Recommended Placement Practices for LEL and H2S Sensors 2

[BizzyCon]

Hello all,

I am working on placing some LEL and H2S detectors in a petroleum refining process unit. I can think of several ways that I think would work. But I would like to know if there are any official or standard procedures for designing the location of these types of sensors.

Clearly I need to place them by equipment that will be likely to emit hazardous gases. Also the gases we are worried about are more dense than air so they should be close to the ground.

What kind of spacing should I shoot for? For some furnaces is it enough to place 1 LEL detector nearby or do I need to surround it with say 4 sensors just in case of mild to moderate winds?

Hopefully my questions make sense, and someone can give me some guidance. If you need more information let me know.

- Bizzy

 

[danw2]

Here's what Honeywell Analytics (formerly Zellweger) says:

Location of Sensors
‘How many detectors do I need?’ and ‘where should I locate them?’ are two of the most often asked questions about gas detection systems, and probably two of the most difficult to answer. Unlike other types of safety related detectors, such as smoke detectors, the location and quantity of detectors required in different applications is not clearly defined.

Considerable guidance is available from standards such as EN50073 Guide for selection, installation, use and maintenance of apparatus for the detection and measurement of combustible gases or oxygen. Similar international codes of practice e.g. National Electrical Code (NEC) or Canadian Electrical Code (CEC) may be used where applicable. In addition certain regulatory bodies publish specifications giving minimum gas detection requirements for specific applications. These references are useful, but tend to be either very generic and therefore too general in detail, or application specific and therefore irrelevant in most applications.

The placement of detectors should be determined following the advice of experts having specialist knowledge of gas dispersion, experts having knowledge of the process plant system and equipment involved, safety and engineering personnel. The agreement reached on the location of detectors should also be recorded.

Detectors should be mounted where the gas is most likely to be present. Locations requiring the most protection in an industrial plant would be around gas boilers, compressors, pressurized storage tanks, cylinders or pipelines. Areas where leaks are most likely to occur are valves, gauges, flanges, T-joints, filling or draining connections etc.

There are a number of simple and quite often obvious considerations that help to determine detector location:

To detect gases that are lighter than air (e.g. Methane and
Ammonia), detectors should be mounted at high level and preferably use a collecting cone.

To detect heavier than air gases (e.g. Butane and Sulfur Dioxide), detectors should be mounted at a low level.

Consider how escaping gas may behave due to natural or forced air currents. Mount detectors in ventilation ducts if appropriate.

When locating detectors consider the possible damage caused by natural events e.g. rain or flooding. For detectors mounted outdoors it is preferable to use the weather protection assembly.

Use a detector sunshade if locating a detector in a hot climate and in direct sun.

Consider the process conditions. Butane and Ammonia, for instance are normally heavier than air, but if released from a process line that is at an elevated temperature and/or under pressure, the gas may rise rather than fall.

Detectors should be positioned a little way back form high pressure parts to allow gas clouds to form. Otherwise any leak of gas is likely to pass by in a high speed jet and not be detected.
Consider ease of access for functional testing and servicing.

Detectors should be installed at the designated location with the detector pointing downwards. This ensures that dust or water will not collect on the front of the sensor and stop the gas entering the detector.

When siting open path infrared devices it is important to ensure that there is no permanent obscuration or blocking of the IR beam. Shortterm blockage from vehicles, site personnel, birds etc can be accommodated.

Ensure the structures that open path devices are mounted to are sturdy and not susceptible to vibration.

http://www.jjstech.com/gasdepr.html

 

[JLSeagull]

My instantaneous thoughts outside of the codes and client standards follow:

In addition to individual point infrared or catalytic sensors you may want to consider the open path sensors. These may be different technologies for your refinery HC than the H2S technique. Some companies can do both; others can do neither.

Some standards apply such as ASTM E 1865 and E 1982; and ISA 12.13.04, etc.

Several suppliers exist including Detector Electronics, Honeywell Analytics, MSA and others.

If staying with individual HC points consider about three outside each serious pump pair location and perhaps others in the area about every 50-foot if the budget is OK. And yes, consider something similar for H2S where potential sources exist.

BTW for multiple single and multiple levels you want all sensors to be easily serviceable for testing monthly (to quarterly) depending upon the client intentions.

Regards,

John

 

[BizzyCon]

Thank you both for your feedback. It gave me quite a bit to consider. I understand that there are a lot of engineering decisions that still need to be made based off of the specific process that I am dealing with.

Thanks!!

- Bizzy