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15 Juin 2021

The importance of periodic ventilation and air quality measurements

In confined spaces, air quality control is essential. This is possible through knowledge of pollutant emissions, measurements of the concentration of harmful gases in the air and the proper functioning of the ventilation system.

Knowledge of pollutant emissions

The possible pollutants present in the confined environment are identified beforehand in the ventilation study. The most common ones are :

Gases concerned by the VLEP
Dust
VOCs
Asbestos

To evaluate the emissions of polluting gases from diesel-powered machines, it is recommended that measurements of exhaust gas content be taken every 500 working hours (RGIE).

Measurements of dust levels in the air

Inhalable and respirable dusts are among the specific pollutions.

Monitoring the presence of dust allows the protection of workers in the short and long term. Their exposure to dust can be measured by a CIP10 individual dust sampler.

Inhalable and respirable dusts are among the specific pollutions.

Monitoring the presence of dust allows the protection of workers in the short and long term. Their exposure to dust can be measured by a CIP10 individual dust sampler.

Concentration measurements of harmful gases in the air

These pollutants are measured or analyzed following individual or ambient sampling at various points and in particular in the work areas and workstations. The main measuring device to evaluate the air quality is the multi-gas analyzer.

Depending on the problematic of the building site, SUB’ROCA can recommend the installation of the SENSOR’BOX, a fixed multi-gas beacon which controls the air quality, signals alerts of exceedance and allows to export the measured values for fine analysis.

Checking the operation of the ventilation system

The correct operation of the ventilation system can be checked on different points:

Control of the good condition of the ventilation system
- Measurement of the power consumption
- Presence of vibration
Measurement of the air speed in the gallery and calculation of the air flow
Measurement of the air velocities in the duct and calculation of the air flow rate
Performance of a dust collector in its capacity to filter dust
Continuous measurements by measuring devices installed on the ventilation equipment

The correct operation of the ventilation system can be checked on different points:

Control of the good condition of the ventilation system
- Measurement of the power consumption
- Presence of vibration
Measurement of the air speed in the gallery and calculation of the air flow
Measurement of the air velocities in the duct and calculation of the air flow rate
Performance of a dust collector in its capacity to filter dust
Continuous measurements by measuring devices installed on the ventilation equipment

The main measuring devices for checking the ventilation system are the propeller/hot-wire anemometer and the liquid or electronic pressure gauge. Smoke tests should be carried out, if necessary, in order to identify and correct any « dead » zones and to analyze the air flows.

Planning of periodic inspections

Controls at the operator’s expense

The periodic measurements are carried out at the expense of the operator who appoints an internal person in charge of the subject of the ventilation.

SUB’ ROCA proposes trainings of the personnel to help the operator in this mission.

These periodic measurements are to be carried out at least at the beginning of the building site and each time there is a change of nature of ground, of section, of process of digging, according to the following periodicity (to be adapted to the case by case):

– Gas: daily and instantaneous,

– Dust/fibers: monthly at least (including % free silica),

– Asbestos: weekly at least in potentially asbestos-rich soils or after proven identification.

The supply and exhaust air flows, air velocities and duct pressures shall be measured regularly. These measurements should be carried out in particular when there are doubts about the performance of the ventilation, when the installations have been modified or significantly extended. For this purpose, it is recommended to install anemometers, flow meters, manometers on the ventilation network with direct reading.

Controls carried out by an external consulting firm

In assistance to the realization of periodic controls of ventilation and air quality SUB’ROCA will establish :

- The strategy of samplings,
- The realization of the samplings,
- The analysis of the samples with respect to the limit values of professional exposure,
- The necessary preventive measures.

SUB’ROCA recommends to its customers the following periodic controls:

- Measurements of exhaust gas content of combustion engine machines every quarter or every 500 working hours of the machine
- Annual updtae of of the ventilation network
  - Control of the ventilation equipment
  - Taking into account the geometrical modifications of the structure (dimensions, modified air passages, …)
  - Measurement of the main and secondary air flows
  - Control of the tightness of the dams, SAS, doors

- Annual measurements of dust exposure on operators
  - Measurement campaign with CIP10 dust samplers
  - Analysis of samples in laboratory

Controls carried out by an external engineering office

In assistance to the realization of periodic controls of the ventilation and the quality of the air SUB’ ROCA will establish:

- The strategy of samplings,
- The realization of the samples,
- The analysis of the samples with respect to the limit values of professional exposure,
- The necessary preventive measures.

SUB’ROCA recommends to its customers the following periodic controls:

- Measurements of exhaust gas content of combustion engine machines every quarter or every 500 working hours of the machine
- Annual updtae of of the ventilation network
  - - Control of the ventilation equipment
    - Taking into account the geometrical modifications of the structure (dimensions, modified air passages, …)
    - Measurement of the main and secondary air flows
    - Control of the tightness of the dams, SAS, doors

- Annual measurements of dust exposure on operators
  - Measurement campaign with CIP10 dust samplers
  - Analysis of samples in laboratory

The SUB’ROCA engineers are at your disposal to assist and advise you in the dimensioning of your ventilation installations. Whether your project concerns a meshed network in an underground mine or a complex digging project in another environment, our teams have the technology to calculate the air requirements and performance necessary to ensure ventilation that complies with the health and safety of personnel.

Contact our engineering office

7 Mai 2021

Admin SUB'ROCA Non classé

What type of refuge chamber for explosive environments?

If the underground environment is a complex one, comprising a plurality of risks in a sometimes important coactivity, the major danger is that of fire. For example, it is reasonable to consider a fire in a digging machine (or mariner), in electrical equipment or in fuel storage facilities, which can be triggered by an electrical fault, overheating or an accident. The power and duration of the fire can vary according to the nature of the fuels, their quantity, or the presence or absence of oxidizer.

When analyzing fire risks, especially in underground work, the objective of the priorities established by prevention rules based on experience is to deal as quickly as possible with incipient fires by means of automatic extinguishing devices. These measures are in addition to the well-known evacuation measures in all public places. The particularity of underground works is that the underground space can hinder the free movement of people as well as the evacuation of smoke and heat. It will not always be possible to find an escape route, and in these situations, only the installation of pressurized shelters can increase the chances of survival.

In the last decades, unfortunate examples have proven that the consequences of a fire in an underground environment can be dramatic, which have led to awareness and have been the origin of regulations, or equipment intended to safeguard the lives of operators working underground.

If the underground works are very complex, the consideration of an atmosphere potentially charged with explosive gases or dusts is an additional constraint which intensifies the risks related to fires.

In coal mines, considered as « explosive », or other mines or galleries classified as such, the risk of causing an explosion is permanent or frequent. This is why, when an operator equips himself with materials, he must ensure that he is equipped with equipment certified by the manufacturer as non-explosive.

This is an intrinsically safe refuge chamber, i.e. a refuge chamber in which no spark or thermal effect, produced under the test conditions prescribed by the standard, is capable of causing the ignition of an explosive atmosphere.

Principle and operation of refuge chambers in explosive atmospheres

Underground explosive mines such as coal mines do not always have emergency exits or escape routes that are suitable for all reasonably foreseeable disaster scenarios and that allow workers to escape from the disaster or risk area. It is the responsibility of the operations manager to propose a place of safety for the personnel as close as possible to the work zones, such as the emergency and rescue shelters which will allow the personnel to take shelter in a secure space.

Faced with these constraints, SUB’ROCA designs its refuge chambers in a spirit of mobility and modularity so that they can be easily moved and create fallback areas as close as possible to the underground works. These shelters allow, thanks to their mobility and modularity, to make the evacuation plan evolve according to the progress of the digging.

This solution ensures a fallback and thus frees up precious time for the organization and intervention of the emergency services.

The « explosive » quality of mines or underground workings is given to sites that show permanent or frequent accumulations of flammable gases and/or combustible dusts mixed with air and which, in the presence of an activation energy, would cause a sudden explosion. Although much progress has been made in the prevention of explosions and disasters in coal mines or underground workings in explosive atmospheres, explosions still occur, undoubtedly causing many deaths.

The dangers due to the presence of coals, flammable gases and dusts in the working atmosphere have led to the development of a special class of emergency shelters for use in coal mines.

SUB’ROCA offers a complete range of refuge chambers adapted to the specific constraints of mines with explosive atmospheres using compressed air to operate its equipment. These equipments are non-electrical, so there is no risk of producing a source of activation that could explode the atmosphere or ignite the dust in suspension.

These refuge chambers are called intrinsically safe refuge or rescue chambers.

Underground refuge chambers solutions

Standard (ITA/EN)

The standard range complies with ITA and EN standards, offering the necessary features for personnel safety.

See more details

Narrow

The narrow range has a modular design to integrate the refuge chamber into small spaces such as narrow tunnels and veins.

See more details

Compact

The compact range is characterized by easily movable refuge chambers with a limited number of people.

See more details

Sur-mesure

The customizable range can be adapted to your specific needs, for example to be integrated on a specific equipment (tunnel boring machine or rescue vehicle).

See more details

Regulations and compliance of the equipment of intrinsically safe refuge chambers

The regulations in force in various parts of the world indicate that intrinsically refuge chambers must comply with the following points in particular:

The chambers must maintain a breathable atmosphere inside the refuge chamber for a period of between 24 and 96 hours (depending on the country). In the event of a fire or if there is a risk of explosion, mine rescue teams may not be able to enter the mine and immediately rescue the miners in the refuge rooms.
Refuge chambers shall be equipped with an oxygen supply and chemical scrubbers that absorb CO 2 and CO from the refuge air. An oxygen supply is provided to compensate for human respiration and leakage losses that occur.
The access of the personnel in the refuge chamber is a critical moment which requires the restoration and the maintenance of the quality of the air. For this reason, the shelters are equipped with a SAS at the entrance that can be purged.
The air quality inside the refuge chamber must be carefully monitored to keep oxygen, CO and CO 2 levels within acceptable limits according to the regulations of the country where the refuge will be installed.
Since most gas sensors are limited in their ability to function properly at a relative humidity of less than 90%, it is necessary that the refuge chamber be equipped with a drying device to keep the relative humidity below 90%. Some studies have shown, in experiments, that the relative humidity inside the chamber would increase to 93%.
Refuge chamber must be equipped with an artificial cooling system to maintain a temperature and humidity in accordance with the regulations in force in the country where the refuge chamber is installed.
A human of average build, at rest, produces about 117 W of heat. In addition, the CO2 scrubbing process involves an exothermic chemical reaction that adds 40 to 50 W per person present to the heat generated inside the chamber. The ability of the chamber to dissipate this heat depends on the temperature of the air and rock inside the mine.
Some mobile refuge chambers are equipped with an air conditioning cooling system that requires battery power. This system has the advantage of maintaining the atmosphere inside the chamber at a more comfortable temperature and humidity level. But other refuge chambers can be equipped with an intrinsically safe cooling system which is a circuit in which no spark or thermal effect, produced under the test conditions prescribed by the standard, is capable of causing the ignition of a given explosive atmosphere.

Conclusion

The emergency and refuge chambers intended to be installed in explosive atmospheres must have the « Intrinsic Safety » quality, i.e. they must be non-electrical, so as not to produce any activation energy that could cause the atmosphere to explode or ignite the dust in suspension.

This is why SUB’ROCA offers a complete range of refuge chambers adapted to the specific constraints of mines with explosive atmospheres.

26 Fév 2021

Admin SUB'ROCA Blog

The ideal autonomy of a refuge chamber?

The issue of autonomy is paramount in defining the need for refuge chambers. Beyond the number of people to be sheltered, it is in fact the determination of the time they will have to spend inside, until the intervention of the rescue services or the end of the incident, that influences the dimensioning of the shelter. This time must be assessed as part of the project risk assessment.

The risk assessment is carried out according to several criteria such as the configuration of the work site, the type of work carried out, the number of people underground, the number and type of motorized machinery operating underground, the fire risk, and the means of evacuation.

A shelter is in autonomous mode when it is deprived of outside resources such as air and electricity. In this case, the shelter operates autonomously thanks to emergency batteries, a reserve of breathable air or an air filtration system.

Other resources essential to life, such as water or food, must also be stored in sufficient quantities for the duration of the shelter’s autonomy.

The autonomy of a shelter is generally between 24 and 36 hours.

One of the world references is the Western Australian Government’s compendium of recommendations, Department of Mines and Petroleum, Refuge chambers in underground mines. This recommends a minimum autonomy of 36 hours based on a worst-case scenario.

Underground refuge chambers solutions

Standard (ITA/EN)

The standard range complies with ITA and EN standards, offering the necessary personnel safety features.

See details

Narrow

The narrow range has a modular design to integrate the refuge into small spaces such as narrow galleries and veins.

See details

Compact

The compact range is characterised by refuges that are easily movable and can accommodate a limited number of people.

See details

CUSTOMIZABLE

The customizable range adapts to your specific needs, for example to be integrated on specific equipment (tunnel boring machine or rescue vehicle).

See details

Feedback from incidents around the world suggests that most shelters are used for less than 10 hours. However, no single event in one mine can be compared with another, making standardization of a duration of autonomy impossible.

Below is a list of the different types of mines with the recommended battery life.

UNDERGROUND MINES

The ideal duration of a rescue and rescue shelter for underground mines can vary from 8 to 48 hours. In this environment the most frequent dangers are fires, rock falls and the propagation of dangerous gases or fumes, it is in these cases that rescue shelters have a vital role.

In terms of safety standards for underground mining operations, many countries use Western Australia’s DMP standards for emergency and rescue shelters as a reference. These standards are based on a worst-case scenario, so the ideal autonomy of a shelter has been set at 36 hours.

Although longer incidents and extreme cases are less frequent, they are still common.

Based on the diverse and varied incidents that have already occurred in underground mines around the world, a reference in terms of the autonomy of a shelter cannot be clearly determined as an ideal duration. This ideal duration of autonomy of a shelter has nevertheless been suggested to be between 2 to 10 hours, despite the fact that extreme cases are less frequent, they can occur at any time.

COAL MINES

The duration of a coal mine rescue shelter can vary from 48 to 96 hours. In the coal industry, the most common hazards include fires, explosions, rock falls, dangerous gases and fumes, and flooding.

Coal mines contain large quantities of fuel that can burn at any time and for several days, so safety, rescue and evacuation standards are different from those of hard rock mines.

The Mine Safety and Health Administration (MSHA) has determined that there is a need for emergency and rescue shelters within the coal operations and that these shelters must be able to sustain the lives of the operators for at least 96 hours. This time frame is intended to facilitate rescues and allow rescue workers time to gain access to trapped miners. In addition, prior to any rescue intervention, it is necessary to monitor the atmosphere in a mine beforehand in order to allow the risk to be determined and also to allow measures and precautions to be taken by the rescue teams.

The most important autonomy is therefore for coal mines because coal is a good fuel and a fire can last for several days.

TUNNEL EXCAVATIONS

The autonomy of a rescue and rescue shelter for tunnel projects can vary from 24 to 72 hours. Risks that may involve the use of a refuge in the tunneling and underground construction industry are fire, explosion and dangerous gases.

Internationally, the International Tunnelling and Underground Space Association (ITA) is the leading industry organization that represents and sets health and safety standards for the industry. The need for emergency and rescue shelters must be assessed as part of the risk assessment of the tunneling project. As in the mining industry, the duration of a tunneling emergency shelter is evaluated based on the time required to perform a rescue. A minimum of 24 hours is recommended by the agency, with the possibility of providing additional resources to extend this period if required by the risk assessment.

21 Juin 2020

Admin SUB'ROCA Blog

What are the specific pollutions ?

Category C accreditation concerns premises with specific pollution.

It allows you to :

measure dust concentration (inhalable and respirable)
measure the efficiency of filtration or dust removal
control dust collectors and ventilation monitoring systems

Inhalable and alveolar dusts: possible health effects?

Inhalable dust enters the human body through the respiratory tract. Particles larger than 5µm in diameter are predominantly deposited in the nasopharyngeal region before being removed. Alveolar dust is finer and reaches the deepest areas of the lungs (the alveoli); it generally represents « 10 to 20% of the inhalable dust fraction ».

For this reason, exposure to alveolar (and to a lesser extent inhalable) dust can cause lung overload. This acute or chronic mechanism is related to the limited capacity of the lung to clear the lungs.

What are the means of prevention and protection?

In relation to the provisions of the Labour Code, we can point out the following:

technical means (capture, filtration, fogging);
organisational measures (monitoring and control: installation control system, camera);
behavioural practices and habits (regular implementation of training and information sessions on preventive measures and protection to be respected) to be deployed in the extractive industries.

How to control the risks related to dust?

For each of the Homogeneous Exposure Groups (HEG) whose risk assessment concludes that there is a « high risk » *, the employer is expected to perform at least one measurement per year.

This measure complies with the following conditions:

carried out at a fixed or mobile station (sampling equipment carried by the worker) in each of the premises with specific pollution that are characteristic of the GEH ;
sampling duration representative of the average daily activity in the specific pollution area(s) ;
sampling and analysis of the samples taken :
- or by an accredited or approved Category C body;
- either by an internal or external laboratory following a protocol recognised by the profession (e.g. IMA-Europe’s Dust Monitoring Protocol); in this case, the degree of reliability of the results (uncertainty,…) is known.

Updating the risk assessment as soon as: changes in operating methods and/or conditions, changes in the characteristics of the deposit, new workstation, new dust-generating machine, etc. are made. (R. 4412-5 and R. 4121-2).

Is the use of bodies approved by the Ministry of Labour the result of regulatory provisions?

This recourse is not compulsory for all periodic checks carried out in companies; however, it will be prescribed in the case of checks carried out at the request of the labour inspectorate, or to check compliance with the limit values for the concentration of certain substances in the atmosphere.

With its experience in the field of ventilation and aeration, SUB’ROCA, is accredited to carry out all the measurements concerning premises with specific pollution and, in the event of detection of a danger for your employees, is at your disposal to accompany you in your reflection and to offer you a complete solution that will adapt to your current situation without delay.

The notion of « low risk » and « high risk » is not defined by the Labour Code.

It is considered low risk if:

All historical measurements < VLEP ;
The average of the historical measurements < 1/4 of the OEL (or 1/10^e of the OEL for respirable silica dust) ;
All means of prevention and protection defined in the past are properly maintained, implemented and effective;
No history of respiratory disease or disorder.

? otherwise a « high risk » is considered for further risk assessment

Contact our engineering office

20 Juin 2020

Admin SUB'ROCA Blog

What are the limiting climatic conditions of a construction site?

Climatic conditions to be respected

The thermal equilibrium of the human body is ensured by physiological regulation mechanisms that keep the internal temperature constant despite the variation of the main parameters of the heat balance, i.e. :

Heat exchanged with the ambient air by convection,
Heat received by radiation from walls and machines,
Heat evacuated by evaporation of sweat,
Internal release of heat linked to the power of the work carried out.

As the possibilities of physiological mechanisms are limited, it is necessary to ensure climatic conditions compatible with the work to be carried out.

The parameters that influence climatic conditions and possible policy options to improve them are given below:

Convective exchanges depend on air temperature and air speed.
Exchanges by evaporation of sweat depend on the value of the :
- Dry temperature ts in °C,
- Wet bulb temperature th in °C
- Air velocity v in m/s.

Acceptable boundary conditions can be evaluated by the resulting temperature recommended by various standards or recommendations.

tr = 0.7*th + 0.3*ts – v

In any case, the physical parameters necessary to evaluate the climatic conditions of a construction site are :

ts in °C
th in °C
v in m/s

Representation of the parameters characterising the state of the air

The psychrometric diagram

The state of the air can be characterized by :

Its temperature
Its humidity

The temperature can be measured by an ordinary thermometer or by the dry bulb thermometer of the psychrometer.

This temperature is called the dry temperature.

Humidity can be assessed in several ways:

Absolute humidity x or mass of water vapour in kg per kg of dry air,
Relative humidity F (%) (ratio of the vapour pressure in the mixture to the saturation pressure of water vapour at the same temperature),
Temperature of the wet bulb of the psychrometer called wet wet temperature th (°C)

To assess the thermal effects relative to air, enthalpy is defined.

Or a mixture of air and water vapour comprising per kg of air, a mass x kg of water vapour.

Enthalpy is the enthalpy of the heat necessary to bring this mixture to the temperature t from 0°C.

**J = 0.241t + x(597 + 0.44*t)**

J : enthalpy in kcal/kg dry air t : dry temperature in °C x : absolute humidity in kg water/kg dry air

0.241 : specific heat at constant air pressure (kcal/kg °C)

(*) 0.44 : specific heat at constant water vapour pressure (kcal/kg °C)

597 : heat of vaporization of water in kcal/kg

(*) Values corresponding to a pressure close to normal atmospheric pressure

To assess the thermal effects relative to air, enthalpy is defined.

Or a mixture of air and water vapour comprising per kg of air, a mass x kg of water vapour.

Enthalpy is the enthalpy of the heat necessary to bring this mixture to the temperature t from 0°C.

**J = 0.241t + x(597 + 0.44*t)**

J : enthalpy in kcal/kg dry air t : dry temperature in °C x : absolute humidity in kg water/kg dry air

0.241 : specific heat at constant air pressure (kcal/kg °C)

(*) 0.44 : specific heat at constant water vapour pressure (kcal/kg °C)

597 : heat of vaporization of water in kcal/kg

(*) Values corresponding to a pressure close to normal atmospheric pressure

It is convenient to use the psychrometric diagram on which one can easily read these different values

The following values correspond to a point in the diagram:

Dry temperature ts (right in blue)
Wet bulb temperature th (straight lines in green)
Relative humidity (%) (black curve)
Absolute humidity x (g/kg dry air) (black vertical line)
Enthalpy J (kcal/kg dry air) (straight red)

Knowledge of any two of these values is sufficient to determine the other air state characteristics.

Contact our engineering office

20 Juin 2020

Admin SUB'ROCA Blog

How to measure an airflow ?

Determining the air flow rate

The knowledge of the air flow Q, implies the knowledge of the section S and the average velocity av in this section. There are different methods for determining the cross-section and average speed.

Section measurement

Tables according to height and width

These tables only give correct results when the frames have their original shape and the floor is flat and horizontal. It is the simplest procedure but it leads to huge errors when applied indiscriminately to very irregular sections.

Decomposition of the cross-section into simple geometric surfaces

A juxtaposition of simple figures is determined that corresponds as well as possible to the section that is to be estimated. We measure the dimensions that make it possible to calculate the surface area of these simple figures: semicircles, rectangles, trapezoids, triangles.

This process allows a quick evaluation of sections of any shape; however, no great accuracy is to be expected.

Evaluation based on a grid of the section

This method, which the surveyors know how to implement, allows a very good precision if the points surveyed are tight enough; it is necessary to spend a rather long time at each station.

Note that polar coordinates are often more convenient than cartesian coordinates to perform the « grid », they allow the calculation of the section from measurements without planimetry.

Photoprofile

This is the method used for measurements for ventilation studies. The operation at the bottom is fast and repetitive.

A photoprofile projector powered by a hat lamp is used for this purpose. The photo is taken with a camera.

We find on the photograph the luminous perimeter of the gallery as well as a square represented by 4 luminous points representing a surface of exactly 1m².

You have to start again if a photo was not successful, which is very rare, but under no circumstances can you get false results.

This method is very well suited to carrying out a series of measurements, two men can, in one position, survey up to 40 sections if the trips are not very long.

Rectangular shaped gallery

The section S of a perfectly rectangular shaped gallery is calculated as follows :

TH shape gallery

Section S of a « TH frame » shaped gallery is measured and calculated as follows :

Let K be a shape coefficient of the gallery
S = H*B*K

Special values of K :

Speed v measurement in galleries

Speed distribution in a section

The speed at a point in a section depends on the position of that point and in particular its distance from the walls.

The areas close to the walls, where the speed is low, are of great importance as they represent a large proportion of the total section. Thus, the area between the outer storyteller of the section and a homothetic curve of it in the ratio 4/5 is equal to 36% of the total section, in a gallery framed in TH 420 the width of this crown is only 40cm; this important zone where the speed is low usually escapes the classical sweeping.

Section choice

For a measurement to be accurate, a well-defined section should be chosen where :

- The flow is almost permanent, i.e. the velocity measured at a point varies little over time. Measurements should therefore be avoided when there are door manoeuvres or gear movements.
- The speed profile is regular. The proximity of obstacles such as equipment depots or stopped trains and doors that leak in the form of high-speed jets should be avoide.
- No fraction of the flow rate will escape the measurement. In particular, sections where there is a large gap between the lining and the ground should be avoided.

In principle, a straight, unencumbered gallery section should be chosen, with frames that are well aligned and well lined. Carefully mark the selected section for use in subsequent measurements.

« Polar » measurement method

Measurements are made by an anemometer equipped with a telescopic rod.

This method consists of 21 measuring points divided into 3 radii r and a centre P.

The points P1, P2, P3 are distributed on a radius such that :

P3 = ½ r P2 = ¾ r P1 = 9/10 r

Note: to locate the measuring points, a rubber band marked with the marks P1 to P3 can be stretched.

Simplified Polar Measurement Method

We will use the same material as for the polar method.

This method consists of 5 measuring points distributed 1 radius r and a centre P.

The P2 points are distributed over a radius such that:

P2 = ¾ r.

Operator positioning

If the operator is in the flow whose velocity he is measuring :

He must ensure his own position in relation to the device
Placed in the same section; it increases the air speed
Placed immediately downstream airing, it reduces it

It must be placed between 1m and 1.50m downstream of the anemometer:

Under these conditions, it changes the air speed little,
This position also maintains the reel plane perpendicular to the direction of the airflow

If the measurement is carried out by two operators :

Whoever does not hold the reel should stand as far as possible downstream of the measuring point and against the facing.

The analysis

The samples taken must be returned to the specialised laboratory for analysis.

SF6 can be dosed on site using an electron capture device.

ANEMOMETERS

Air velocities are measured using electronic devices called anemometers.

There are mainly 2 types of anemometers: With reel or Bare wire

These devices can be accompanied by a processing software that allows the measurements to be exported to a computer.

Reel anemometer
Sensitive electronic devices.
Measurement possible between 7 and 40 m/s at a temperature of 60°C or less, giving a direct or average reading if the instrument is equipped with this function.
These devices are fragile and deserve regular maintenance to avoid clogging the reel, deforming the blades and thus disturbing the measurement.

There are different reel diameters:

Bare-end anemometer
- Very sensitive electronic devices.
- Measurement possible below 7 m/s at a temperature less than or equal to 70°C, giving a direct or average reading if the instrument is equipped with this function.
- These devices are very fragile and deserve special attention to the bare wire at the end of the handle.

Calibration

Whatever the type of anemometer, it is necessary to calibrate it periodically; indeed, wear, dust, shocks modify the response curve.

An annual calibration and a calibration following any damage should be scheduled.

Contact our engineering office

20 Juin 2020

Admin SUB'ROCA Blog

How to design your ventilation network?

Our aero© software

Mesh network modelling made possible thanks to the AERO© Suite

AERO© is a suite of mesh network calculation software based on the HARDY-CROSS method. It allows to determine the distribution of air flows and pressure drops in the network, calculates the operating point of the fans and takes into account the effects of natural ventilation.

Accepted & modelled constraints :

Presence of fire(s) according to established scenarios
Production and propagation of gases and fumes per unit of time
…

The aeraulic modelling of meshed networks, a unique competence

Base network modeling

The diagram above shows an example of a ventilation network consisting of an air inlet shaft, an air return shaft and two connecting galleries between the shafts.

This network was modeled by calculation by introducing in the software :

– The aeraulic resistances of the branches,

– The altitudes and temperatures of the knots,

– The characteristics of a main fan located at the top of the return air shaft,

– Daytime air pressure.

The following tables contain the input data for the calculation :

It can be seen that the total air flow rate of 62.9 m³/s is shared between the northern and southern links with values respectively equal to 35.3 m³/s and 27.6 m³/s.

The main fan operates with a total pressure of 32.7 mmWE and a manufacturer’s air flow rate of 63.3 m³/s.

With efficiency assumptions equal to 0.8 for the fan and 0.95 for the motor, the electrical power consumed by the fan is equal to 27 kW.

The diagram below shows the results obtained when the fan speed is increased to 980 rpm.

It can be seen that the total air flow rate of 62.9 m³/s is shared between the northern and southern links with values respectively equal to 35.3 m³/s and 27.6 m³/s.

The main fan operates with a total pressure of 32.7 mmWE and a manufacturer’s air flow rate of 63.3 m³/s.

With efficiency assumptions equal to 0.8 for the fan and 0.95 for the motor, the electrical power consumed by the fan is equal to 27 kW.

The diagram below shows the results obtained when the fan speed is increased to 980 rpm.

It can be seen that the total air flow increases from 63.3 to 84.4 m³/s and is shared between the north and south links with values respectively equal to 47.3 m³/s and 37.1 m³/s.

The main fan operates with a total pressure of 65.8 mmWE and a manufacturer’s air flow rate of 85.4 m³/s.

With efficiency assumptions equal to 0.8 for the fan and 0.95 for the motor, the electrical power consumed by the fan is equal to 73 kW.

Consequence of the accidental opening of the SAS in the daytime

From the previous ventilation network (fan at 980 rpm), the accidental opening of the SAS during the day (branch 2-40) shows that the air flow rate of the SAS increases from 1.4 m³/s to 26 m³/s.

At the same time, the total air inlet flow rate decreases from 84.4 m³/s to 63 m³/s.

Calculation example of a network with a secondary ventilation line

The diagram above shows the results of the calculation:

– With the fan at 980 rpm

– With a secondary suction ventilation line ventilating the excavation of gallery 13-17.

The air flow rate of the excavation in this example is equal to 10 m³/s (600 m³/min).

The secondary ventilation fan operates with a total pressure of 233 mmWE and a manufacturer’s air flow rate of 10.2 m³/s.

With efficiency assumptions equal to 0.8 for the fan and 0.95 for the motor, the electrical power consumed by the fan is equal to 31 kW.

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20 Juin 2020

Admin SUB'ROCA Blog

Is natural ventilation sufficient?

Natural ventilation

Natural ventilation is equivalent to a fan with a pressure equal to :

**H = ? * ?z**

H_(AF) = H_(AB) + H_(BC) + H_(CD) + H_(DE) + H_(EF)

Operating principle

To explain how natural ventilation works, we can use the example of a mine according to its exposure to the sun.

CASE N°1

In case N°1, the sun is on the air inlet side. The air inlet is heated by the sun. Since the return air is cooler than the air inlet, the air flows from the return air to the air inlet.

CASE N°2

In case N°2, when the sun is on the return air side, the opposite effect occurs and the air will circulate from the air inlet to the return air side.

CAS N°3

When the air inlet and return air are at the same temperature there is equilibrium between the two air columns and there is no more air circulating in the galleries.

Conclusion

The air circulation in a mine not equipped with a fan is governed by natural ventilation and depends on the weather conditions outside.

Under these conditions, natural ventilation is not a stable ventilation because the ventilation regime changes over the course of a day according to the exposure of the sun and during the different seasons (depending on weather conditions).

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19 Juin 2020

Admin SUB'ROCA Blog

How to size a main fan ?

Ventilation studies are carried out, for periods that best characterize the evolution of the mine, in order to determine the values of air flow rate and total pressure corresponding to the points where the fan will have to operate.

For example, the case of a ventilation study that led to the operating points shown in the figure below:

P₁ at the start of the installation in year A

P₂ et P₃ accidental operating points

P_M final points from year A+2 onwards

If we set as a requirement

To have in PM a yield close to the maximum (84-85%)

To make sure that P1 does not enter the pumping

A range ABCDEA can be defined, the characteristics of which must be guaranteed by the manufacturer.

P₁ at the start of the installation in year A

P₂ et P₃ accidental operating points

P_M final points from year A+2 onwards

If we set as a requirement

To have in PM a yield close to the maximum (84-85%)

To make sure that P1 does not enter the pumping

A range ABCDEA can be defined, the characteristics of which must be guaranteed by the manufacturer.

On the basis of this information, the manufacturer will supply a fan whose characteristics are such that the ABCDEA zone is not amputated:

Nor by the pumping limit curve (along the ABC line)
Neither by the curve corresponding to the nominal power of the motor (along the BCD line)

Installation of measurements of aerodynamic characteristics

These measurements relate to the two main characteristics of the fan from the ventilation point of view, i.e. the air flow rate and the total pressure.

A distinction must be made between the types of use of the measuring probes :

Recording of flow rate and total pressure during operation,
Detailed measurements for checks and calibrations, upon acceptance of the installation.

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19 Juin 2020

Admin SUB'ROCA Blog

Explanation of the main (or primary) ventilation

Main ventilation network

Main airflow is defined as the totality of the air flows that flow through the open lanes of a network of galleries from an access called MAIN AIR INTAKE to an exit called MAIN AIR RETURN.

The main air inlet can be a shaft, a downspout or a horizontal gallery.

The same applies to the main air return.

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Principle of operation

A preliminary study determines:

The total air flow rate required in the gallery system,
The desired distribution in the galleries,
Depending on the equivalent resistance of the ventilation network and the desired total air flow, the main fan vacuum to be installed and the adjustment brakes to be set up to obtain the desired air flow distribution are determined.
The distribution of air flows is achieved by the installation of brakes and air locks.

A preliminary study determines:

The total air flow rate required in the gallery system,
The desired distribution in the galleries,
Depending on the equivalent resistance of the ventilation network and the desired total air flow, the main fan vacuum to be installed and the adjustment brakes to be set up to obtain the desired air flow distribution are determined.
The distribution of air flows is achieved by the installation of brakes and air locks.

Ventilation brakes

A ventilation brake corresponds to a total or partial closure of a gallery to limit the passage of air flow.

There are different types of brakes:

Masonry or wooden brake with doors for the passage of vehicles

Rubber or plastic bands

Diaphragm characterized by:

s = open area free to air passage

S = gallery section

R = brake resistance depends on the s/S ratio

Diaphragm characterized by:

s = open area free to air passage

S = gallery section

R = brake resistance depends on the s/S ratio

Ventilation SAS

Consists of two successive closures equipped with doors for the passage of equipment and personnel.
They are characterised by the pressure difference ?P existing at their terminals and by the leakage rate Qf.

They are installed to separate the air inlet from the return air and thus avoid the short ventilation circuit.

The main fan

Definition

The main ventilation fan is a device designed to convey the air needed for ventilation throughout the mine.

If it has been found that the required air flow Q must overcome a pressure drop ?P, a fan must be built into the circuit, which will provide the flow with a load increase H=?P when the same flow Q passes through it.

The basic characteristics of the fan (H and Q) are therefore determined by studying the ventilation system.

R_e is the equivalent resistance of the mine

Figure 1: Ventilation network with a single input and a single output

Operating characteristic

Either a system of coordinate axes :

Flow rate Q on the x-axis
Total pressure or total load H in coordinates

On the same graph (H, Q), the fan characteristic curve H=f(Q) and the parabolic curve H=Re.Q² are plotted representing the pressure drop of the network.

The point of intersection of these two curves represents the operating point of the fan.

Its abscissa is the total flow rate of the mine and its ordinate H is the fan head

Operating characteristic

Either a system of coordinate axes :

Flow rate Q on the x-axis
Total pressure or total load H in coordinates

On the same graph (H, Q), the fan characteristic curve H=f(Q) and the parabolic curve H=Re.Q² are plotted representing the pressure drop of the network.

The point of intersection of these two curves represents the operating point of the fan.

Its abscissa is the total flow rate of the mine and its ordinate H is the fan head

Ventilation solutions

Ventilation of underground mines and quarries

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Posts by Admin SUB'ROCA:

Knowledge of pollutant emissions

Measurements of dust levels in the air

Concentration measurements of harmful gases in the air

Checking the operation of the ventilation system

Planning of periodic inspections

Principle and operation of refuge chambers in explosive atmospheres

Underground refuge chambers solutions

Regulations and compliance of the equipment of intrinsically safe refuge chambers

Conclusion

Underground refuge chambers solutions

UNDERGROUND MINES

COAL MINES

TUNNEL EXCAVATIONS

Inhalable and alveolar dusts: possible health effects?

What are the means of prevention and protection?

How to control the risks related to dust?

Is the use of bodies approved by the Ministry of Labour the result of regulatory provisions?

Climatic conditions to be respected

Representation of the parameters characterising the state of the air

The psychrometric diagram

J = 0.241*t + x*(597 + 0.44*t)

J = 0.241*t + x*(597 + 0.44*t)

Determining the air flow rate

Section measurement

Speed v measurement in galleries

P2 = ¾ r.

Operator positioning

The analysis

Calibration

Our aero© software

The aeraulic modelling of meshed networks, a unique competence

Base network modeling

Consequence of the accidental opening of the SAS in the daytime

Calculation example of a network with a secondary ventilation line

Natural ventilation

H = ? * ?z

Operating principle

Conclusion

Installation of measurements of aerodynamic characteristics

Main ventilation network

Principle of operation

Ventilation brakes

Ventilation SAS

The main fan

Ventilation solutions

**J = 0.241t + x(597 + 0.44*t)**

**J = 0.241t + x(597 + 0.44*t)**

**H = ? * ?z**