Inert gas analysis to stand the test of time

The measurement of oxygen in inert gas is vital in ensuring the safety of a tanker. But what gas analysis technologies are available and why is robust instrumentation and accurate measurement so important?

There is a well-used adage that in order to manage anything, you first need to be able to measure it. With the risk of explosions being a real possibility if such systems are not properly managed, accurate and reliable gas analysers are therefore key to ensuring the safety of a tanker and her crew.

The recent shift by many shipbuilders in the Far East towards ?lowest cost? equipment has encouraged the use of less robust analysis technologies. But, as with other kinds of marine equipment, many owners and operators are beginning to see the benefits in specifying the equipment they want at the outset.

Technology explained:

Before equipment can be specified, it is important for owners to appreciate that there are a number of technologies available to measure gas concentrations on ships. Vibration, tilt and changes to ambient temperature are all factors that affect the technology that can be used successfully in a marine environment. Outlined below are the three principal technologies used for the measurement of inert gas oxygen concentrations on ships.

Electrochemical sensors

Electrochemical sensors

are based on the principle that cells can be constructed that react with the measured gas and generate an electric current. The current can be measured and the amount of gas determined as a result. These sensors are low cost and small enough to allow several to be incorporated into the same instrument easily, making them ideal for simple detection applications such as ambient monitoring in pre-checked enclosed spaces. Typical combinations in this application are sensors for oxygen deficiency and traces of hydrogen sulphide and carbon monoxide.

The disadvantages of using electrochemical sensors are that, like batteries, they need replacing once all the chemicals inside them are expended. Because they are chemical based the sensors are affected by temperature and the presence of background components (which may alter the reactions taking place). The other significant problem is that it is not easy to determine whether a low reading may be due to low levels of the measured gas or expended chemicals (especially with oxygen).

It is for this reason that electrochemical sensors are often not accepted for use on critical low oxygen monitoring applications on land-based facilities.

In the marine industry, electrochemical are commonly used in portable personnel protection devices and the small size of these makes them very attractive. Electrochemical analysers were also once commonplace on inert gas systems, where alarm points of 5% and 8% oxygen are typical. However, exposure to the risk of falsely low readings, the need to replace cells every six months to two years and the administration of having to obtain spares and show inspectors a check is kept of use-by dates on stored cells, means most inert gas system (IGS) suppliers are being asked to move away from these for fixed systems.

Zirconia

Fixed analysers for oxygen monitoring on IGS?s are now generally either zirconia based or paramagnetic.

Zirconia

analysers make use of the unusual properties of zirconium oxide, ?zirconia?. If placed between two different concentrations of oxygen and heated to around 650°C this material allows oxygen ions to pass through it. If an electrode is attached to each side of the zirconia, the flow of ions can be detected as an electric current. This current is proportional to the difference between the two concentrations. Analysers based on this principle expose one side of the zirconia to air and the other to the sampled gas in order to determine the oxygen present.

There are three key advantages of using a zirconia analyser: it offers a fast response, it will not suffer the ?false low? characteristics of electrochemical sensors and requires minimal sample gas preparation. Sophisticated variants of this analyser are commonly used on land for controlling combustion processes in power plants.

However, the high temperatures involved with zirconia based measuring techniques mean that limited cell lifetimes are inevitable. The most expensive, high-grade zirconia cells for power stations typically last around 3-5 years but those used in marine applications usually last just long enough to cover a shipbuilder?s warranty period. Lifetimes can be maximised by turning a unit off when not in use but a warm up period of around an hour must be allowed when it is next switched on (Ion flow properties are highly temperature dependant so allowing the zirconia to stabilise before calibration is essential). Care must also be taken to ensure that the air side of the zirconia cell is always open to hydrocarbon free air, otherwise these will combust on the hot zirconia, decrease the oxygen contacting it and affect the reading.

The high price of spare zirconia cells, the rather unpredictable nature of their failure and the high cost of being off charter when the inert gas system is out of use, means that many owners and operators now opt to specify paramagnetic analysers for IGS control. These analysers make use of the magnetic properties of oxygen and as a result the sensors are non-consumable. This enables them to be left continually ready for use and if appropriately designed gives them a typical lifetime of over ten years.

Paramagnetic

Not all paramagnetic analysers are suitable for use in marine applications however, as certain techniques are more susceptible to vibration and tilt than others.

An obstacle to use of paramagnetic oxygen analysers is initial price: the precision engineering involved in manufacturing the sensors makes them up to £5,000 more expensive than zirconia or electrochemical units.

This makes it unattractive to shipyards and IGS manufacturers who are asked to supply ?lowest cost? equipment. However, shrewd owners and operators are increasingly taking the long view by specifying them and the extra cost is easily recouped in reduced maintenance time. Alternatively, owners wait until the original equipment fails and then replace the units with paramagnetic ones as a matter of course.

A summary of these technologies would conclude that there are effectively two acceptable routes for inert gas analysis on board tankers. Those looking for the lowest initial cost are likely to opt for zirconia based IGS monitors whilst those looking for maximum reliability opt for paramagnetic IGS analysers.

Electrochemical sensors

on the other hand are now generally seen only as an option for personal monitors. Whatever the decision, the wrong choice of technology, or equipment supplier, can be extremely costly.

When it comes to working with suppliers of gas analysers, there are a few large and many smaller players, though only a few have specific marine products and global service capability. Servomex is one of these and offers paramagnetic oxygen analysers for inert gas system control (the 1800 Marine) and portable use (the Servomex 262). It is also obtaining marine approval for the Servomex 1900, a version of the 1800 designed for use in areas at risk of flammable gases such as those around vapour recovery systems.