Steam turbine: typical outlet velocity

1. Mar 14, 2013

Sunfire

Hello,

would someone know a real-life example value of the steam velocity at the turbine outlet? Also, an example mass flow (kg/s) at outlet? The typical pressure at outlet?

If you know these for both high-pressure (HP) and low-pressure (LP) stage turbines, that would be great

I have tried to find these myself, some people quote 160 m/s outlet velocity at 20 kg/s mass flux and 160 kPa exit pressure; I am unsure if these are realistic though

Thanks

Last edited: Mar 14, 2013
2. Mar 14, 2013

Simon Bridge

There won't be a "typical" outflow - steam engines come in such variety.
http://books.google.co.nz/books?id=...am velocity at outlet to steam engine&f=false
http://archive.org/stream/steamturbineprin00crof/steamturbineprin00crof_djvu.txt

You can look up specs for specific steam engines, i.e.
http://www.cussons.co.uk/education/products/steam_engineering/steam_power_plant_and_steam_benches/p7676r_steam_engine_steam_bench.asp [Broken]

Usually they give you the pressure at the outlet rather than the speed.

Last edited by a moderator: May 6, 2017
3. Mar 14, 2013

Astronuc

Staff Emeritus
The numbers will of course depend on the size/capacity of the turbine.

See this for the largest (Installed capacity, gross (MWe) 1750) turbine.
http://www.alstom.com/Global/Power/Resources/Documents/Brochures/flamanville-3-epr-turbine-island-construction-editorial.pdf [Broken]

One can take the volumetric flow rate and multiply by the appropriate density and assume continuity, or consider the blade tip speed as a ballpark figure.

Last edited by a moderator: May 6, 2017
4. Mar 15, 2013

Sunfire

The blade tip speed approach would be perfect The document lists the rpm (that is, the rad/s),... then there is one mention of the diameter (8m), unsure if this is at exhaust; there is the exhaust area, (155m^2), quite a big number to determine exit radius from here (that would be 7m radius)

Exit velocity is a good indicator of what can be done with the exhaust gas... High-pressure, low velocity would mean putting additional nozzle to accelerate the gas when it goes into the next stage

5. Mar 20, 2013

Sunfire

Some "engineering individuals" are giving me grief for assuming the outlet gas speed being equal to ωR (blade tip speed at exit radius R); they claim no turbine manufacturer would allow such high-energy gas to be disposed off. They also claim thermal energy (that is, internal energy) at exit is appreciable, while kinetic energy is small.

While this makes perfect "engineering" sense, one can wonder how it is achieved with the exit turbine blades whipping around with velocity of ωR.

Could someone shed light on this issue? Is it through the blade angle that a high-velocity parcel is decelerated isothermally... And, how is this parcel decelerated to a low exit velocity, considering that it has just been in a contact with a blade, moving with speed ωR

6. Mar 20, 2013

SteamKing

Staff Emeritus
7. Mar 20, 2013

Sunfire

SteamKing, thank you for the link. indeed, this article quotes quite high exit speeds " When the steam finally leaves the low-pressure turbine, its velocity is about 600 to 1,000 feet per second."

This is what seems to be happening: In the first stage, very high pressure gas expands through a nozzle and imparts kinetic energy onto a row of blades (an impulse stage);
Then, it is directed towards the next turbine stages (reaction stages).
The concept is: the gas keeps expanding, thus gaining speed, while its pressure keeps dropping. This way it can keep imparting energy onto blade rows, which rotate with higher velocity where the diameter of the turbine is higher. Because of these pressure differences, there are seals in the turbine stages to keep the gas from leaking in the wrong direction.
The outer turbine shell acts as a diffuser, thus raising the pressure of the gas and slowing it down as it proceeds towards the exit.
But all in all, the outer shell and the processes of expansion on the inside are carefully balanced to extract max work from the gas.

At the last stage, the gas leaves with the highest velocity, but its pressure has dropped below atmospheric. Then it is slowed down, its speed drops, its pressure rises just above one atmosphere and it then leaves with negligible velocity. This ensures high efficiency of the turbine.
This makes sense

8. May 18, 2016

Gudiya kumari

for supercritical Steam turbine exit velocity for HP turbine is nearly 150 m/sec: For IP turbine 200 m/sec and for LP Turbine exit velocity is nearly 300-350 m/sec, these are actual figures