Producers and users of natural gas, as well as those operating pipelines, need to know the hydrocarbon dew point of the gas flow for a variety of reasons.

Buyers of natural gas need to know that the product meets the minimum contractual specification, and sellers want to be sure they are not exceeding the specification and losing money by delivering a more highly processed product than is necessary. Just as important, however, is the need to avoid hydrocarbons condensing out in the gas stream, as the resulting liquid can combine with any moisture present to form hydrate emulsions. If these amalgamate and form a slug that blocks the pipeline, the slug will be forced along the pipeline at high speed, propelled by the pressure of gas behind it. With pipelines measuring from 0.5to10m in diameter, such a slug can cause catastrophic damage when it arrives at a compressor or other equipment.
While this may sound far-fetched, such an event could easily result in the temporary closure of a pumping station or even an intercontinental gas pipeline such as those bringing gas into the UK. If this were to happen, the resultant costs would potentially run into millions of Euros.
Water consists of only one compound, so, for a given pressure, there will be a single temperature at which condensate starts to form. Natural gas, on the other hand, contains a wide range of hydrocarbons, from light, short-chain aliphatics to heavy, long-chain molecules, making it difficult to measure the dew point. Each hydrocarbon will have a slightly different dew point temperature for a given pressure -- though the important temperature is that at which the first molecules start to condense out, as other molecules will condensate out soon after.
The dew point of water is often measured using achilled mirror' technology, in which the surface of a mirror is slowly cooled until condensate forms, at which point there is an easily distinguished change in the reflectivity of the mirror. Water forms a multitude of tiny droplets when it condenses -- due to its surface tension -- thereby scattering light that is directed onto the mirror, but hydrocarbons form a planar dew, which is typically as reflective or more reflective than a condensate-free mirror surface. This makes hydrocarbon dewpoint difficult to detect using traditional cooled mirror techniques.
To get around this problem, one approach is to use gas chromatography to analyse the gas constituents, then use a model to calculate the theoretical dew point. A limitation of this approach, however, is that gas chromatography equipment is not normally capable of analysing natural gas with the degree of sensitivity required to accurately predict the hydrocarbon dewpoint temperature. An alternative system involves passing a sample of the gas through a condenser and weighing the condensate. Although this gives accurate results, it is time-consuming and too slow for such instruments to be used as part of a process control feedback loop.

Dark-Spot detection technique

Around 20 years ago Shell Research invented and patented a technology based on the chilled mirror principle. The difference, however, is that the amirror' has an etched matt surface and a central conical depression. When there is no condensate present, the light from a collimated visible red light source is scattered and an optical detector picks up a low-level base signal. As soon as condensate starts to form, the conical mirror is coated with liquid and reflects an annular ring of light (Fig.1). Because of the position of the optical detector inside the annular light pattern, there is a corresponding dramatic reduction in the light intensity.
Michell Instruments purchased an exclusive licence to develop this Dark Spot detection technique and used it at the heart of its Condumax hydrocarbon dew point analyser, which was successfully sold for almost 20 years. Nevertheless, although the Condumax remained possibly the most popular instrument of its type, the market requirements evolved and the instrument started to become less attractive. For instance, pipeline operators are now building unmanned pumping stations, and the Condumax needed a certain amount of manual intervention; also the Condumax electronics had to be located in a designated safe area, but safe areas are not as common or popular as they were on older sites. Also the cost of making intrinsically safe electrical connections from hazardous to safe areas is becoming prohibitive.
It was apparent that a replacement for the Condumax was required, so Michell Instruments embarked on one of its most important ever product development programmes. The company recognised the CondumaxII had to be market-led, not just technology-led.
Once the feedback from VOC (voice of the customer) exercises and focus groups had been collated, a Product Definition and Product Specification were drawn up, using a weighted prioritisation list to distil the customer requirements. Normally there would be no design work until these two documents were complete, but due to the foreseen complexities in this case, it was felt prudent to make an early start on the core sensor technology, the optical component design, the core microprocessor architecture and the sensor mechanical design.

Certification

In order to meet its design objectives, the complete CondumaxII analyser itself had to be certified for hazardous area use so that it could be installed adjacent to a pipeline, for example. Practically this meant that the analyser enclosure had to be explosion-proof (Eex d classification). However, the designers were faced with a significant problem because, in order for it to function, the CondumaxII required process gas to be continually present inside the instrument -- effectively in intimate contact with the electrically-powered devices -- which is not normally the case with Eex d equipment.
Nonetheless, the potential benefits were felt to be so great as to make the development worthwhile because, being an Eex d rated product with the sensor and instrumentation contained within a single enclosure (other than for its output signals) the CondumaxII would not need to be wired back to a safe area -- which would provide customers with considerable savings on the cost of installation. Considerable effort and the services of a consultant were then required to convince the certifying Notified Body that it was acceptable to take a sample of the process gas inside the analyser and then return it to the vent line without making the inside of the enclosure effectively a hazardous area. Michell Instruments believes the CondumaxII may be the first instrument of its type which has been successfully certified to operate in this way.
It is anticipated that the majority of the units sold will be for use in Europe, at least initially, so the approach taken was to first gain approvals and CE mark the CondumaxII in line with the EMC and ATEX Directives. It is hoped that much of the documentation generated during this process will then be directly transferable for CSA (Canadian Standards Authority) approval and, consequently, CSA (US) approval for use in the USA.

Chilled mirror

One of the requirements for the CondumaxII was that it should be able to operate simply from an 85/265Vac mains voltage supply. The design team was therefore drawn to using a Peltier device, which could also act as a heat source for raising the temperature of the mirror to clear the condensate after a measurement had been taken, but using a Peltier creates its own difficulties. Firstly, the Peltier has to operate at the pipeline pressure, which would normally be around 27barg, but could be as high as 100barg. Peltiers are relatively delicate devices mechanically, and several were damaged during early tests. In addition, it was found that a bespoke three-stage device had to be employed to deliver sufficient cooling, and a special drive circuit had to be developed to operate the Peltier efficiently without generating excessive waste heat within the instrument enclosure.
To make it suitable for use in hotter climates, the Condumax II has to be able to operate in ambient temperatures of up to 60°C. Electronic components are normally rated for use at temperatures up to 70°C, so it was essential that excess heat from the Peltier and its driver -- as well as other electrical and electronic components -- did not cause the temperature inside the enclosure to rise significantly. There is, however, no forced cooling inside the enclosure, rather the aluminium alloy enclosure acts as a heatsink.
The final sensor cell design employed an alternative mirror mounting system, with O-ring seals, an acetal insulation ring and a mirror disc that is thinner at the outside edges to minimise heat transfer between the mirror and the rest of the sensor cell (Fig.2).

Temperature measurement

This sensor design also makes use of a low-mass thermocouple; problems of over-reading the temperature had initially been traced to the use of a PT100 thermocouple that had been measuring the temperature over a short length, instead of at its tip. Although the first tests with the low-mass thermocouple also showed slight over-reading, this was found to be due to heat being conducted along the thermocouple wires. By careful consideration to the routing of these wires, this effect was negated and the thermocouple was able to give accurate temperature measurements.
Aside from being able to cool the mirror to a low enough temperature (-34°C), in hydrocarbon dewpoint measurement the rate of cooling is also important. Once condensate starts to form, the temperature must be held constant while the reading is taken. If the rate of cooling is too fast, the instrument will overshoot and give a reading that is too low; yet the rate must be fast enough to achieve a reasonably short cycle time (typically the instrument will take a measurement every ten minutes, with a cycle time of less than three minutes).
During start-up the CondumaxII performs a measurement cycle at a standard cooling rate in order to arange find' the hydrocarbon dew point level. This value is used in the subsequent measurement cycle to optimise the cooling so that there is initially fast cooling, then the rate reduces to 0.05°C per second as the dew point is approached. This gives a measurement accuracy of ±0.5°C.

Operator interface

Although the CondumaxII was to be capable of operating at unmanned locations, there was a need for an operator interface during commissioning and on-site servicing. Rather than use an LCD display, it was decided to install a vacuum-fluorescent graphics display that would provide superior contrast and viewability, as well as a wider operating temperature range. To ensure that the instrument remained explosion-proof, the display had to be mounted behind a 20mm thick glass window, which made incorporating pushbuttons or switches difficult.
One possibility was to use Hall-effect sensors behind the glass, with a magnetic pointer for use by the operator. Tests showed that this was functional, but far from elegant. Instead, a bespoke proximity detector was developed that detects changes in capacitance if the glass in front of the switch is touched. This is easy to use, and works equally well with a bare or gloved finger.
To aid factory set-up and servicing, the operating panel, complete with display and proximity switches, can be dismounted from the instrument and temporarily mounted to one side (Fig.3).

Design for manufacture

Michell Instruments was already certified to manufacture intrinsically safe products, and has since become certified to manufacture Eex d products, but did not wish to undertake the lengthy and costly process to become itself a manufacturer of explosion-proof enclosures and related equipment. It was therefore decided to use an off-the-shelf explosion-proof enclosure, suitably machined to accept the sensor cell, electronics and other hardware.
Despite being cast from aluminium alloy, the empty enclosure weighs around 20kg. To minimise the handling required during product assembly, the CondumaxII was designed so that the sensor cell and electronics can be built and tested as separate sub-assemblies before being installed in the enclosure.

Pre-production prototypes

In many product developments the prototypes only resemble the final product in some respects. However, the prototype CondumaxIIs had to be as close to the final product as possible in order to prove the supply chain, manufacturing instructions and almost every aspect of the design.
Four fully-functional prototypes were produced to enable the final software developments to be completed and for the initial product functional test programme. Six pre-production PCBs were built to test the production process and supply chain and to be used in Beta site tests.

Successful completion

The final product meets all of its design objectives, even though some minor design compromises were required in order to gain the necessary certifications. One of the most important aspects of the design, which did not raise any particular problems during the development, was the incorporation of Modbus industrial fieldbus communications capabilities. The requirement for Modbus resulted from studying the trend towards unmanned sites for which automatic, remote data collection and instrument configuration are essential. With this in mind, the CondumaxII is supplied OPC-enabled to allow simple and straightforward integration with SCADA and DCS systems as well as other industry-standard control systems. Moreover, with the incorporation of ActiveX tools, customers can monitor and control the CondumaxII from anywhere in the world via a telephone line (Fig.4).
This also means that Michell Instruments is now in a position to offer customers 24-hour support for the first time, with the ability to diagnose faults remotely. Problems with the instrument itself -- or problems with other equipment identified by the instrument -- can therefore be resolved more quickly, and possibly without the need for a site visit. With the high cost of downtime in the natural gas industry, the potential savings are vast. From Michell Instruments' point of view, it also presents a new business opportunity, enabling the company to expand from offering products alone to complementary support services as well.
The purchase cost for the CondumaxII is similar to that for the previous Condumax model, but the installation costs and operating costs are significantly lower. These factors, together with the CondumaxII's ability to operate for long periods without any human intervention, mean that the new model is suitable for a far wider range of applications than its predecessor. Indeed, it is estimated that the market could be several times larger, including applications such as monitoring the feed gas for power stations, monitoring natural gas as it leaves contactors to check for excess glycol (a drying agent that can cause hydrate formation if it is picked up by the gas stream), and monitoring gas as it leaves privately-owned underground storage facilities.
Taking all of this into account, the CondumaxII is one of the most import product development programmes that Michell Instruments has ever undertaken. The project was completed with financial assistance from a UK Government-funded Smart award, and Michell Instruments anticipates that sales of this new product will make a significant difference to the company's turnover in years to come.