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Why vacuum may be lost or not maintained in steam turbine system?

04 Aug '17, 09:55

Aug. 4, 2017, 9:55 a.m.
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Answer:

The greatest trouble in running the steam turbines is the loss of vacuum caused by air leaking into the surface condenser through the joints or packing glands. Another trouble spot is cooling water leaking into the steam space through the ends of the tubes or through tiny holes in the tubes. The tubes may also become plugged with mud, shells, debris, slime or algae, thus cutting down on the cooling water supply. Corrosion may be uniform, or it may occur in small holes or pits.

  1. Excessive air leakage into the vacuum system
    a) Sealing steam is not adequate to glands of turbine and stop valves
    b) Gland sealing steam is not exhausted into the condenser
    c) Gland steam makeup and spill controllers are defective
    d) Gland steam return needle valve not adjusted correctly
    e) Air leaks from other lines connected to vacuum condenser
    f) Leaking exhaust bellow at condenser inlet
    g) Condenser safety valve or any other penetration leak
    h) Air leaking into system reduces the condenser efficiency
    i) Air leaking system increases insulation to condense steam
    j) Air bubbling may be observed in vacuum condenser sight glass
    k) Gland steam inlet steam traps are fouled
    l) Function of a gland drain is to draw water from sealing-gland cavities created by the condensation of sealing steam to keep clear to vacuum condenser
    m) Gland steam blowing should be avoided because the steam usually blows into the bearing, destroying the lubrication oil in the main bearing. Steam blowing from a turbine gland also creates condensate, causing undue moisture in plant equipment.

  2. Improper functioning of the air removal equipment
    a) In case of vacuum pump unit, dirty nozzle in ejector unit
    b) Condenser cooling is not adequate
    c) Condenser is leaking
    d) Multiple pumps inter connection and N/R valves leaking
    e) Pump running directions are not correct
    f) Line valves may be not correctly set
    g) Dirty circulating water- corrosive, corroded tank and piping system
    h) The nozzle provided is very small, necessary to maintain clean system
    i) The suction line from condenser obstructed
    j) The air discharge line from the ejector to tank is restricted
    k) The tank vent line obstructed

  3. Improper drainage of condensate from condenser
    a) Condensate pump suction line vent restricted
    b) Condensate pump re-circulating controller erratic
    c) Air leaking from the Nonrunning unit mountings
    d) The condensate temperature may be too hot
    e) Condensate pumps clearances may be high
    f) The valves to and from hot well are throttled
    g) Flooding of vacuum condenser and g/glass defective
    h) Condenser level controller is erratic

  4. Insufficient flow of circulating water
    a) Dirty condenser
    b) Division plate leaks
    c) Ideal temperature of sea water outlet is 5-8 degrees centigrade below the corresponding vacuum temperature of steam

  5. Boiler water level controller to be correctly operating

  6. LP water separator water level controller to be in correct operation

  7. Vacuum condenser level controller unit operation is to be satisfactory

  8. Dump steam line from HP / LP valves should be leak free
  9. HP/LP dump controllers to be operating correctly and no leaks of valves ensured
  10. When vacuum lost suddenly from running condenser, shut down immediately. The condenser cannot stand the steam pressure, the condenser tubes may leak from excessive temperature. Excessive pressure will also damage the shell, the exhaust and the low-pressure parts of the turbine.

Identifying leaks:
First, find the leak by passing a flame over the suspected part while the condenser is under vacuum. Leaks in the flange joints or porous castings can be stopped with asphalt paint or shellac.

Ideal Conditions of vacuum condenser is following:
1. The maximum amount of steam condensed per unit area of available heat transfer surface.
2. Minimum quantity of circulating coolant required.
3. Minimum heat transfer surface required per kW capacity.
4. Minimum power is drawn by the auxiliaries.

The importance of the vacuum being maintained in the steam condenser:
1. By maintaining a vacuum in the steam condenser, the efficiency of the steam-power plant can be increased as greater the vacuum in the system, greater will be the enthalpy drop of steam. Therefore, more work will be available per kg of steam condensing.
2. The noncondensate (air) can be removed from the condensate-steam circuit by pulling and maintaining a vacuum in the steam side. Therefore, the condensate can be used as boiler feed.

Ideal conditions of surface condenser used for steam power plants 1. Uniform distribution of exhaust steam throughout to the heat transfer surface of the condenser
2. Absence of condensate sub-cooling
3. There should not be any leakage of air into the condenser
4. There should not be any tube leakage
5. The heat transfer surface in contact with cooling water must be free from any deposit as scaling reduces the efficiency of heat exchangers.

Important precautions:
1. High-pressure steam in contact with sub cooled condensate is an unstable and potentially explosive mixture.
2. Don't admit steam into a line filled with sub cooled condensate. In fact, always be wary of admitting steam to any cold steam line if you cannot be absolutely certain that the line's been completely drained.
3. Allowing sub cooled condensate to flow into a stream filled line is more dangerous than admitting steam into a line with sub cooled condensate.
4. If you suspect a pressurized steam line is filled with sub cooled condensate, don't attempt to drain the condensate. Shut the steam off first, then drain the condensate. If you do open a drain, and the line hammers, close it and get the steam off. The line may continue to hammer until you get the steam off.
5. A mixture of steam above sub cooled condensate can sit dormant in an isolated steam line like a loaded gun awaiting a triggering event. Opening a valve to admit steam or opening a bleeder to drain condensate can trigger an event. Don't let yourself or those you supervise inadvertently pull that trigger without first making sure the gun is unloaded.

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04 Aug '17, 10:21

Aug. 4, 2017, 10:21 a.m.
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