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Anonymous Poster

Effect in Steam Turbine Due to High Vaccum in Condensor

01/04/2011 3:10 PM

hello to all.. my question is that what is the effect or losses in Turbine due to high vaccume in steam condensor, whta's limit

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#1

Re: Effect in steam turbine due to high vaccume in condensor

01/04/2011 4:31 PM

The vacuum should be maintained within the parameters that are set fourth in the turbine documentation. Boiler, turbine and condenser have to work together.

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#3
In reply to #1

Re: Effect in steam turbine due to high vaccume in condensor

01/05/2011 5:21 AM

ok i agree with you that values are as pr manual instruction, but when vaccum drops its creat back prss which causes to reduced work done of turbine, so the efficiency will redued , but when vaccum will increases to high ie -0.99 or more then what will affects ie my question

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#4
In reply to #3

Re: Effect in steam turbine due to high vaccume in condensor

01/05/2011 6:07 AM
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#11
In reply to #3

Re: Effect in steam turbine due to high vaccume in condensor

01/11/2011 12:53 PM

Dear Mr.SHSHKNTVRM,

A lot of explanation has been posted by learned CR4 Members.

I want to add my point, IF VACCUUM IS INCREASED BEYOND THE DESIGN VALUE, the SPECIFIC VOLUME OF STEAM WILL INCREASE - REFER THE STEAM TABLE and the VELOCITY of EXHAUST STEAM will tend to increase for the given Quantity of Steam, since the Sp.Vol will ncrease.

This, the increase in VELOCITY, may lead to vibrational probem, low temperature of Steam tending to change to minute water droplets due to low temperature, and the kinetic energy of this particle may damage the turbine.

One gram of water may create a force few Kgs. of force. It will be 4.99 Kgs. force at a velocity of 700 Metres/Second and the ratio for the water particle weight to the dynamic force will be 4990 times and lead to damage. This is similar to BIRD HIT for the flying Air Craft and this will result in severe problem for the Turbine.

DHAYANANDHAN.S,

CR4 MEMBER, INDIA.

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#2

Re: Effect in steam turbine due to high vaccume in condensor

01/04/2011 5:14 PM

Increasing the pressure in the condenser reduces the pressure across the turbine, which will reduce its output.

The converse is also true.

The only way to reduce the pressure in the condenser is to make it colder.

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#5

Re: Effect in Steam Turbine Due to High Vaccum in Condensor

01/05/2011 2:45 PM

There should be no adverse affect from increasing your vacuum. The greater the vacuum the more pressure drop you are achieving hence the more energy is removed from the steam. Anyone who has been in the engine room of a ship can attest to the reduction in speed when entering the Gulf Stream warmer waters from the North Atlantic or conversely when leaving the Gulf Stream into the North Atlantic the speed increases often dramatically just from the difference in temperature of the main condenser inlet water temperature and the greater resulting vacuum.

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#6

Re: Effect in Steam Turbine Due to High Vaccum in Condensor

01/05/2011 3:45 PM

ok sir ur example is to clear but one more question arising in my mind is that last stage blades of turbine will affect due to very low exhust temprature

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#7
In reply to #6

Re: Effect in Steam Turbine Due to High Vacuum in Condensor

01/05/2011 4:10 PM

Define "very low exhaust temperature". Unless you're using liquid N2 or some other exotic cooling medium it is doubtful that you will come within 10 to 20 degree F approach temperature of the cooling water in your condenser which means that the region in the area or the last stage turbine blades will probably never be lower than 110 degree F. If that can cause harm to the turbine then return it ASAP to wherever you got it and get a refund.

Turbines probably suffer more damage with entrained water in steam than due to inadequate vacuum. If you lose your vacuum and high pressure steam gets through to your LP turbine you've got real trouble.This is something that occurs more on cold start-ups than anywhere else. This is why you have your drains open until you get steam roaring through them and your casings gets warmed up.

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#9
In reply to #6

Re: Effect in Steam Turbine Due to High Vaccum in Condensor

01/06/2011 5:42 AM

If last stage blades are having very low exhaust temperature means the steam will tends to become water. As the turbine is running nearly 1500rpm even a small water droplet lead the turbine blades to be broken due to the impact of water droplet.

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#8

Re: Effect in Steam Turbine Due to High Vaccum in Condensor

01/05/2011 6:19 PM

we are using cooling water,inlet temp about 26C & at full load, exhust temp is near about 46c & vaccume is -0.94, I have seen that when cooling water inlet temp become 23C then exhust temp 42 & vaccume -0.99

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#10
In reply to #8

Re: Effect in Steam Turbine Due to High Vaccum in Condensor

01/06/2011 5:45 AM

Vacuum is created in condenser due to volume changes from steam to liquid. If your CW inlet temp is less means there is a possibility of more steam will converted to liquid. So automatically vacuum will increase.

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#12
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Re: Effect in Steam Turbine Due to High Vacuum in Condensor

01/11/2011 2:30 PM

The vacuum is created by removing the air in the condenser by a steam ejector or vacuum pump. If one does not first remove the air, no amount of steam being condensed will "create" a vacuum. The pressure inside the condenser would still be atmospheric. An air leak into a condenser will break the vacuum and the condenser pressure will rise to atmospheric, greatly reducing the turbine output.

Once the vacuum has been established, the temperature of the cooling water and the design approach temperature of the condenser and the effectiveness of the ejector system will determine the amount or degree of vacuum that can be maintained. I would imagine that a turbine designer would have to make the assumption that theoretically the turbine could be made to operate in a near perfect vacuum and the necessary design criteria would be incorporated into the design.

Ships operate in the North Atlantic, the Bearing Straight and in Antarctica and do not suffer loss of turbines. The vacuum on those ships (providing a tight system is maintained) has to be pretty low.

I am not a turbine designer but I seem to recall that the increase in diameter of the rotor blade assembly plus the greater amount of blades would insure that based on the increased volume of the steam, due to the pressure drop across each stage, that one would be maintaining a constant velocity throughout the turbine.

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#13
In reply to #12

Re: Effect in Steam Turbine Due to High Vacuum in Condensor

02/13/2011 6:42 AM

thanks to all members who posted me answer, But sir i want to edit one of the topic in this section is that Is this a subcooling problem in condensor?

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#14
In reply to #13

Re: Effect in Steam Turbine Due to High Vacuum in Condensor

02/15/2011 10:58 AM

This should answer your question.

Condenser vacuum should be maintained as close to 29 inches Hg as practical. This allows maximum expansion of the steam, and therefore, the maximum work. If the condenser were perfectly air-tight (no air or non-condensable gasses present in the exhaust steam), it would be necessary only to condense the steam and remove the condensate to create and maintain a vacuum. The sudden reduction in steam volume, as it condenses, would maintain the vacuum. Pumping the water from the condenser as fast as it is formed would maintain the vacuum. It is, however, impossible to prevent the entrance of air and other non-condensable gasses into the condenser. In addition, some method must exist to initially cause a vacuum to exist in the condenser. This necessitates the use of an air ejector or vacuum pump to establish and help maintain condenser vacuum.

After the steam condenses, the saturated liquid continues to transfer heat to the cooling water as it falls to the bottom of the condenser, or hotwell. This is called sub-cooling, and a certain amount is desirable. A few degrees sub-cooling prevents condensate pump cavitation. The difference between the saturation temperature for the existing condenser vacuum and the temperature of the condensate is termed condensate depression. This is expressed as a number of degrees condensate depression or degrees sub-cooled. Excessive condensate depression decreases the operating efficiency of the plant because the sub-cooled condensate must be reheated in the boiler, which in turn requires more heat from the reactor, fossil fuel, or other heat source.

The above answers were found by simply Googling "sub-cooling condensate." If you do the same you will find many other explanations and engineering examples.

Good luck.

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dhayanandhan (1); kramarat (2); Natarajan (2); PWSlack (1); shshkntvrm (4); Spinco (4)

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