As part of a fire protection upgrade at the 320-mw Roxburgh hydro project in New Zealand, owner Contact Energy Limited divided the 150-meter-long powerhouse into separate fire zones by installing interior firewalls and doors. The firewalls and doors were intended to prevent the spread of fire from one section of the powerhouse to the next.
However, while solving one problem, the doors created another. Closing the doors restricted air flow through the building and raised temperatures inside the powerhouse by as much as 20 degrees Celsius (C) in the summer. These high temperatures forced Contact Energy to implement machine de-ratings to keep the units’ operating temperatures within acceptable limits. Obviously, this was not an acceptable long-term situation.
A magnetic door retainer installed on a fire door between generator compartments at the 320-mw Roxburgh powerhouse allows the doors to be held open to improve ventilation, without compromising fire security.
After experimenting with several solutions, Contact Energy installed electro-magnetic door retainers on the fire doors to hold them open. These retainers are connected to the fire alarm and electrical system. When the fire alarm is activated or there is a power failure, magnetic plates on the door retainers release, and the doors automatically close. In this way, the doors between sections of the powerhouse can stay open without compromising fire safety. This solution, combined with others to improve ventilation, has reduced temperatures in the powerhouse to acceptable levels.
How Roxburgh was set up
The Roxburgh powerhouse contains eight vertical turbine-generating units, as well as two horizontal auxiliary units. The machine hall consists of three main floor levels, with cable galleries on the downstream side that run the length of the building. The unloading bay and workshops are on a level about 3 meters higher than the main machine floor, but the unloading bay and machine hall are all one big open space. The generator stators, located on the second floor level, are enclosed in thick-walled octagonal concrete housing. A makeup air intake is located in an upstream corner of each housing. The turbines are on the bottom floor level.
Atmospheric temperatures at Roxburgh range from 35 C in the summer to -10 C in the winter. To help compensate for low temperatures in the powerhouse in the winter, machine designers arranged the generator “closed circuit” cooling system so that warm air could be bled off and used to heat the powerhouse. This warm air is discharged into the main machine hall or at the higher workshop and unloading bay level.
Fire protection work at Roxburgh
When Roxburgh was converted to a state-owned enterprise in the late 1980s, ECNZ part of which later became Contact Energy had to obtain commercial insurance coverage. As part of obtaining the coverage, the risk of a station fire had to be mitigated. So, in the mid-1990s, ECNZ implemented major fire protection upgrades at the station. This included compartmentalization of the powerhouse into several separate fire zones to prevent the spread of any smoke or fire. The stator floor was one of these zones, and the generator stators are in individual housings within this zone.
The compartmentalization simply involved constructing fire-rated barriers separating the various zones. The larger openings required double-sided firewalls with doors, while the smaller openings between zones only required insertion of approved firestopping products. The most significant fire barrier is the one between the stator floor and the cable gallery on the downstream side of the powerhouse. In most cases, the other zones only required installation of fire-rated doors.
The compartmentalization work was carried out by a local facilities contractor over a three-month period in 1995. All doors were fitted with heavy-duty adjustable closers, supplied by a local aluminum joinery company.
Two unfortunate, although predictable, side effects of this work were restricted airflow and higher temperatures throughout the powerhouse. After the powerhouse was compartmentalized, summer temperatures reached the mid-30s C on the machine hall floor and the mid-40s C on the stator floor. Generally, temperatures inside the facility peaked at about 8 p.m. each evening.
During these periods of high temperatures, Contact Energy had to implement machine de-ratings to keep the unit operating temperatures within acceptable limits. De-rating, even on a temporary basis, is not a preferable alternative because it limits operational flexibility and can result in lost market opportunities or spilling of water. The utility had to develop a long-term solution.
Ventilation solutions attempted
Traditional summer practice at Roxburgh, even before the fire protection upgrades, was to open the generator air vents and use modified ducting to discharge the hot air outside the building. It also was normal practice to open the main powerhouse door and start the extraction fans installed high in the wall at the other end of the machine hall.
These two actions were necessary because of declining stator air cooler effectiveness. This decline accentuated deficiencies in the generator cooling system. The manufacturer had modified the cooling circuit design after the factory test in order to comply with its guaranteed windage and friction loss limits. This modification, which involved blocking the through-rotor air movement, meant the cooling system was less than optimum. Effectiveness was not tested during commissioning because of chronic power shortages.
In 1997, Contact Energy commenced a program to address the generator cooling issue. Personnel no longer open the air vents because the configuration of the vents effectively takes one of the eight air coolers within the generator out of the circuit. The practice is acceptable when the replacement air is colder than the output from the cooler, but this is not the case with a compartmentalized station. Everything just gets hotter.
Personnel still open the main powerhouse door on really hot days, but this practice only provides some relief at the unloading bay level and, to a lesser extent, at the main machine floor level. One disadvantage is the introduction of airborne grit, which causes increased wear on the exciter commutators of the first few machines inside the door.
Obviously, the simplest way to reduce the temperature on the stator floor was to open the doors between the compartments (fire zones). This would create a larger area for the heat to disperse and enable the hot air to mix with the cooler air in the galleries. Contact Energy initially attempted this practice, using simple wooden wedges to hold open the doors. However, when the next building services/ insurance survey was carried out, all wedges had to be removed because they compromised fire security.
Installing electro-magnetic door retainers
The solution proved to be simple. In 1999, Contact Energy replaced the wedges with electro-magnetic door retainers and automatic door closers supplied and fitted by a local fire systems servicing contractor. The retainers consist of a plate affixed to the back of the door, near the top, and a solenoid unit positioned on the wall at the same height as the plate on the door. When the door is opened, these two pieces touch and form a magnetic seal that holds the door open. The solenoid unit is controlled by the fire alarm system so that it is continuously energized.
In the event a fire alarm is activated or there is a power failure, the solenoid releases its magnetism, allowing the door to close. In addition, the units feature a manual release button on the solenoid to allow the door to be closed without activation of the fire alarm system. This is a failsafe system that allows Contact Energy to maintain the integrity of the fire separation barriers at Roxburgh, while allowing access to the cooler air in the galleries.
Ventilation of the stator floor was further enhanced by introducing cool air from a dam drainage gallery. Work, carried out in 2002, involved drilling a hole through the concrete upstream side of the powerhouse into a vertical duct from the drainage gallery and installing a fan to pull the cool air into the powerhouse.
The combination of reducing the heat source and improving ventilation at Roxburgh has reduced temperature on the stator floor level to a very acceptable mid-20s C range.
All the work now being performed relates to the generator coolers. Seven of the eight machines have been modified, and the final stator air cooler will be reconfigured as part of a current machine refurbishment program.
By Colin McDonald, generation engineer, Contact Energy Limited, Fruitgrowers Road, P.O. Box 25, Clyde 9341 New Zealand; (64) 3-4400324; E-mail: email@example.com.