Enviro Process Engineering (HK) Co. Ltd.
SMARTOX-O GAS DETECTION SYSTEM DESCRIPTION
The purpose of the ‘SmarTox-O’ gas monitoring system. is to monitor H2S levels (or other specific components as requested for detection) in ‘foul air and the signal from the sensing equipment is transmitted via 4-20mA proportional current loop to a compatible data collection system or to drive the extractor fan or to tune to HIGH SPEED for ventilation.
The ‘SmarTox-O’ module has a single sensor that is ranged 0 to 100.0 PPM H2S or custom required range for whatever the chemical required to detect.
It is housed in IP65 SS316 enclosure.
It is also supplied with a weather hood that provides protection from water ingress.
In addition to the sensors there is also a low-flow switch, mains to DC power supply
And a lighting protection and loop isolation unit are also provided.
Electrical, mechanical and pneumatic set-up of the ‘SmarTox-O’ system
The system is designed for the monitoring of the concentration of hydrogen sulphide (H2S) entering the inlet side of a ‘foul air’ gas sample.
Typically the inlet sampling gas will contain condensing moisture, along with various other corrosive solutions and gases. The ‘SmarTox-O’ system has as its functional core a H2S electro-chemical sensor.(or to whatever the chemical customer desires to detect). The sensor has been ranged at a nominal 0 — 50PPM maximum range. Other concentration ranges are also available.
To make servicing of this gas detection system simpler and more cost effective the gas transmitter has been built into a module that contains all the equipment needed to support the sensor. Consumable items of the system are contained within the module for ease of servicing as one unit. The module is called ‘SmarTox-O’.
An effort has been made to construct the ‘SmarTox-O’ modules with materials that will maximise the resistance to corrosion, thus ensuring longer life.
‘SmarTox-O’ modules are available as single gas with one sensor or dual gas with two sensor modular monitoring units.
The dual gas module has all the features of the single in duplicate however due to the sample being drawn from the same source, only a single pump and sample conditioning unit are employed similar to the single model.
The ‘SmarTox-O’ module itself has three quick connections to make to operate.
( Fig 1: Connections — from left to right, sample inlet signal and power connection and sample exhaust )
These connections have been designed to make installation of the module quick and easy for a non-technical but trained person. The gas sample inlet connection is a knurled 316 stainless steel nut to allow quick connection.
The electrical Power/Signals connector is a twist type multi-pole circular connector.
The sample outlet connector is a quick disconnect fitting manufactured from plastic Delrin®.
All tubing and materials in contact with a gas sample prior to it being measured by the sensor will only be either 316 Stainless Steel or Teflon®. These materials offer the best seal against leaks, the least chemical resistance to good gas flow and are also inert to contamination, minimising the possibility of dilution by reactive gas adhesion or absorption onto tubing walls.
Each ‘SmarTox-O’ is supplied with a ready made electrical plug and multi-core lead for terminating into a 24VDC supply and control loop system.
WHY PRE-DEHUMIDIFIER IS REQUIRED FOR ALL GAS SENSORS
The sensors used within the ‘SmarTox-O’ system are selected for best selectivity to their target gas in the presence of suspected interfering concentrations. However an important requirement is that each must operate continuously in relative humidity conditions typically above 90% non-condensing exiting the scrubber. These conditions unless modified will cause rapid premature failure of the cell which will then provide nil output
In order to achieve this in the normally saturated condition that are typically found in scrubbed air it is strongly recommended that an upstream moisture trap be used to remove condensate from the sample prior to entering the ‘SmarTox-O’ module.
In addition ‘SmarTox-O’ modules contain a sample drying and conditioning circuit that presents the sample to the sensor at a constant temperature and a controlled relative humidity.
Inclusion of this also reduces the effect of zero drift on the sensor (typically caused by temperature swings) and prolongs sensor life.
After exiting the sample conditioning circuit gas is drawn to the sensor (G, Fig 2) wherein a nano-amp signal is typically generated which is proportional to concentration, amplified then re-transmitted. The housing of the ‘SmarTox-O’ sensor assembly (C & D, Fig 2) is 100% machined virgin PTFE, another form of Teflon®. This material provides the same sample integrity benefits as that of FEP tubing. Each gas sensor is equipped with a transmitter (I, Fig 2) that outputs a linear industry standard 4-20mA signal.
Sample exhausting from the gas sensor is then drawn via Nalge® type tubing through a Rota meter (adjustable flow controller) with a stainless steel float for regulating sample flow to optimum levels of 0.4 to 1.0 LPM.
Before exhausting to atmosphere sample is drawn through a high duty miniature diaphragm pump.
The current signal from the sensor transmitter described above will be displayed in ppm’s on a backlit LCD for local checking of gas concentration.
Scope of Supply
The ‘SmarTox-O’ system contains peripheral devices and an enclosure for the ‘SmarTox-O’ module.
( 1 ) The Stainless Steel enclosure housing the module and a weather hood is also provided for further protection. The cabinet has a front panel sight glass that allows the user to view the ‘SmarTox-O’ module display and rotameter without the need to open the cabinet.
( 2 ) The moisture trap located on the side of the cabinet collects condensing moisture, preventing it from entering the ‘SmarTox-O’ module. Twice daily the moisture trap is drained by means of a timed solenoid valve. This negates the need to manually check the condensate level in the moisture trap.
The parts of the solenoid valve exposed to the highly corrosive condensate have been manufactured from 316 stainless steel for corrosion resistance.
All tubing to and from the moisture trap are also ¼” 316 stainless steel.
( 3 ) The interior of the cabinet is a stainless steel back plate on which two lengths of DIN rail are mounted. All internal components are mounted on these DIN rail lengths.
The ‘SmarTox-O’ module requires 24VDC for operation. Adjacent to the power supply is the timer for the solenoid valve control. The timer has been set to be on, providing drainage for 20 seconds twice daily.
This lightning protection for the sensing equipment in the ‘SmarTox-O’ module is provided in the event of a direct lightning strike to the cable linking the ‘SmarTox-O’ system and the data collection system. Earthing is provided. A low flow switch is provided to detect the gas flow out. Flow impediment could be a result of a failed pump a tubing blockage or a substantial leak in the pneumatic system. The signal provided by the switch is simply a contact that is open on flow (>150m1/min) or closed if flow drops below the set point (<150m1/min).
The flow switch may be susceptible to ‘clogging’ up if the sample being fed through the system contains particulate matter.
The float is manufactured from a billet of stainless steel and as such is resistant to corrosion. The problem arises with the small gap between the float and the wall of the switch.
A weekly check of the flow switch operation will show up a stuck float. If a switch does become clogged it may be cleaned by removal from the system.
The built-in pump is capable of pulling a sample through the tubing with out causing pressure transients.
The sample exhaust tubing is to remove the sample from the area of the system to a remote point. This eliminates possible contamination of the sample from H2S as well as removing possibly harmful concentrations of H2S from the immediate area.
The power requirement of the system can be supplied by 220 VAC or 24 VDC.
The “SmarTox” system provides 4 output signals :-
1. 4-20mA signal out proportional to gas concentration
2. Temperature alarm for signal conditioning unit
3. Pump and conditioner control signal
4. Low flow dry contact
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