14 replies to this topic

[viba0124]

There are four heaters in CCRU, viz Charge Heater (F-001),  Inter stage Heater-1 (F-002), Inter stage Heater-2 (F-003) & Inter stage Heater-3 (F-004). There are two fire boxes, one for F-001 & F-002 and one for F-003 & F-004. Both fire boxes have two parallel convention banks and a common stack. 

Flue Gas / Stack Temperature at the Outlet of F-001/F-002 Convection Banks was 267 DegC against the design of 160 DegC.

Flue Gas / Stack Temperature at the Outlet of  F-003/F-004 Convection Banks was 255 DegC against the design of 160 DegC.

HP Steam generation from Convection Banks are ~34 MTPH against the design of 92 MTPH.

Maldistribution of BFW is also observed in Convection banks of each Fire boxes. Thoroughness of coils were checked and found to be okay.

Heat absorption duty of Radiation zone  for F-001/F-002/F-003/F-004 (MMKcal) is lower @ 21.71/32.69/22.27/16.93 against the design of 24.54/46.01/31.79/19.44.

What could be the reason for high stack temperature and the remedial solutions.

Sketch attached for configuration of different coils in the heaters.

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[Pilesar]

A thorough heat and material balance for the system will identify where the inefficiencies are located. The flue gas duty and the coil duty should match for each section. Determine the effective heat transfer coefficients from the data to compare to design values.

[viba0124]

A thorough heat and material balance for the complete system was carried out. Thermodynamically, the system doesn't show any inefficiency. What looks to me, is that something has been missed out in detailed engineering. Like when we isolate the BFW coils in one of the two banks, flow goes through the other bank and stack temp reduces to the range of ~ 230 degC but not to the design values. isolated bank side stack temp rises to ~260-265 degC. It was cross checked with common temperature indicator in BFW outlet line which was equal to the steam drum temperature indicating no flow in the coil.

[PingPong]

It is already difficult enough to do throubleshooting while being in the plant, let alone at home behind a PC with minimal information.

I suggest you ask yourself:

 

When was this unit started up for the first time?

 

Was there then a test run to verify whether the guaranteed performance was met? Was it met?

 

If the unit operation was OK after start-up, when did the problems that you describe occur?

Did the performance of the convection banks deteriorate gradually, or suddenly?

 

What was changed in the operation of the unit just before the problems started? Different feedstock? Different fuel? Different excess air? Different reactor inlet (radiant coil outlet) temperatures? Different ..................................?

 

What changes were made to the unit itself just before the problems started?

[viba0124]

That's true. However, heaters have been  with us for ages. Due to technological upgradation, we rely mostly on softwares to do the detailing and hence we land up in such issues. That's why I want to know the possible reasons for the deviation.

 

Regarding test run, yes during the first test run also, stack temp were near the current value. What I could find out that during that time also, heater radiation section absorption duty was lower than design as process temperature (COT) requirement was less than design (528 against design of 549 degC). Now the required process temperature is 510-515 degC. However, it was more than current value (~ 10-12 % higher). Due to this steam generation was also higher than current (~ 50-52 MTPH). However, we could not achieve the design steam generation flow or stack temperature.

 

Hence, question lies, if enough surface area is available for heat pickup by convection coils, why the flue gas temp from radiation section is not getting cooled? I expected that stack temperature should have been much lower than design and close to acid dew point but fact is different.

 

Moreover, in regular Utility boilers, BFW coils and steam generation (or evaporator) coils don't join and enter the steam drum. They are independent lines joining directly at the steam drum. However, I have checked with Reformer heaters of different Licensors, they are of same design except for few licensors. There, they are not facing any problem.

 

Is there any maldistribution in flue gas path or BFW flow or any other reason?

[PingPong]

What was the original heavy naphtha feed flowrate when the reformer unit was built?
What is heavy naphtha feed flowrate nowadays?

You mention "technological upgradation". Exactly what changes were made to the furnaces?
Modifications to radiant coils?
Modifications to convection coils?
Modifications to burners?
Any other modifications?

You mention "acid gas dewpoint". Does this mean that the heaters run on burning fuel oil?
If so, was that already so long time ago when the reformer unit was built, or has there been a switch from fuel gas to fuel oil since then? Burning fuel oil creates soot that will deposit on the fins and studs of the convection coils. Sootblowers are to be used to regularly to remove soot, provided the sootblowers were actually installed and connected.

Are TV0018 and TV0032 closed during present operation? If not that would increase stack temperature. Check the temperature setpoints of the controllers of these valves.

[viba0124]

Thanks for the Reply...

The naphtha feedstock has not changed over the years. Only RIT was being maintained in lower range for production of MS blend component reformate. However, it was designed for providing feedstock for downstream Paraxylene (PX) unit wherein RIT to be maintained is higher. As PX plant is yet to be commissioned, CCR is running in Refinery mode of operation rather than Petrochemical mode.

Regarding technological upgradation, I meant process simulation softwares (steady state & dynamic), PV Elite, other engineering softwares, where we rely mostly on software output rather than cross checking with manual calculation of formulae.

Acid dew point is mentioned as Refinery Fuel Gas (~ 20-100 ppmv H2S) is used as fuel in heaters. However, soot formation/sulphur deposition has not been observed on the coils.

TV0018 & TV0032 are in full open condition to maintain design BFW temperature of 120 degC.

[PingPong]

All four heaters are operating at much too high stack temperatures. That could mean that there is a common cause.

 

Hence my original question whether the heaters were modified to burn fuel oil instead of fuel gas, but you say that is not the case.

 

Then we need to look at another common cause: the BFW and its treating, the steam drum water and its treating, and the steam drum blowdown flowrate.

 

It could be that the steam generation as well as BFW preheat coils suffer from scale build-up (deposits of CaCO3, MgCO3 and other salts) inside the coils which, over time, severely reduces heat transfer.

Caused by insufficient water treating and/or insufficient blowdown, in combination with lousy distribution of BFW over parallel heaters.

 

On the flow scheme the condition in steam drum V-007 is indicated as 257 oC at 42.5 kg/cm2.g.

42.5 kg/cm2.g is equal to 42.7 bara which would correspond with a temperature of 254 oC, not 257 oC.

(257 oC would correspond to a pressure of 44.7 bara or 44.5 kg/cm2.g, not 42.5 kg/cm2.g).

I would consider the pressure measurement more reliable than the temperature measurement.

 

Lets look at F-003: according to your flow scheme the preheated BFW has an actual temperature of 319 oC. If that TI is correct than it would mean that the BFW preheat coil outlet is superheated steam instead of liquid water, resulting in scaling inside the coil.

 

Look also at F-001: according to your flow scheme the preheated BFW has an actual temperature of 258 oC, which is higher than the saturated temperature of 254 at actual steam drum pressure. If that TI is correct than it would mean that the BFW preheat coil outlet is steam/water or steam instead of only liquid water.

 

At other periods in the past years it may have been that F-002 and F-004 had these problems due to lousy BFW distribution between the heaters.

The BFW distribution can be adjusted by globe valves in the lines to each heater. I will call them GV1 through GV4, from left to right.

GV1 should be fully open and GV2 partly closed such that F-001 and F-002 have roughly the same stack temperatures and roughly the same BFW outlet temperatures.

GV3 should be fully open and GV4 partly closed such that F-003 and F-004 have roughly the same stack temperatures and roughly the same BFW outlet temperatures.

In general: the GV should be fully open for the heater with the highest stack and BFW temperature and partly closed for the other heater.

 

By closing TV0018 and TV0032 (by reducing their temperature setpoints) the stack temperatures should drop somewhat. In view of the high stack temperatures there is presently no need to increase BFW supply temperature.

 

The design convection duty of F-001 / 002 is roughly 57 % steam generation, 19 % BFW preheat and 24 % steam superheat.

In Heaters F-003 and F-004 steam generation is roughly 76 % and BFW preheat is roughly 24 %. No steam superheat.

Too high stack temperature is likely caused by insufficient performance of the steam generation coils as these represent by far the biggest duty. That could mean that they might suffer from scaling. Flue gas maldistribution is not likely to occur in four different heaters as that could not result from a common cause.

Less steam generation automatically results in less BFW drawn in by the steam drum, and less steam to be superheated, so less flue gas heat absorbed in their coils as well.

 

There is noting that you can do about scale until the next plant shut down when it can be removed. You may need to hire a specialized company for that.

Until then you can only prevent it from getting worse by having a close look at BFW and drum water chemical treatment, drum blowdown flowrate, analysis of BFW, drum water and steam from drum.

[viba0124]

 Thanks for the reply.

 

All four heaters are operating at much too high stack temperatures. That could mean that there is a common cause.

Reply: Could be possible.

 

Hence my original question whether the heaters were modified to burn fuel oil instead of fuel gas, but you say that is not the case.

 

Then we need to look at another common cause: the BFW and its treating, the steam drum water and its treating, and the steam drum blowdown flowrate.

Reply: There is common BFW generated at Captive Power Plant (CPP) and distributed to all process units including NHT-CCR unit. So, BFW quality seems not the reason.

 

It could be that the steam generation as well as BFW preheat coils suffer from scale build-up (deposits of CaCO3, MgCO3 and other salts) inside the coils which, over time, severely reduces heat transfer.

Reply: We had opened the manifold at entry of BFW coils and pressurized water was forced back to the coils through the hydrotest nozzle in outlet. Water was coming out in pressure. However, any scaling inside coil can't be ruled out. We will get in cleaned in next turnaround.

 

Caused by insufficient water treating and/or insufficient blowdown, in combination with lousy distribution of BFW over parallel heaters.

 

On the flow scheme the condition in steam drum V-007 is indicated as 257 oC at 42.5 kg/cm2.g.

42.5 kg/cm2.g is equal to 42.7 bara which would correspond with a temperature of 254 oC, not 257 oC.

(257 oC would correspond to a pressure of 44.7 bara or 44.5 kg/cm2.g, not 42.5 kg/cm2.g).

I would consider the pressure measurement more reliable than the temperature measurement.

Reply: Agreed. We will get the temperature and pressure indicator checked.

 

Lets look at F-003: according to your flow scheme the preheated BFW has an actual temperature of 319 oC. If that TI is correct than it would mean that the BFW preheat coil outlet is superheated steam instead of liquid water, resulting in scaling inside the coil.

Reply: We have checked through strap-on ultrasonic flowmeter in the inlet side, the bank wherein temp is 319 degC, flow was NIL.

 

Look also at F-001: according to your flow scheme the preheated BFW has an actual temperature of 258 oC, which is higher than the saturated temperature of 254 at actual steam drum pressure. If that TI is correct than it would mean that the BFW preheat coil outlet is steam/water or steam instead of only liquid water.

Reply: We have checked through strap-on ultrasonic flowmeter in the inlet side, the bank wherein temp is 258 degC, flow was NIL. There can be some error in temperature indication, but it was close to steam drum temperature, so didn't doubt it. 

 

At other periods in the past years it may have been that F-002 and F-004 had these problems due to lousy BFW distribution between the heaters.

The BFW distribution can be adjusted by globe valves in the lines to each heater. I will call them GV1 through GV4, from left to right.

GV1 should be fully open and GV2 partly closed such that F-001 and F-002 have roughly the same stack temperatures and roughly the same BFW outlet temperatures.

GV3 should be fully open and GV4 partly closed such that F-003 and F-004 have roughly the same stack temperatures and roughly the same BFW outlet temperatures.

In general: the GV should be fully open for the heater with the highest stack and BFW temperature and partly closed for the other heater.

Reply: We had gone for the same solution in 2022 S/D and provided Globe valves in each bank inlet due to time constraint as providing GV in outlet meant we had to provide a PSV for safety. But even after providing the GVs, until the GV is completely closed in the bank wherein stack temp is on lower side, the temperature on other bank would not come down. So to avoid dry run of coils, we generally keep the GVs on both the banks fully open.

 

By closing TV0018 and TV0032 (by reducing their temperature setpoints) the stack temperatures should drop somewhat. In view of the high stack temperatures there is presently no need to increase BFW supply temperature.

Reply: Noted. BFW inlet temperature to Economizer was tried to be kept 14 degC higher than corresponding acid dew point temperature to avoid such acid condensation in cold zone. However, We will look into this aspect.

 

The design convection duty of F-001 / 002 is roughly 57 % steam generation, 19 % BFW preheat and 24 % steam superheat.

In Heaters F-003 and F-004 steam generation is roughly 76 % and BFW preheat is roughly 24 %. No steam superheat.

Too high stack temperature is likely caused by insufficient performance of the steam generation coils as these represent by far the biggest duty. That could mean that they might suffer from scaling. Flue gas maldistribution is not likely to occur in four different heaters as that could not result from a common cause.

Reply: The circulation flow rate in steam generations is ~ 84-85 % of design flow rate. margin still exits in pump to circulate more. But we didn't get any benefit on increasing the flow rate from earlier ~60 % to current 84 %, so we have held it at 84 %. However, we will try to come close to design flow rates and see if it increases steam generation. Further, we will again check for scaling removal from SG coils.

Earlier it was observed that even at lower circulation rate in steam generation coils, with increased fired duty, steam generation was high.

 

Regarding Flue gas maldistribution, it also looks to be a reason, you have also mentioned that major of the convection duty is picked up by SG coils. So, if overall current fired duty is lower than/near to turndown rate, can't flue gas maldistribution be a reason. I'm attaching the GADs for reference.

 

 

Less steam generation automatically results in less BFW drawn in by the steam drum, and less steam to be superheated, so less flue gas heat absorbed in their coils as well.

 

There is noting that you can do about scale until the next plant shut down when it can be removed. You may need to hire a specialized company for that.

Until then you can only prevent it from getting worse by having a close look at BFW and drum water chemical treatment, drum blowdown flowrate, analysis of BFW, drum water and steam from drum.

 

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[PingPong]

You need not worry about the circulation rate through the SG coils. The heat transfer coefficient on the tubeside is very high for steam generation, provided there is no scale. Water circulation rate has little impact on that, provided it is not extremely low (not less than six times the steam production rate, corresponding to maximum 20 % vapor at coil outlet).

The overall heat transfer coefficient U of each coil is mainly determined by flue gas side, hence the use of fins or studs on the outside of the tubes, except probably the lowest tube rows (shock tubes) in the convection bank.

 

The resolution of the drawings is too low to properly see whether flue gas bypassing is possible around some of the sections. It does however look like it is possible in the lower section of F-001/F-002 and the upper section of F-003/F-004. Other sections are too unclear to see the distance between refractory and tubes

 

Usually the refractory is shaped in such a way that flue gas bypass is not possible, like in this typical sketch:

 

 

You and your team should verify whether that is the case with all convection sections in all your heaters.

 

 

It is not always clear to me what you wrote here before, but if the high stack temperatures already occurred immediately after the very first start-up of the heaters when they had just been built then there may be mistakes in the design calculations of the convection coils. In that case however your company should have held the heater designer and/or contractor that built the CCR responsible and force them to rectify the problem. This is not something that you can now work around by adjusting operating parameters.

 

If the stack temperatures were OK when the unit was new, and the high stack temperatures occurred later, then you should figure out what was changed just before the stack temperatures started rising as that is then likely the cause.

Edited by PingPong, 17 December 2024 - 06:53 AM.

[viba0124]

Thanks for the reply.

I agree that the company should have been held responsible. Due to a design change requested by owner to Licensor, it was clarified by Licensor that the heater duty requirement is lower from design duty. Hence, no change is envisaged in the heaters. However, during detailed engineering, any change required due to design change should have been taken care by company responsible for detailed engineering.  Now, history can't be rectified. Hence, I'm looking for any design deficiency or operational deficiency that can be rectified in upcoming turnaround.

 

One thing that I observe is that the bridgewall duct arrangement in heaters are not similar. Can that be a reason?

Secondly, is the two pass flow configuration in BFW coils the reason as it leads to uneven no of tubes in each pass?

 

I'm attaching the GADs in legible format.

Attached Files

•  F-03-04-merged.pdf   1.55MB   32 downloads
•  F-01-02-merged.pdf   1.91MB   16 downloads

[PingPong]

From the GAD's it is now clear that flue gas bypassing is not the cause of the problem, although at a few locations the shaping of the refractory could have been better.

 

Note that there is space for additional BFW preheat tube rows at the top of each convection section:  "2 NOS. ROWS PROVISION OF FUTURE TUBES".

That would slightly lower the stack temperatures and slightly increase steam production.

 

 

One thing that I observe is that the bridgewall duct arrangement in heaters are not similar. Can that be a reason?

You mean that the cone towards the convection section is not symmetrical? Don't worry.

 

Secondly, is the two pass flow configuration in BFW coils the reason as it leads to uneven no of tubes in each pass?

No, look again, BFW pass #1 and pass #2 have the same number of tubes.

 

I estimate from the data above that in the design case with a stack temperature of 160 oC the efficiency of each heater is about 93 % (excl. heat leak), so the fired duty is about 114 MMkcal/h for F-001 plus F-002 and about 84 MMkcal/h for for F-003 plus F-004.

 

In the actual operation (data from your flow scheme) I estimate that the average efficiency of the heaters is about 89 % and the fired duty is only roughly two-thirds of design.

 

It seems to me that the main reason for the much-lower-than-design steam production is the much-lower-than-design fired duty, resulting moreover in a higher-than-design radiant efficiency, leaving much less heat available for the convection section.

That still does not explain why the actual stack temperatures are higher-than-design instead of somewhat lower, what one would expect.

The deviation in stack temperature is however only equivalent to a few percent efficiency. Even if you would somehow manage to improve the performance of the convection section that still would not result in a steam production close to design simply because of the much lower fired duty.

 

I still think that the stack temperature problem is inside the tubes. If not scale due to miss operation during initial start-up, than possibly due to insufficient cleaning or pickling of the tubes during pre-commissioning before the initial start-up.

[viba0124]

Thanks for the reply.

 

From the GAD's it is now clear that flue gas bypassing is not the cause of the problem, although at a few locations the shaping of the refractory could have been better.

 

Note that there is space for additional BFW preheat tube rows at the top of each convection section:  "2 NOS. ROWS PROVISION OF FUTURE TUBES".

That would slightly lower the stack temperatures and slightly increase steam production.

 

Reply: Noted. However as BFW is still low and non-symmetric in both banks, putting flow through additional tubes may not be of much help.

 

 

viba0124, on 19 Dec 2024 - 3:13 PM, said:

One thing that I observe is that the bridgewall duct arrangement in heaters are not similar. Can that be a reason?

You mean that the cone towards the convection section is not symmetrical? Don't worry.

 

Reply: Noted. However in CFD study, it was observed to have some cold spot due to this non-symmetry. Is the CFD report correctly depicting?

 

viba0124, on 19 Dec 2024 - 3:13 PM, said:

Secondly, is the two pass flow configuration in BFW coils the reason as it leads to uneven no of tubes in each pass?

No, look again, BFW pass #1 and pass #2 have the same number of tubes.

 

Reply: Kindly check the snapshot of tube & pass flow configuration for each bank in attachment. Does this configuration have any effect on stack temperature?

 

I estimate from the data above that in the design case with a stack temperature of 160 oC the efficiency of each heater is about 93 % (excl. heat leak), so the fired duty is about 114 MMkcal/h for F-001 plus F-002 and about 84 MMkcal/h for for F-003 plus F-004.

 

In the actual operation (data from your flow scheme) I estimate that the average efficiency of the heaters is about 89 % and the fired duty is only roughly two-thirds of design.

 

It seems to me that the main reason for the much-lower-than-design steam production is the much-lower-than-design fired duty, resulting moreover in a higher-than-design radiant efficiency, leaving much less heat available for the convection section.

That still does not explain why the actual stack temperatures are higher-than-design instead of somewhat lower, what one would expect.

The deviation in stack temperature is however only equivalent to a few percent efficiency. Even if you would somehow manage to improve the performance of the convection section that still would not result in a steam production close to design simply because of the much lower fired duty.

 

Reply: Noted. We are also trying to go for design mode of operation of plant to see the effect of increased fired duty of heater on stack temperature.

 

I still think that the stack temperature problem is inside the tubes. If not scale due to miss operation during initial start-up, than possibly due to insufficient cleaning or pickling of the tubes during pre-commissioning before the initial start-up.

 

Reply: Noted. We will try to do proper cleaning during next turnaround in FY-26-27.

 

Further, as BFW flow is leaner compared to significant high SG coils flow and BFW coils is joining with SG coils, can the pressure profile of both streams play a role in the maldistribution of BFW flow in coils?

[PingPong]

Reply: Noted. However as BFW is still low and non-symmetric in both banks, putting flow through additional tubes may not be of much help.

 

Both BFW passes (in each convection bank) have the same number of tubes, so what do you mean by "non-symmetric" ?

 

Note also that BFW flow follows amount of steam steam generated. Less steam generated automatically means that the steam drum level control will draw-in less BFW.

 

One thing that I observe is that the bridgewall duct arrangement in heaters are not similar. Can that be a reason?

You mean that the cone towards the convection section is not symmetrical? Don't worry.

 

Reply: Noted. However in CFD study, it was observed to have some cold spot due to this non-symmetry. Is the CFD report correctly depicting?

 

Where exactly are those cold spot(s) according to the CFD study ?

 

This is highly turbulent flue gas flow that is moreover repeatedly split and mixed again by multiple staggered finned tube rows, so I doubt that the non-symmetrical shaped cone from radiant to convection would cause a problem. Each convection section acts as a gigantic static mixer.

 

 

Secondly, is the two pass flow configuration in BFW coils the reason as it leads to uneven no of tubes in each pass?

No, look again, BFW pass #1 and pass #2 have the same number of tubes.

 

Reply: Kindly check the snapshot of tube & pass flow configuration for each bank in attachment. Does this configuration have any effect on stack temperature?

BFW 2-pass flow configuration.jpg

 

Once again: pass #1 and #2 have the same number of tubes, therefore the BFW flow in each pass will be 50 %, unless one pass is partly plugged.

 

 

Further, as BFW flow is leaner compared to significant high SG coils flow and BFW coils is joining with SG coils, can the pressure profile of both streams play a role in the maldistribution of BFW flow in coils?

You now mean maldistribution of BFW between the convection banks of F-001 and F-002, and between the convection banks of F-003 and F-004 ?

The globe valves are there to adjust BFW distribution such that BFW coil outlet temperatures are roughly the same. However you said before that that does not actually work in practice, so you can't really influence anything then.

 

To get an idea of impact of combination of BFW and SG outlet lines you could calculate the pressure drop over the BFW passes as function of BFW flow, and also pressure drop over the supply lines (including fully open GV) and pressure drop over return lines from BFW coil up to steam drum.

[viba0124]

 

Reply: Noted. However as BFW is still low and non-symmetric in both banks, putting flow through additional tubes may not be of much help.

 

Both BFW passes (in each convection bank) have the same number of tubes, so what do you mean by "non-symmetric" ?

 

New Reply: I meant that in both the passes, BFW passes through 5 tubes in 1st row and then passes through 4 tubes in 2nd row and thereafter. So, does this non-symmetry play any role in maldistribution?

 

Note also that BFW flow follows amount of steam steam generated. Less steam generated automatically means that the steam drum level control will draw-in less BFW.

 

One thing that I observe is that the bridgewall duct arrangement in heaters are not similar. Can that be a reason?

You mean that the cone towards the convection section is not symmetrical? Don't worry.

 

Reply: Noted. However in CFD study, it was observed to have some cold spot due to this non-symmetry. Is the CFD report correctly depicting?

 

Where exactly are those cold spot(s) according to the CFD study ?

 

This is highly turbulent flue gas flow that is moreover repeatedly split and mixed again by multiple staggered finned tube rows, so I doubt that the non-symmetrical shaped cone from radiant to convection would cause a problem. Each convection section acts as a gigantic static mixer.

 

New Reply: Noted. However, in  the CFD study conducted, low velocity contours & static temperatures were observed in F-003/004. Snapshot attached.

 

 

Secondly, is the two pass flow configuration in BFW coils the reason as it leads to uneven no of tubes in each pass?

No, look again, BFW pass #1 and pass #2 have the same number of tubes.

 

Reply: Kindly check the snapshot of tube & pass flow configuration for each bank in attachment. Does this configuration have any effect on stack temperature?

BFW 2-pass flow configuration.jpg

 

Once again: pass #1 and #2 have the same number of tubes, therefore the BFW flow in each pass will be 50 %, unless one pass is partly plugged.

 

New Reply: We also had doubt of one pass plugging. But recently, we had a leak in one of the BFW coils in one bank of F001/002, wherein as leakage increased, BFW flow through both the banks in F-001/002 also increased, nullifying plugging issue in BFW tubes.

 

 

Further, as BFW flow is leaner compared to significant high SG coils flow and BFW coils is joining with SG coils, can the pressure profile of both streams play a role in the maldistribution of BFW flow in coils?

You now mean maldistribution of BFW between the convection banks of F-001 and F-002, and between the convection banks of F-003 and F-004 ?

The globe valves are there to adjust BFW distribution such that BFW coil outlet temperatures are roughly the same. However you said before that that does not actually work in practice, so you can't really influence anything then.

 

New Reply: Maldistribution is in flow in each bank of both F001/002 and F-003/004. One of the bank gets flow while the other bank gets no/minimal flow. Thereby no flow bank shows high flue gas temperature. Same side bank in F-003/004 also faces the same problem. We have provided isolation valves in each bank inlet (earlier) and outlet side now to completely isolate the bank in case of future leakage.

 

To get an idea of impact of combination of BFW and SG outlet lines you could calculate the pressure drop over the BFW passes as function of BFW flow, and also pressure drop over the supply lines (including fully open GV) and pressure drop over return lines from BFW coil up to steam drum.

 

New Reply: Could you help out with calculation of pressure drop across the circuit. What is the tool that can help out?