Energy Efficiency in aeration control
Part 2: Control valves
Control valves are used for the control of the air flow rate into one aerated tank or zone on one hand. On the other hand, control valves organize the distribution of air between the different aeration tanks in the sewage treatment plant with a common header system and a central blower station. They typically should close gas-tight for the use in flexible zones.
By throttling the orifice of the valve, a pressure drop is created, which can be used for control and distribution of the air flow. From technical point of view there is a minimum pressure drop for a stable control system necessary, depending on the number of valves in the system as well as the local pressure situation (water level in the tanks, type and age of aerators, pressure drop in the pipping system). The minimum value is ranging at approx. 8-12 mbar, but can be much higher. In multistage aerated plants the water level can vary between the first and last aerated zone of 10 cm and more. This will increase the minimum level accordingly. Total pressure balance shows following structure regarding counter-pressure, which will determine the header pressure in the system: Header pressure = Water level above aerators (1 cm ≈ 1 mbar) + pressure drop aerators + pressure drop of piping system (outlet blower to the inlet 1st diffuser) + pressure drop of the control valves + pressure drop of further BFV + some safety distance One of the most important requirements on the valve is a low pressure drop of the control valve, but not only at 100% stroke (fully open) also in the typical range of control mode.
Each Millibar or Hekto-Pascal, which can be saved, will give a direct energy saving. 30 mbar reduction of header pressure at 600 mbar header pressure will lead to a direct saving of 5% of the blower´s power consumption (assuming same efficiency). [1] defines the typical pressure drop of the piping system from blower outlet to first diffuser inlet incl. all fittings and control valves, which should not exceed 40-50 mbar in total.
Beside pressure drop there are further properties influence the efficiency of the valves and can generate indirectly up to another 10% energy savings and less maintenance costs: – As much as possible linear operational characteristic curve which will lead to a constant amplification factor – Small mechanical slag of the valve, slow running and very precise actuators which will lead to a high positioning accuracy / repeatability and so improve control accuracy, respectively reduce deviations from DO-setpoint A common characteristic value of a valve is the Kv value. The Kv value corresponds to the water flow through a valve (in m³/hr) at a pressure difference of about one bar (exactly 0.98 bar) and a water temperature of 5°C to 30°C. A Kv value only applies to the associated stroke (opening degree) of a valve. The Kv value of a valve at rated lift (100% stroke) is called the Kvs value. The determination of the Kv value is regulated in Technical Standard Paper VDI / VDE 2173 [2] . A comparison of control valves based on Kv value only is not meaningful. Neither pressure drop nor control accuracy are considered. Especially manufacturer of valves, developed originally for water and used for aeration air, supply kv values and try to use it for comparisons. But good water valve design is not ideally for air control valves. Butterfly valves are suited very well for ON/ OFF operation, but shouldn´t used any more for control purpose. [1] At the moment there are world-wide only two valves, especially developed and optimized for aeration air as requested in the German Standard paper DWA-M 229-1[1]: Binder´s VACOMASS® jet control valve and Binder´s VACOMASS elliptical diaphragm control valve. Both fulfil all requirements of the new standard paper.
Literature: [1] DWA-M 229-1: Systems for aeration and mixing in activated sludge plants – Part 1: Planning, tender and execution. Beuth Verlag September 2017 [2] VDI/ VDE 2173: Fluidic characteristic quantities of control valves and their determination. Beuth Verlag September 2007
VACOMASS® jet control valve
Unique control valve – best in performance
The VACOMASS® jet control valve is an innovative, flow-optimized stainless steel control valve. In terms of control range and product quality, this development represents a milestone in the evolution of air control valves and fittings. It is unique and with worldwide patent applications. It fulfills all requirements defined in the Advisory Leaflet DWA-M 229-1 (September 2017) and the new “Handbook of Energy North Rhine-Westphalia” (January 2018).
VACOMASS® elliptic diaphragm control valve
Control valve with lenticular shaped orifice
The VACOMASS® elliptic diaphragm control valve is a technically optimized control valve, especially developed for aeration air, closing gas-tight with a lenticular control aperture. Low pressure drop in the whole control range and high flow rate capacity for flexing and cleaning the diffuser with very low pressure drop are two of the main features (design according to DIN EN 60534-2-3).
VACOMASS® square diaphragm control valve – Control valve with square shaped orifice
The VACOMASS® square diaphragm control valve is a technically optimized sliding gate control valve with a falling flow axis and gas-tight shut-off using a square shaped control aperture. It is used for precise and low-loss control of airflow and distribution in the aeration tanks of a wastewater treatment plant (design according to DIN EN 60534-2-3).
VACOMASS® flexcontrol
Field housing with modular electronic modules for control of air supply and air distribution
The special feature of VACOMASS® flexcontrol is the complete flexibility and intuitive operation of the open system, which allows easy adjustment to the plant requirements – not a “black box” that spreads uncertainty, but modular with standardized building blocks and based on control algorithms that have been successfully used worldwide for many years, simple and clear for every control loop.
VACOMASS® flow meter
Thermal mass flow meter for aeration air
The VACOMASS® air flow meters use thermal mass flow principle and measure the gas mass flow respectively standard volume flow directly, independent of pressure and temperature of the medium at standard conditions according to DIN 1343.