Wärtsilä’s dual-fuel engine technology is widely recognized as an enabler to the use of gas as a marine fuel. The Wärtsilä Gas Valve Unit – Enclosed Design™ is delivered as standard with these engines, and is part of the company’s comprehensive gas propulsion package.
Fig. 1 – The Wärtsilä Gas Valve Unit – Enclosed Design™ come as standard with a built on control system, factory tested before delivery.
The traditional choice of fuel for ships has either been marine diesel oil (MDO) or heavy fuel oil (HFO). However, for a long time already natural gas has been used to fuel LNG carriers. Another niche market where this is happening is with vessels operating along the coast of Norway.
Soon more strict emission regulations will be applied in Emission Control Areas (ECA). In order to comply with these new regulations, it will be necessary to invest in new technologies. Installing an exhaust gas after-treatment system is one alternative, while another solution is to use fuels other than diesel , for example liquefied natural gas (LNG). By converting to LNG no exhaust gas after-treatment system is required. Together, the new regulations and the growing availability of LNG have made the use of gas as a marine fuel a very attractive option.
The predominate method for storing LNG in gas fuelled vessels is pressurized LNG storage tanks. Before the LNG can be burned in an engine it has to be evaporated and heated to the correct temperature. The evaporated gas has a fixed pressure depending on the LNG-storage tank pressure.
The gas is transferred from the tank below decks to the engine via what is known as a double walled pipe . The inner pipe is fully enclosed within an outer pipe, and the annular space between the pipes is mechanically ventilated.
In case of a gas leakage from the inner pipe, the outer pipe will form a second barrier to contain any leak and prevent gas from entering the engine room. Gas leaking from the inner pipe can be detected with gas detectors in the ventilation pipe before entering the extraction fans. Upon detection, the affected gas supply line can be shut down.
The main functions of the Gas Valve Unit are to regulate the gas feeding pressure to the engine, and to ensure a fast and reliable shut down of the gas supply. The latter requirement is specified in the Interim Guidelines on Safety for natural gas fuelled engine installations on ships (IGF code), which state that each item of gas consuming equipment has to be provided with a set of “double block and bleed” valves.
The double block and bleed valves consist of two quick closing valves and a ventilation valve between the quick closing valves. The two block valves provide full redundancy as they are installed in series. In addition, the pneumatically actuated valves will close in case of the loss of either power or air supply. The ventilation valve is always open when the blocking valves are closed. In a unique test sequence developed by Wärtsilä, the functionality of the valves is tested at each engine start and stop.
For each engine, the gas supply pressure has to be adjusted within a narrow, load dependent, pressure range. The adjustment is made by means of a pressure control valve located within 10 m of the engine. A smaller gas volume between the pressure control valve and the engine improves the response time of the system during transient conditions, such as engine load fluctuations.
Maintenance and the inerting of supply lines
Before maintenance work is commenced on the engine and/or the GVU, it is required that any remaining natural gas is removed by substituting the natural gas with an inert gas, for example nitrogen. The GVU inerting process ensures that natural gas cannot leak to the surrounding areas, thus eliminating potential risks.
Fig. 2 – The GVU-ED™ can be installed directly in the engineroom with minimum footprint and "plug and play" commissioning."
Fig. 3 – GVU-ED™ installed into Viking Grace.
Limitations with existing designs of Gas Valve Unit room arrangements
If a single walled gas pipe passes through a room below deck, the complete room will become a gas hazardous area. The traditional concept to fulfill the safety requirements has been to build a room exclusively for the GVU, completely separated from other rooms.
The room has to be a gas tight enclosure and must meet the following requirements.
Fulfilling all these requirements is a daunting task, especially for smaller vessels where the engine room space has been fully utilized.
However, due to the number of different valves required in a GVU, when made from standard off-the-shelf components, the physical size of the unit has prohibited any alternative design than to install the GVU inside a dedicated room.
On the market, control systems are not commercially feasible for installation in a gas hazardous area. Instead, an ATEX certified connecting box has until now been installed for the GVU’s electric system. The actual control system for the GVU must be installed in a separate room. Connection to the automation system has not been possible prior to final installation in the vessel. Therefore, verification and functionality tests of the complete system is not possible until the most hectic days during the vessel’s commissioning, which is typically just before the sea trials.
The innovative Wärtsilä GVU-ED™ is distinguished from earlier designs by having a gas tight enclosure around the process components, such as the valves, actuators and sensors. Because of the enclosure design, the same principles can be applied as for double walled piping. The enclosure forms a gas tight, second barrier against any unforeseen gas leakages. Hence, the GVU-ED™ as such is just part of the larger ventilated double wall piping system.
The piping and components are coated on the outside in accordance with marine specific colour schemes.
Since the gas related equipment is contained within the unit, the Wärtsilä GVU-ED™ can be installed next to the engine, in a similar fashion as other auxiliary equipment. As no dedicated GVU room is required, the space and cost savings for both the yard and owner are evident. Similarly, savings are achieved in the control system architecture, where the control and solenoid valve cabinet is mounted directly onto the unit. The complete GVU-ED™ functionality is controlled by the built on control system. The control system is based on the Wärtsilä Unic hardware, and the same reliable components as used on the Wärtsilä Dual Fuel (DF) engines themselves. Based on the signals from the control system logic, the solenoids control the pneumatically actuated valves inside the enclosure.
Fig. 4 – Operational experience with complete propulsion solutions for gas fuelled vessels have been gained in several configurations and ship types.
Furthermore, a full colour HMI panel (Human Machine Interface) is mounted on the control cabinet, from where the following parameters can be monitored
The complete unit is Factory Acceptance Tested with the built on control system before delivery from the factory, thereby safeguarding a high quality and trouble free commissioning
The volume to be ventilated is significantly reduced, compared to large dedicated rooms, enabling faster and earlier detection of even the smallest gas leakage. The capacity of the fans required for the ventilation can be reduced in equal proportion. By selectively installing the gas detection sensors in the supply lines, identification of the source and location of possible leaks is simplified.
To summarize, the following benefits are derived from the GVU-ED™:
The latest generation of GVU-ED™ has a new set of alternative configurations that change the layout and design to fit the installation circumstances in the most beneficial way. The second generation GVU-ED™ are vertical designs, meaning that the largest dimension is in a vertical position to give the smallest possible footprint.
GVU-ED™ matches the complete Wärtsilä engine range and consists of four different sizes, depending on the gas consumption required for the engine installation. The GVU-ED™ sizes are DN50, DN80, DN100 and DN125 referring to the inner gas pipe dimensions. The internal and external basic design is kept the same for all four sizes. The differences are found in the size and piping of the components.
Depending on the natural gas properties, such as Lower Heating Value (LHV), the GVU-ED™’s hardware is optimized to allow for a higher volume flow. This compensates for gas with a lower energy content or vice versa.
3.1 – GVU-ED™ stainless steel version
The standard version always comes with good corrosion protection. In addition to the above, there is now an optional completely stainless steel version, i.e. all components in direct contact with the natural gas are made from stainless steel. The recommendation is to choose this version whenever the GVU-ED™ will be operating on humid gas. This is the case, for example, with all Wärtsilä Gas Reformer installations.
3.2 – GVU-ED™ with Mass Flow meter
As a new standard option, a mass flow meter to measure the natural gas can be installed inside the GVU-ED™ enclosure. The Coriolis mass flow meter gives a far more precise indication of the energy supplied to the engine compared to more indirect volume flow measurements, even when they are temperature and pressure compensated. This is the case as long as the gas does not contain substantial amounts of inerts, such as nitrogen and carbon dioxide. Based on the continuous gas mass flow measurements, the individual engine energy consumption can easily be calculated and used as input in engine or vessel optimization programmes.
3.3 – GVU-ED™ mirror versions
All sizes of the GVU-Enclosed Design™ can be installed as a mirror like design in order to optimize the engine room design. The GVU-ED™ requires maintenance space on the enclosure cover opening side for servicing the components inside. Therefore, the control cabinets can be either mounted on the left or right side of the enclosure. Alternatively, the cover can be lifted from its hinges, in case space is very limited. This option is pre-configured to suit the installation conditions. The internal component arrangement is the same regardless of the mounting side.
Wärtsilä is today recognized as a leader in propulsion solutions for gas fuelled vessels. The company's strong and early commitment has created an in depth knowledge of the use of natural gas and LNG. The Wärtsilä DF engine is globally recognized in the marine industry. The most recent products, the LNGPac™ and GVU - Enclosed Design™, further exemplify the cutting edge technology and commitment to the Wärtsilä strategy of reducing the overall investment costs and lowering the threshold for switching from liquid fuel to gas in EC areas. Starting from 2010, the GVU enclosure has been delivered as standard together with Wärtsilä DF engines. Wärtsilä is today in the unique position of being able to offer complete gas propulsion packages that include everything from LNG bunkering, storage and engines, to power-train propulsion.