Open-Path Gas Detection - Philosophy of Useor The Story of Clouds

People who are familiar with point type detectors ask about open-path gas detectors:-

How can we know the concentration, at any point, is not above the flammable limit?

and

The reading will vary with path length. Dont we need to know the length of the path to obtain a true reading of flammability?

These and similar questions may be important for some applications but not for offshore safety.

They do not need to know the path length because they are not interested in the point concentration.

They are not even directly interested in an average measurement

- their aim is to detect a significant hazard

In safety applications, gas detection should be used to warn that a hazardous amount of gas might be present in an area.

We are not trying to detect leaks as such because leaks are often not hazardous.

Many leaks will either be too small to threaten safety or may result in flash fires, which can be dealt with by fire hazard management.

Generally speaking, gas detection in production areas is aimed at explosion threats/hazards with a general principle that the degree of hazard should never be under-estimated.

Not all gas clouds are hazardous only if a flammable gas cloud or plume is wide enough to allow flame acceleration to speeds greater than 100m/sec, does it become a significant threat.

Just as an athlete performing the long jump needs a run-up distance, so too a flame front needs distance to reach the velocities which cause the damaging effects of over-pressure, pressure pulse and windage.

This distance is mainly controlled by the confinement and congestion of the area but 5 metres diameter is typical of offshore plant (less in some areas and more in others).

If a gas cloud of this size were at stoichiometric concentration (about 2 x the LEL of the gas) - then ignition could lead to the consequences we are keen to avoid.

Although detection of small releases would be nice, it is not critical to safety if some are missed - the aim must be to detect ANY cloud or plume of significant size.

Limitations of point type grid layoutsTraditionally, gas cloud monitoring was achieved by installing many point type detectors, in a 3-dimensional grid formation and correlating their signals.

One detector seeing gas would cause a warning, prompting inspection, while a second would cause automatic action.

Limitations of point type grid layoutsThe grid layout of point type gas detectors achieves the aim of volumetric hazard monitoring provided there are sufficient detectors (a tight enough grid) to give a representative picture of the whole area, but this is rarely the case!

The plume of a significant gas leak can (and has) passed undetected between point detector positions.

Limitations of point type grid layouts

There are two problems:

1. Not only has gas to reach the point detectors 2. But it must also do so at a concentration high enough to cause alarms.

This means that the gas must reach a point detector before it dilutes below the alarm level in any wind conditions. (Actually, the ventilation in the area will have been specifically designed to prevent this by diluting gas as rapidly as possible).

Limitations of point type grid layoutsThe number of point detectors in an area is limited by the practicalities of installation and maintenance and also by the inherent restriction on the cost-benefit of gas detection, due mainly to lack of an effective action to control explosions.

The number of detectors allowed by these limits is often somewhat less than an ideal grid and designers have many problems trying to site point detectors effectively.

PhilosophyAll leaks result in a plume (or cloud), which is concentrated at source and becomes dilute as it disperses in the airflows in the area.

Diluted to 15%LELOpen pathPoint type5 metres spacingHigh concentrationNo point detector reads more than 15%LEL below alarms

A point detector has to be close enough to the source that the gas concentration is still strong enough to be detectable but if too close, the plume will be very narrow and detectors would have to be close together.Open-Path reads 8 x 15%LEL = 1.2 LEL.m = above alarms Here is an open-path detector and some point detectors in a 5m grid layout

PhilosophyOpen-path gas detectors remove this problem.

Whether the open-path is close to the source of leak or far away, provided that the whole width of the plume is within the path, will produce a similar response.

This is partly due to the way the detector works and because air currents primarily govern the movement of gas in offshore modules so the plume does not usually spread as it would in free-field flow.

Philosophy

The challenge for siting gas detectors is to predict where the gas will go, instead of trying to predict where it could leak or collect.

On the whole, this is a much easier task the gas will go with the flow. A great deal of work offshore with video cameras and smoke generators convinced the offshore oil companies that the old notion of light gases rising and heavy gases falling is not likely for gaseous releases.

Local air movements have the only detectable effect. (Flashing liquid releases are an exception, while they remain cold)

Airflows that carry the gas tend to be concentrated in the open, unobstructed areas of the plant, just the right sort of area to install open-path detectors

Open-path measurement rangeRemember we discussed the 5 metres diameter plume/cloud re flame front velocities and a gas cloud of this size at stoichiometric concentration (about 2 x the LEL) being the limit this would integrate as 10 LEL.metres (2 LEL x 5m).

For safety margins on the open-path gas detectors full scale range, we halve this to 0 - 5 LEL.metres with alarms normally set at 20% and 60% of this scale (equiv 1 & 3 LEL.m resp.).

Safeye even has the sensitivity for certain distances to be set at 0 - 2 LEL.m full scale

The ResultOpen-path gas detectors offer the way forward to good coverage for a justified cost.

The use of the point detector grid approach can be limited to congested areas while open-path detectors monitor the major airflows.

The reduction in the number of devices, cabling, support equipment and maintenance is substantial in most plant.