Need condensing turbine vacuumize before cold startup

In this article we discussed about the importance of building vacuum inside the turbine during cold start up of turbine and what the problem could be occurred if we are not vaccumize the turbine.There are much air inside the turbine before cold start, if the vaccumize procedure does not performed.

Before starting  the discussion we should also know about the how vacuum building starting in turbine and what are the function of vacuum inside the turbine what are the role of main parts of steam turbine which helps for the vacuum building inside the turbine 

Labyrinth glands 

In order to prevent the leakage area where the turbine rotor shaft passes through the casing , non contact glands are provided. The fins are positioned to match grooves truned in the rotor shaft and gland holder. The assembly present a tortuous , labyrinth path against leakage. The many restrictions and corresponding space drop the pressure of the steam thereby increasing its volume and limiting the quantity that can pass through the final restrictions

At the low pressure exhaust end , we are primarily concerned with preventing air leakage into the turbine and condenser , destroying the vacuum and thereby reducing the output and efficiency of the unit. The low pressure labyrinth is divided along its length into three separate group of fins. The spaces between the groups are called pockets. The oppose the air leakage , a quantity of gland sealing steam is introduced into the inner pocket. This flow inward into the turbine through a labyrinth and outward through a labyrinth to a second pocket.

The quality of sealing steam supplied is regulated to keep the pressure in this outer pocket just above atmosphere. This keep air out and limits the leakage of sealing steam to atmosphere through the outer labyrinths.A connection from the outer pocket is led to a gland vent condenser. The supply of steam should always be regulated so that the pressure is between 0.10 – 0.20 kg/cm2 this ensure that the gland is sealed using the maximum amount of steam  . Similar labyrinths , pockets and connections are provided at the high pressure inlet end of the turbine . The wheelcase pressure increases progressively with the load on the turbine. At and near full load , this pressure is considerably in excess of atmospheric pressure.

To break this pressure down , and to reduce steam leakage outward from the wheelcase , the high pressure gland is provided with additional inner labyrinth rings and one additional pocket. The labyrinth rings are supported in the gland housing and are held concentric with the turbines shaft by radial leaf springs. After erection , the turbine parts are carefully aligned to that the labyrinth packing clearances and consequent leakage will be small. Providing that the turbine is warmed through and run up to speed slowly the clearances will remain small and little or no labyrinth wear will occur.

GLAND STEAM SEALING SUPPLY

Low pressure steam is supplied to gland sealing through a valve , a constant supply pressure of 0.10- 0.20 kg/cm2 as maintained. Desired gland steam pressure can be adjusted by operating manually the valve provided in each of the line going to steam and exhaust side.

GLAND VENT CONDENSER

the gland vent condenser is used to condense the steam discharged from the rotor shaft glands . The unit consists basically of a heat exchanger divided into suitably baffled compartments and fitted with tubes located in the tube plate.

Condensate from the unit is taken through a trap to the condenser

The following dangers will be resulted:

• A great deal of steam quantity was in need to cope with the friction resistance and rotor inertia from the shaft so as to increase the driven power to the blade;
• Because of the air existence within the cylinder, the steam outlet temperature was raised up by the end blade blast friction;
• Because of the air existence within the condenser which weaken the heat-water transformation efficiency and cause the temperature raise and the cylinder metal deformation and condenser copper pipe leakage;
• Because of the non-condensing air, the pressure was raised for the turbine steam outlet and caused condenser to safety adjusted.
• In conclusion, the vacuumize procedure shall be performed before condensing turbine started.

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