Our combine cycle power plant set includes two gas turbines, two heat recycling boilers (HRB) and one steam turbine. The HP/LP drum of HRB is force recirculation by circulation pumps. During plant cold startup, the high pressure drum level is swelling so abruptly and finally tripped the HRB by over the highest drum level setpoint. We solved this problem by raising the drum re-circulation water flow rate step by step according to drum level during cold startup phase. We solved the problem but we don't know why it works. My question is, does anyone know strongly theory for this control strategy or can explain why this happened? And any comments to improve drum level swelling control would be appreciated! Thanks!

 

Please explain in detail:
1- Please send us the procedure of start up details before & after solving the problem. (The B.C. Pump rate)
2- How old is your boiler? Who supply/ design? Which country?
3- Does your drum include 'Perforated Plate'?
4- Does your Feed (inlet) water injection to drum injected via smooth distributed holes in pipe?
5- Did you mean 'bubbling' water in drum or exactly 'swelling'?
6- Have drum levels in both sides of drum shown same?
7- What is temperature difference between feed water inlet to drum and water inside drum?

Send the answers to [email protected],
Thanks
Saeid Masoudi,
Head Engineer
Mechanical Dept.
Monenco Iran Consultant Engineer

 

This takes me back to the days where I served as a boiler technician in the United States Navy.

All pressure vessels work on the pressure = volume X temperature (P=VT) law.

Term FIRING RATE The amount of heat applied the the surface of the pressure vessel via tubes, etc.

Term SWELL Sudden increases in boiler firing temperature cause the drum level to INITIALLY increase due to the sudden increase in nucleate boiling.

Term SHRINK Sudden decreases in boiler firing temperature cause the drum level to INITIALLY decrease due the the sudden decrease in nucleate boiling.

SHRINK and SWELL are compensated by a three element feedwater control system.

In a 3 element feedwater control system, the drum level transmitter, feed flow transmitter, and steam flow transmitter are used to calculate the amount of feedwater actually allowed.
Steam flow and feedwater flow are used to allow the system to predict drum level based on feedback from the drum level transmitter, thereby compensating for the SHRINK and SWELL factors caused by sudden changes in FIRING RATE.

A sudden change in FIRING RATE can be caused by sudden change of demand upon the boiler. The steam flow transmitter will pick up on the sudden change in demand. The feed flow transmitter noticed that feed flow has not been changed. Before the water level transmitter can detect that water level is CHANGING. The system has already compensated. The drum level transmitter is still the SUPERVISERY signal and can override the BIAS provided by the steam flow and feed flow transmitters to prevent a low or high water condition or trip.

I felt that I must explain the basics of the relationships in a natural circulation system to assist in the understanding of a forced circulation system.

In a natural circulation boiler, the system depends on the water in the lower drum (mud drum) being cooler than the water in the steam drum. Thereby natural circulation is achieved because hotter; less dense water that is heated in the tubes (firing section) will rise to the steam drum.

Forced circulation boiler are different. Forced circulation boilers were designed to allow for increased FIRING RATES that a natural circulation boiler could not compensate for.

Basically a natural circulation boiler would MELT DOWN the tubes because the water could not replace the steam being generated in the heating surface (tubes).

Boiler basics terminated!

The reason what you did worked is:

During cold startup, you are SUDDENLY increasing the firing rate of the boiler. The water level WILL SWELL!!!!

The changes of the circulation rate by the forced circulation rate changes have reduced the amount of nucleate boiling actually occuring in the boiler tubes, thereby eliminating the cause of SWELL in a FORCED CIRCULATION system. You have replaced hot boiling water with cool water.

In a natural circulation system, goodbye tubes.

Although the increased circulation rate has prevented the trip, the boiler is being STRESSED.
Anytime you heat and/or cool something rapidly, Stress will cause welded and/or rolled tube joint to fail. The coefficent of linear expansion laws published in most physics books are observed laws, not theory.

Recommendation:

Please try to keep the boiler water temperatures as high as possible during down time. Use boiler layup methods such as the Steam Blanket. The steam blanket uses a external steam source to provide pressure on the boiler while it is not being used. The key to the steam blanket is waste. Many drains on the superheater, etc must be "cracked open" to vent steam and keep the entire heating section HOT. You are wasting steam (BTUs) using this method, but saving the boiler heating surface. I cannot evaluate maintanence dollars, not my job.

The CT(Combustion Turbine) exposes the boiler to a 1000 degree exhaust temperature if it is allowed to start and immediately go to full load. The 600 degree exhaust temperature of the CT at full speed no load could be used to slowly warm the HRSG for several hours before startup. That is still too hot.

Exhaust gas diversion can control the temperatures that a introduced to the firing section of the boiler. Basically the exhaust is diverted to a seperate exhaust stack.

I have not witnessed a CT with a successful exhaust gas diversion method. Although they may exist! The exhaust gas diversions methods work good at startup but cannot seal off the exhaust gases when the CT is loaded and all of those Good exhaust BTUs go up the wrong stack.

I hope I have explained why your "FIX" worked. I also hope I have explained why your "FIX" will eventually destroy your boiler.

CTTech

 

Dear CTTech,

Your answer is interesting. Please let me know your email address, I wanna discuss this further.
My email address: tpc.joe @ msa. hinet. net. Thanks!

 

Please address all replies CONTROL in the subject matter or they might be deleted by my spam filter.

[email protected]

I hope I can help.

 

A couple of questions on your problem. Has the recirc control valve been operating correctly. Has the feedwater regulating valve been operating ok. What is the HRSG damper doing on a cold start, ie is it following its opening sequence correctly. Do you have a steam bypass v/v to condenser for HRSG start up. What is the design rate of rise of steam saturation temp for the drum and is this rate of rise controlled by the by-pass v/v or damper position or combination of both,or has this set point been altered. What GT MW loading are you using for start up of the HRSG and do you start-up with IGV'S in temp control or not. What are the drum blowdown v/v positions on start-up. The only reasons the drum can swell like this is because of boiler ciculation pump flow control, boiler feedwater control or high firing rate. Has the damper position calibration changed hence damper opening more than set point indicates.

Cheers
BAC

 

The following is the description of the case we are dealing with:

Basically, we have a divert damper between the gas turbine and HRSG. We can start up the plant successfully under cold condition by using the damper to adjust the exhaust gas into the HRSG section. But this way means so much energy produced by gas turbine will be lost due to the
damper's position, which is not fully opened. So we try to improve this control strategy. We want the damper fully open during the cold start up
phase so that no hot exhaust gas lost. And the result before we do any modification is the HRSG tripped by HP drum level over the higest setting even the drum blowdown valve is 100% opened.

What we have modified is:
1--> Lower the exhaust gas temperature by using the variable inlet guide vant (VIGV) of the gas turbine. We decrease the temperature of the exhaust gas from 340 deg.C to 310 deg.C, and this also decrease the gas turbine generator load from 18MW to 12MW.

2--> We maintained the temperature of the exhaust gas at 310 deg.C until the HP drum pressure is greater than 3.5bar.

3--> The HP drum level range from 0 to 800mm. It is going to be set between 140mm to 210mm during HRSG start up and We change the HP drum circulation pump water flow rate. The HP drum circulation water flow rate is fixed at 154 kg/s originally and we change it at 110 kg/s when drum level is under 300mm, then raise it at 125 kg/s when drum level exceeds 300mm and then 140 kg/s-->330mm, 155 kg/s-->360mm, 175 kg/s-->390mm, 195 kg/s-->420mm, 215 kg/s-->450mm, 220 kg/s-->480mm.

After these modifications we can reached the target. We can start up successfully under cold condition without any damper adjustment. The cold start up time was greatly reduced. But sometimes it still failed and we think it maybe caused by leakage of the feedwater control valve.

Regards,
Joeliu

 

The boiler is being stressed by not using the exhaust gas diversion.

As with everything, the exhaust gas diversion was placed/engineered there for a reason (protect the boiler).

A decision was made to NOT divert the exhaust gas to save the BTUs associated with performance.

The only recommendation I can recommend at this point is a cost analysis.

Will it cost more to replace the boiler than the lost performance BTUs during startup?

The FIX will not extend the life of you HRSG, if fact it will probably shorten it.

Cost/Lifetime analysis is the bottom line here. I suggest that a detailed analysis be performed.

I am of the old school. Do not stress it, maintain it, and it will last forever. Sometimes this is not the most profitable/planned method.

CTTech

 

If you have two towers with a specific level and you start circulating that level fluid between them, you will notice that the level goes down. This is due to incresing the pressure with in the circulating line. Also in the boiler's case,with increasibg the flow, more volume and colder water will be heated by the furnace heat in a specific time. This will heat up the water faster and reduce the flue gas temp during that time. Also the steam drum vent will act as an ejector to the steam drum level and show a false higher level than it is. You might want to try to throttle a bit on the vent during the start up of the boiler.

 

The purpose of economiser recircultion line is to ensure water availability in the economiser coils during boiler start up operation as the feeding of the boiler will be done intermittently in the initial stages of firing, till the stage of boiler swelling is crossed. To explain swelling it can be witnessed if one follows carefully the drum level behaviour that is abnormal and fast rise in the drum level which neccessitates opening of the EBD valve to reduce the drum level and avoid flooding in to super heaters

hope this'll be useful for yo.

courtesy:Scribd training material

 

...and you can trip the boiler due to low-low level! There are some important things to be taken into consideration: boiler starts to produce steam; swell effect is diminishing at a rise of pressure...If you respect the start-up level of the boiler, no need to open EBD!

 

In which sequences the tripping occurs and what are the HRSG Protection criteria?