Partial arc versus full arc machine 2

[ninhn]

I have different opinion about the full arc and partial arc admission steam turbine. I would appreciate if someone can tell me the main advantages and disadvantages of full arc versus partial arc.

Thanks,
NN

 

[byrdj]

Full arc provides uniform thermal for the nozzle and impulse stress for the buckets, but at a loss of efficency below full load.

Partial arc allows maximun efficienct for a nozzle segment to improve efficency below full load, but at an increase in bucket impulse and unequal thermal.

At Full Load, there is little differance. PA provides a minor control benifiet in more linear incremantal regulation from MCR up to CVWO.

There are several ways to change between admision mode, and may have FA limited to low loads for startup only. These usually have a transfer from MSV to CV Where some of the newer my switch admission modes at any load and only use CV.

Whats your different opinion?

 

[byrdj]

Was this sufficient, or was there a less or more specific question?

 

[ninhn]

Thanks for response.
For supercritical coal fired steam cycle, will the partial arc or full arc admission is better off? Some people said that since there are no steam drum so full arc make more sense.
Thanks,
NN

 

[byrdj]

Therefore you want the ability to go to full load with all CVs opening together, instead of the CV opening seqential (one admission CV timing). If the boiler requires the CVs to control flow (pressure control), then all CVs will be modulating (throttling losses and controls stabilty).
Another advantage of FA is that the turbine first stage temperture differance from low load to full load is less than in PA (miltiple admission CV timing). (low load FA is hotter) Therefore, with the same ramp rate, the loading rate can be faster.

With the electronic controls, it is possible to have the ability to run with both one admission and multiple admission timing (Admissision Mode Selection). For example with 4 CVs, all 4 open together for startup and loading ramps, then a transfer to PA is used to fully open CV 1, 2 and 3 (no throttling losses for effeciancy) and only CV4 throttling for flow controll.

With the unit details, the expected efficancy losses (and $) for operating FA can be calculated.

 

[byrdj]

I found an old correction curve for a 4 CV unit that shows an expectected 3 1/2% change in heat rate for 50% flow from 3 admissin to one admission (with heat rate same at CVWO)

These curves and thermo predictions should be in the material discussed in thread605-47965 (Power Generating facility forum)

 

[marada93]

Most of the units (both super and sub-critical) employ full AND partial arc admission.

Full arc admission was utilized on startup when you need even heat distribution around the circumference of the nozzle block. This allows for uniform warming of valves, turbine and associated piping. This is usually specified by the manufacturer.

Once the turbine is under load and at or near rated inlet steam conditions, it is then desirable to select partial arc to avoid unneccessary throttle pressure losses across all turbine control valves. In this mode the throttle loss is over one or two control valves only. This mode improves your heat rate over a wide range of load.

 

[abeltio]

full arc admission is a requirement for reaction type 1st stages.
partial arc admission can only be used with impulse type 1st stages.
the reason is that in case of reaction type, partial arc would cause recirculation in the sections that are not facing the admission arc.


saludos.
a.