen.
b)as far as I know from the last normal operation to now, modification or adjustment has not been made to CGC antisurge control or setting. However, it should be checked.
c) CGC stage 4 & 5 were opened. Every thing is ok.

Beyond these points, some questions have still remained:
1. Why the discharge temp stage 5 increased if ASV stage 5 is 100% opened?
2. The capacity of orifice RO in stage 5D to flare is 40 t/h and the FE in stage 5D represented 100 t/h. ASV stage 5 is fully open. Temp stage 5S is almost constant but temp & press stage 5D rise.
What happened to the other 60 t/h flow? If ASV is fully open, T&P stage 5 should decrease and CGC moves from surge line to right.
3. Why DP of the caustic tower is fluctuated? Why the check valve is chattered?

RE: Problem in crack gas compressor start up

Re your questions:
1. If stage 5 suction pressure and temp and cracked gas mol wt is all within spec, then stage 5D temp should be within spec also,since you say there is no fouling of the impellers/diffuser vanes on stage 5. So I cannot explain this also.
2. FE on stage 5D is within stage 5 recycle loop, so it will read higher than gas to flare. It just means that 60t/hr is going through stage 5 recycle line.
3. These are signs that stage 5 is operating inside the surge line for stage 5, if you ask me.

Presumably the steam turbine speed is operating in a stable manner. You also said earlier that stage 5D cooler was checked and there is no sign of leaks or fouling on water side.

Presume you are not over injecting BFW and wash oil into the compressor to the extent that it is interfering with compressor operation. Is there some injection quill for these?

Have you checked the TT on stage 5D? When was it last calibrated?

RE: Problem in crack gas compressor start up

(OP)
TT on stage 5D and other transmitters are checked. All calibrated.

There are some paradoxes in operating manual and process data sheets documents:
1- When 3 furnaces are fed with feed stock and one in hot stand by conditions with all effluents diverted to quench water tower, the crack gas compressor can be started.
The capacity of each furnace is 30 t/h, for 3 furnaces is 90 t/h.
2-The CGC shall be designed to operate satisfactorily at a 40% turn down of the feed capacity i.e. 102.88 t/h. This condition will occur during start-up, shutdown or cracking furnace worst operating condition. This value is accordant to the compressor process datasheet for turn down case with 94.45 t/h as recycle flow in stage 1.
3-In the compressor datasheet, for start-up case it is clearly stated 175.8 t/h feed to stage 1 CGC and 55.1 t/h recycle flow.
4- Composition of crack gas for start-up case is 22.95% mol. H2, 42.78% mol. Ethylene (!) and 26.2% mol. Ethane. and for normal operation case is 35% mol. H2, 32.62% mol. Ethylene and 20% mol. Ethane.
In one of the previous posts I noted that " The recommended values by vendor are 22.14 (MW) and 31.9 mol% (H2)." but the corrected value is 22.95% mol during start-up. While the H2 composition from lab is 32.53% mol.

Here is the questions:
a) why the conc. of ethylene during start-up is high and H2 is low compared to normal ope. ?
b) The CGC shall be started by which flow, 90 t/hr or 102.88 t/h or 175.8 t/h ?

RE: Problem in crack gas compressor start up

a) Think this has to do with the average tubeskin temp in the cracking furnace : tubeskin may be cooler during startup and hotter during normal operation ( to get from low thermal duty to high thermal duty). Hence further cracking of ethylene at the tubeskin surface may be significant during normal operation (conversion of ethylene to propylene and C4, C5 = / C4,C5 dienes), which results in further formation of H2, despite the lower residence time. Hence H2 increases and ethylene decreases when going from startup to normal ops ( and as a result other cracked gas products increases).
b)Since you have a stage 4 to stage 1 recycle which is also controlled by low stage 1S pressure, in practice, you should be able to operate down to close to zero net feed flow from the quench tower ( even when you have only one furnace with say 50% design flow ie 15t/hr), assuming the stage 4 ASV is suitably sized to provide this recycle flow. You may pls check that the ASV on stage 4 can do this (ie. be able to recycle approx say 170-200t/hr when at say 90% of full speed-also refer to the performance curve for the compressor). The same should be checked for stage 5 ASV. There is a delicate balance between this high recycle flow during emergency ramp down in cracker furnace output vs the low normal operating recycle flow - turndown on control valve is approx 20:1 and can be stretched to 30:1).

RE: Problem in crack gas compressor start up

Based on this recent info, think the prime suspect for this is now the ASV on stage 5 - it is too small to allow stable operation at full discharge pressure with a net fresh feed of 40t/hr. Reasons for these are :
a) The furnace total effluent of 90t/hr seems completely out of order with the compressor capacity of approx 230-260t/hr.
b) Operation of stage 5 close to or inside surge line has resulted in high stage 5D temp due to low polytropic eff close to the surge line and possibly also low pressure on stage 5
c) Surging of stage 5 is evident from chattering of exit valve on caustic tower.

To address this problem we have 2 possible solutions
1) Live within the recycle capacity constraints of the current stage 5 ASV. So in case you wish to operate with a startup flow of 40t/hr, operate the flare valve such that 5D pressure is deliberately kept low and stage 5 ASV is not fully open. You can only ramp up pressure on 5D when there is sufficent fresh feed from the furnaces.
2)If you wish to ramp up pressure at 40t/hr and lower flow, then it is obvious we have to change out the trim on stage 5 ASV to allow more recycle. Ask a local consultant to resize this valve for you for the lowest furnace feed flow you wish to operate at. It is also better to enable a control scheme similar to stage 4 ASV ie. allow the ASV to recycle back to 5S also on detection of low pressure on 5S. Then stage 5 ASV will be acting as both capacity recycle and antisurge service.

It is also strange that though stage 5 is operating very close to or inside the surge zone, the compressor has not tripped-some one has disabled the surge trip on stage 5?

RE: Problem in crack gas compressor start up

(OP)
Dear georgeverghese
After opening and checking the CGC, opening and fully cleaning the caustic tower, the plant was started. This time the compressor started with no problem and the plant is in normal operation now.
since no obvious evident has been detected in the CGC, I checked the trend of caustic tower DP. During the normal operation the DP in middle of the tower (DP=0.2bar) is above the spec (DP=0.05bar). Before cleaning the tower and during the startup when the speed was increased to min. speed the DP rose to 0.152 barg & fluctuated trend (First start-up), 0.151 barg & fluctuated trend (Second start-up). After using wash oil to clean the tower, DP rose 0.304 barg & smooth trend(Third start-up), 0.492 barg & smooth trend (Fourth start-up). After opening the tower ad fully drain the tower with water and wash oil, in the recent successful start-up the DP rose to 0.04 barg & normal trend.
It seems the middle of the tower was fully plugged during normal operation and using wash oil could not remove the plugging and more ever compact the fouling inside the middel packing. The channeling of crack gas inside the tower was still remained. When the tower was opened, and fully drained by water and wash oil, the significant amount of polymer was removed and the packing was cleaned and the channeling problem was solved.
Do you agree with this hypothesis?

RE: Problem in crack gas compressor start up

It is good you have found this fouling of the packing in the tower, and have cleaned up the packing. But what is the worst case total dp you observed previously when this packing was fouled? Even the worst case dp you have now quoted ie 0.5bar is small in comparison to the stage 5 compressor normal dp, so it cannot be the reason why the compressor was operating in the surge zone for stage 5, it is only a small contributing factor.
Suspect that in this recent smooth startup, the plant operators must have waited longer till there was sufficient flow ( ie >40t/hr)from the furnaces before they ramped up pressure on stage 5D at the flare valve.

RE: Problem in crack gas compressor start up

(OP)
Dear georgeverghese
I checked the specifications of the ASVs and found these valves are suitably sized for recycling 170-200 t/h gas flow at 90% of full speed.
It should be noted that the plant has been producing since 2004 and the CGC has successfully started many times. I prepared the trends of speed, flow, P, T and dP of these previous runs which may help us to better undrestand the problem but because of some limitations I could not upload here publically. I was wondering if you could give me your email to send you these trends.

RE: Problem in crack gas compressor start up

I am interested to have the trends, gas composition and the maps for each section, if at all possible!

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
rotw,
How can I give you the data?

RE: Problem in crack gas compressor start up

At the bottom of "Reply to this Thread" you'll see "Attachment"
next
"Click Here to upload your file to ENGINEERING.com"

you will have a search window to the files

RE: Problem in crack gas compressor start up

I made a request file to dropbox:

https://www.dropbox.com/request/pKL7lKMiB2IR5h4EJB...

As for the construction aspect, I would also need the following additional information; if you could provide it, it would be appreciated:
- Number of impellers per each process section and their tip diameters ;
- Which case is the design case and/or the certified case;
- The design speed (rpm).

PS: please also blur / make invisible any reference to client / company / project location, etc.
So this remains anonymous as a sort of 'academic' exercise only.
Thanks!

RE: Problem in crack gas compressor start up

(OP)
georgeverghese
here is the data for ASV 1 & 2:
ASV 1 : 187 t/h (min.) , 277 t/h (normal)
ASV 2 : 80 t/h (min. at 80% speed) , 277 t/h (normal)
Do you believe that these specifications may cause problems in start-up of the compressor?
And can you please describe more your previous note "You can only ramp up pressure on 5D when there is sufficent fresh feed from the furnaces." ?

RE: Problem in crack gas compressor start up

Mohammad Ka
First off, thanks for sending out the docs. Well received. Also thanks for the work to consolidate all the info. It is presented clearly.

This goes far beyond my expertise, but just tentatively so we explore new causes: Could it be the case that controllers are fighting each other; the performance controller and anti-surge controller must be decoupled. If not, when performance controller provide new speed set point to turbine to ramp up, anti-surge controller and performance can have antagonist actions. So the gains and other parameters are tuned so both are dis coupled. I think otherwise this may lead to instabilities. Possibly due to process reasons the process was too much upset for the dis-coupling to be robust enough and be able to cope with that. Again, liked I said, one more speculation here.

By the way, would be able to provide on top of the documents you send already the following map:

Outlet pressure and temperature vs. flow for the design case?
Thanks again for your efforts in preparing and sending out the material.

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

Your Item 4

"Fluctuation of flow was observed in all five stages of the compressor."

What is happening in the Gas Cracking Furnaces?

RE: Problem in crack gas compressor start up

(OP)
hacksaw
No abnormal conditions were observed in furnaces during the runs. Outlet temperature, pressure and gas composition all were in the normal range.

RE: Problem in crack gas compressor start up

(OP)
rotw
The turbine speed controller and antisurge controller has been decoupled and no interference has reported yet.

RE: Problem in crack gas compressor start up

yet, the quick opening and closing of the valve is a matter to question.
Reference to your OP: "fast opening and closing of a valve was observed".

Few possibilities I can think of:

- Process upset could be coped with by control system;
- Process upset exceeded control system capabilities (instruments, calibration, characterization, not designed for the signals fed to system);
- Process upset triggered control system to behave unexpectedly which in return made process situation much worse;

I just have the impression that the control system was 'fighting' to keep the train in operation, that is attempting to fulfill the set points value from master control (DCS or operator). Thus we see quick oscillations of the parameters, until it finally trips.

As precision, I meant performance controller not speed controller. The performance controller is the one that provides speed set point value to speed controller that further actuates the turbine governor. The performance controller should be 'the PIC on stage 1S'; its set point should be train suction pressure. Anyway your response is noted.

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
rotw
Thank you for your response
In case of "fast opening and closing of a valve ...", a valve here refers to the check valve after the caustic tower and not a control valve.
Appreciating for clarify the subject, the performance controller does not interfer the antisurge controller.

RE: Problem in crack gas compressor start up

Any update about the following map - if possible:
Outlet pressure and temperature vs. flow for the design case?

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

Will take a closer look at these in a few days time. Am travelling overseas at the moment on a personal matter.

RE: Problem in crack gas compressor start up

Back from overseas trip. The dropbox link is not accessible to me, but maybe we can still manage without them? My email address is apachen01@hotmail.com if you would want me to see them anyway.
Your earlier response says each furnace has a capacity of 30t/hr each for 3 furnaces, but your company website says 1267 e3 t/annum, which converts to 50t/hr/furnace based on 350days run time per year. So total compression capacity for CGC would be approx 150t/hr. ASV 1 and 2 appear to have sufficient recycle capacity even when there is zero fresh feed from the 3 furnaces.
However, you also said earlier that ASV 2 is not activated by low pressure on stage 5. A PIC on stage 5S should feed its output signal to the antisurge algorithm to open the ASV 2 through an auto signal selector. This will avoid having the ASV 2 going too near the surge line and waiting for the antisurge algorithm to act.

RE: Problem in crack gas compressor start up

Mohammad Ka

I did some work on the material, but due to time limitation I could not do more at this time.
Have modeled your 1 stage only to do some tests.

- You appear to have a double flow machine as first stage (3+3 impellers) while your map is for the total flow. Is this correct?
- I have some doubts on the gas composition for normal operation. I doubt the heavy HC's would lead to a MW=18.6 g/mol. Please check?

Anyhow, I have assumed a gas composition so to arrive to the same MW=18.6 g/mol as for Normal Case (this is not accurate but just as first attempt). I do not get close to the discharge temperature you've provided, either because of assumed compounds or this provided temperature data may need to be reverified.

After I model Normal case, I run a different case, for which I have made a sensitivity and used gas composition such as to obtain MW=20 g/mol (same as your failed start up gas composition).

Preliminary Findings:

If I keep the operating speed identical as Normal and inlet conditions identical as Normal, except gas set to MW=20g/mol; then:
The discharge pressure goes slightly up but the point moves near surge (I got it on a control line set at 12%).
Because stage #1 is not equipped with individual anti-surge valve, it is needed to do the exercise for each stage 1 to 4; but I believe the overall map is more stringent, as this will have shorter margin to surge than the case where each stage would have its own anti-surge valve.

So, I suspect that a gas of MW = 20 g/mol instead of 18.6 g/mol will cause surge.
To get a confirmation, it is needed to model accurately all gas compounds and then run simulation for stages 1 thru4 and also stage 5 separately.


Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

As continuation of my previous post....

Some notes:

- Something looks strange in the trends. The temperature indication downstream of stage #5 does seem to be erratic and departing from the value we would expect based on the compression ratio. I suspect a possible erroneous indication from the temperature transmitters. For example, on 2nd unsuccessful start-up, the temperature cannot be 85 deg C. that is too low; it could be that the temperature at inlet of the stage#5 is not stable which may have cause such behavior but this hypothesis was previously discarded if I am correct. Note that I assumed for all unsuccessful start ups, a gas of relatively constant gas molecular weight. So please check this: temperature transmitters and acquisition downstream stage 5.

- It does not seem possible to ramp-up this compressor train to nominal speed with a gas MW=20 g/mol. This would put stages into surge, anti-surge valves would open and even then - the maximum casing allowable working pressure would be exceeded on stage #5 and possibly excess of power requirement.

So, the plausible start up scenario to me is; start up with heavier gas (MW=22) should be okay until train has reached at minimum speed, then stabilize, then start to build new gas in the loop, and this new gas would need to have lower MW so that it permits to ramp up to nominal speed (MW of that gas should be around 18~19).

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
georgeverghese,
Just three furnaces are used for start up. During the normal operation the number of furnaces in service shall be increased to meet the design capacity.
As far as I remembered, I noted that " The ASV controller uses suction temperature and pressure, discharge flow, temperature and pressure to adjust the valve opening." at 13 Sep 19 05:53. However, It could be misunderstood or misspelled that ASV 2 is not activated by low pressure on stage 5. Already there is a PIC as you mentioned on 5S with the same function.

RE: Problem in crack gas compressor start up

(OP)
rotw,
Thank you for your effort to simulate the phenomenon.
1- Yes, the first stage is a double flow machine.
2- I checked the MW and it is valid based on the vendor, design and laboratory data.
3- I have the same problem with discharge temperature in simulation of the CGC and it could be solved by adjusting the temperature data somehow.
4- Generally speaking, the heavier gas, the lower force is required for compression. It does mean that the CGC would be operated in far away from the surge line with higher MW. If we supposed that the higher MW gas moves the operating point to the surge line, 100% opening of ASV 1 & 2 would solve this problem. Please consider that MW from 18.6 g/mol (in normal operation) to 22 g/mol (in start up) is recommended by vendor.
5- The TTs, PTs and FTs have been checked and all OK.
6- Normally from the start up to the normal operation the gas MW is decreased but I do not understand that how the lower MW could permit to ramp up the compressor while the higher MW NOT ?

RE: Problem in crack gas compressor start up

2- I checked the MW and it is valid based on the vendor, design and laboratory data.
The total MW is certainly valid, but as example on stage#1, normal gas: 49.439% C2H4 + 32.51% C2H6 cannot lead to a MW=18.6 g/mol.
Therefore, I think the gas composition must be re-checked.

3- I have the same problem with discharge temperature in simulation of the CGC and it could be solved by adjusting the temperature data somehow.
Please clarify what you mean by adjusting temperature data?

5- The TTs, PTs and FTs have been checked and all OK.
Noted, yet I cannot resolve the conflict between compression ratio and resulting discharge temperature in the trends.

4- Generally speaking, the heavier gas, the lower force is required for compression. It does mean that the CGC would be operated in far away from the surge line with higher MW. If we supposed that the higher MW gas moves the operating point to the surge line, 100% opening of ASV 1 & 2 would solve this problem. Please consider that MW from 18.6 g/mol (in normal operation) to 22 g/mol (in start up) is recommended by vendor.
6- Normally from the start up to the normal operation the gas MW is decreased but I do not understand that how the lower MW could permit to ramp up the compressor while the higher MW NOT ?

Yes, what you say is correct, if you start up away from surge it is not problem but when you say 100% opening of surge line will solve the problem, I think generally speaking, this depends on the sizing of the anti-surge valves (CV's). In your case, I suspect CV is sufficient if MW=22g/mol was already taken into account for start-up sequence by OEM. If you start along surge control line, then you cannot ramp up.

Yet, when you reach the nominal speed line with heavier gas, you cannot build up pressure by closing the anti-surge valves with a high gas MW because at some point you will exceed design limits, so you would need to have enter into the process loop a lighter gas and build a lower MW (18.6 g/mol) to further reach the nominal operating point. Do you agree?

In the trends you have provided, I cannot read clearly the pressure, temperatures. The start up process is dynamic in itself but I suppose there was enough time for variable to stabilize so the readings should be static-like when I look at the trend and the duration. Yet, there is conflicts between measured temperature and expected values based on compression ratio which I could not comprehend.

Let me know your thoughts.

Added:

- You have indicated the check valves have been checked, this includes downstream Stage#5. This points remains very unclear (check valve chattering).
Maybe downstream stage#5 is a sort of 'feedback loop', so that the flow must feed forward immediately with downstream system quickly relieved from hot gas to ensure temperature is stabilized. From trend, it looks temperature increases exponentially. Anti-surge valve might not be sufficiently fast to relief downstream system (for instance like a hot gas by pass valve would do) versus fast train speed ramp-up. Not sure then how gas temperature would effect check valve proper functionning, if at all.

RE: Problem in crack gas compressor start up

Will assume the ASV on stage 5 IS activated by low pressure on 5S without waiting for the antisurge algorithm to act.

In most of these trends you have sent, compressor speed is approx 4600rpm = 85% of max normal speed, and stage 5S pressure is approx 17barg in most cases. Pls check with your instrumentation engineer if the ASV2 has sufficient recycle capacity at this operating condition. We see from the compressor design case startup performance trends that the operating point at 13000m3/hr for stage 5 is quite close to the surge point for this case at 11000m3/hr.

RE: Problem in crack gas compressor start up

(OP)
rotw,
Thanks for your notes.

1- The mentioned gas composition is based on weight percent so in mole percent is 32.6% C2H4 + 20% C2H6
MWavg= 0.326*28.05 + 0.2 * 30.07 = 15.16
It sounds the data is correct.

2- For simulation of the compressor I used the ploytropic head and efficiency from the performance curve by reading the graph. I believe that reading the data from a graph may produce some error. So I adjusted these data in a narrow range to meet the discharge condition of the CGC. However, there is still deviation between simulation discharge temperature stage 5 and design value!

3- Can you please explain more about this conflict?

4- Certainly as long as the compressor is in surge control line, it could not be ramped up and this problem was seen in this case. However, I checked the CVs of both ASVs for different MWs. Increasing MW from 18 to 22 g/mol would grow the passing flow rate in ASV 1 and 2 by about 10 %. It means that the CVs are sufficient for this range of MW. Its should be noted a gas with lower MW has higher amount of light components and it tends to occupy more space in the valve rather than heavier component which causes to reduce the passing flow rate. So the higher MW is better than the lower MW for start up.
As long as you do not exceed design pressure limit, where this is valid in our case, a higher MW gas could build up higher pressure in comparison to a lower MW.
5- I mentioned that the check valve in upstream of 5S (Stage 5 suction) was chattering (between tower and the CGC stage 5) and NOT in downstream.

RE: Problem in crack gas compressor start up

Mohammad Ka,

1- Noted. Thanks for your explanation.
2- Noted. Which type of simulation are you using? do you have a vendor simulator for your cracked gas train or third party one or else?
3- If I look at 2nd unsuccessful start-up in your trends, I use same data at inlet flange of stage#5
Pin = 10.22 bara (I assumed 1 bar pressure drop between stage 4 and stage 5)
Tin = 323.15 K
Massflow = 89805 kg/h

I consider Speed = 4650 rpm and MW= 19.9
Pout (predicted)= 18.7 bar-a (from trends = 18.9 bar-g)

Outlet temperature (predicted) = 373 degK (from trends = 358 degK) -> that is 15 degK !!
This is what I meant by conflict. By the way, the point is very much positionned on control line.

Anyway, I modeled all 5 stages now.
If you like me to check a particular scenario, I can do (if this is useful for you). You could also use the dropbox and I could give you access to dropbox if needed.

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

One question:

Is the maps you have provided 'as predicted' or 'as tested' ?

That might solve my 'conflict'...

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
rotw,
- The maps are predicted. Can you please tell me how this solve the conflict?
- I am using a third party simulator , AspenHysys.
For sure, I do like to access the dropbox.

RE: Problem in crack gas compressor start up


- Predicted map reflect vendor prediction model of performance. When machine is installed in the field, the performance deviate from the predicted model. The magnitude of the deviation depends case by case. Up to ~5 to 10% deviation have been reported. Thus it is recommended to base the analysis upon 'as tested' maps. But I understand, this information is not available.

- You could give me your email and I will add you as member in the dropbox. But this would only be useful to you, if you require me to analyse a particular scenario. I could then share for example the results in dropbox if this is more suitable.

- By the way, if there is any chance to receive the trends in a more readable state, it would be also better.

Life is not about waiting for the storm to pass. It's about learning dance in the rain.

RE: Problem in crack gas compressor start up

(OP)
georgeverghese,
Talking with instrumentation engineers of the plant, it is revealed ASV on stage 5 could be activated by discharge pressure or FIC or MV by DCS operator or anti-surge algorithm internal controller as soon as reaching the min. governor speed. On the other hand, the activating factors on ASV stage 4 are suction pressure / FIC / MV by DCS operator / internal controller. Each PV is compared with relevant SP and the maximum difference would be selected to activate the ASV after min. speed.
So, ASV on stage 5 is activated by high pressure on 5D and NOT low pressure on 5S while high pressure on 1S my activate ASV on stage 4 without waiting for the algorithm.
I suppose 17barg is stage 5D pressure and not 5S. If we assume high pressure activation on 5D (based on these data)and the characteristic of ASV is Cv=463gpm, the valve will show about 84 t/hr ASV2 recycle capacity at 17barg on 5D and about 10barg on 5S. The result is same when discharge and suction of stage 5 are 15.37barg and 9.71barg, respectively (This is the successful run). Do you believe this recycle capacity is sufficient?
In all failed starts, the discharge flow of stage 5 is less than 139t/hr while in the normal one is higher than 150t/hr. Is this suspicious or not?

RE: Problem in crack gas compressor start up

In theory, ASV2 should be operated by low set PIC on 5S also.
So what is the min governor speed which allows these controllers to take over? We see most of these trends indicate speed at about 4600rpm during early part of startup when surging occured.
Yes, you are right, 17barg is the 5D pressure I was referring to, not 5S.
The high set discharge PIC which operates ASV2 would be set at >35barg or so, so it cannot be the one which operated ASV2 in all these startups.
The full Cv of ASV2 is stated to be >600, so we should use say 90% of the wide open Cv of ASV2 for the rating check of ASV2 for these failed startups.
In any case, it is obvious that good operating procedure would be to
a)Ensure that neither ASV1 or ASV2 should be operated say more than 90% open during startup / pressure ramp up.
b)If ASV2 is approaching 90% open, then more fresh feed from the furnaces must be enabled. Running the compressor with ASV2 at 100% open during pressure ramp up is bad operation. c)Speed also should be sufficiently high so that compressor stage 5D discharge pressure can be developed.

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