Mechanical Auxiliaries

The Mechanical Auxiliary Equipment comprises all the mechanical systems needed to operate and maintain the powerhouse. (It does not include the Ancillary Equipment, which are needed for maintenance and operation of the turbine-generator units.)

The mechanical auxiliaries and the electrical auxiliaries are often provided under a separate “Balance of Plant” contract.

Water Systems

Raw Water System

The raw water system supplies water to all the other water systems. It is connected to the penstocks or to the reservoir by a separate intake and delivery pipe. The incoming water is untreated, or “raw”. It passes through an isolation valve, typically a gate valve, which enables the system to be isolated from the high-pressure penstock water for maintenance.

A strainer is connected downstream of the isolation valve to remove coarse impurities. If the raw water has a high concentration of vegetal matter (such as algae), an automatic duplex strainer is used. Duplex strainers use two strainers, one in service and one on standby. When the first strainer becomes clogged, the second strainer is automatically switched into service while the first strainer is back-flushed to clean the screen. The flushing water discharges to the drainage system.  When the second strainer becomes clogged, the process reverses and the first strainer is automatically switched into service.

Cooling Water System

The cooling water system is usually supplied as part of the generating equipment package. It connects to a raw water pipe, screens or filters the water further, circulates it through the generator heat exchangers and discharges the warmed water to the tailrace.

Bearing Water System

The bearing water system is also usually supplied as part of the generating equipment package. It takes strained water from the raw water system, passes it through a screen to remove all but the finest particles, and supplies it to the lower turbine bearing gland.

Service Water System

The service water system distributes the strained raw water to hose connection points throughout the powerhouse where it is used for washing, cleaning and flushing toilets. If the water is safe for bathing, it is also used to supply showers and wash basins in the change rooms and washrooms.

Potable Water System

Many powerhouses use bottled water for drinking. Bottled water dispensers supply hot and cold water at stations in the lunch room, offices and workshop. Manned stations often have a potable water system. Such systems are connected to the service water system. The water is screened, filtered, decontaminated and disinfected to meet government standards for safe drinking water. The potable water system distributes the water to drinking fountains, kitchen, showers, wash hand basins and toilets.

Eye Wash Station/Decontamination Shower

The station direct current (DC) system is equipped with a battery room. In most stations, the batteries are lead-acid type, and acid splashes are a potential hazard. The primary emergency treatment for an acid splash is immediate dilution and flushing with water. Facilities are provided outside the door of the battery room to provide this emergency treatment. These comprise an eye wash station and sometimes a decontamination shower. These are connected to the potable water system, if present. 

Fire Protection Water System

The fire protection system is controlled by the fire detection system and supplies water for fire suppression. Design of the system is governed by Fire Codes, and is built of components certified in accordance with the fire safety regulations. The fire protection water system requires at least two independent water sources for redundancy. At least one of these systems should be a gravity supply. When one of the sources is a penstock (which might be dewatered for inspection and maintenance), a valved connection should be made to another penstock that will not be out of service at the same time. Where one of the sources is pumped, specially-certified fire water pumps are used.

The water is supplied to zoned sprinkler systems, hose reel stations and deluge systems. The two primary deluge systems are installed in each generator and around each generator transformer.

Station Air System

The Station Air System comprises compressors, oil cleaners, drying filters, accumulators, and the air distribution piping system, which delivers compressed air to all points in the powerhouse where air might be required to operate tools or clean equipment.

HVAC System

The HVAC (heating, ventilating and cooling) system is not connected to the Station Air System. While the latter distributes compressed air in small bore pipes, the HVAC system delivers low pressure air in large ducts.

The HVAC system draws fresh air from outside the powerhouse via a bird-proof screen and filter pack, heats or cools it, and distributes it throughout the powerhouse. Ventilation is carried out according to a consistent philosophy. For example, based on the fact that warm air is less dense and therefore rises, it is common to supply air to the lower floors of the powerhouse and remove heated air from the ceiling of the powerhouse.

The HVAC maintains a slight overpressure in the powerhouse (typically 1” of water) to ensure that any leakage through outside doors and the building envelope will flow from the inside out. This helps to keep the powerhouse equipment dust free.

In some powerhouses, the HVAC system delivers cool air to the primary sources of heat such as the control room, exciter transformers and station service transformers, and  exhausts the air separately so that the heat does not enter the powerhouse air space.

The HVAC system also targets areas with potentially contaminated air, such as workshop welding hoods (to exhaust any welding fumes) and battery rooms (to exhaust acid fumes and the hydrogen gas formed when lead-acid batteries are charged). 

The HVAC system is supervised by a control system, but the Fire Detection System overrides this system when a fire is detected, and stops or reverses air flow in parts of the HVAC system to exhaust smoke from the affected zone while providing safe egress to personnel. 

Draft Tube Suppression System

The Draft Tube Suppression air system is optional ancillary equipment that might form part of the Generating Equipment supply package. (See here.

Cranes and Hoists

Powerhouse Crane

The main hoisting equipment in a powerhouse is the Powerhouse Crane. This is an overhead bridge crane supported on the powerhouse structure (or walls of an underground powerhouse). Because it can only be installed once the powerhouse structure is substantially complete, it is not usually installed in time for assembling the units. However, it is used during routine unit maintenance for dismantling and reassembly of the generating components. 

Oil Handling System

Most larger stations have a dedicated oil handling system with connections near the transformers and the bearings of the generating units. When servicing one of these components, the oil is removed and pumped to a “dirty oil tank” in the oil room. Then the component is refilled with oil from the “clean oil tank” in the same room. A portable oil conditioning system is then connected between the tanks and the dirty oil is filtered to approximately 20 micron, free, emulsified and dissolved water are removed and the conditioned oil pumped into the clean oil tank.

Station Drainage System

The station drainage system includes floor drains, turbine pit drains, and deluge water drains, all drainage piping, the powerhouse wet sump, an oil/water separator, oil skimmer, sump pumps and piping to the tailrace.

Sanitary System

This system collects effluent from the powerhouse changerooms (sinks, showers and toilets) and either processes it to a high standard for discharge to the tailrace, or stores it in a sewage sump at the lowest level of the powerhouse and pumps it to a service holding tank at ground level. This sump is pumped out at intervals by a sanitary waste contractor and treated in a remote facility.

Unit Unwatering System

The purpose of the unit unwatering system is to remove water from the turbines when required for inspection or maintenance. The unwatering pumps are located in the powerhouse dry sump and is connected by piping to a low point in the draft tubes of each turbine. 

The pumps are sized to unwater any one unit in a specified time. If there is a turbine inlet valve and gates at the end of the draft tube, only the volume of water in the turbine needs to be pumped. If there is no TIV, the penstock may be drained by gravity to tailwater level. The remaining water in the penstock must then be pumped out in addition to the volume of water in the turbine. Similarly, the pumped volume increases if the draft tube gates are located far downstream (which in not uncommon in underground powerhouses).

The large unwatering pumps discharge directly to the Tailrace (downstream of the isolation gates).

In the event that the drainage pumps in the wet sump are unable to handle the drainage inflows (as happens, for example, when a powerhouse floods), the unwatering system has a connection to the wet sump. Opening the connection valve allows the large unwatering pumps to provide a large boost to the pumping capacity available for the wet sump.


Most modern powerhouses are equipped with an elevator from the control room level to the lowest floor level in the powerhouse. This is not a luxury; it provides safe evacuation in case a worker is injured, it transports small parts and tools, and reduces downtime for some unit outages by efficiently transporting maintenance staff between floors.

The elevator normally has hospital doors – the doors open flush with the interior walls of the elevator so that tools carts and stretchers can be moved in and out easily.

Maintenance Workshop and Stores

Many older stations, especially those in remote locations, have well-equipped mechanical and electrical workshops that can be used for major refurbishment of equipment. These days, it is more common to have few staff at hydro stations, and workshops are only equipped for minor repairs. When larger jobs are required, teams of maintenance staff visit the station. Usually parts are removed and replaced. When refurbishment of major parts is necessary, they are removed and shipped off to a mechanical contractor. 

Diesel Gen-Set

Most stations are equipped with at least one diesel generator (diesel gen-set) to provide station power in the event that there is no connection to the transmission line. This generator is equipped to start up automatically upon loss of station service power. It then provides voltage to an Automatic Transfer Switch (ATS). This switch closes when there is no power on the station service system, but power available from the diesel gen-set. Non-essential supplies are turned off when the power fails, so the gen-set only needs to power the essential services to run and control the station.

When the power returns to the station service system, the ATS disconnects the generator and it shuts down.A day tank is usually provided to supply the gen-set with fuel for 24 hours. To prevent diesel spills, the day tank has a double wall, or the tank foundation is surrounded by a berm to catch any spilled fuel.

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