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FEED BACK NO. 46

SUBJECT

Delayed opening of LPBP stop & control valves leading to Boiler trip outs.

Details

In one of the 500 MW sets proble of Boiler tripping on either of the follo!ing

protections !as reported !henever turbine tripped"

#. $eheater protection

%. Boiler load inde ' (0)

*nalysis

#. Whenever turbine trips and boiler troubles are not present+ latter should continue

to operate through ,P-LP bypass syste. nder such conditions $eheater is to be

protected against starvation. /o above t!o protections are provided. $,

protection has a tie delay of seconds !here as BLI has ( seconds only.

%. /12 & trends analysis confired that !ith turbine trip outs instant coands

!ere issued by ,P BP as !ell as LP BP controllers !ere issued by ,P BP as !ell as

LP BP controllers to go to full open states. ,P BP valves follo!ed the coands

siultaneously and !ere opening as per coand !ithin # sec. of issue of 

coands. ,o!ever+ LP BP valves !ere ta3ing ore than %0 seconds to open.

(. 4he syste !as thoroughly chec3ed and dapers provided in spray !ater

pressure ipulse line to liiter of LP BP rac3 !ere found to be !elded in !rong

direction.

2planation

* 4o protect condenser against daages caused by hot stea entering

through LP BP syste condensate spray syste is provided. 4o ensure

that sufficient spray !ater is available before stea is duped in

condenser+ the spray !ater pressure at the do!n strea of !ater in6ection

valves is sensed and counicated to liiter !hich acts as the perissive

providing tool for LP BP stop and control valves. 4his signal is also used as

LP BP tripping signal and acts on in6ection !ater pressure slide valve in the

rac3. 7Pl. refer the enclosed diagra.

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B 4he echanis has sensing of spray !ater pressure at do!n strea of 

WI valves via daper 7(. When LPBP syste is not in use+ no !ater !ill

be available in do!n strea. 4o avoid undesirable thrust on the bello! of 

liiter+ it is necessary to ensure positive filling of ipulse line upto the

bello!. 4his is achieved by optii8ing orifices 7# & 7%. Daper 7( also

acts as 9$: and stops bac3 flo! of fill-!ater !hen LPBP is not in use.

; 4he dapers 7( !ere installed in the reversed direction. 4his not only

!as resulting into draining of spray !ater pressure ipulse line but also

the liiter !as providing perissive to LPBP control and stop valves

opening due to in6ection !ater pressure ahead of the !ater in6ection

valves 7due to stoppage of !ater flo! at daper on pressuri8ation of 

ipulse line !ith spray !ater at do!n strea of WI valves.

4his !hole process of counication of spray !ater pressure 7that+ too of upstrea

header to liiter !as ta3ing considerable tie 7< %# seconds.

Action Taken

4he dapers 7( !ere cut and re-!elded in the right direction. :arious trials !ere ta3en.

4he tie gap bet!een issue of coand by controller 7*s3ania and eecution of valve

stri3ers could be reduced of # second.

4hus+ the proble of Boiler trip+ on turbine tripping due to above protections could be

resolved.

 

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