Recent events in the Mont Blanc Tunnel and the resultant loss of life has again emphasised the importance of effective tunnel ventilation. Failings highlighted by the disaster led the French government to examine other tunnels, and Prime Minister Lionel Jospin launched a security audit into 37 tunnels across France to investigate, among other things, the effectiveness of ventilation.
Currently, there are no regulations governing tunnels ventilation in Europe, although this could change. This would result in the need to upgrade existing tunnels to incorporate the latest technology.
Fire and smoke dampers should form an important part of ventilation systems and be installed primarily to prevent the spread of fire and smoke. Consequently, they provide safe routes for passenger evacuation and allow fire crews in to tackle the fire. This is achieved by opening dampers local to the fire source, and closing other dampers in the tunnel to cause smoke to be exhausted through ventilation shafts. This enables smoke extraction fans in transverse ventilation systems to be more effective as air is extracted from near the fire.
Dampers are therefore key elements in metro systems as well as road and rail tunnels. Dependent upon the tunnel ventilation strategy, dampers are either installed in the tunnel roof, in the side wall of the running tunnels or in the ventilation shafts.
Criteria for damper selection can be summarised as:
– The damper period of integrity under fire conditions
– The damper leakage
– Ability to operate at a high temperature
– Damper structural integrity at high pressure drop
– Actuation
– Installation strategy
Fire testing
BS 476 Part 20 is generally accepted as the standard for fire testing. It requires the damper to maintain integrity for a predetermined time when subjected to a maximum furnace temperature of 1050o C. Integrity is defined as the damper’s ability to prevent the passage of fire from one side of the closed blades to the other.
The test is typically conducted at a specialist fire testing laboratory such as the UK’s Warrington Fire Research Centre and should be verified by an independent authority such as Lloyds.
Both horizontal and vertical testing of a damper should be conducted to ensure that the fire damper will operate and maintain its integrity when installed in the tunnel roof or side wall. The period of integrity required will depend on a number of factors such as the anticipated evacuation time. The most frequently specified fire rating is 2h, although in Hong Kong 4h is specified, due to the population density. The maximum size single damper module should be tested with its actuator. Calidair has fire tested a 2.1m x 2.5m module with blades incorporating the ‘firelock’ principle. This ensures blades remain closed in a fire without loss of integrity.
Leakage
Smoke inhalation is the major cause of fire fatalities. Minimising smoke leakage through the closed damper is critical for life preservation. It is expressed in m3/s/m2 of damper surface area at a given differential pressure. Low leakage rates are achieved by flexible sprung steel sides between the damper casing and well-engineered blades.
Leakage figures should be verified by independent test at a certified laboratory such as BSRIA. A leakage rate of 0.1m3/s/m2 at a differential pressure of 2kN/m2 is generally accepted as the maximum although Calidair has achieved 0.056m3/s/m2. Leakage tests should also be conducted at the manufacturer to check damper performance prior to delivery. A typical leakage test involves setting up a test rig with a sample damper mounted on a transformation plate. This is connected to a plenum and an extract fan via flow straighteners.
Sensors measure static pressure in the plenum/dampers assembly and volume flow. With dampers closed the fan speed can be adjusted to develop the correct pressure according to the test procedure. Leakage should be measured across a band of different pressures with a sensor in the plenum measuring flow.
High temperature operation
Dampers for smoke and fume control must operate at high temperature. Specifications call for operation at 250o C for up to 2h, and in some cases temperatures as high as 400o C are specified. Dampers have to be opened and closed during this period to control the spread of smoke to ensure safe evacuation routes.
Testing to verify operation at high temperature has to be undertaken in a furnace using a fully operational test damper complete with actuator and microswitches. For this test the actuator is inside the furnace which is rapidly brought up to temperature, after which the damper is operated from the open to the closed position every 15 minutes for the duration of the test. The damper’s microswitches should be used to ensure the damper has fully opened and closed.
It is imperative that the damper, actuator and micro switches are tested as an assembly to ensure the actuator can cope with any increased loads imposed by the damper due to expansion. Practical tests have shown it necessary to increase the normal actuator torque output to ensure the efficient operation of the damper at high temperature.
Specially designed pneumatic actuators with viton seals and high temperature grease will accommodate the high temperature operation without insulation, whereas the majority of electric actuators will require a thermal enclosure.
Structural integrity
In metro applications, where trains are now travelling at high speeds and with greater regularity, dampers must be designed to accommodate pressure differentials of up to 6kPa. The pressure created by the piston effect of the passing trains causes a cyclic load on the damper assembly. This can cause fatigue due to the frequency of the trains and must be considered when selecting dampers. As well as unreliability, failure could cause an accident if parts were to fall into the path of a train.
Calidair damper blades have been subjected to analysis and fatigue testing for 7.8 M pressure cycles at 6kPa. The structural fatigue testing was carried out at Hong Kong University using a specially designed test rig for long-term cyclic tests. This verified the integrity of the welds and overall blade assembly. Finite element analysis has also been undertaken to qualify the damper for these high pressure piston effect differentials.
Specifications now require dampers to accommodate a differential pressure of 6kPa. As part of a test procedure the specification will also now call for criteria on measured deflection of the blades at a given pressure differential. This type of test can be carried out in-house. Conventional measuring equipment can be set up to measure the maximum deflection at specific points on the damper structure.
Following this, a visual inspection of principal welds should be undertaken together with a series of functional tests, to ensure the correct operational function.
Materials
Tunnels can experience highly corrosive environments emanating from vehicle fumes and the surrounding geology. Materials of construction should be either stainless steel or galvanised mild steel. Moving parts such as damper shafts and linkage should always be of stainless steel. The galvanised coating should be a minimum of 600g/m2 which is generally acceptable for electrified train tunnels. Stainless steel should be considered where diesel trains are present. Aluminium is not considered suitable for tunnel dampers used for fire protection due to its poor performance at elevated temperatures.
Improvements in the safety of road tunnels, leading to reliability demands, have led to many installations being supplied with fire dampers manufactured from 316TI stainless steel, which is particularly resistant to the corrosive effects of vehicle emissions. Both 304 and 316 stainless steel are commonly used in road and rail tunnels, which increases the purchase price of the dampers by approximately 15 per cent.
The use of galvanised mild steel has been qualified for a 25-year life; the preference is to use pre-galvanised sheet rather than hot dipping the final damper assembly. The hot dipping process can present problems with distortion of the casing and blades during the process and this should be avoided. To ensure the
25-year life, the galvanised thickness has to be at least 600g/m2.
Dampers manufactured from pre-galvanised sheet are treated with galvanised paint on the cut edges and welds to ensure that corrosion will not occur in these areas.
Painted dampers are not recommended as the paint can be a source of toxic fumes and ignite in fire, transferring the fire across the dampers. On the moving parts, the paint surface can easily be damaged during installation, which could in time lead to corrosion.
Operational speed and safety
Damper operating times are an important factor for improved operational safety of the overall ventilation system.
Consultants are now stipulating damper opening and closing times as low as five seconds, where as many specifications in the past stated operating times in the region of 10-20 seconds.
In a fire, time is of the essence, either to contain the fire or to manage the movement of smoke or fresh air. In this respect the damper should be designed to respond rapidly to the control signal. Its operation should be smooth and controlled as the blades open or close against their mechanical stops.
The choice of actuation dictates the damper operating time; a single acting device with a mechanical spring to either open or close the damper is often selected to achieve a fail-safe operation with rapid response time.
Pneumatic actuators generally operate at higher speeds than electric. They are cost effective when compared with electric, however the system will require compressed air at between 5 to 8 bar. If a pneumatic compressor system is already installed to operate platform doors or points in a metro station the choice of pneumatic actuators to power the fire and control dampers would be the logical solution.
Total system failure can be accommodated by the installation of an uninterrupted power supply (UPS) system and in the case of pneumatic actuators an in-line air reservoir with solenoid valve.
Typically, dampers may be required to operate five times in one hour as part of the system safety strategy. As an alternative to springs, electric actuators can be supplied with a battery back-up to provide this facility. These can be mounted within the actuator casing or in a remote location. If the specification calls for an operating temperature of 250º C for one hour the batteries have to be protected with a thermal enclosure. From the aspect of reliability, battery life needs to be considered, as actuator suppliers recommend batteries are changed within a two- to five-year period.
Damper actuation
Both rotary and linear actuators can be used to operate ventilation dampers. The 90º rotary type is often the first choice, as the moving parts are normally self contained within the actuator casing, whereas the linear type will at times be static with the piston rod exposed to the environment.
The latter have a lower reliability as the exposed piston rod can be subjected to corrosion or damage during installation and operation.
Propriety actuators, designed for use across a wide range of industrial applications have a high level of reliability. Many of the semi-rotary types are used in the process industry to actuate valves.
Pneumatic actuators of the semi-rotary type often use a rack and pinion to change the linear movement of the pistons into a rotary action.
These units are simple in their construction and operation, easily maintained, low cost and highly reliable. Pneumatic actuators are available to operate at high temperature without thermal enclosures by using viton seals and high-temperature grease. There will, however, still be a loss in performance of the spring at high temperature and this must be considered when selecting the actuators. Actuators must be oversized to compensate for this and consultants often specify a 1.5x safety factor on the operating torques.
As part of safety strategy, electric actuators are often required to be fitted with a manual override so that the damper can still be operated during a power failure. The manual override must be accessible, an important issue when installing the damper.
If the actuator is insulated for operation at high temperature then the manual override must stand clear of the insulation. Microswitches must be provided to monitor the damper position at the control panel and can be integral within the actuator casing or mounted direct to the damper linkage mechanism. Again they should be rated for high temperature use or insulated to protect them.
Consultants’ specifications now call for components such as actuators to be tested to up to 300,000 cycles. In reality, the damper may only operate two to three times per day, but operators are seeking at least 25 years’ life. From the aspect of improved safety and reliability, it is important to select actuators with a proven record from a competent manufacturer.
There is a wide choice of manufacturers of double-acting electric actuators, but for single-acting spring return the choice is quite limited. Add the requirement to operate at high speed and the selection is further reduced.
Some rotary electric actuators can be bulky and in some cases more difficult to install than linear. This is particularly the case where an actuator is required to be spring return and fitted with a manual override and battery back-up.
The linear electric actuator has a particular advantage in respect that the drive is usually direct via a ball screw device. This design has a minimum number of components and moving parts leading to lower maintenance and increased reliability.
The conventional rotary electric actuator drives through a gearbox and drive train, although alternative designs utilise a combination of an electro hydraulic drive package powering either a vane type or piston type actuator. These can be provided with a mechanical spring return device as fitted to pneumatic actuators.
The later type can offer high response times in a cost-effective package with high levels of reliability, the electro hydraulic power pack is simple in its operation and is compact for ease of installation.
The principal is to use a hydraulic gear pump driven by an electric motor. A small hydraulic reservoir supplies oil to the pump which powers the piston actuator via a solenoid valve.
The system operates at low pressure in the region
of 10 bar. Encased in a thermal enclosure the
electro hydraulic actuator is also suitable for high temperature use.
Damper installation
Typically, dampers will be installed direct to
structural openings or to steel ventilation ducts. In the case of fan isolation dampers mounting will be direct to the fan transition piece.
For structural openings the damper can be mounted either on the face or inside the opening using approved frames or clamp plates. Duct mounting is via flanges provided on the damper case drilled to accept standard bolts and nuts.
Experience has shown that the correct installation of the damper is a major contributory factor to its efficient operation and therefore the safety of the overall system.
Contractors must ensure that the damper is mounted flat on the ventilation opening face. Any distortion and misalignment may cause the damper to lock up resulting in a malfunction.
Dampers for large openings are made up from modules assembled on site. The Calidair damper incorporates a special damper casing design that enables the modules to be bolted together without a separate mounting frame for many installations.
The integrity of the overall structure is critical to the efficient operation of the damper. For ceiling-mounted damper on a road tunnel it is recommended that the damper be mounted “on top” of the opening rather than inside the opening.
This simple installation enables easy access to the damper’s actuator, maximising the free cross-sectional area, which reduces pressure drop. It also removes the risk of the damper breaking free of its mounting and falling down into the path of vehicles below.
Dampers can be mounted inside concrete openings, although they will have a reduced free area and the actuator will require a special mounting arrangement to elevate it above the damper casing. This will mean that the actuator will be exposed to the airstream which is not an ideal situation but one that can be accommodated by Calidair.
Operation safety analysis
The damper design and all associated components should be subjected to operation safety analysis in order to provide recommendations on reliability, maintainability, availability and safety (RAMS). All the analyses should consider all the components, both moveable and static, that will affect the dampers operation.
Analysis should be based upon quantitative data as far as possible and demonstrate that the dampers meet the operator’s requirements. Typically operators are seeking availability of over 99.8% and Mean Time to Repair (MTTR) of 10 hours.
Analysis of the Calidair damper has identified that the event ‘damper fails to open/close’ falls into the ‘rare to occur category – one in every 10 to 100 years’, while the event ‘damper failure in case of fire’ and ‘fails to indicate damper failure’ falls into the ‘incredible to occur category – one in every 10,000 years’.
Maintenance
Equally important to ensure high levels of safety and reliability is the maintenance of the damper and actuator.
Dampers should be operated and inspected regularly. Foreign bodies drawn through the ventilation system can foul the damper blade mechanism causing failure, which could be fatal in a fire.
The damper unit itself is a highly reliable device with rare operational problems. However, the actuation system should be the engineer’s focus of attention.
Pneumatic actuators must receive clean and dry air at between 5 to 8 bar. These units are usually of the piston or vane type with minimal maintenance. Regular operation will ensure an acceptable life. Piston and shaft seal replacement can be done on site.
Electric actuators are more complex and require their own specific maintenance procedure. Electric drive motors and control boards can be replaced on site, but usually the complete actuator will probably be replaced as a unit.
Accessibility to the actuator on site is an important factor, not only for maintenance but also in the case of an emergency the damper may be required to be operated manually.
Summary
There can be no compromise where safety is concerned. Fire dampers may only be expected to operate once in their life, so long-term reliability is paramount.
Damper selection should therefore be based on certified products of proven quality and performance.