Steam Turbine Condenser Vacuum

The "highest" theoretical vacuum would be 29.92 inches Hg (± a tad depending on barometric pressure). I would guess that a practical limitation would correspond to about 60°F saturated water vapor pressure, maybe somewhat cooler if sailing a steam engine in Antarctic waters.

Sir, thank you for your reply. I understand that indeed absolute zero is the highest attainable/theoretical vacuum. However, this is not applicable to steam turbine since high vacuum means greater exit velocity and higher moisture content at the exhaust which is detrimental to last stage blades. I would like to solicit answers if there is any limit on the highest vacuum a unit can operate yet safe to operate. Since, most manufacturers only provide the lower limit. If none, may be, a typical expansion ration (Pinlet/Pexhaust) will do or a typical range of vacuum operation for condensing turbines.

Another thing, how about maximum velocity at exhaust Sir? is there any typical safe value that would not endanger the blades to erosion?

Thank you, your reply would be appreciated.

Dear lito there is no limit for high vaccuum. First of all let me tell u that the turbine exhaust areas are designed in such a way that the vaccuum does not exceed the normal value which is typically arpund 0.3 bar absolute. If in some case the pressure drops below the said value, the water in condenser which is normally at 50 - 60 oC will start boiling and the pressure inside the condenser will normalise again due to the formation of water vapours SB

Sir, appreciate your reply. i understand "boiling" will occur only when pressure go below liquid saturation. Also, in my experience 0.3bar absolute value is too "low" already, and i have encountered turbine operating as low as 0.05bar abs. I am more concerned on the erosion of last stage blades and the corresponding point of exit (in the wall casing). In turbine overhauls, i have observed that the erosion found on the wall casing is more significant than in the last stage blades. i suspect that the culprit is the excessive moisture at the turbine last stage blades. However, calculations based on operating parameters the exit moisture doest not yet go beyond allowable design. Another cause is the very high velocity of the moisture (within allowable) at this point of exit. All of these can be attributed to the condenser vacuum, so i'm thinking that certain limits on high vacuum might be established depending on the turbine inlet pressure, if none, may be then exit velocity or expansion/pressure ratio?

Your opinion is well appreciated. Thanks.

I've operated steam turbines with condenser cooling water temperatures of 27F. The really high quality vacuum caused no turbine damage and yielded a extremely good overall efficiency. But to get the the sea water temps down that low, the arctic ocean is a better choice than the southern ocean, IMHO. Good idea to keep the bilges pumped dry!

"I've operated steam turbines with condenser cooling water temperatures of 27F. The really high quality vacuum caused no turbine damage and yielded a extremely good overall efficiency."

--you're right high vacuum means high efficiency but maintenance will suffer. in your case, no damage observed may be because you have very high turbine inlet superheat temperature and exhaust moisture is within minimum. Have you witnessed the overhaul and observed the last stage blades and casing? or checked the actual expansion process in a mollier diagram?

I did observe the annual inspection of the blades (including last stage) and case that revealed no damage. We frequently used the Mollier diagram for training and review. One salient point is that the turbine was probably over-engineered and over-built with plenty of margin. It had over 70 forward stages. This is approaching the limits of my memory as it has been about 40 years ago. :-)

Thank you so much Sir. looking forward for your ideas on future discussions:)

Nobody else seems to mention your proposed problem. This makes me wonder why you asked the question in the first place, since you already seem to know (and insist on) the answer.

Sir, my knowledge in turbine o&m is only limited and asking on the operational limits of condenser vacuum, if there is or any good practice. if there is really none, i'm asking how about on the pressure ratio or may be exit velocity limits? with these data established, we can prevent undue erosion and extended maintenance works, thus optimizing plant efficiency in general.

I have search also on various plants, manufacturer's, books and the net, not one provided the answer. so, i join here thinking that may be experts and designers can help me on this, even with the prudent practice.

and i'm not insisting on my answers, i am only trying to expand the ideas presented here cause i think, it does not point to my questions.

Thank you.

In some quick looking around, I found a recommendation of 29"Hg vacuum (~0.03 bar absolute), another recommendation for as deep a vacuum as possible, and an example of 0.4"Hg absolute (~0.013 bar absolute); but so far only one other mention of a droplet problem in turbines.

I'm guessing that droplets don't tend to form until the steam exits the turbine and enters the condenser. From what I can tell thus far, the literature does not support the OP's concern.

Thank you Sir. i am referring here on low pressure saturated steam turbine and in one of my inspections, found water marks on the casing right after the last stage blades.

May be we need to further study on this.

Thank you.

That could be. Was any erosion evident on the final stage blades, and how significant were the water marks on the subsequent casing?

erosion in the last stage is evident though still minimal, erosion is more significant in tha casing that merits attention.

No matter how low the vaccuum in the condenser, as long as the Exiting stage steam is still hot above the saturation temperature at that pressure, droplets should not form. That is why there is no real need to mention a limit.

Your problem is probably that the cooling is reaching the last stage exit and droping the temperature enough to start condensing there! therefore, try and keep the temperature of the last stage steam exiting at high enough above the condensing point ... (even if you need to jacket heat that zone(?).

Since the Vaccuum in the condenser is mainly due to the steam condensing there, ...

Hi Lito,

Some of your worries are unfounded.

First you need to have the lowest vacuum possible for efficiency (assuming a fully condensing unit). It is limited by barometric pressure and your cooling water temp. The lower both are the better.

Steam pressure and temp is converted to steam velocity, and then extracted as shaft work at every stage. so there are no excessive velocities. You design for just enough velocity axial, at full load, to help move the steam out to condenser.

Some point in the last stages you get saturated steam, with water droplets. There you have blading with special tips to minimise erosion.

Getting the most energy out of your steam is paramount. The design is a compromise of various aspects, with the ultimate aim of getting the best financial return on your investment.

Thank you sir. operation point of view, there is no limit for the vacuum even reaching the absolute zero of possible, however, in view of ensuring safety to the turbine unit, limit to prevent erosion in last stage blades is of paramount importance.

may be, i have the wrong question, the correct should be, is there a typical condenser vacuum with respect to inlet presure, i mean typical pressure ratio for low and saturated steam turbine?

thanks much!

moreover sir, i beg to disagree with your statement that ther are no excessive velocities, as i understand, the lower the pressure the higher the velocity would be... this is the basic principle of bernoulli's. pressure is inversely proportional to the velocity...

thanks.

This question is for design engieers. Vacuume depend on the cooling water temperature at particular site. This is not common for all the operating sites. The amount of heat to be removed from the condensing steam and the coresponding load on the cooling towers are the important critaria which designer's take in consideration for the amount of vacuume which shall be suitable for the site.

As far as erosion on the turbine casing is considered, that may happen due to leakage through the parting plane of the turbine. Even at 0.3 bar absolute vacuume, turbine casings had been reported having erosion but problem was solved by making the parting plane square and arresting leage at the vacuume end.

I've never personally overhauled a steam turbine that showed any blade erosion, but I've trained with a guru. He passed on some wisdom that made perfect sense to me. It doesn't have anything to do with vacuum. He told me that back end erosion issues can be typically be traced to improper warm up procedures, and if not that, then inlet steam quality or a combination of both.

As an former operator/manager I can see that corner being cut out of impatience or improper direction from management. The plant is warming up, steam quality is low, but the turbine needs warming so it can be fully loaded when the boilers are at full temperature. So poor quality steam gets admitted. Or the powers that be dictate to fully load the turbine as soon as the steam is available rather than "loose" X thousand dollars an hour while blowing steam out the roof.... and of course the thousands extra they make by not properly warming it up is later recouped in the overhaul repairs.

There is offcourse a limit on minimum condenser pressure (or what you call higer limit on condenser vacuum). In exterem winters, we switch off certain cooling tower fans just to mainatain the consenser pressure above a minimum limit.

I can't get the figure off hand , but as you said it is very much goverened by the moisture content in the last stage buckets, exit velocity and annulus pressure loss.

You need to control the last stage vacuum falling too low.

thanks! i have on my side one person that vacuum need to have limits!:)

any idea if there is typical pressure ratio for low pressure saturated steam turbine?

In my opinion, there is no such ration that will dictate the condenser vacuum pressure relative to the main Inlet steam pressure. The Idea in the 1st place is to avoid having water droplets at the last stages in the turbine while maximizing the total pressure drop for efficiency (maximum power from the steam).

therefore, the setting of the last stage pressure must be in relation to the Steam Temperature at tha stage exit point.And since temperature/Pressure are related, the emphasis is to make sure the steam does not condense until it gets inside the condenser.

Your problem is to do with the connection between the turbine and the condenser: it has been suggested to you in previous comments that you might be having some leaks at these joint or connection area ... it could be!? Or some heat insulation problem at these points?. You could certainly reduce the cooling and therefore the vacuum in the condenser if that helps to eliminate droplets, but the end of it is that there are no rules or strict formulae that will link the inlet pressure to the vacuum level. this is a compromise to reach according to the prevaling conditions of the site etc.

Maybe I am wrong and someone has a formula that he has experimented ???

Yes, the outlet pressure needs to be limited to a lower limit such that steam quality at the last stage of the turbine does not fall below a limit.

Since you asked for typical pressure ratio, I have quickly calculated the minimum outlet pressure for a given inlet pressure, assuming its a staurated steam inlet, and the minimum qulity is 78%. Note that in superheated steam turbines we do not operate below 88% quality. You can easily recalculate if you want to change these assumptions, or let me know.

What do you do then when operating at part load, when condenser is operating much below capacity?

In extreme winters you do not need so much cooling in the cooling towers. The reason is because overcooling the condensate coming out of condenser is energy wasteful since you will have to heat it again in the low pressure feed heaters. That is energy you can convert to electricity instead. You avoid that by cutting down on tower cooling.

But the point where you condense the steam in condenser still has to be as low as possible for efficiency.

Dear Mr. lito,

At Sea-Level, the Max. Vaccuum obtainable is 29.92 Inches of Mercury Column or 760 MM of Mercury Column. For Turbine operation a vaccuum corresponding to 0.1 Kg/cm^2 of ABSOLUTE PRESSURE is MOST ECONOMICAL. Even Higher Vaccuum can be created and maintained but more energy is to be spent to creat that vaccuum which may not be economical.

There is no specific ratio for inlet and outlet pressure and it is purely Designer's Choice

Thanks,

RAJESWARI

To all my collegues in profession, thank you so much for sharing your thoughts. I have somehow some enlightenment got from all the ideas presented.

Thank you for your unselfish sharing:)

Here are the calculations based on your requested steam parameters. Hope that will help you!

Thank you so much sir:)

More Power!

There IS a certain limit on how deep condenser pressure may drop: if the exhaust annulus of the LP turbine becomes "choked" , there is a risk of excessive sub-cooling of the condensate because there is too much moisture in the steam which does not condensate but simply cools down to the lowest "cold-end" temperature. This, in it's turn, sub-cools the hotwell. Thus, more energy (fuel) should be added afterwards to regain for the lost heat by sub-cooling. This has perhaps no immediate effects on operations, but it certainly reduces the gain in heat rate (KJ/kWh) and even inverses it e.g. beneath 20 mbars absolute pressure. See the upward slope in picture, which depicts the inversed tendancy in the heat rate gain - most visible at high load: