Gland sealing steam

[Drexl]

Hi,
Could someone describe in reasonable detail why a high pressure steam turbine needs gland sealing steam supply?

Drex

 

[ScottyUK]

The gland becomes self-sealing once the internal steam pressure is sufficiently high. Initially there is no internal steam pressure and the external gland steam supply is required to prevent air ingress at the glands and thus allow condenser vacuum to be established prior to rolling the unit.

A typical control scheme may have a supply valve regulating steam from an external source during startup which is closed during normal running, and also a back pressure regulator letting down to the gland steam condenser controlling gland pressure during normal operation.

 

[byrdj]

Here is a typical control scheme that was in a recent post. only shows the header, not the turbine.

it shows the SSFV being the supply of main steam for startup. As load is applied to the turbine, the internal pressure at the HP seals will start providing steam to the header and the SSFV closes. at the load goes higher, the feed from the HP seals is more than required for the LP seals and the SPUV will dump the header to maintan its pressure

http://files.engineering.com/getfile.aspx?folder=602d5a54-1689-4f36-9da6-84b4cc7ef948&file=SSS.JPG

 

[Drexl]

ScottyUK, I don't follow. You mean at startup (without steam admission to turbine) while establishing condenser vacuum, the turbine is also emptied from air, e.g. you are pulling a vacuum in the turbine and air would leak into the condenser through the turbine. Instead you supply gland steam which leaks in and is condensible. Correct? If so yes I would understand it for the low pressure turbine.

But the high pressure turbine is not connected directly to the condenser. Are you saying it is the case that you draw a vacuum in the high pressure turbine also during start-up and thus the same argument holds as for the LP-turbine? Then once the pressure in the glands is above atmospheric the steam will leak towards atmosphere and there is no need to supply gland steam?

 

[byrdj]

Drexl,
what is your turbine configuration, ie
Does your turbine have reheat?
if yes, does your turbine have an HP and IP turbine or are they combined?
how many LP turbines?

If you know the OEM and turbine Code, they might identify

 

[ScottyUK]

Yes, you're correct that without steam flow the HP turbine casing is also under condenser vacuum, unless there are other factors involved. Any gland capable of leaking air needs sealing steam during startup. If you can post a copy of the P&I'D we can probably talk you through it. I might have a copy at home, currently visiting family so will be a few days until I have a look.

 

[byrdj]

I have an illustration of a couple GE code units, one with HP / LP and another with HP / IP / 2 LPs that shows the paths from condensor to the boiler, if you would like to use it to walk Drexl through how the condensor is connected to the HP turbine

 

[Drexl]

byrdj, question is general but in my current case the setup is HP Turbine - Reheater - LP Turbine - Condenser.

Your answer is: Sealing steam is used for the purpose of keeping air out of the turbine - nothing else.

I didn't think that it would be normal to pull vacuum through the reheater and HP-turbine at startup. If that is the typical case, it is in line with the answer above.

However this current HP turbine is also supplied with sealing steam during normal operation when the pressure is >8 bar and there no possibility of air leaking into the turbine - so there must be some other reason also.
It is not possible to supply P&ID or similar. I'm out of office and don't have access to any documentation.

 

[byrdj]

For every steam turbine I have actually seen or drawings off, pull vacuum all the way to the main control/governor valves (main stop valves too)
besides the rotor shafts that exit the turbine, there is sealing steam to the CVs since they do not backseat and if not provided to the SVs that do back seat when reset, there will be vacuum leakage till the unit is reaset and they are opened.

if you look at the illustrations, you will see that the vacuum is pulled on the condensor, but the LP steam path will allow the vacuum to pulled to the crossovers, then the full vacuum as seen at the condensor will be in the exhuast of the reheat turbine. the RH steam path will allow the vacuum to be pulled all the way to the hot heat piping. then the full vacuum as seen at the condensor will be in the reheater section of the boiler. thus the vacuum in the boiler reheat section will pull on the cold reheat piping all the way to the HP turbine exhaust. the steam path of the HP turbine will all vacuum to be pulled all the way to the SV/CVs.

One could make the aurgument that the CIVs will be closed when the unit is tripped and thus the vacuum in the reheat turbine would not be pulled on the reheater section of the boiler, BUT how about all the drains on the HP turbine like for the extraction piping, the CV/SV below seat drains, they are piped to the condensor so they will pull vacuum even with the CIVs tripped closed.

then, as the unit is being rolled to speed, the CIVs are opened, and the amount of steam needed to roll the unit is so small there will be minumn pressure in the HP section that it will be below atomsperic till there is suffficent load on the unit.

Not sure what you are saying...
"However this current HP turbine is also supplied with sealing steam during normal operation when the pressure is >8 bar and there no possibility of air leaking into the turbine - so there must be some other reason also."

 

[Drexl]

I'm saying there must be some second reason for supplying sealing steam other than to prevent air leakage. During operation the HP turbine pressure is > 8 bar so the steam must be leaking out from the turbine towards atmosphere = air can't leak in. I'm sure the HP turbine is not supplied with sealing steam by mistake during operation so it must have some function. I would like to understand what this function is.

Regarding your description about pulling vacuum all the way to the HP turbine inlet valves it got through. Thanks for this information!

 

[byrdj]

You are absolutely correct, There is no need to SUPPLY STEAM TO the HP turbine seals once there is pressure inside the HP casing.

Even more so, once the HP casing pressure is above about 1/3 to 1/2 rated, there is no way to supply steam since the pressure trying to escape from the ends will be greater than the sealing pressure. Once the turbine is loading, the steam that would be forced out of the ends of the HP turbine will now flow INTO the steam seal header and the startup steam seal feed valve needs to be closed. But instead of letting the steam blow out of the ends of the HP turbine, that steam can now be used to supply the steam to the LP turbine seals.

so for a design purpose discription, the HP seals are connected to the steam seal header that also provides steam to the LP turbine during startup and low load.
Once at load, the HP seals are connected to the steam seal header so that the waste steam that would blow from the ends can be used to seal the LP turbine and the original main steam used for sealing during startup is no longer needed and used to make MWs. So you are also correct in that there is another function

Given that the design has an uncontrollable supply of sealing steam from the HP turbine at load, the steam seal header MUST have a means to control the pressure to the LP seals. the steam seal controls will close the feed as the pressure increases and then open a DUMP valve to releive the excessive flow from the HP turbine seals. this dump can go directly to the condensor (and thus be wasted) or it can be routed to a low pressure heater.

the link in my first post shows a control scheme for the steam seal header

http://files.engineering.com/getfile.aspx?folder=602d5a54-1689-4f36-9da6-84b4cc7ef948&file=SSS.JPG

the startup feed valve - SSFV
the at load dump valve - SPUV

 

[ScottyUK]

Gland steam systems allow bi-directional flow. When casing pressure is lower than atmospheric, external steam is supplied via a letdown valve and de-superheater. When casing pressure is higher than atmospheric the gland sealing steam passes outward toward the gland steam condenser through a spillover valve. See the attached Westinghouse P&ID for an example of this type of control scheme. Note the flow direction arrows.

 

[ScottyUK]

Try again with the attachment

 

[byrdj]

Scotty, please clarify WH drawing practices for me.
Is Hex 062 indicating a connection from a steam source and
Hex 063 indication a connection to a steam sink

Are you saying WH uses a seperate condensor for the excessive seal steam when it dumps?

 

[Drexl]

To sum it up:
-We are supplying external sealing steam to HP turbine during operation when the pressure in the turbine is > 8bar
-According to byrdj the function of the sealing steam is to keep out air, so sealing steam is not required once the pressure in the turbine section is above atmospheric

-> Either the plant operation principle is wrong OR I have misunderstood the operating instructions OR byrdj is wrong. I don't know which at the moment.

 

[byrdj]

Assuming you are suppling steam to HP turbine when the internal pressure is >8 bar;

do you provide sealing steam to the HP turbine when it is <8 bar?

Is your sealing steam to the HP turbine seperate than the sealing steam supplied to the LP turbine? Or is the same sealing steam header used for both

what is your sealing steam pressures? how are these pressures controlled?

does your steam seal design use a Gland header (seperate condensor with few inches of water vacuum that draws air and escaping sealing steam?

can you give us the brand name, possible model discription?

 

[ScottyUK]

Hi byrdj,

Machine is an early 1990s vintage and probably considerably older in design.

Yes, there is a separate gland steam condenser. It's a fairly small tube & shell exchanger fed from cooling water tapped of the supply to the main L/O cooler bank. Hex 062 is a source upstream of the MSV's on the HP steam header. Hex 063 goes to the gland steam condenser. The two valves operate with a split range control scheme so at mid-span both valves are closed. In practice either the supply valve is delivering steam to the gland or the backpressure valve is allowing flow to the G/S condenser. It's a pig of a system to set up accurately unless the positioners are in A-1 condition.

 

[byrdj]

thanks for the education of that "other brand"

are you positive hex 063 goes to "gland condensor" and not main condesor or to the last heater?

Given you went to all the effort to post a WH P&ID, I cleaned up a GE P&ID. (removed unit specific and heat balance data) 1970s early GE EHC.

I wanted to post one of my favorite design, a MHC, but the detail of the representation of the feed/dump controller was reduced to a box

Being an old mechanical/hydraulic type of guy, I though it was going too complex to apply pnematics like the one I'm posting, but I see the WH has signals from DEH! Pretty easy to get the overlaps of the feed/dump when it is just one MHC actuator controlling both. and It is still amazing those MHC could be absolute isobaric where the pnematic has to use a pressure range.

This P&ID is the same control scheme as shown in my first illustration

 

[byrdj]

Scotty
is the grammer of my narritive discribing the transition from HP feed to HP bleed comprehensiable?

 

[Drexl]

Assuming you are suppling steam to HP turbine when the internal pressure is >8 bar;

do you provide sealing steam to the HP turbine when it is <8 bar? YES (both during start-up and operation)

Is your sealing steam to the HP turbine seperate than the sealing steam supplied to the LP turbine? Or is the same sealing steam header used for both Separate. There are two systems one LP and one HP sealing steam systems/headers

what is your sealing steam pressures? how are these pressures controlled? HP-header about 8,5 bar (HP sealing steam control valve take live steam and lower the pressure to HP-turbine outlet pressure + 0,5 bar) and LP-header a bit over atmospheric (controlled by a external supply valve during start-up. When pressure in the turbine is high enough for it to supply itself with gland steam, the external supply valve closes

does your steam seal design use a Gland header (seperate condensor with few inches of water vacuum that draws air and escaping sealing steam? Yes

can you give us the brand name, possible model discription? Don't have documentation available at the moment