Keeping vapours at bay

Liquid cargo is transferred into tankers via ship-to-ship transfer or from shore-based facilities, including refineries and terminals.

This liquid not only displaces any vapours present within the cargo tank, but also generates vapours of its own through evaporation.

Typically, these vapours contain volatile organic compounds (VOCs) which are flammable and combustible, creating dangers such as cargo tank overfilling, cargo spillage, and the risk of fire or explosion. During cargo tank filling, large volumes of vapours are generated. These must be either vented or recovered. Gas monitoring is an essential part of this process to ensure the safety of the vessel, terminal, and personnel, and also to reduce harmful emissions.

MVR regulation

Vapour control also introduces its own hazards, including the possibility of over or under pressuring the tanker, and introducing extra sources of ignition.

This raises the importance of accurate gas monitoring even further, and has led to the creation of stringent rules to govern the gas analysis systems involved.

In the US, strict regulations for marine vapour control systems (MVCS) are enforced by the US Coast Guard (USCG), operating under the authority of the US Clean Air Act. All MVCS operating in the country require USCG certification.

These rules, designed to safeguard personnel and equipment by containing and controlling vapour emissions from a vessel’s cargo tanks during loading, have also been adopted by many other countries. USCG regulations are currently the most comprehensive standards being enforced, so those MVCS which meet these criteria will be suitable for use around the world, subject to minor changes for local requirements.

In addition, the US remains the largest market for MVCS – although South America is becoming increasingly important as key US companies invest in the region – so compatibility with USCG rules is a highly desirable quality for any MVCS.

Among the VOCs present in vapours generated during loading are benzene, crude oil vapours, and gasoline blends. These vapours are either returned to the plant and used as fuel or raw materials, or taken to a safe area and incinerated.

In either case, it is necessary to monitor the return lines for the ingress of air, in order to avoid the creation of explosive conditions.

Under USCG regulations, this monitoring must be carried out by a redundant system – in case one analyser fails – so typically a pair of paramagnetic oxygen analysers, certified for use in hazardous areas, are used.

Paramagnetic sensing offers important benefits to reliability, as it is a non-depleting technology. This means there is no uncertainty about how long the sensor will last before needing replacement.

Electrochemical sensing

Prior to the introduction of regulations in 1990, oxygen monitoring was performed by electrochemical sensors.

These sensors, also called a galvanic sensor, typically consist of a small, partially sealed cell containing two dissimilar electrodes immersed in an aqueous electrolyte (commonly potassium hydroxide). Oxygen molecules diffuse through the semi-permeable membrane installed on one side of the sensor and are reduced at the cathode, forming positively charged hydroxyl ions. These ions then migrate to the sensor anode, where oxidation takes place.

This generates an electrical current which is proportional to the oxygen concentration in the sample gas. The current can be measured, conditioned with external electronics, and displayed on a digital panel meter to show either percent or parts per million (ppm) concentrations.

Electrochemical cells are initially inexpensive and do not consume much power. However, they have a relatively short lifespan, and may cease operation without warning, delivering a low false reading. They require frequent calibration due to sensor drift, and must be replaced regularly.

Advantages of paramagnetic sensing

Non-depleting paramagnetic technology has a longer lifespan than an electrochemical sensor. It operates by relying on the relatively high magnetic susceptibility of oxygen compared to other gases. The paramagnetic cell contains a small glass dumbbell, filled with nitrogen and suspended on a taut platinum wire within a non-uniform magnetic field. The dumbbell is suspended from the wire so it is able to move freely.

When a sample gas containing oxygen passes through the sensor, oxygen molecules are attracted to the magnetic fields. This displaces the dumbbell, causing it to rotate – a precision optical system, made up of a light source, photodiode, and amplifier circuit, is used to measure the degree of rotation.

In order to restore the dumbbell to its normal position, Servomex introduced the use of an opposing current. This current is directly proportional to the partial pressure of oxygen, and is represented electronically as percent oxygen.

Paramagnetic sensors offer a very good response time, and use no consumable parts, typically providing many years of service. While initially more expensive than electrochemical cells, the long lifespan and infrequent calibration needs make them good value in the longer term.

Emissions monitoring applications

Reducing emissions is a key requirement for MVR monitoring equipment. Operational performance of recovery methods, such as activated carbon adsorption or refrigeration condensation, is monitored by a hydrocarbon analyser.

Depending on the site, the control and monitoring of different hydrocarbons (e.g. methane or non-methane hydrocarbons) will be required. An infrared analyser is used for this application and, again, depending on site conditions, may need to be certified for hazardous areas.

This analyser can be located in a control loop at the vapour recovery equipment or, for an emissions application, at the vent of the recovery system.

Alternatively, instead of hydrocarbons being captured, they can be removed from the vent stream by destruction, typically through the use of a thermal oxidiser. This may also require the monitoring of combustion by-products such as carbon monoxide (CO) and sulfur dioxide (SO₂).

To perform this gas monitoring, a multi-gas infrared analyser is commonly used, as it will function effectively in air or inert gas environments.

In order to meet USCG requirements, a system performing both vapour recovery and emissions control must be calibrated before each loading operation.

For the most effective operation, the system will also require robust sampling components, remote calibration capabilities, and a redundant analyser system in a single enclosure. The analysers should be designed for batch loading with varying background gases, and suitable for corrosive marine environments.

Case study: oxygen monitoring for MVR on a loading dock

Servomex has experience working alongside many MVCS manufacturers, supplying reliable and accurate gas analysis solutions that are compliant with international regulations, and are supported by expert systems knowledge.

A typical example involved the supply of an oxygen monitoring system to a US-based manufacturer specialising in environmental and combustion applications.

The manufacturer had previously purchased two oxygen analysers for a MVR system on a loading dock, but was unhappy with the response times and accuracy.

Its system used a dilution system to add nitrogen, ensuring any combustibles were kept below their lower explosive limit (LEL) in the presence of oxygen.

To make this system operational, the voting system needed to read within 1% of the measured range. However, the analysers were reading 3% apart. Step changes took place, because the flow and pressure was not balanced going to both analysers.

In addition, the magnetic wind technology used by these analysers was subject to errors in the presence of hydrocarbons or hydrogen, and due to their plumbing configuration could not be calibrated simultaneously.

As such, the MVR system failed to comply with USCG regulations that stipulated a response time of not more than 30 seconds.

To resolve the issue, Servomex supplied a system using the SERVOPRO Oxy 1900, a digital paramagnetic analyser certified for Zone 1/Div 1 hazardous area applications. Two analysers were used, to comply with the redundant system requirement. Both of these analysers can be calibrated at the same time, with a single control point for the gas inlet for calibration and sample.

The new redundant systems were designed to ensure equal length tubing from the sample take-off to sample inlet on both analysers, avoiding the reading errors inherent in the previous solution. The coalescing membrane for this system was plumbed, so that the water did not collect, and the filter was draining continuously.

The successful, regulation-compliant performance of this system immediately led to a second order from the manufacturer.

Benefits of expertise and experience

The technology used in MVR is federally mandated, so there is little – if any – variation in the design and engineering of these systems. Every dock skid has the same construction and components, with the only difference being the scale, which changes according to the volume of vapour being collected.

Competition between suppliers is therefore highly dependent on factors such as reliability and expert support.

The redundant system means operators can continue to load if they lose one analyser, but must cease operation once loading is complete, to repair or replace the sensor. Downtime means that docks sit idle, costing logistics firms significant sums of money.

Reliable technology is, therefore, vital to MVR. However, occasional dock skid breakdown is inevitable, so rapid and knowledgeable service support from a company that understands the MVR sector is essential.

For example, a global service network is a major benefit for end users, who can rely on rapid, expert support if their dock skid requires maintenance or repair. Continuous uptime is essential for logistics companies, so it is important to have the structure in place to get processes up and running again fast.

Furthermore, companies should look for suppliers who can provide a range of service products that are scalable and customisable to suit user requirements and budgets, from a cost-effective, reactive approach to a full-service partnership that delivers regular servicing and preventative maintenance.

For oxygen monitoring, Servomex provides either the SERVOTOUGH Oxy 1900 or SERVOTOUGH OxyExact 2200, which both meet the necessary certification for use in hazardous areas. These analysers use paramagnetic technology, which ensures reliability and availability for operation at all times. This is vital, as unscheduled breakdown means operations may have to stop immediately.

To control emissions during vapour recovery or destruction, the SERVOTOUGH SpectraExact 2500 Infrared analyser is used to monitor hydrocarbons, while the SERVOPRO 4900 Multigas monitors combustion by-products such as CO and SO₂.

Conclusion

Reliable, high-quality components and expert support are the key differentiators in gas analysis for MVR.

Extensive systems build expertise, and the support of highly trained service personnel is an essential element in delivering consistent, accurate measurements across a long life of operation.

This means MVR operators can be confident that they fully comply with global regulations while running a safe, efficient process.

Written by Keith Warren, Servomex, UK.

Read the article online at: https://www.hydrocarbonengineering.com/gas-processing/12022021/keeping-vapours-at-bay/