Turbine bleed pressure

[Drexl]

Hi,

I was wondering how high of a pressure the turbine bleeds, extraction piping and pre-heater can experience in case of blocked passage after the pre-heater (assuming no safety valves on the pre-heater shell sides).
To formulate the question in another way, what would be a good design basis for selection of design pressure of the bleed/piping/preheater.
I mean the worst possible situation would be all blocked bleeds and open turbine valves in which case the pressure would leak through and pressurize the whole system with the turbine inlet pressure. This situation is however not reasonable - what would be a reasonable worst case?

Drex

 

[rmw]

Huh????

The only thing that is going to see turbine inlet pressure is the first stage nozzle block. Anything downstream of that sees a continuously declining pressure which follows your turbine expansion line on a Mollier diagram.

rmw

 

[Drexl]

If the inlet pressure is 20 bar and all passages out from the turbine (bleed/outlet) are blocked the pressure in the whole system will eventually reach 20 bar. There is only pressure drop over the nozzles if there is a flow. As you can see in my post I clearly explain that this is not a reasonable design scenario for the bleeds(in most cases).

Lets say during normal operation the pressure in the inlet/bleed/outlet is 20/4/0,1 bar and i block the bleed. This means the steam that escaped through the bleed will have to continue through the rest of the turbine. Intuitively I would assume the pressure in the bleed would go up, how much - I don't know. Say it would be 6 bar. So then the question is, would this pressure be a reasonable design pressure for the extraction piping and pre-heater?

I'm thinking about what scenarios I would have to consider in order to select a suitable design pressure/temperature for the extraction piping/preheaters.

 

[EdStainless]

If the turbine is running and you block the bleed the pressure in that line does not change. The pressure at that location in the turbine does not care about you bleed.
If the flow through the turbine is restricted the throttle will close or pressure relief will open long before you reach high pressures in the extraction lines.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[stgrme]

First of all, please note that typically, pressures given on a manufacturer's heat balance can vary by plus or minus 5%. This variation is due to manufacturing and assembly tolerances on the turbine's components.

If extraction are shut off the pressure in that extraction zone will rise. The pressure in the extraction zone is essentially linear with the flow to the following blade group.

Best of luck!

 

[dougarthur42]

I can agree with the last 2 posts(ed stainless, and stgrme)but i will add some more. First our unit has an inlet pressure of 1350 psi and 955 degF. We have an uncontrolled extraction (HP bleed)that runs at any where between 245-400 psi, depending on how much load is on the unit. At full load (110MW) we have about 392psi on the bleed and at 60Mw we have about 265psi. This steam goes to a FW heater to increase efficiency. Steam line and fw heater safety is set at 500psi, if we shut (isolate) the bleed steam then this pressure goes down not up. The internal pressure at the bleed chamber will increase slightly but not alot, however the MW will increase. On our next extraction (140psi)we control extraction pressure. if we need more extraction the valves opens increasing flow not pressure. However the internal pressure in the turbine extraction area runs 20 to 30 psi higher, but not the extraction piping. The internals of the turbine is designed to handle varying pressure. I would say go by the recommended documnetation.