Pressure effects upstream of a control valve

[MattNCSU03]

I am working on 1200MWe turbine-generator (1 HP, 3 LP) set and evaluating the effects of going from full-arc to partial-arc steam admission. The unit has four control and stop valves governing steam admission to the HP. Looking at the drawing, I noticed that between SV1 and CV1 and SV4 and CV4 is are pressure transmitters that feed different control loops.

In full arc mode there should be the same steam flow and pressure drop through each valve so the PT would sense the same pressure but in full arc assuming CV1-3 are full open and CV4 is modulating, would the PT just upstream of CV4 see a different pressure than the PT upstream of CV1 because the changing steam flow through the CV or would both PTs still have the same pressure as the source?
[tt]
HP---CV1-PT1-SV1------\_____
HP---CV2-----SV2------/
HP---CV3-----SV3------\_____
HP---CV4-PT4-SV4------/
[/tt]

 

[byrdj]

Hello Matt NCSU03 from Joe NCSU78
When you say FA, do you mean CVWO and MSVs throttling through SVBP for low load operation OR
do you mean all 4 CVs throttling the same (1 addmission) then going to CV1,2,3 opening together then CV4 (2 addmission, PA)
Isn't there an equalizing header between the CVs and the SVs. thus all MSVs feed the CV chest header, So that if SV1 is tested closed, the flow to CV1 will come through the other 3SVs. So You might think that both PTs would see the CV chest pressure, but I would bet the PT near CV4 would be seing some variance and possible pulsations. The control loop might need a filter (steam line resonance compensation)

1200MW? is that a nuclear N2 or a fossil S2 unit

 

[MattNCSU03]

Its a 4-loop nuclear unit. Looking at the main steam P&ID and GE's schematic, there is no crossconnect downstream of the stop valve. I haven't been to the site to lay my own eyes on the valves yet. I would have to talk to the client to see if they currently have the CVs wide open and are throttling with the stop valves and exactly how many admissions they plan on having. From discussions it sounds like 2 admission but I never asked specifically (this is my first project dealing with turbine controls). I thought typically turbine controls modulated with the stop valves at low power and then transitioned to controlling with the control valves at high power

 

[byrdj]

The GE MSV CV I have seen are the "angled bodied valve chest" design (1960s up. The four MSVs have a seperated short lead that attaches to the upper side of the CV chest. The area above the CV seats are common, then the 4 CV leads go to the quadrants of the HP nozzle.

the last new GE designs I've seen were 1990's so if they now have individual SV and CV leads it it news to me and I would like to know for sure.

My terminology might be old too. The pahrase FA to me means the low power operation where the CVs are wide open and the SVs are throttling. Then the sequencing of the CVs is then referred to by its "admission". When I left GE, they had introduced a MII system that varied the CV admissions from 1 addimssion for SU (all 4 CVs opening in time) than at any load the addsmission could be changed to sequential CV opening for efficiency.

Back to the PT question, If they are used for pressure control for a BWR, I would be worried that the very slight differances would cause a problem for the control loop (if each PT is the input to a loop) The PT arrangements I've seen for BWR use a tap on each MSV lead, then connect them to a common pipe header to average (pressure averaging manifold) and then connect the two PTs for the two pressure control channels to it.

I only did a couple PWR GE controls and I recall the main steam pressure was used for a Throttle pressure compensator network, that would slightly decrease the CV demand signal at low load since the PWR pressure started slighty high and reduced to rated when load came up. That circuit had a lot of electronic damping in it. I personally didn't see its need either.

Is this a new unit?

 

[MattNCSU03]

I don't remember the year built without looking it up but it began commercial ops in the late 80s. I do know they have GE Mark II turbine controls. When I get in to work I'll call the client and see if there is any kind of common CV equalizing header

 

[MattNCSU03]

There are common drain lines to all four control valves but they are 1/2" and 1." I'm not sure if that would be large enough to make up the flow difference if one valve was throttled.

 

[byrdj]

That is the steam lead drain system and will not have any substantial flow. Also, when going from 1 addmission back to sequential, an restriction orfice should be installed in the CV4 drain. the drains for the other valves will be supply just a little warming steam to the CV #4 lead, which was not needed when 1 admssion. The common for all 4 CV drains going to condensor is closed above 15% or so load

I've search the net with no luck looking for a drawing of a modern GE CV / SV valve chest. I am sure a GE 1980's vintage N2 will have the area above all 4 CV seats common. If not they would get a load swing when they performed the daily SV test (nuclear frequency of the daily SV test can be much greater)

If Possible Vol I of the "GE Black Books" will have a section for steam valves and should have a GEK discribing. Sorry I can't give you a specific GEK#

I also looked over my list of GE manufactured and was surprised at the number of ~1200MW units out there (but my list only had a few with inservice dates since it is a few decades old.

As part of your project, are you aware of the history. Units in servcie prior to 1980 were sequential, but 1st stage bucket design limitations required them to go to 1 admission. Going back to squential should address the cyclic loading forces.

 

[byrdj]

Oh...
You stated it was a four loop, thus I assume that means a PWR. If so, I recall MI PWR used the throttle pressure as input for a throtle pressure compensation circuit that decreased EL when throtle was greater than rated. It only was a single channel, thus only one PT. There was an option to have chest pressure meter on the operators pannel but I'm sure it came from that same PT. I don't recall ever working on a MII PWR, but I can't think of a reason to have two throttle pressure channels in the controls.

Also, I didn't see the need of the TPC circuit, It looked unstable back then, but a never heard any thing after commissioning. If you find out what they are used for, please let me know.

Could you give the 170X # ?

 

[MattNCSU03]

You are correct in that it is a PWR, the turbine number is 170X615.

The PT between CV4 and SV4 is labeled EHCS and the one between CV1 and SV1 is labeled QMBC if those acronyms ring a bell.

 

[byrdj]

sorry those acronyms don't ring any thing. EHC-6 is the PT when it used just for indication. The QMBC sounds like it might go some where besides EHC.

I've looked and still unable to find any thing on the net that shows the common chest configuration.

I've never been to that station (and very little for that utility) so I can't recommend a contact person

 

[byrdj]

If you have any part with the recamming of the MII EHc CVs positioning loops, I would recomend new digital cards (from my company)

 

[byrdj]

Are the CVs in a chest configuration or in individual leads?