Can anybody expain me why we open condenser vaccum breaker in case of turbine trip? or what are the conditions under which we open & close condenser vacuum breaker? Kindly reply with reference to Coal fired power plant.
my basic understanding was that during a turbine trip, not a normal shutdown, most control systems will break vacuum in the condenser to equalize pressure in the steam turbine. This will have the effect of slowing the machine down, or prevent or minimize an overspeed event due to the loss of vacuum. This assumes you understand the basics of a condensing steam turbine where the vacuum in the condenser helps to create the differential pressure across the high pressure to low pressure areas. By breaking vacuum you reduce the energy available to drive the machine.
I know others might explain this better, but I hope this helps you somewhat.
In the plants I've been in, a trip did not cause any time of "vacuum breaker" to open. In fact it would be the other way...loss of vacuum (or low vacuum) would trip the turbine.
It's tough to give a solid explanation without understanding what type of equipment the original poster has. Unlike gas turbines, steam turbines are often customized to each plant.
Regardless, most turbines usually have some "blow off" valves that open when the unit trips. There is often a valve that connects just past the control valves and vents steam directly to the condenser, and sometimes another that vents packing steam and maybe IP steam directly to the condenser. These valves may open during a trip or may only open during an overspeed trip. It all depends on the manufacturer, control system design, etc.
Breaking vacuum when a turbine is rolling down isn't really a desired thing to do because it will raise the saturation temperature of the the exhaust steam and could possibly cause some pain on the last stages as steam condenses to water.
I suggest looking at the control diagrams and piping and instrumentation diagrams from your plant to see how the systems are designed to interact. Each manufacture has many of the same functions, but each may incorporate it in a unique way.
the purpose of Vaccum Breaker , as my few friend suggested, is to break the vaccum suddenly. This provides <b> Breaking effect</b> to the turbine speed i.e. brings tubine to zero rpm as soon as possible whatever the reason.
As per plnat,BHEL supplied, in 55 MW STG, the vaccume breaker OPENs in following events:
1. Lube oil pressure Low i.e. no lubrication oil for shaft bearing, take machine to zero rpm to avoid further damage
2. Axial Thrust Shift Hi Hi..i.e. Already machine expansion is abnormal,take machine to zero rpm
Vacuum breaker valve normally remains closed in case of turbine trips and is opened when turbine reaches at turning gear speed (zero rpm) because opening of vacuum breaker valve at higher speeds results in increased thermal stresses on turbine as cold dense air enters into the turbine.
vacuum breaker valve is opened only in case of emergency conditions like loss of lube oil, high abnormal sound from turbine, fire event,high exhaust pressure, high vibrations or thrust bearing failures so that turbine could be coast-down quickly.
there are certain cases when we need to open vacuum breaker of condenser, like turbine trip on axial shift, lube oil pr. very low and differential protection of generator.
AXIAL SHIFT-We need to open the vacuum breaker to minimise the coasting down time of turbine and bring it to to standstill quickly to avoid any damage done to the turbine.
LUBE OIL PRESSURE VERY LOW-We need to open the vacuum breaker to minimise the coasting down time of turbine and bring it to to standstill quickly to avoid any damage done to the TURBINE BEARING.
DIFFERENTIAL PROTECTION-To reduce the voltage of generator to avoid overheating.
> there are certain cases when we need to open vacuum breaker of condenser, like turbine trip on axial shift, lube oil
> pr. very low and differential protection of generator.
> AXIAL SHIFT-We need to open the vacuum breaker to minimise the coasting down time of turbine and bring it to to
> standstill quickly to avoid any damage done to the turbine.
> LUBE OIL PRESSURE VERY LOW-We need to open the vacuum breaker to minimise the coasting down time of turbine and bring
> it to to standstill quickly to avoid any damage done to the TURBINE BEARING.
> DIFFERENTIAL PROTECTION-To reduce the voltage of generator to avoid overheating.
Additionally, steam turbines hasn't got any braking elements. However you need don't keeping the turbine speed slowly coasting down and spending too much time running on critical speed ranges. So, the best solution is braking with the vacuum breakers fastly wide opened, until reaching one "partial" vacuum value, and then partially closing them maintaining it until completing the rest of the coast down (about one half of nominal speed). You have not this critical speeds slow passing problem on running the turbine up, as you have control over the running up speed gradient.
> Can anybody expain me why we open condenser vacuum breaker in case of turbine trip? or what are the conditions
> under which we open & close condenser vacuum breaker? Kindly reply with reference to Coal fired power plant.
Ans. mostly overall the condensing turbine vacuum breaker opening automatically in 2 (two) cases, one of them if the turbine trip due to lube oil failure, 2nd case if the turbine trip due to shaft axial displacement.Basically in both cases main aim to reduce the run-down time and safe the the turbine. In some other cases as well as fire any other abnormalities h append, vacuum breaker can be open for the same purpose.
During Turbine Trip Critical Speed Zone IF Vibration Level IS high during that time Vacuum Breaker Will Open (That is called partial Vacuum Breaking) for that exhaust pressure setting (example 150 mbar vale open 100 mbar valve close).
Other case IF Fire protection activate at any load Vacuum breaker will open.
> Can anybody explain me why we open condenser vaccum breaker
> in case of turbine trip? or what are the conditions under
> which we open & close condenser vacuum breaker? Kindly reply
> with reference to Coal fired power plant.
