Turbines

Force is rate of change of momentum. Momentum is mass multiplied by velocity, and water is a couple of orders of magnitude higher than steam in terms of mass per unit volume.

Force is one of the two factors comprising bending moment in the turbine blades, therefore a similarly-dimensioned turbine needs to turn slower with water in order to survive the fluid passing it.

Further, an economic maximum pipe velocity for steam might be in the order of 10m/s, whereas with water it is in the order of 3m/s; if the fluid is entering more slowly in the case of substituting a liquid for a gas, then the turbine will turn more slowly as a result.

In summary, a steam turbine and a water turbine are two different animals and must be designed and selected accordingly.

That means the steam velocity would become less for producing lesser speed. Then would the mass flow rate of steam become more?? How the steam velocity effects the performance?

Speed and flow rate of steam are directly proportional, i.e. an increase of one is an increase of the other.

The reduced velocity of the turbine will yield less volume, speed and certainly pressure as the turbine is not "hermetic" like a compresor; you will have important amounts of your precius steam flowing back.

Why do you ask this?.

Let's hope he is a curious student, and not employed in a power plant.

The past questions would indicate so.

Dear Mr Dhyanandhan,

Suppose in a 2-pole machine, to get 250MW load we need 760 t/hr steam, 147kg/sq. cm of particular pipe diameter. Then if we replace 2-pole machine by 4-pole machine turbine will run at 1500 rpm.

What are the problems of making the turbine run at reduced speed??

One of them is getting sufficient material through it to turn it at the lower speed without stalling or breaking.

• Has the turbine manufacturer's manual been consulted to determine the acceptable operating envelope?
• Has the manufacturer responded to a request for guidance on this proposal and if so, what was the nature of that guidance?

The same as making a Ferrari race on unpaved roads, it is simply not built for that purpose, likewise a Jeep will do very poorly in Le Mans for the same reason. Turbine design is similar, each is built for a specific purpose. Comparing a slow-speed, 12/16 pole hydro-generator with a 2/4 pole steam-turbine generator is the same as comparing apples to oranges, they are both fruits but the comparison ends there.

Take a look at the rotors, one is wide and short, the other is thin and long, then think about the differences in the forces on the materials of construction if they were both subjected to the same rotational velocity. Then compare the velocity at the outer edge of the rotor, and in both cases you will see that the thin and long one is better suited to the higher speeds and subsequent rotational forces than the wide and short one. There are many, many other considerations that can be found by Googlng on the different types. Good luck in your studies.

Dear Mr. ksathpathi,

If you decide to replace 2-Pole Machine, by 4-Pole Machine, you have to change the Gear Box Ratio. For Example, If Turbine Speed is 11,000 RPM, for 2 Pole-Alternator, the Alternator will have to run at 3000 RPM, hence Gear Box Ratio is 11,000/3000 = 3.6666.

If you want 4 Pole Alternator, the Speed of Alternator will be and should be 1500 RPM, hence the Gear Box Ratio will be and should be 11000/1500 = 7.3333. Hence Ratio Increase will be 7.3333/3.6666 = 2.00003, which means DOUBLE THE RATIO when compared to 3000 RPM Alternator.

Hence the Gear Ratio has to be doubled. The Speed of Turbine cannot be lowered as we think, since it involves Several Factors. If Lower Speed is desired for the Turbine, the size will be bigger.

DHAYANANDHAN.S

As this thread seems to be a desktop study rather than have practical applications, that would be a viable solution. Good point.

poles resist rotation lower speed = more stand/still-friction to overcome

http://www.energybytesla.com/?hop=wealthybiz (not shure the steam has anything to do with it)

I seem to remember from my training 20+ years ago, that the steam turbine maximum efficiency was when it operated close to ideal blade speed which is closer to, if not faster than 3600 rpm. Since the speed also controls the resultant frequency, the turbine should be operated at 3600 (60 Hz) or 3000(50 Hz) depending on the grid frequency. I think that frequency control is better at higher speeds as well due to the inertia.