Drainage & Dewatering System

By Amit Jha

Drainage system

• The water in the power house is to be drained outside power house.

• This is to be done through proper routing of water which is to be routed finally into drainage sump through gravity drain.

• Water in the sump is to be pumped out at proper intervals.

• For this a drainage sump is to be there whose bottom elevation should be lower than the lowest elevation of floor.

• Also volume of the sump is designed so as to cater the seepage volume for at least 3 minutes.

Equipments

1. Pump2. Valves-Motorised/manual3. NRV4. Flow Indicator5. Float Switch6. Pressure gauge/Indicator

Dewatering

• It is done so that maintenance work can be done in the tail race .

• All water is to be dewatered after closing the tail race gate in the time as mentioned in the contract.

• Accordingly pump is to be designed.

• The drainage system is more critical in the case of Francis turbine and not in the case of Pelton Turbine.

•Why?

Sainj HEP Drainage sump section

Teesta III HEP Drainage Scheme

Drainage System

Drainage System comprise of two types:

1. Station Drainage – The Leakage water from the bushings of guide vanes, Shaft seals, MIV flanges, CW System etc. has to be evacuated out of the Powerhouse.

2. Structure Drainage – This is the seepage water from rocks / soil which enters the cavities such as an underground powerhouse due to gravity and high pressure of rock mass.

Drainage System

The Drainage System comprises of the following components:

1. Drainage Water Sump/ Pit – May be located at different Levels.

2. Drainage Water Pumps (Vertical Turbine or Submersible) with Starters.

3. Valves

4. Pipes and Fittings

5. Level Sensors.

Leakages in Turbine PitThree Methods of removing leaked water from Shaft Seal,

Guide Vane Bushes etc in Turbine Pit:

1. Gravitation Method: Leakage water is lead through embedded pipes by gravity drainage sump. This water is then pumped out to the tail race.

2. Ejection Method: The water is made to pass through a nozzle, which creates a low pressure area, thereby sucking the leaked water to Drainage sump, from where, it is pumped to the tail race.

3. Submersible Pump Unit: In case leakage water is not controllable with above said means, submersible pump sets are sometimes deployed for draining Turbine Top Cover.

Reaction Turbine Cross Section

Dewatering System

Dewatering System is of very high importance in respect to the Powerhouses equipped with Reaction Turbine, as the Runner Centerline is below the Tail Water Level.

The Water present in the draft tube is removed by directing the water into a Dewatering Pit, from where it is pumped out to the downstream of the Tail Race Gate, to avoid back flow.

Dewatering System

The Dewatering System comprises of the following components:

1. Dewatering Sump/ Pit – Is located at the lower most level of the draft tube/ turbine pit.

2. Dewatering Pumps (Choice as per Silt Content in Water)

3. Hand Operated Valves

4. Pipes and Fittings

5. Level Sensors.