Valves in action -
Back pressure regulator UV 3.5
Ammonia control in the SCR system of a large ship diesel engine
Around 90 % of the world's cargo ship fleets are powered by heavy fuel oil. Among other things, its combustion produces sulfur oxides (SOx) and nitrogen oxides (NOx), which cause illnesses and damage ecosystems.
In order to prevent marine pollution and improve air quality by controlling emissions from ships, the international environmental agreement MARPOL was concluded as early as 1973 and is still valid worldwide in its 1978 amended version. The International Maritime Organization (IMO), a special office of the United Nations, acts as the secretariat for the enforcement of the MARPOL Convention.
Its emissions standards have been gradually implemented at international and national level over the past decades. Since January 2020, marine fuel may not contain more than 0.5 percent of sulfur worldwide. The currently valid IMO Tier III emission level of MARPOL applies to newly built ships whose formal keel laying took place from 2016. These must not exceed the strict nitrogen oxide limits of between 2.0 and 3.4 g/kWh, depending on engine speed, in the coastal NOx Emission Control Areas (NECA) in America and - since 2021 - in the North Sea and Baltic Sea.
Shipping companies are optimizing fuel consumption in their newbuildings through streamlined hull shapes and consumption-optimized speed management. Although high gas temperatures during combustion in the cylinder reduce the specific fuel consumption of large diesel engines, this significantly increases the formation of nitrogen oxides (NOx) and of other substances.
In the last century, the focus was still on internal engine measures to reduce NOx by optimizing design and operational parameters. However, the stricter IMO guidelines of the new millennium required a switch to exhaust gas post-treatment by Selective Catalytic Reduction (SCR), which has no effect on engine performance. The efficiency of this technology can exceed 90 %.
TASK // Constant pressure upstream of the dosing pump
A leading Asian container shipping company uses an SCR system in its newbuildings, that is installed between the exhaust manifold and the turbocharger.
Here, urea is pumped off by a dosing pump and injected into the exhaust gas stream as aqueous ammonia. While the water evaporates and the urea is converted into ammonia, this combines in the catalyst module with the nitrogen oxides in the exhaust gases. The toxic components are thus converted into inert nitrogen gas and harmless water vapor and released into the atmosphere.
Depending on the requirements, the pressure at the nozzle should be kept constant at 9 bar. Otherwise the urea would not mix completely with the exhaust gas or too much urea could be consumed. The Mankenberg back pressure regulator UV 3.5 ensures constant pressure upstream of the dosing pump.
SOLUTION // Back pressure regulator UV 3.5
The pressure of the dosing pump is usually 9 - 10 bar. The backpressure regulator is located on the bypass line to return the excess medium to the urea tank. The shipping company chose Mankenberg's UV 3.5 because it is made entirely of stainless steel and has high corrosion resistance in the aggressive atmosphere of the SCR system. The valve is very compact and, with its non-rising adjusting screw, it offers a space-saving solution for the ship engine room. The precise regulation of the upstream pressure ensures the greatest possible effectiveness of the SCR system.
