The IMO Ballast Water Management Convention will come into force on September 8, 2017.
Steven Matthew, regional manager – Asia, at Calgon Carbon UV Technologies and Hyde Marine, offers advice on minimizing the impact of installation of BWTS on a ship’s operation with proper planning and by following engineering principles.
The spread of invasive species through ships’ ballast water has been documented as a significant vector in the decrease of biodiversity and significant economic impacts to coastal communities around the world. Because of this threat, the International Maritime Organization (IMO) introduced the 2004 International Convention for the Control and Treatment of Ships’ Ballast Water and Sediments. As this Convention is now approaching Entry Into Force, there is a lot of chatter in the market about whether or not systems actually perform when installed on ships.
Although much research and effort has been put into examining the cost impacts of BWTS installation, much less has been put into the real impacts that installation of a BWTS may have on vessel operations. Further exacerbating the lack of fundamental knowledge about how ships will be impacted by these systems has been both the relative lack of vessels having installed systems and the low proportion of those ships with installed systems installed routinely using those systems.
Talking to experts with years of experience in the design, installation, and integration of ballast water treatment systems has revealed three primary issues that need to be addressed during the design phase to ensure that systems are successfully installed.
Step 1: Optimize ballast pump performance
The first concern is the impact that the system will have on the performance of the ship’s installed ballast pump. Ships are primarily designed at new construction with ballast pumps capable of matching the loading capacity of the ship. Ballast pumps are then installed with a discharge pressure designed to fill the vessel’s ballast tanks but also designed not to over-pressurize the ballast tanks if the tanks are inadvertently overfilled.
This balance can easily be upset if a ballast water treatment system is installed on the vessel since the BWTS will invariably introduce some change to the design head loss of the overall ballast system. The simple addition of a small amount of additional pressure may increase ballasting time for a vessel from a design of 8 hours to potentially nearly 13 hours due solely to the decreased flow at the new position on the pump curve.
Interestingly, many pumps are offered with multiple impeller options. By closely examining the pump curve, alternate impellors can be identified and installed as part of the overall scope of the BWTS installation, reducing greatly these impacts.
Step 2: Take care with your filter
The single most common initial problem following the installation of a BWTS is the unacceptable performance of automatic backwashing filters. These filters form an integral part of the overall treatment system and are the primary barrier for large organisms, shells, fish, and other treatment-resistant life stages of organisms entering the ballast water tanks. BWTS filters are very fine mesh filters and while they are capable of automatic cleaning they often rely on the differential pressure between the outlet and the drain line to clean effectively.
Issues arise because filter outlet is heavily dependent on a number of critical factors, including screen cleanliness, system pump pressure, back pressure on the piping system, and head losses inherent to the drain line connection. As the filter begins to load, fouling increases differential pressure across the filter and decreases pressure on the clean side, robbing the filter of the pressure it needs to clean itself.
To prevent the eventual erosion of this differential pressure there are two different engineering solutions: use dirty side pressure for establishing backflush flow and use of a suction pump drain line.
The using of inlet pressure is a good engineering solution because as the filter loads, unlike the clean side pressure, the dirty side pressure increases up to the dead-head pressure of the pump. This allows cleaning at much greater filter element fouling than standard filters. Using a filter backflush pump is another solution that works by inducing suction across the filter and ensuring that the drain line is evacuated.
Step 3: Account for piping losses
It is critical to consider the additional piping as well when considering the impact to the performance of the ballast pump. When installing a BWTS, the ballast piping needs sometimes extensive modification. Steven Matthew, Asia Regional Manager of Hyde Marine commented, “Ships not designed from new construction with BWTS need to find space to install the system and often this space is less than ideal for the piping configuration. It can get very complex, very quickly.” This piping is often be done during a shipyard period or underway where speed is critical and may field welding and fabrication often preclude proper internal protection for the pipe materials.
As these pipes are installed, the head loss through fittings, open valves, and check can quickly add a significant additional impact to the overall head loss in the ballast system following the installation of a BWTS.
By engineering around the effects that BWTS installations will have on a ship’s ballast pump, optimizing the filter’s backflush arrangements, and properly accounting for pipes, fittings, and new arrangements, ship owners can have the best chance to successful install BWTS onboard their vessels. Failure to follow these basic engineering principles and placing too much reliance on the manufacturers and installers to simply “make the system work” on a ship may result in more systems that unable to meet the ship’s expectation.