HRSG back pressure

[Dave001]

In a combined cycle power plant , the gas turbines load are constant and the hrsg produces certain steam flow rate and pressure from drum, if the steam turbine valves closed or the bypass or anything that increases the pressure so a back pressure occurs on the hrsg and drum so does the steam flow rate rom drum decreases while the gas turbines load are constant or what will happens ?

 

[MFJewell]

It seems you are mixing up the steam side pressure and gas side pressure. The gas side should be constant (unless you have a stack damper that is throttled, which is NOT what a stack damper is for) for a given flow rate. The HRSG manufacturer provides datasheets with many cases that show what the dP for each section is. You talk about turbine valves closing, I assume you mean to the steam turbine. You would either have to dump steam or overheat and damage the HRSG really quick. If the steam side has a turbine bypass, the HRSG still produces steam, but it is dumped to the condenser, which has to be designed to take that load.

 

[Dave001]

Yes assume that the steam moving from boiler drum to the steam turbine pressure increases due to throttling the control valve so the back pressure on the drum increases , so what happens to the steam flow rate from the drum to the steam turbine ?

 

[MFJewell]

The control valve is used to control mass flow to the turbine based on loading of the generator.

 

[Dave001]

my question is about the flow rate of steam coming from the drum as I know that the turbine has a control system

 

[davefitz]

If the gas turibne remains at a fixed output and the steam turbien inlet valve were to throttle, then the hp steam drum pressure would increase. This increase is usually addressed by opening the HP bypass valve . If the steam cycle is a reheat cycle and the plant has a steam to reheater bypass system, then the steam will pass from the reheater into the IP turbine via the intercept valve.If the HP bypass discharges directly to the condenser, then the condenser pressure will increase and eventually trip the plant if the condenser backpressure exceeds trip limits.

If the pressure in the HP main steam line exceeds its design pressure , then the rules of B31.1 require the control system to automatically reduce heat input to the HRSG, whihc implies runback of the duct burners followed by runback of the gas turbine load. However, many designers of the DCS logic are not aware of this code requirement.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[georgeverghese]

If there is no where else the excess steam can be diverted to, it seems obvious that the turbine flue gas will have to be routed direct to exhaust through a bypass line. This turbine flue gas to exhaust control would most likely be set up as a high set steam drum pressure controller modulating a bypass low dp damper. This need not be set up as part of turbine controls.
If you need fast response on this bypass, typically the low dp control valve would have quick open style trim and also include a pneumatic volume booster ( downstream of the I/P signal convertor) at the control valve.

 

[Dhurjati Sen]

Dave001,

The steam from the HRSG goes to the HP steam header. If the steam turbine stops taking steam from the header, then HRSG load has to be reduced by controlling the supplementary firing in the superheater zone. If that is not done, then the header PSV might pop !!!

Regards.

DHURJATI SEN

 

[crshears]

Dave 001, your question seems very elementary...

Gas turbine exhaust [plus duct firing, if any] is producing ten units of steam per minute from HRSG; turbine uses ten units of steam per minute; flow is balanced, HRSG steam outlet pressure is steady.

Same scenario except turbine uses only nine units of steam per minute due to slightly closed-in steam valves reducing the flow; HRSG steam outlet pressure will rise at a rate inversely proportional to the amount of space available upstream of the control valves to hold the excess steam production of one unit per minute...

Now, how this gets handled is the question, and others have already provided some responses; here's another.

If the HRSG is of the once-through type [ OTSG ], "nothing" happens; provided the OTSGs are rated to withstand the maximum gas turbine tailpipe temperature, the feedwater supply can simply be reduced, steam production will fall, any outlet header pressure rise or condenser back-pressure issues will be resolved, and the HRSG exit temperature will rise. Note that all of this will take place at the sacrifice of HRSG / OTSG efficiency, but no equipment damage will occur; OTSG steam outlet temperature will tend to rise, so desuperheating will be required to hold the steam turbine admission temperature steady to preclude steam turbine differential expansion issues.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davefitz]

crshears,

for the OTSG, something will happen. As all fluid in the hrsg heats up toward the gas turbine exhaust temperature, the fluid specific volume increases and cannot flow backwards due to the feedwater inlet check valve, so the safety valves must lift or a bypass valve must open.Without such pressure release the tubes would burst.

Most OTSG's that are designed for "dry firing" are fabricated with inconel tubes , using a single circuit that uses tiny inlet orifices for providing flow stability . This desing has found a market niche for very small hrsg's, but larger hrsg's would have difficulty with this design.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[crshears]

Hi Dave, I agree that "it depends"; but I stand by my answer that if the conditions I specified are met, the answer I provided is valid.

[edit - added material]

The Greater Toronto Airport Authority operates a combined cycle power plant using two LM 6000 gas turbines @ 43 MW each; the associated once-through steam generators were manufactured by Innovative Steam Technologies of Cambridge, Ontario, Canada, and produce enough steam to drive a 31 MW steam turbine. Perhaps this installation is "niche," but one could hardly describe it as small-scale.

CR

 

[crshears]

Another place that uses an OTSG by IST is the Whitby Co-Gen station; steam is supplied to a 60 MW steam turbine, the exhaust from which supplies process steam to Atlantic Packaging. Again, possibly niche, but definitely not "very small".

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]