191

What are the advantages of Super-critical units?

v  Increase in cycle efficiency

v  Less fuel consumption

v  Less auxiliary power consumption

v  Less emission

v  Less infrastructure investments per MW

v  Less water consumption

v  Reduced land requirement

v  Reduced manpower requirement

v  Shorter unit start-up time

v  Suitable for variable pressure operation

v  Better temperature control and load change flexibility

192

What are the advantages of Combined HP-IP steam turbine?

v  Shorter longitudinal span

v  Smaller differential expansion

v  Easy maintenance

193

List some conditions for Boiler trip.

v  Both FD Fans Off

v  All ID Fans Off

v  Inadequate Water wall Circulation

v  Loss  Of Reheat Protection

v  De-aerator level Lo-Lo 

v  All Boiler Feed Pumps Off

v  Elevation Power Failure For > 2 seconds.

v  Furnace Pressure High-High 

v  Furnace Pressure Lo-Lo

v  Loss Of  All Fuel

v  Unit Flame Failure

v  Loss Of Critical Power For > 2 seconds

v  Loss Of  220 V DC For >  2 seconds

v  Loss Of ACS Power >  2 seconds

v  Loss of AC at any Elevation in service

v  Air Flow < 30 %

v  Both Emergency Push Buttons Pressed.

194

What are the primary causes for losses in a steam turbine?

The four primary causes of losses in a steam turbine, and to its efficiency and performance are:

v  Chemical deposits in the steam path

v  Nozzle and bucket surface erosion

v  Mechanical damage to nozzles and buckets due to foreign objects, and

v  Steam leakage through the unit's shaft packing, tip seals, and inlet steam pipes (with packing and tip seal losses accounting for more than 50% of a steam turbine’s efficiency losses).

195

By lowering the condensing temperature, the backpressure on the turbine is lowered, which improves turbine performance.

A condenser degrades primarily due to fouling of the tubes and air in-leakage.

Tube fouling leads to reduced heat transfer rates, while air in-leakage directly increases the backpressure of the condenser and degrades the quality of the water.

196

When unit loads vary throughout the year, the benefits of using VFDs on large-size equipment, such as FD or ID fans, boiler feed water pumps and condenser circulation water pumps, can be justified.

197

Boiler scale lowers heat transfer due to low thermal conductivity. Heat transfer may be reduced as much as 5-10% by the presence of scale.

More important than the heat loss is that scale can cause overheating of the boiler tube metal and can result in subsequent tube failures, leading to costly repairs and boiler outages.

198

What is a Langlier Scalability Index (LSI)?

The scaling tendency of water can be estimated based on the pH, alkalinity, calcium concentration, and temperature of the water using a scaling index called Langlier saturation index (LSI).

The LSI indicates the number of pH units the solution must be lowered in order to prevent scaling.

The change in pH typically is affected by injection of an acid, such as sulfuric acid.

199

All three contaminants - deposition, scaling, and biological fouling, contribute to plugging of cooling tower fill.

200

Where in a thermal power plant is Sodium Hypochlorite (NaOCl) used?

Sodium Hypochlorite (bleach) is the industry-standard chemical for biological fouling control.

Sodium Hypochlorite is injected either continuously at 1-2 ppm or injected at a shock dose of 5 ppm for 90 minutes 3 times per week.