Question & answers on vacuum troubleshooting in steam turbines

Understanding  the term Vacuum.

The earth's atmosphere exerts pressure upon us known as atmospheric pressure which can be measured in number of ways & methods.At the seal level the pressure measured is 760 mm of Hg, 1 Torr or 14.7 PSIa or 1 bar.Because the barometric pressure varies above seal level pressure is used as reference point.

As there is 1 bar pressure exerted on our body and also there is 1 bar pressure within our body. So as per Newton's law action &* reaction are equal and opposite, we do not feel any discomfort. That is there is no any pressure difference between inside & out side of the body.

Evacuating air from a closed volume develops a pressure differential between the volume and the surrounding atmosphere. If this closed volume is bound by the surface of a vacuum cup and a work piece, atmospheric pressure will press the two objects together. The amount of holding force depends on the surface area shared by the two objects and the vacuum level.

Because it is virtually impossible to remove all the air molecules from a container, a perfect vacuum cannot be achieved. Of course, as more air is removed, the pressure differential increases, and the potential vacuum force becomes greater.

The vacuum level is determined by the pressure differential between the evacuated volume and the surrounding atmosphere

Is it possible to have a negative absolute pressure?

No, absolute pressure is measured with reference to a perfect vacuum so it is impossible for it to go negative. You can only measure negative pressure between two different pressures. For example if you allow atmospheric air to gradually flow into a vacuum vessel and measure pressure inside relative to outside it will show a negative pressure reading.

What type of problems do you face in steam turbines related to vacuum?

Problems such as:

• Low vacuum

• High exhaust pressure

• High exhaust temperature

• Higher specific steam consumption

• More cooling water circulation

• Hot well level variation

How do you create vacuum in steam condensers?

Vacuum is created in condenser by steam jet ejectors, where high pressure 8–12 kg/cm2 steam is passed through nozzle which is connected to air line from condenser. This creates high negative pressure there by evacuating air from condenser.

Generally there are Two Types of Ejectors:

Hogger Ejector: Initially this ejector is used for pulling vacuum. It has steam and air lines connections, steam is vented directly into atmosphere. It consumes more steam than main ejectors. It requires 20–30 minutes to create 85% of operating vacuum.

Main Ejector: It comes with first stage and second stage. Air line from surface condenser is given to 1st stage then again air from 1st stage is collected and discharged into 2nd stage. 2nd stage ejector has air vent line.

It consumes less steam than hogger ejector. Generally an ejector come with 1W + 1S i.e. one working and one stand by.

Also vacuum pumps called liquid ring vacuum pumps are used to create vacuum in condensers. Which consume less energy than steam jet air ejector

How does low vacuum affect on turbine speed?

Lower vacuum creates back pressure on turbine blades and rotors. So in emergency, vacuum breaker valve is opened to bring down the turbine speed to zero in minimum time to avoid any further damages.

What is the effect of low vacuum & high exhaust pressure on steam turbine performance?

Low vacuum or high exhaust pressure & high exhaust temperatures lead to more steam consumption to generate unit power.

Steam condenser,vacuum & calculations....

What are the potential reasons for lower vacuum in steam condenser?

• More condenser load than design

• Lesser amount of cooling water circulation in condenser

• Higher atmosphere temperature

• Location of the steam condenser at higher elevations.

• More exhaust temperature

• Air leakages in the system

• Lesser efficiency of steam ejector or vacuum pump

• Ejector inter condense (1^st stage) condensate seal break

• Lesser pressure & temperature of motive steam at ejector inlet

• Worn out ejector nozzles

• Improper quality of motive steam

• Variation in condenser inlet & outlet cooling water temperatures

• Operation of Turbine at lower load

• Lower gland seal steam pressure

Why does vacuum drops in steam condensers??

Reasons for increase in Turbine SSC

What are the effects of air leakage in condenser?

Following are the major effects due to air leakage into condenser:

Lower Thermal Efficiency: The leaked air in the condenser results in increased back pressure on the turbine this means there is loss of heat drop consequently thermal efficiency of plant will decrease.

Increased Requirement of Cooling Water: The leaked air in the condenser lowers the partial pressure of steam due to this, saturation temperature of steam lowers and latent heat increases. So it requires more cooling water to condense more latent heat steam.

Reduced Heat Transfer: Due to poor conductivity of air heat transfer is poor.

Corrosion: The presence of air in the condenser increases the corrosion rate.

What is the function of vacuum breaker valve?

Vacuum breaker valve is used to bring down the turbine speed quickly to zero in case of emergency trip of turbine. Valve can be manually or auto opened.

What are the different conditions on which vacuum breaker valve opens?

30-things you must know about steam Turbines

On following emergency or fault cases vacuum breaker valve will get open

• High bearing vibrations

• High bearing temperature 

• High axial displacement of rotor

• High differential expansion

How can you identify the air leakage into the system?

If there is air leakage into the system, then this should be vented out though ejector system. Rota meter of ejector shows the increase in air quantity than the normal air flow.

How do the motive steam pressure & quality affect on ejector performance?

If the motive steam pressure is below design by more than 5%, or above design by 20%, poor performance may occur with a resulting increase in the condenser pressure.

Motive steam quality - Wet motive steam will cause poor performance as well as ejector wear. Super heated steam having a temperature greater than 10° C above the saturation temperature will also cause poor performance if not considered in the design.

What will happen to hot well level, if condenser vacuum drops suddenly?

Hot well level rises up

What are the common problems associated with steam jet ejectors related to vacuum?

Common problems are:

• Low or high motive pressure due to improper sized nozzles:If the cross section area at those locations is greater than 7% above the design values, performance problems are likely.

• Wet motive steam

• Failure of vacuum trap

• Larger pressure drop at shell side: If the shell side pressure drop is greater than 5% of the absolute operating pressure, then either shell side fouling or flooding of the condenser could be present. Check the trap or loop seal on the condensate outlet for proper drainage

• Breaking of 1st stage condenser U loop seal

• Air leakages through safety valve & flanges

• Non operational rota meters

Why U loop and float valves are used in steam ejector 1st stage (inter condenser) and second stage (after condenser)?

U loop and float valves are used for sealing purpose between 1st stage and 2nd stage ejectors and condensers. As there is a pressure difference between these two and turbine steam condenser.

U loop is around 2.5 to 3 meter, it depends on pressure difference between 1st stage and steam condenser. If there is pressure difference of 0.25 kg/cm2 between 1st stage and steam condenser then the U loop height should be 2.5 meter. So it is very must to seal the U seal (filling DM water in loop) before pulling vacuum.

A steam Turbine's exhaust steam temperature gauge is showing 60 Deg C & vacuum gauge is showing pressure -0.75 Kg/cm2, then what do you think, is the pressure gauge showing  correct reading?

As discussed earlier, condenser vacuum depends on the atmospheric pressure, as the atmospheric pressure is more, vacuum can be maintained more. Hence the steam condenser installed at higher elevation have lower vacuum than that of condensers installed at lower elevations.

In this case at temperature 60 deg & considering atmospheric pressure 1.033 kg/cm2 the gauge  pressure in the condenser should be around 0.81 kg/cm2.

There might be error in vacuum gauge or might be installed at some higher elevation around 600 mm causing  lower pressure due to head difference.

A steam power plant is installed 580 meters above  the seal level, then what will be the atmospheric pressure in that area?

Atmospheric pressure = P = 1.033 X (1-2.2557 X 10^-5 X 580 m)^5.2558

Atmospheric pressure = P = 0.9638 kg/cm2

What are the potential reasons for a Steam jet ejector consuming more steam for creating particular vacuum in steam condenser?

Potential reasons are:

• Improper design of ejector

• Improper pipe line layout from & to the ejector

• Worn out steam nozzles

• Steam quality is wet

• Higher steam pressure

• Air leakage into the system

• Steam line leakages

• Fouling in ejector shell

• Insufficient quantity of cooling water
• 
  
• Water tubes leakage

What can cause,If ejector's motive steam pressure & temperature are higher than design?

• Ejector capacity gets reduce

• Ejector performance gets reduce

• Steam wastes

What will be the hogger ejector capacity as compared to main ejectors?

Hogger ejector should create 60-70% vacuum in 15-20 minutes

What is the steam steam consumption for Hogger ejectors as compared to main steam jet ejectors?

It is usually 30-40% more than main ejectors

What is the reason for high exhaust pressure in steam Turbines?

Or

What is the reason for low vacuum in steam Turbines?

Common reasons are;

1-Higher load on Turbine or over load or operating the turbine at lower load i.e < 20% for long time

2-More exhaust steam flow to condenser

3-Low cooling water circulation in steam condenser

4-Poor performance of cooling tower

5-Poor performance of steam ejectors

6-Less pressure & temperature of motive steam at ejector inlet

7-Air ingress in vacuum steam

8-Improper heat transfer in condenser i.e scaled tubes

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