A Paper to Highlight System Requirements for Reliable Electrolytic Process based Ballast Treatment System Operation
Operating costs for large vessels in the Aframax and Suezmax Class and larger are substantial. Daily operating costs can be between $12k - $20k per day. Therefore, it is in the best interest of the owner/ operator to have reliable, robust, and properly operating equipment on their vessel. The other consideration for proper operating equipment during ballasting or deballasting periods is that the task demand is very high on the crew during these periods. Simultaneous loading and unloading of cargo and ballast requires coordination with shore facilities and ship operation; the crew does not have time to conduct repairs to equipment during this period when all crew members have designated tasks. Failed equipment in need of immediate repairs only extends the time dockside and adds to overall operating costs. The Ballast Water Management System (BWMS) is no exception. The ship owner that selects the unreliable BWMS can be inviting an economic disaster if the system causes interruptions of cargo operations and forces repeated off-hire periods for repair and re-ballast. These interruptions can significantly impact operational budgets. These costs can easily exceed the differences in capital expenses of initially purchasing a better, more reliable BWMS.
Minimizing BWMS Impact on Vessel Operations
The worst time for a crew to discover a BWMS outage is during ballasting or deballasting operations. The BWMS must support operations at all times, even if with limited capacity. For vessels with high-paced schedules to avoid a total outage, the BWMS must employ redundant auxiliaries. A single fault should not put the whole BWMS out of commission.
Modern electronics including power supplies and computer-based controls tend to be highly reliable, but have limitations. Marine vessels subject these sub-systems to intense vibration and elevated temperatures. Some BWMS employ on-deck components exposed to seawater. The BWMS manufacturer should conduct detailed design reviews for possible conscience failures and examine the mean time between failures of the hundreds of interrelated components. Standby auxiliaries must be already installed to eliminate downtime. Some vessels should have fully redundant systems to support port and starboard ballast header operations with crossover capability.
Features of the BWMS that improve reliability and robustness must be evaluated when making vendor and technology selections. Important considerations include simplicity of operation, automation, long life-cycle of components and operational reliability.
In addition, the ability to quickly get port state control inspectors on and off the vessel is important. Systems that provide electronic data logging of performance indicators of treatment efficacy will prove valuable when enforcement begins to impose fines for ship-owners.
It has been recognized that on-site generation of hypochlorite to treat large volumes of ballast is cost effective, simple, and a reliable method to comply with the IMO D-2 standard and United States Coast Guard (USCG) standards (33 CFR Part 151 and 46 CFR Part 162). But the design of the system must be fit for purpose to sustain that reliability. Along with reliability, safety of operating equipment as it relates to the crew, the ship, and the environment is just as important. Electrolytic systems that generate hypochlorite for disinfection of ballast seawater also generate hydrogen which when handled properly eliminates any risk to the ship and crew. It is the intent of this article to provide insight to a properly designed electrolytic ballast water treatment system that provides the required reliability and safety.
The Electrolytic System for Ballast Water Treatment
The following sections provide details on components within an Electrolytic System to treat ballast water. These sections also include references to a slip-stream based BWMS which offers unique features to enhance the reliability, simplicity, and safety during operation.
Slip Stream Approach
There are two basic types of Electrolytic Systems used to treat ballast water: 1) Direct electrolysis 2) Slipstream.
Direct electrolysis requires the full ballast stream to pass through the electrolytic cells (also known as Electrolyzers). This requires the main ballast flow to be split into multiple parallel streams to accommodate the flow through the Electrolyzers. Typically a single Electrolyzer can accommodate only up to 50 cubic meter per hour. Unfortunately since the entire ballast flow is directed through the Electrolyzers, this also means that the hypochlorite concentration exiting each Electrolyzer is at the operating concentration (>5 ppm). Therefore this technology is better suited for low flow ballast and non-hazardous ships. This also means in the case of a 5,000 cubic meter per hour ballast stream for a crude oil tanker, numerous Electrolyzers will need to be located in the hazardous area pump room. The result is a very difficult design leading to a very expensive and potentially dangerous installation. Other considerations for this type of system are that seawater requirements to conduct the electrolytic process are very difficult because any adjustment to salinity or temperature must be done to the entire ballast stream since the Electrolyzers are in the main ballast flow.
The Slip Stream Electrolytic System based BWMS design is well suited for high flow ballast and / or ships with hazardous areas. The slip stream treatment approach allows the BWMS to be remotely mounted away from the ballast lines and can be split into small sub-assemblies to maximize use of typical dead space off decking. This is especially useful for retrofit systems. The Slip Stream design allows utilization of any seawater source; engine room sea chest, engine cooling loop seawater, or aft peak ballast tank. Typically, the seawater flow requirement is only 1% or less of the full ballast flow (50 m3 versus full ballast of 5,000 m3 for a tanker). This reduced seawater requirement also has many advantages related to handling the seawater as feed to the Electrolytic Cells. Typically, an Electrochlorination Electrolytic System requires saltwater with a minimum salt concentration and minimum temperature to operate at optimal performance. Should the ballasting operation take place in a fresh water harbor, open ocean seawater stored in the aft peak tank can serve as a source to feed the Electrolyzers. In the case of ballasting operation in a cold water port at less than 15 deg. C., a small heat exchanger during initial installation of the entire system can be installed and can heat the small seawater stream to the optimum temperature. One more advantage of the Slip Stream System relates to tankers carrying hazardous material. All major equipment can be installed in the engine room and only a few low energy (suited for Zone 0) control devices need be in the pump room. This minimizes space required in the pump room (hazardous area) and reduces alterations during retrofits. This design also helps to treat waters from aft peak tank or any remote tank, often being on a separate circuit can otherwise require its own BWMS to meet IMO D-2 and USCG standards.
Electrolytic Cells
The electrolytic cells used in Ballast Water Treatment Systems traditionally have been used in costal, marine, and offshore applications for many years by leading BWMS vendors. The historical experience of using seawater to generate sodium hypochlorite with electrolytic cells (a.k.a. Electrolyzers) has led to the development of safe operating practice to eliminate fouling leading to electrical arcing. Electrolyzers are used in the process to generate hypochlorite by passing direct current through and across electrodes. Proper maintenance is required to insure stable operation and eliminate arcing within the electrolytic cell which could cause damage to the hypochlorite generation unit.
Typical Electrolyzers accumulate hardness from the seawater with time. The calcium and magnesium in the seawater accumulates on the electrodes, bridges the gap between electrodes, causing poor seawater flow, reducing cell efficiency, overheating, and possible arcing. The Electrolyzers must be acid washed (circulate mild (or diluted) hydrochloric acid through the Electrolyzer) when not in use to remove the deposits and establish proper and safe cell operation. This acid wash operation is required after a few hundred hours of operation using an optional acid cleaning system.
An optional self-cleaning feature is provided by few BWMS vendors for automatic cleaning of electrolytic cells without requiring an acid cleaning. This self-cleaning feature is provided by the unique electrode coating and power reversing modules installed in a switched power supply. For the Electrolyzers installed with a self-cleaning feature, PLC reverses the polarity output of the power supply after each ballasting cycle. This action dissolves the small amount of hardness (Ca & Mg) accumulated after each cycle and eliminates the need for acid washing for the life of the electrode. The self-cleaning electrolytic cells significantly increase the safety of operation, eliminating any chance of bridging and shorting. It also eliminates necessity of bringing concentrated Hydrochloric acid onto the ship and associated safety issues.
Typically, an electrolytic BWMS does not have a spare set of Electrolyzers as back-up. But most slip-stream electrolytic BWMS systems use two or three Electrolyzers. So if one Electrolyzer should fail after a few years of operation, the BWMS can operate at a reduced capacity until the failed Electrolyzer can be serviced. Typically, the Electrolyzers age slowly and fail slowly whereby the amount of hypochlorite production diminishes with time while the operating voltage increases. The PLC of BWMS has alarm set-points to warn the crew of the failing electrodes within the Electrolyzer. The outside shell of most Electrolyzers are made from inert materials which last much longer than the electrodes.
The electrolytic cells are typically installed in the ship’s engine room, making access for maintenance and monitoring easy.
Hydrogen Management
The historical experience of using seawater to generate hypochlorite with electrolytic cells has led to the development of safe operating practice to handle and manage the byproduct hydrogen. Hydrogen gas is generated along with hypochlorite to create a multiple phase product as it exists the Electrolyzer: Liquid, foam (mixture of gas and liquid), and gas. Good design practice is to separate the gas from the foam and liquid as quickly as it exits the Electrolyzer. This allows the small amount of gas as it is produced to be processed easily, quickly, and safely rather than pushing the hydrogen throughout the ship and risking it to accumulate to an explosive amount. Typically the hydrogen is simply separated in a holding tank and vented to atmosphere, while the liquid hypochlorite is pumped (injected) into the ballast stream. This method requires space for the tank and does not completely eliminate hydrogen accumulation in the tank.
A few BWMS use a “de-gas” vessel to dynamically separate the gas from the liquid in a small space. The pressure of the liquid stream is sustained eliminating the need for extra pumps to inject the hypochlorite. The amount of gas is relatively small and can be mixed with air in a vent stack exiting to a safe space on deck. Typically, the air is added to the vent stack before the hydrogen by a forced air non-sparking blower. This eliminates the chance for ignition of the hydrogen twofold: 1) Immediate dilution of the hydrogen to below 1% concentration in air or 25% of the LEL of H2; 2) Hydrogen never enters the blower and is confined to the sealed vent stack. Normally a good design has two blowers, one in operation and one in ready stand-by that immediately starts should the primary fail. Also good controls practice is to alarm if the primary blower fails, and alarm with immediate shutdown of the Electrolyzer (interrupt power) if the second blower fails. This mitigates any risk of hydrogen leaking into the engine room. Also as a tertiary safety feature; a hydrogen sensor placed above the unit will alert the crew should any hydrogen seep into the engine room.
Power Supply for Electrolytic Cells
The electrolytic cells operate with direct current (DC) power. This requires converting the ship’s alternating current (AC) to DC power. There are two basic types of power supply technologies that can be used to accomplish this task: 1) Linear Power Supply (Transformer / Rectifier (TR)) and 2) Switch Mode Power Supply (SMPS). Historically, the TR unit is large and weighs a lot. If it fails, the unit is non-functional until repairs can be made. These repairs take time, are costly, and normally can only be done by a trained electronic technician. The SMPS is modular with multiple self-contained units of a given Kilowatt power rating. These units are connected (ganged) together to provide the required Volts and Amperage to operate the electrolytic cells. As a result, the total SM unit is compact and low weight. The other advantages to the SMPS: 1) Should one module fail the other units will still operate delivering the same voltage and proportionally slightly less amperage 2) The SMPS units lend themselves to reverse power (bidirectional) 3) Should one module fail, it can be removed as a cassette and another snapped into its place (plug and play) to resume full power output. This can be done by a crew member. BWMS vendors have found that these features provide a more reliable and an easy to maintain unit; incorporating the SMPS into the BWMS.
Booster Pumps
Most Electrochlorination systems require a means to increase the pressure and control the seawater flow within the system to inject the produced hypochlorite into the main ballast line. Most systems rely on some sort of pump to facilitate delivery where it is required. There are several ways to accomplish this injection: 1) Centrifugal pump with fixed speed motor coupled with a flow control valve 2) Centrifugal pump with variable speed motor 3) Metering pump. All three styles of pump have been used in industry for many years and reliability is related to the manufacturer. To enhance operating reliability of the system, a few prominent BWMS vendors provide an in-line spare pump plumbed in parallel with the primary pump (redundant). Switch over can be handled with automatic valves, alarms, and immediate switch when the primary pump faults. Manual switch over can be designed through alarms and immediate crew response as dictated by the Ballast Water Management Plan. There is no safety concern to the equipment, crew, or environment if there is a very short suspension in operation of the Electrochlorination system.
Strainer(s)
It is common practice to strain the seawater feed to the electrolytic cells. Typically a simple 600-800 micron strainer is sufficient to remove large particles that might lodge between the plates of the Electrolyzer. This obstruction could reduce seawater flow, lead to lower hypochlorite production, and ultimately lead to shorting between plates and damage to the Electrolyzer. To reduce the probability of the Electrochlorination system shutting down, few prominent BWMS vendor installs a duplex strainer; should one strainer foul, the system automatically switches to the alternate strainer to provide continuous seawater feed to the electrolytic cells. As with the booster pumps, this switch can be done manually through alarms and immediate crew response as dictated by the Ballast Water Management Plan. There is no safety concern to the equipment, crew, or environment if there is a very short suspension in operation of the Electrochlorination system.