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Installation Tips

How many Gas Sensors, how to locate them, etc.

There is NO ABSOLUTE RULE in determining the number of gas sensors and their location.

However, there are a number of simple guidelines, which will help you to make a decision.

Keep in mind that sensors monitor a point as opposed to an area.  If the gas leak does not reach the sensor then no alarm will be raised. Therefore, it is extremely important to carefully select the sensor location.

There are two approaches to gas sensor location: perimeter protection or point detection.  With perimeter detection you place sensors all around the perimeter of the space in question to make sure you monitor the whole space. This is North, South, East and West.

With point detection you locate a sensor at a particular position where you are concerned about a leak e.g compressors, expansion valves, mechanical joints or cable duct trenches. Locate the sensors no farther than 1.5m from the potential “leak source”.

Regarding the number of gas sensors, there is no legal guideline on sensor coverage. The British Standard that preceded EN378 specified 36m² per sensor in a machinery room. This can be increased to 50 m2 if there is a discernible airflow.

In refrigerated spaces e.g. cold store with evaporators coverage can be increased to 50/100 m² due to the strong air movement. This applies also in car parks with ventilation or in air conditioned spaces with good airflows.

When selecting the number of sensors for your installation, remember not to try to economise by using the minimum possible.  A few extra could make ALL the difference if a gas leak occurs.

CROSS SENSITIVITY

In all cases the sensor supplied is designed for maximum sensitivity to a particular gas. There are a number of simple guidelines, which will help you to make a decision.

However, in certain circumstances false alarms may be caused by the occasional presence of sufficiently high concentrations of other gaseous impurities. If such a situation is likely to arise installers should check with our

Technical Department so that sensor (s) of suitable cross sensitivity can be supplied.

Examples of situations where such abnormalities may arise include.

• Plant room maintenance activity involving solvent or paint fumes or refrigerant leaks.
• Plant rooms in fruit ripening/storage facilities because of accidental gas migration (bananas – ethylene, apples – carbon dioxide)
• Heavy localised exhaust fumes (carbon monoxide, dioxide, propane, LPG) from engine driven forklifts in confined spaces or close to sensors.

With most models a response delay may be selected to minimise any problems that arise.

SENSOR LOCATION

a-Gas Density:

Any sensor used to detect a gas that is heavier than air such as butane, propane, LPG or ozone, where the relative density to air is greater than 1, should be located near ground level.  Care must be taken to protect the sensor from any flooding, excessive dust or in a wash down area.

Conversely, for any gas lighter than air such as hydrogen, methane, and ammonia, the sensor needs to be located higher up, in the roof space or on the ceiling. (NB at very low temperatures, such as in refrigerated cold stores ammonia becomes heavier than air).

In high temperature rooms hot air may be trapped under the ceiling and may act as a buffer keeping gases away from the ceiling mounted sensors. In this case, sensors should be mounted on the walls about 30cm below ceiling height.

For equal density gases, sensors should be mounted almost at face level.

Finally, take into account any room ventilation, since it alters the airflow pattern.

b-  Outdoor areas:

In the open, environmental conditions take on more importance.  Sensors need to be located downstream of the prevailing winds and protected against rain and snow.  Tropical rains can splash more that 30cms off the ground so sensors for heavy gases like LPG should be raised accordingly.  Take into account the wind effects of buildings, tanks and other obstructions and remember that gas can collect in roof voids, pits and trenches.  If possible, anticipate where maximum concentrations would be likely to occur in main airflows.

Locations requiring the most protection in an industrial plant would be around gas boilers, compressors, pressurised storage tanks, cylinders or pipelines, vent pipes and pressure relief valves.  Most vulnerable are valves, gauges, flanges, T-joints, filling or draining connections, etc.  Sensors should be positioned a little way back from any high-pressure parts  such as pressure relief values to allow gas clouds to form. Otherwise any leakage of gas is likely to pass by in a high-speed jet and not be detected by the sensor.

c- Accessibility:

Standards or Regulations may recommend annual or more frequent testing or full calibration of the equipment.

Care should be taken to install the sensors in a way that facilitates access for maintenance, repairs or calibrations.

d- Standards and Regulations:

Installers and operators should be aware of existing regulations and standards on gas detection installation and maintenance in their particular industry/country as these may vary considerably.

These notes are only intended as a guide.

WIRING / CABLE LENGTH

Gas Sensors must be located within the appropriate cable lengths from a central control unit( if relevant to the model being used).

For each range we have specified a standard type of cable and corresponding cable length (see table below).  However if longer distances are required you can use a heavier cable of any length once its resistance does not exceeds the value stipulated in the instructions.

This condition mainly effects the MGD range, because of the product configuration.  For the 1,2 Sensor -Controller the resistance value is MAX. 3.52 Ohms, and for the 4,6 Sensor-Controller it is MAX. 8.8 Ohms.

Contact Murco for detailed suggestions on senor locations

Table 1. Guideline to Murco Wiring

REFRIGERATION SPECIFICS

a- Machinery rooms

Gas Sensors monitor a point as opposed to an area. If the gas leak does not reach the sensor then no alarm will be raised. Therefore, it is extremely important to carefully select the gas sensor location. Also consider ease of access for maintenance. The size and nature of the site will help to decide which method is the most appropriate to use. Locations requiring the most protection in a machinery or plant room would be around compressors, pressurised storage vessels, refrigerant cylinders or storage rooms or pipelines. Most vulnerable are valves, gauges, flanges, joints (brazed or mechanical), filling or draining connections etc.

When mechanical or natural ventilation is present mount a sensor in the airflow. In machinery rooms where there is no discernable or strong airflow then options are:

• Point Detection, where sensors are located as near as possible to the most likely sources of leakage, such as the compressor, expansion valves, mechanical joints or cable duct trenches, and,
• Perimeter Detection, where sensors completely surround the area or equipment.
• With heavier than air gases such as halocarbon and hydrocarbon refrigerants such as R404A, propane, and butane sensors should be located near ground level
• With lighter than air gas e.g. ammonia, the sensor needs to be located above the equipment to be monitored on a bracket or high on a wall within 300 mm of, or on the ceiling provided there is no possibility of a thermal layer trapped under the ceiling preventing gas reaching the sensor. (NB. At very low temperatures, such as in a refrigerated cold store, ammonia gas becomes heavier than air).
• With similar density or miscible gases, such as CO or CO2, sensors should be mounted about head high – say 1.5m.
• Sensors should be positioned a little way back, 1.5-2m, from any high-pressure parts to allow gas clouds to form. Otherwise any leakage of gas is likely to pass by in a high-speed jet and not be detected by the sensor.
• Make sure that pits, stairwells and trenches are monitored since they may fill with stagnant pockets of gas.

Where there are deep beams and lighter than air  refrigerants it is recommended that the  detectors are mounted between pairs of beams and also on  he underside of the beams.

If a pressure relief vent pipe is fitted to the system, it may be a requirement to mount a sensor to monitor this vent pipe. It should be positioned about 2 m above the PRV to allow gas clouds to form.

With racks or chillers pre-fitted with refrigerant sensors, these should be mounted so as to monitor the compressors or if extract ducts are fitted the airflow in the duct may be monitored.

b- Cold Rooms

In refrigerated spaces gas sensors should be located in the return airflow to the evaporators on a sidewall, below head high preferred, or on the ceiling, never immediately in front of an evaporator. In large rooms with multiple evaporators, sensors should be mounted on a central line between 2 adjacent evaporators, as turbulence will result in airflows mixing.

If located in a strong airflow or if condensation is a possibility , a Protective Housing e.g. IP66, should  be used.

AIR CONDITIONING SPECIFICS

Air Conditioning – Direct Systems VRV/VRF

EN378 requires installation of a detector when the concentration of the refrigerant may exceed the practical limit in accordance with  EN 378-3:2008, Annex C.
EN 378-1 Annex C also states that in the case of AC systems for human comfort in Category A occupancy other methods of ensuring safety  in the event of a sudden major release of refrigerant are permitted. Such methods should ensure that the concentrations will not rise above the practical limits given in the informative annex E or to give adequate warning to occupant(s) in the space of such a rise so that they may avoid excess exposure time. General practise is to  use fixed  leak detectors,  refer to  Part 3, 7.1.

Section 8.2 stipulates that the location of detectors shall be chosen in relation to the refrigerant and they shall be located where the refrigerant from the leak will collect – low down in the case of heavier than air gases. In the case of Category A occupancy   e.g. in a hotel room, detectors should be mounted at at less than bed height.

Do’s

• mount in-room sensor at less than the normal heights of the occupants e.g in a hotel room this is less than bed height – between 200-500mm off the floor.
• away from draughts and heat sources like radiators etc.
• avoid sources of steam

Don’ts

• Do not mount sensors
  - under mirrors
  -at vanity units
  -in or near bathrooms

b- Chiller Rooms:

For small size chiller rooms, perimeter protection is suitable. Mount four sensors: North, South, East, and West.

If refrigerant is heavier than air then mount sensors about 300mm off the floor.

If the chiller room has an extraction fan, place a sensor downwind of the equipment being monitored or below the fan at about face level.

In bigger chiller rooms, with more than 1 chiller it is recommended to mount sensors on each chiller, see below.

c- Chillers:

In the case or small water or air-cooled enclosed chiller units mount the gas sensor so as to monitor airflow to the extract fans. With larger models also place a sensor inside the enclosure under or adjacent to the compressors

In the case of outdoor units:

• such as enclosed air-cooled chillers or the outdoor unit for VRV/VRF systems mount the sensor so as to monitor airflow to the extract fan. With large units also place a sensor inside the enclosure under or adjacent to the compressors

In the case of non-enclosed outdoor units

• If there is an enclosed machinery section then locate a sensor there.
• In the case of units with enclosed compressors, mount sensors in the enclosures
• Where you have protective or acoustic panels mount the sensor low down under the compressors where it is protected by the panels.
• With air-cooled chillers or air-cooled condensers with non-enclosed condenser sections it is difficult to effectively monitor leaks in the coil sections. With some designs it will be possible using an airflow sensor to monitor airflow to the start -up fans in the front or rear sections.
• If there is a possibility of refrigerant leaks into a duct or air-handling unit install an Airflow/duct mount sensor to monitor the airflow.

Weatherproof gas sensors should be used for unprotected outdoor applications.

Refer to the Murco Chiller leaflet pdf in the  Applications section of the Murco website here (http://www.murco.ie/?id=5&section=5 ), or contact Murco for our recommendations.

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