EFFECT OF LOW MAIN STEAM TEMPERATURE

[powering2002]

One of Our Steam Power Plant Unit was in trouble. Originally the main steam temperature was set 540 C and the main steam press was set 170 kg/cm2. But, there was a problem. We reduced the steam temp setting to 515 C. After we reduced the setting of main Steam Temp, the turbine bearing vibration increased. Our Queston are:
1. Was Our action correct?
2. Is there any correlation between the main steam temperature and bearing vibration?

 

[unclesyd]

Can you comeback with a little more information?

What was the type of vibration before and after the incident?
Did you experience any pressure drop or flow in the main header?
When the boiler recovered did the vibration change?

Did you notice any other physical parameter changes?

 

[hacksaw]

Not a simple issue, but with the generator controls engaged, dropping the steam temp reduces the energy content of the steam, so the controls will tend to increase the throttle valve opening to hold frequency etc. , so you are going to see increased mass flow through the turbine for the same kw delivered.

Is it enough to cause a problem? It depends on the turbine curves.

 

[rmw]

Not much description of the type of unit was given, but to hacksaw's point, once you make that temperature change, you not only change the mass flow, but that changes the pressure drops through each stage and changes all the extraction pressures and temperatures assuming of course that it has extraction stages for feedwater heating.

All in all, I am with UncleSyd in that not much real information was presented that would have allowed for any real type of analysis.

What type of trouble was the unit in that caused the decision to be made to reduce the inlet temperature? Which bearing (or is it all) is vibrating?

rmw

 

[ione]

powering2002,

Just making a hypothesis with the few information passed on to us.

Initially you were dealing with superheated steam at 170 kg/cm2 (gauge I presume) at temperature of 540 °C. With these conditions the density of steam is 50.38 kg/m3.
Reducing temperature at 515 °C, keeping pressure constant at 170 kg/cm2 (gauge) density becomes 53 kg/m3 (a 5% increase). If the mass flow rate is unchanged, the steam velocity will increase, and this possibly could lead to bothers.

 

[hacksaw]

If the turnine design is set up for 540C the discharge steam is just above condensation (design constraint)

lowering the temperature increases the likelihood of wet steam in the exit and thus high bearing vib., so lowering the SH is not a good idea.

need to engage the TG supplier on the issue in advance

 

[ione]

The reduction of steam temperature described by the OP represents a partial desuperheating: at 170 kg/cm2 gauge pressure and 515 °C temperature, steam is still superheated (saturation temperature is 351.2 °C). The detrimental effects of desuperheating are a reduced turbine efficiency and an increased steam consumption.

 

[rmw]

The OP doesn't seem to want to give us any additional information, but to make some basic assumptions, for power plant turbines, which typically exhaust into a vacuum condenser, with any reasonable efficiency and at reasonably cool cooling water temperatures (2-3 in. hg. BP), it would be exhausting about 14-16% moisture. This turbine would have to have a VERY inefficient expansion line to exhaust without any moisture at all, doubtful.

To ione's point, if anything, the inefficiencies created by the lowering of the SH would shift the tail end of the expansion line to the right somewhat and that would result in less moisture, not more as the end point climbs up along the 2-3"hg line.

It does as ione has noted reduct the overall delta h of the expansion line, hence requiring more steam flow for the same amount of power output.

What it does do is change the volumetric flow rate through all stages, and it is possible that that could have caused some deleterious effects. But without knowing which bearing....... who knows.

rmw

 

[dcasto]

Without the extra superheat, there could be condensing in the turbine that it wasn't designed for.

 

[hacksaw]

we had to concern ourselves with wet discharge constraints in the case of back pressure TG. Blade erosion, etc. We kept inlet SH at the max allowed by the piping code.

 

[rmw]

dcasto,

Not according to the expansion line drawn on a Mollier diagram. The moisture lessens, not increases with the efficiency loss as the slope of the expansion line decreases.

Hacksaw, that would be something that would work with a BP turbine as long as the exhaust pressure was above the saturation line. But BP turbines are not very common in steam power plants.

rmw

 

[dcasto]

rmw,

I ran a high efficiency turbine with very low outlet pressure and I hit two phase. Until we get exact conditions of outlet and turbine eff, we don't know for sure.

 

[unclesyd]

I'll bet if you were standing by the turbine when this happened your pucker factor was very high.

 

[rmw]

Most of the turbines I was ever involved with involved wet steam exhaust conditions. Until the OP gives us some actual data, we are just conjecturing about where it is occurring along the expansion line and/or turbine steam path.

rmw