1898

Ventilation of long tunnels

R.K. Khanna Anand Mincons, New Delhi, India

SYNOPSIS: It is essential to design, install and maintain a proper standard of ventilation of tunnels for achieving improved productivity with due regard to safety. Every tunnels is unique in its size and length, and method of construction and engagement of construction equipment. Therefore it is essential to consider all the relevant details to assess the various parameters of ventilation like the required airflow at the face-end, fan discharge, fan pressure etc. so that a suitable combination of type and size flexible ducts and the ventilation fans are selected for specific installation. The paper highlights the scope and application of flexible ducts and fans for ventilation of tunnels, besides method of calculating & measuring these parameters.

1. INTRODUCTION

Adequate quantity of air-flow is necessary for safe and efficient progress of long drivages. Because of the increasing size of lengths of tunnels in the hydro-electric projects, it becomes essential to design and maintain a proper standard of ventilation of the face-ends. The traditional method of using metal ducts has now, become obsolete because its' inherent disadvantages of higher leakage rates, heavier weights, installation of metal ducts at height etc. Flexible Tubings offer an excellent scope for ventilation of long headings which are efficient and cost-effective. A suitable ventilation system must be provided commensurate with the type of drivage and method of construction. As the face advances, the air quantity reaching the face gradually decreases due to both increasing resistance of the ductline and increasing leakage. When the air-flow at the face gets reduced to a minimum acceptable level. one or more fans are installed, in series, to maintain the desired flow of air at the face. It is, therefore, essential for ventilation system of long tunnels that the system is appropriately designed, operated and maintained to provide a reliable, efficient and cost-effective solution.

2. SYSTEM OF VENTILATION

Most systems fall into one of the following categories:

1. Simple forcing & exhausting

- With a single Fan (Figure -1a).

- With two or more fans outside the drivage (Figure -1 b).

- With fans spaced along the duct-line (Figure -1 c).

2. Forcing main duct-line plus exhausting overlap duct-line i.e. a forcing overlap system (Figure 1d).

3. Exhausting main duct-line plus forcing overlap duct-line i.e. exhausting overlap system (Figure 1 e).

The comparative merits & demerits of forcing and exhausting systems of auxiliary ventilation are:

l. The high velocity of the discharge air at the face from a forcing system produces a scouring effect, breaking up any tendency of accumulation of fumes at the face. It also results in a cooling effect by lowering the effective temperature.

2. With a forcing system. the diesel fumes emitted out bye in the heading are carried away from the face along the tunnel. With an exhausting system, it is taken out through the duct out bye.

3. With a forcing system, unsupported flexible ducts offer (lower resistance, lower fan duty, lower cost and easier transport) With an exhausting system, either rigid or supported, flexible tubings have to be used to withstand the high suction pressure developed by the auxiliary fan.

World Tunnel Congress 2008 - Underground Facilities for Better Environment and Safety - India

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4. With a forcing system, the air reaching the face usually takes up less heat and moisture. Leaks in the duct-line are more easily detected.

5. With an exhausting system, airborne and blasting fumes are taken away from the heading. It maintains fresh air within the heading and it is possible to filter the airborne dust. The main disadvantage is that there is little control of the air flow reaching upto the face of the heading.

In nutshell, forcing system in combination with Overlap System of Ventilation offers an ideal choice for proper and effective system of ventilation mainly because face can be defumed quickly after the "blast" and provide fresh air to the persons during drilling, mucking, charging operations etc.

3. PRINICIPALS OF DESIGN

3.1 Criteria for asssessment of air flow at face

The client, normally, specifies the criteria of ventilation of the tunnels. The standard, generally, adopted are:

(i) The ventilating system shall be of such efficiency that the average air velocity in the largest excavated profile, in the return airway, is not less than 0.3 meter per second. In case the presence of methane is detected or suspected, this value shall be increased to 0.5 meter per second.

(ii) The main ventilation system shall ensure that following minimum Fresh air volume requirements are satisfied at all times:

(a) 4.25 cubic meter per minute for each person employed in the underground works at one time.

(b) A minimum of 2.00 m3 of air per minute shall be supplied for each horse power of diesel powered equipment deployed in the underground works at any one time.

(c) A minimum clearance of 200 mm between the duct and maximum height of mobile equipment should be maintained.

Even after assessing the required air-flow based upon the criteria for assessment as on minimum standard mentioned above, it is essential

to further consider the effect of velocity of air in the return air-way. A minimum return airway velocity of 0.3 m per second may be adequate for short-lengths of tunnels upto about 500 m but the same velocity will not be adequate for tunnels of lengths more than 500 m, especially when Drill I Blast I Load method is adopted for construction of tunnels. Therefore, it may be necessary to maintain air-flow so as to provide return-airway velocity of about 0.4 to 0.6 m per second for tunnels having lengths more than 2500 m.

4. SELECTION OF SIZE & TYPE OF FLEXIBLE TUBINGS

After assessing the air-flow at the face, it is necessary to consider the following for selecting the size and type of flexible tubings out of the following :

(a) Lay Flat Flexible Tubings.

(b) Semi Rigid Flexible Tubings with normal I close pitch.

(c) Light Duty Semi Rigid Flexible Tubings.

(d) Elliptical Shape Flexible Tubings 1 Twin Flexible Tubings.

Each of these tubings has special application to suit site specific conditions, so that the overall resistance of the duct-line is reduced. This would help in containing the rating of fan motor vis-a-vis effect appropriate reduction in the overall cost of ventilation. Lay Flat Flexible Tubings are best suited for primary ventilation. These are light in weight and are available in standard lengths of 10 m, 20 m, 30 m & 100 meters with diameter ranging from 200 mm to 3000 mm. Longer lengths are recommended as the number of joints can be reduced. Semi Rigid Flexible Tubings with normal pitch are suitable for both" Pull & Push" system of ventilation. These are generally used for secondary ventilation where it's diameter could be upto 1000 mm. With higher diameters, these tubings become too heavy to handle and install. (900 mm Dia x 4 m long Semi Rigid Flexible Tubings I Twin Flexible Tubings with 75 mm pitch would weight about 82 kg. per length of 4 mtr). As the diameter increases, the suction pressure against the tendency to collapse decreases. However, Light Duty Semi Rigid

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Flexible Tubings upto 1700 mm Dia can be conveniently used where limited head room is available. For example, in an Adit (Size : 6.0 m x 6.5 m Dia D-Shape), the head room does not permit installation of two parallel duct-lines (Lay Flat Flexible Type) having dia in excess of 1200 mm, otherwise the tubings will get fouled with the mobile equipment in the event stoppage of fan causing the duct to collapse fully. In such, a situation, one can use two parallel Light Duty Semi Rigid Flexible Tubings having diameter upto 1700 mm Dia. In case of long but smaller head-room, say 2 m x 2.5 m to 4.5 m x 4.5 m sections of the tunnels of relatively medium to long lengths of tunnels, there is no escape except using Flexible Elliptical Shape Tubings as these permit greater cross-sectional area for reducing the load on the rating of fan-motors as well as in reducing power-cost. These ducts offers excellent scope for an efficient and effective system for mini-hydel projects. After having decided the type of tubings, the next question arises, "what should be the size 1 diameter I shape of the tubings" for a specific area of application. Obviously, the bigger, the better. Again the need to maintain a minimum clearance, say 200 mm between the mobile equipment and duct-line would determine the diameter, shape and type of tubings for primary ventilation. It is not uncommon to experience frequent stoppage of fan due to myriad reasons. If Lay Flat Flexible Tubings are used. one can see it's effect on the vertical clearance between the fully collapsed duct and the mobile equipment. (Fig. 2) . When confronted with such a situation, any of the following configuration/ suspension can be used in limited head-room conditions: 1. Use Light Duty Semi Rigid Flexible Tubings. 2. Use Flexible Elliptical Tubings (upto a max

vertical height of 1m ). 3. Use double 1 triple suspension of Lay Flat

Flexible Tubings & Light Duty Semi Rigid Flexible Tubings.

4. Twin Flexible Tubings.

5. EFFECT OF DIAMETER OF TUBINGS

The static pressure to effect a specific air-flow through a tubings is inversely proportional to it's diameter (D )5 and square of the velocity of air in

ducts. From economic considerations, the size of the ducts should be selected in such a manner so as to permit a velocity of air, preferably, around 10 m per second in the duct. Hence, the size of tubings assumes a very critical importance both in reducing the resistance of duct-line with it's cascading effect on the rating of fan-motor and power-cost. Figure 3 reveals broad the effect. The effect of change in diameter of duct has been shown in the graph & table in Figure 3 and the result, thereof, on the various parameters of ventilation viz required fan discharge, fan pressure, minimum K.W. rating of the fan motor etc. have also illustrated in Fig 3. Following parameters have been considered as an example: 1. Length of Tunnel - 2640 m

2. Size of Tunnel - 3.9 m dia 3. Area of Tunnel - 13.69 sq.m 4. Desired flow, % at Face - 5.2 m3 /sec. 5. Return Air Velocity in tune - 0.37 m/sec.

For a given situation, it is essential to select appropriate size type and size of ducts in context of the mobile equipment (BHP, height, number equipment etc.). In limited roof conditions, it makes sense to use either Flexible Elliptical Ducts or Semi Rigid Flexible Tubings, even though the initial cost may slightly higher as these increase in the cost would be offset much more by the recurrent saving in power-cost, capital investment on the fans and captive diesel generators.

6. LIMITATION OF AXIAL FLOW FANS Axial fans are designed to operate between a certain-range of fan discharge and corresponding fan pressure depending upon its design, diameter, rating, RPM of motor etc. It is essential to avoid build-up of excessive resistance against air-flow in tubings connected to axial flow fans, thereby obviating the tendency of failure of the fans. Figure 4 is a typical group of fan characteristic curves of fan discharge Vrs pressure input power and efficiency. As resistance is increased from zero with the increase of lengths of tubing, the pressure rises between the “stable range” as shown in the above Figure 4. With further increase in resistance beyond the “stable range” the fan delivers still less air but at

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Figure 2.

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Figure 3.

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Figure 4.

a lower pressure. In this “stall zone” the fan delivers still less air but at a lower pressure. In this “stall zone, air separates from the railing edges of the blades, changing the sound level markedly. In this “stall zone”, for the same pressure, the fan has a tendency to deliver varying volumes of air. The manometer and the motor ammeter can be seen to oscillate. The vibration in the blades and in the shaft of fan motor can cause mechanical failure, so fans must not be operated in this “stall zone.” For safe operation of the axial flow fans, excessive resistance against air-flow should be avoided and the size of the duct-line should be planned well in advance to avoid such occurrence, which may lead to failure of fans.

7. ASSESSMENT OF PARAMETERS OF VENTILATION FOR LONG TUNNELS

For affecting a required air-flow at the face of tunnels, various methods are deployed in assessing the required fan discharge, fan pressure, rating of fan motor etc. nomograhs I Tables 1 formulae and software programs are used. Amongst these, widely accepted Swiss Standard: SIA-196, is used by most of the companies engaged in planning and design of ventilation systems for tunnel ventilation. These standards were drawn-up 1 modified 1 revised from time to time after study of actual site studies for numerous applications of flexible ducts in ventilation of tunnels during construction. In nutshell, the standard works out the required static

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pressure, leakage-rate, fan discharge and minimum required rating of fan motor on the following :

1. Type of tubings (Class S. A. B, depending it's condition).

2. Required air-flow, Qo at face.

3. Length of duct-line

4. Diameter of tubings.

5. Length 1 Diameter Ratio.

6. Site Height & Temperature.

A graph showing the above relationship is enclosed in Fig.5. The static pressure, P5, is calculated by the following equation:

PS = ζ x Π1 x U2o / 2 (Pa)

where, ζ = Density of air (Kg/m3)

Π = Pressure Factor.

Uo = Velocity of air in duct at discharge end (m/sec)

Qo = Air flow at face-end, (m3/sec).

Qf = Fan Discharge (m3/sec)

The value of pressure factor (Π1,) & W1 = (Q1 / Qo) are obtained from the enclosed nomo-gram. To arrive at the total pressure, dynamic-pressure is calculated as follows and added to the Static pressure (Ps). Pd = ζ x U2 / 2 (Pa). However, it is essential to add the pressure required to overcome the resistance of the bends, transition pieces etc. in the duct-line to arrive at the final total pressure and rating of the fan motor.

8. K.W. RATING OF FAN MOTORS / POWER COST

Ventilation systems have to be kept operational during all stages of construction of the tunnels. Therefore while planning, the entire gamut of criteria for a specific application considered while selecting the rating and placement of fans, in series, for ventilation of long tunnels so that the investment on fans and ducts is not only minimal but also the subsequent power-cost, which forms the major chunk of the total cost of ventilation. Extreme care is, therefore, needed in selection of the fans and it's

type, capacity, sitting of fan-station (s) etc. Secondly, during the initial stage of construction the load on the fans is very small as the tunnel advances from it's starting point and the resistance against flow increases gradually due to the increase in the length of tubings and the need to effect greater fan discharge for achieving a predetermined air-flow at the face-end. In order to limit the load especially in context of long tunnels. it may be advisable to deploy multiple fans, instead, in series, of using a fan of large capacity. Variable Frequency drives can be effectively deployed to vary the RPM of the fan-motor to suit the desired duty requirement in regard to the varying fan discharge and pressure. Initially, the speed of the motor is reduced in the initial stages of construction and gradually increased with the increase in length of the duct-line. A case study of such an application is given below.

9. SAVING OF POWER WITH VARIABLE FREQUENCY DRIVE (A case study)

9.1 Basic parameters

1. Length of Tunnel - 3200 m

2. Size of Tunnel - 4.25mx4.25m

3. Required air flow at face, Oo - 5.5 m3/sec.

4. Diameter of Duct - 1000 mm.

5. Fan Discharge, Qf - 11. m3/sec.

6. Fan Pressure, Pt - 4000 Pa.

7. Motor Rating - 75 k.W

8. Average Progress - 100 m/ Month.

9. Power Tariff - Rs. 5.00 / unit.

10. Fan Operation 3 - 22 hrs/ Day.

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9.2 Progressive Operational Parameters

Length of face

(m) From - To

Air-flow Qo at the

face (m3/sec)

Fan discharge,

Qf (m3/sec)

Fan Pressure (Pa)

Min. K.W. Rating

Period of Construction (Month)

Power Cost (Rs. Lacs)

0 - 1000 5.5 6.33 791 7 10 2.31 1000 - 1500 5.5. 6.97 1207 11 5 1.82 1500 - 2000 5.5 7.76 1721 18 5 2.97 2000 - 2500 5.5 8.69 2362 27 5 4.46 2500 - 3000 5.5 9.78 3172 41 5 6.77 3000 - 3200 5.5 10.26 3555 48 2 3.17

Total power cost 21.50

Total saving in power cost during 32 months = Rs. 29.19 lacs.

Pay back period = less than 4 months.

Figure 5.

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The recurrent saving in power-cost off-sets greatly the initial costly investment on the Variable Frequency Drives. In case, the power is required to be generated by captive D.G. Sets, the gains will be much higher as the generating cost could vary between Rs. 10.00 to Rs. 15.00 per unit, (KWH).Variable Frequency Drives will help in smooth start of the fans, the starting current will be greatly reduced when compared with the starting-current with star-delta operation, thereby reducing the capacity of D.G. Set and the consequent saving on investment & operational cost. The Lay Flat Flexible Tubings will also blow-up smoothly without undue violent vibrations during "start" & "stop" operations.

10. PRINCIPALS OF DESIGN INVOLVING BOOSTER FANS FOR LONG TUNNELS.

With the advance of faces of the tunnel, tubings are extended. The flow at the face reduces both on account of the increase in resistance as well as the leakage through the tubings. When the air-flow, Qfl, has decreased below the acceptable value, it is necessary to boost the pressure by adding one or more fan, in series. By placing all the fans outside the portal, the pressure and quantity, no doubt, increases, but the leakage against resistance also increases due to excessive pressure. Alternatively, additional fans, so required, may be sited/ placed at specific intervals along the duct-line, but hazard of collapse flexible tubings at or near the suction end of the fan and recirculation could occur in uncontrolled fashion, if the fans are located arbitrarily along the tubings. Location of all the fans outside the portal will avoid the need to carry powercables inside the tunnel and avoid partial collapse 1 recirculation of air Whenever it is impractical to position all the fans at the portal, the fans inside the tunnel should be so positioned so as to reduce 1 eliminate the chances of collapse and recirculation of air. In order to overcome these problems following precautions need to be taken :

(i) Ensure that the fan, in series, as a booster should be sufficient to overcome the resistance of the tubings in front of it.

(ii) The air-flow at the end of proceeding duct-end should, generally, be more than the discharge

of the booster fan by 5% to 10% to avoid re-circulation.

(iii) It is essential to either install a damper on the booster-fan to regulate its fan discharge or use Variable Frequency Drive to control / regulate the air-flow. As the length of duct-line, in front of the booster fan increases, either the damper is gradually opened or the speed of the booster-fan is gradually increased.

Inspite of observing the aforesaid-mentioned precautions, it will be necessary to mount a length of steel duct (equivalent to 10 times the diameter of the tubings) and provide a sliding mechanism to regulate the gap between the Booster-Fan and the duct-end of the preceding fan as shown above in Fig. 6. If recirculation takes place, the location of the booster fan may be shifted slightly out bye towards the portal.

11. SPECIFICATIONS OF FLEXIBLE TUBINGS

The flexible tubings are made from polyester coated on both sides from PVC. These tubigs should fulfill the following criteria for application in ventilation of long tunnels :

• Tensile Strength should, preferably, more than 280 kg / 50 mm width.

• Tear Strength > 100 kg to 150 kg.

• Fire-retardant as per IS, B.S. or DIN Standards.

• Excellent Adhesion between the PVC coating and base polyester fabric for withstanding high dynamic pressure with frequent Start / stop operations.

• Resistance to withstand temperature upto 60oC.

• Strong Suspension Arrangement to withstand vibrations.

• Proper joints / couplings futures to control leakage.

• Anti-static properly in the event of presence of methane gas.

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Figure 6.

12. CARE & MAINTENANCE OF FLEXIBLE

TUBINGS

In order to ensure effective ventilation, especially when constructing relatively long tunnels, efforts should be made and adequate precautions including periodic review for ensuring effective & sustained standard of ventilation , right from the beginning i.e. from the fan installed at the portal to the face. It is, therefore, absolutely essential to take the following precautions while installing flexible-tubings.

• Straightness of duct-line is essential for better results. Sudden deviations should be avoided.

• The joint rings should be properly "strapped-on" at the joints and the tension bolts should be properly tensioned. Coupling should be installed in such a manner that all the tension bolts are on the one side of the duct-line to facilitate subsequent examination 1 tightening whenever required.

• The tubings, after extension, should be stretched fully to avoid any formation of kinks.

• Damaged tubings should be repaired with the help of Cold I Hot Repair Kits or replaced immediately.

• Wires & nails should never be used for repairing.

• Undue sag of the duct-line should be avoided by securing the ductlines with the help of "vertical hangers" at every 3 m to 4 m interval.

• Vertical hangers should be used only for supporting overhead Head-Wire-Rope

Suspension Assembly and not to support tubings directly from it's suspension hooks.

• The haul-roads should be kept clean of any fallen debris so as to avoid reduction in the clearance below the duct-line, thus avoid damage to the ducts.

• In case of vertical 1 inclines shafts, flexible -tubings should be securely fastened to the wire-rope-suspension assemblies.

• A minimum distance of 10 m should be kept between the fan and portal to avoid re-circulation of air.

• Adequate protection should be provided at the suction end of the fans installed at the portal to avoid any entry of rain water 1 foreign material into the duct-line.

• A special team of persons should be trained 1 dedicated for installation, repair and maintenance of the duct-line. Periodic inspection of the duct-line should be carried out by a component person, especially authorized for this job and necessary remedial action should be undertaken to keep the duct-line in proper working condition.

13. MEASUREMENT OF AIRFLOW IN DUCTS

Airflow in the ducts are measured by either Pitot-Tube (Fig. 7) or Anemometer_ Unlike the Pitot-Tube, which gives instantaneous spot reading, the Anemometer gives and integrated cumulative reading.

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Figu

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Since the cross-sectional area of ventilation-ducts is relatively small, the velocity of flow in the tubings will be relatively high, and as such the measuring instruments must be suitable for the high speeds. This requirement is met by pitol tube and by high speed anemometers. In all cases, the velocity at the centre of the duct may be measured by means of pitot-tube and inclined manometer. The average velocity of the airflow in the duct is then obtained by applying centre factor of 0.80. Thus v= 0.80 vc.

Where v = mean velocity of air in ducts.

vc = centre velocity of air at the centre of ducts.

The Pitot Tube may be inserted through the side of the duct (i.e. through an airtight trap door in a steel duct, or through the wail of the fabric duct), or else held at the discharge end of the duct. It should be neither be at the inlet end of a duct nor just downwind of an fan, because here the flow is not uniform and the 0.8 centre factor is not valid For the same reason of non-uniform flow, an anemometer should not be used right at the inlet end of the duct fan. An anemometer may be held at the outlet end ring or through a trap door in the length of the ducting, provided it is at least four meters down-wind of the inlet of any fan.

14. MEASUREMENT OF PRESSURE

Ventilation pressure (i.e. pressure differences are expressed as Pascals (1 Pa = 1 N/m2), pressure

being defined as force per unit area. (1 mm of head of water= 9.806 Pa). The total pressure developed by into developing Static Pressure (Ps) and Velocity pressure (or Dynamic Pressure) as Pa to effect air-flow in a duct. Static Pressure (also called frictional loss) in overcoming the resistance against air-flow is pressure difference between the absolute pressure at a point and the absolute pressure of the surrounding atmosphere.

REFERENCES

1. Swiss Standard "SIA-196" Empfehlung Ausgabe 1998. Zurich.

2. Ventilation in Coal Mines - A Handbook for Colliery Ventilation Officers National Coal Board (U.K).

3. Workshop on Design of Ventilation Systems For Long Tunnels - By Prof. V. S. Vutukuri - At Delhi - April, 2003.

4. Ventilation Systems Given Various Types of Drivages by Prof. Robert Fechtig, Zurich - May, 1984.

5. Flexible Ventilation Systems by R.K. Khanna - International Seminar - Coal, New Delhi. 1988.

6. Design of Auxiliary Ventilation Systems With Particular Reference To Ventilation of Long Tunnels by K. M. Kaiser, H. C. Singh, M.L. Gupta, N.K. Verma & N. Sahay, Scientists. CMRI, Dhanbad.