Steam Turbine Question

Hello Guest,

If a steam turbine is supplied with "Wet Steam", the water causes blade damage, due to corrosion and cavitation.

As the distance from the steam generator increases, the proportion of Wet steam to dry steam in that supply line increases, as the combined steam temperature falls off due to heat loss from the pipework.

Thus the Moisture Separator is always placed as close as possible to the steam turbine, to extract the maximum moisture, and minimise the damage to the steam turbine.

Please also remember that Dry Steam has greater latent energy, than Wet Steam.....

What has been said truly makes sence and I add my own experiences.

In early aircraft turbojet engines, with axial flow compressors, as in a steam turbines, it ocassionally happened that severe damage was done to the compressor blades which actually broke from hitting large volumes of water in severe thunderstorms which then carried back through the engine and reactionary turbines in the hot end of the engine. This was one of the reasons that some aircraft were classed as 'Fair Weather only' operations. Aircraft Turbine engines usually operate around 6,000 to 8,000 RPM which in no higher than many motor vehicle piston engines on the market today. Steam turbine reactionary engines (actually motors as they have an external power energy source) also operate at RPM ranges from 5,000 to 100,000 RPM and a single drop of water is much greater energy than the equivalent of diving from the top of the Sydney harbour bridge into the sea below, which would cause the blade to flex and permanently distort creating vibrations that would cause the whole system to shatter at worst, so delivery of 'DRY' steam via a filter just prior to the delivery to the turbine blades is obligatory.

A particular steam turbine had the filter removed and to just keep production going during a sugar season, the mill crusher was kept running without the filter on the one turbine. The result was horrendous damage due to bent and broken blades costing many 10s of thousands of dollars.

I have had turbo-chargers on race engines disintegrate from 'Slugs of Fuel' from carburetors impinging onto the leading edge of the centrifical turbine.

The moral is that any fluid that has the potential to create a drop 'Slug' must be prevented from entering into fan blades when they are rotating at operational rpms.

The separator is placed as close to the turbine element as possible to remove the maximum amount of water. There are a number of devices and methods to accomplish this. Current convention nuclear steam supplies deliver steam to the high pressure turbine element at 0.15 percent moisture inlet and exhaust from the high pressure element at 14 to 18 percent moisture. Now is the time for a moisture separator and reheater. The moisture separator portion uses chevron vanes and tight turns to force the moisture from the steam flow. The steam flow finishes its path in the separator by flowing through one or two reheater bundles to establish about 160 degrees F of superheat before entering the low pressure turbine elements. The blading of the turbine elements are designed to work with some evidence of moisture, design dependent on stage location. The turbine rotor speed for this type of fuel is 1500 to 1800 rpms constant due to the 44 to 58 inches long last row steam foils.

SCS

A steam turbine converts the steam energy (enthalpy) into mechanical rotational energy. The energy losses of steam causes:

a) Pressure drop. The lesser the pressure, the lower the saturation temperature

b) If you have saturated steam, the losses comes from latent heat of partial steam condensation.

Two phase flow (water and steam) causes the phenomenon known as "erosion-corrosion". So, you need to have the steam as dry as possible and install moisture separators because water drops not only can affect the turbine blades but also the pipe lines at the turbine exhaust (cross-under).

Carbon and low alloy steels are very sensible to this phenomenon and I've see some pipe bends 25mm nominal thickness reduced to 6-8 mm after some period.

For small pipes it is advisable to change to austenitic stainless steel (less susceptible to erosion-corrosion) and greater ones (say 24") use to be internally arc/flame sprayed with austenitic stainless steel.

Dear Sir,

For small pipes it is advisable to change to austenitic stainless steel (less susceptible to erosion-corrosion) and greater ones (say 24") use to be internally arc/flame sprayed with austenitic stainless steel.

Can you please provide refernce of any standard for your above statement?

Many Many thanks n regards,

Hello aaly beatles,

I don't know standards which deal with this matter. It's not a question of standards but facts.

Anyway, its a known fact that two-phased flow (steam-water) causes erosion-corrosion problems in carbon and low alloy steels. It's a common problem of nuclear power plants which produce saturated steam (fossil fired ones produce usually superheated steam).

The solution indicated , change small diameter piping to SS and spray layering greater diameters is just due to economical/practical reasons.

Kind regards

In our case we r using turbine to drive a compressor and we have inlet steam piping of Carbon Steel 6S1 Class 600 corrision allowance 1.5mm.

So is it advisable to change it to SS.

Thanx n regards.

What are the steam conditions at inlet (pressure/temperature)?

1.5 mm for corrosion allowance seems the designer has taken normal corrosion phenomena but not erosion-corrosion.

Kind regards

Steam pressure 42 bar, n temp 371C

At these conditions you have a superheated steam. At 42 bar, the boiling point is 254.7 C, therefore you have more than 116 C superheating and just one phase flow.

No need to make any change in the inlet piping. See http://www.spiraxsarco.com/resources/steam-tables/superheated-steam.asp if you want to check steam properties.

Kind regards

thank u very much

But wouldn't you loose pressure when passiing through the separator and if so will it effect the flowrate through thhe turbine?

Hello "Guest"

<"....But wouldn't you loose pressure when passiing through the separator and if so will it effect the flowrate through thhe turbine?....">

Of course.

Note: You appear to be having a problem with Spell-checking Facility.

Kind Regards....

Due to Super heated steam required for rotor safety.