Purging In Generator PPT.ppt

 A World World of Protecon

CO2 Purging in Generators

BY--------------R.P. Sharma

Power Generation Bulletin

CO2 CARBON DIOXIDE FIRE SUPPRESSION Inerting/Purging Systems

Hydrogen Cooled Electric Generators

When

the generator rotor is turned at high seed in larger steam tur!ine units" the tur!ine must o#ercome the resistance to the turning rotor created !$ the atmoshere in the generator Casing. %his &asted energ$ use is called &indage loss. %o reduce this &indage loss to a minimum" the air in the casing is relaced &ith h$drogen" our lightest gas.

When

the unit is 'rst installed" the casing is full of air. %he air cannot !e directl$ relaced &ith h$drogen &ithout going through the e(losi#e range )&hich is *uite !road for h$drogen+. Similarl$" &hen ta,ing a unit from ser#ice" the h$drogen in the unit cannot !e directl$ !led o and relaced &ith air &ithout again going through the e(losi#e range. %o ,ee from creang an e(losi#e atmoshere" CO 2 is used to inert the air !efore the h$drogen is introduced into the casing. %hen the CO 2  air mi(ture is relaced !$ the h$dro-gen. %o emt$ the casing" the h$drogen is urged !$ CO2 and then the unit #enlated &hen the casing is oened.

%he

generator manufacturer !uilds a CO 2  manifold into the !o/om of the

machine )CO2 !eing hea#$+ and a h$drogen manifold in the to )h$drogen !eing light+. %his hels re#ent mi(ing and reduces the amount of gas needed. %his gas e(change is normall$ done on turning gear. 0ore gas is needed if done at seed. %he generator manufacturer !uilds a CO 2 manifold into the !o/om of the machine )CO2 !eing hea#$+ and a h$drogen manifold in the to )h$drogen !eing light+. %his hels re#ent mi(ing and reduces the amount of gas needed. %his gas e(change is normall$ done on turning gear. 0ore gas is needed if done at seed.

See the diagram on Next Slide which illustrates this inerting/ purging operation

Purging the hydrogen from the machine is usually estimated to require about two volumes of CO . To replace the air before hydrogen is introduced will take 1½ volumes. Thus it can be estimated that it will take !½ volumes of CO to take a unit out of " service open the casing and then return the unit to service. #f done at speed it could take $ volumes. %ears ago it was common practice for the generator manufacturer to provide a CO "   supply of high pressure cylinders manifold together. These were not fire system cylinders but rather cylinders with manual valves at the cylinder vapour spaces. &ith this arrangement CO "  vapour is bled off from the   cylinders into a manifold and then regulated pressure CO "   introduced into the generator through the controls and manifold provided by the machine manufacturer. 'iquid CO"  is not used( vapour is withdrawn which prevents the cooling that occurs when liquid is discharged. )s the vapour from the cylinder is bled off the pressure in the cylinder drops and the flows lows. #n the time usually allotted for purging all the CO"   in the cylinders cannot be recovered unless e*ternal heating is used. #t can be assumed that perhaps +,- is recoverable. #f it/s hot and you allow enough time this  percentage will increase.0

The disadantages o! the cylinder system are" There is only a minimum amount of CO 2  available. Thus, if a second purge is needed, there could be delays until a replacement CO 2 supply is obtained. #

%$#&% The CO"  flow rate is determined by the vapour bleed off rate, which is a function of how many cylinders are bleeding and the heat input to the cylinders. It is hard to increase the rate if this should ever prove necessary. In an emergency, it may be appropriate to purge the hydrogen as fast as possible. #'% Plant personnel have to handle high pressure gas cylinders, which is always a risk. or this reason, the industry trend is to bulk gas systems. !pecial rigs for handling cylinders to prevent an accident " where the cylinder becomes a pro#ectile are common. #( %Cylinder CO2 is more costly and not as readily available as bulk li$uid CO 2 .

Rather, the bulk low pressure system is favored. With the low pressure CO "  system, the CO"  vapor needed is obtained by taking liquid CO "  from the unit and pung it through a direct to  process vaporier. !his adds the heat of vaporia"on #about $%& '!(s)lb.*, plus the addi"onal heat required to get the CO "  temperature up to the desired level. !he CO "  vapor then goes through a +ow control manifold #throling valve, regulator with gauge* to establish the  proper +ow rate. CO"  is usually used at - / 0& psi with a purge "me of an hour or two. !he  +ow control schemes are normally redundant. When the +ow rate is established, it is normally le1 set at that level, but can be easily increased in an emergency. 2f a system with remotely actuated opera"ng valves is required, this is easily accomplished. Refer to the drawing of the Carbon 3io4ide 5urging 6ystem, which illustrates an arrangement where the CO " main control valve is electrically operated, which allows pushbuon opera"on. !he electric vaporier is a demand type with thermosta"c control7 when the valve opens and cold CO "  liquid is introduced into the vaporier, the platen hea"ng elements are automa"cally turned on. !herefore, the unit is always ready to go, but uses no power un"l hea"ng is needed.

6ince the ma4imum +ow rate is a func"on of the capacity of the vaporier, the vaporier is sied to a worst case condi"on. 2ncreases in vaporier capacity do not propor"onately increase the cost. !he advantages of the low pressure system and the ability to use this same storage unit for mul"ple purposes #hydrogen purging, coal silo iner"ng, 84ed 8re protec"on, hand hose line coverage, etc.* has made it virtually an industry standard. 9ore details on speci8c system arrangement are available from Chemetron.

Other Inerting Uses

When there is any applica"on for CO "  vapour, the setup for using the :5CO "  6ystem and the direct to process vaporier to give the required quan""es of vapour are virtually the same as that described above for the hydrogen purge. Each pound of CO "   will expand to about 8.3 cubic feet of vapour at atmospheric condions. The volume of CO  "  reuired is calculated and

Basic units are1 ;- lb.)hour $