Fluid Control Systems Selected product range

348Proportional ValvesProportional Valves/Mass Flow Controllers and MetersBürkert proportional valves, are based on switching solenoidvalves that are closed by a spring when without current. By meansof design changes to the electro-magnetic components ofswitching solenoid valves, a balance can be created between thespring and magnetic force of any coil current. The size of the coilcurrent and/or the magnetic force determines the stroke of thearmature and/or the valve opening. The relationship between themagnetic force and the valve opening is proportional.Proportional valves are controlled using a pulse-width modulatedsignal (PWM). This signal causes the coil current to change accordingto the pre-set value. As both the pressure of the medium andthe magnetic force work against the spring, it is practical to setthe minimum and maximum flow values of the working range underoperational conditions. This is possible using the control electronics.Open and closed control loops can be realised using proportionalvalves. Open control loops drive the valve without feedback of theactual value, while closed control loops regulate the differencebetween the set value and the actual value.Major characteristics of the positioningbehaviour of proportional valves• Hysteresis:The largest difference of the fluidic output signal when runningthrough the complete electrical input signal range in the upwardsand downwards direction; is quoted as a % of the maximumfluidic output signal.• Response sensitivity:The smallest difference in the set value that will lead to a changeof the fluidic output signal. It is quoted as a % of the maximumfluidic output signal.• Repeatability (reproducibility):Range over which the fluidic output value is spread when the sameelectrical input signal, coming from the same direction, is repeatedlyapplied.• Span:Ratio of the k v values (see the calculation of proportional valves) atthe largest and smallest opening. With proportional valves, spansfrom 1:25 up to 1:50 can be realised.Selection guideIn order to be able to guarantee a problem-free control function,proportional valves have to be designed and selected accordingto their specific task. The most important characteristics for theselection of a proportional valve are the k v value and the pressurerange of the application.Valves are compared fluidically by the k vs value (unit m 3 /h), whichis measured with a flow of water at 20 °C (68 °F) and 1 bar(14.505 PSI) relative pressure at the valve inlet against 0 bar atthe valve outlet. For gases, a second flow value is often quoted,the so-called Q Nn value. The Q Nn value gives the normal flow valuein l N/min for air (20 °C) and 6 bar (g) at the valve inlet and 1 bar ofpressure loss over the valve. The reference conditions for gas are1.013 bar absolute and a temperature of 0 °C (273 K) .In addition to the k vs value, the maximum pre-pressure or inlet pressureis the main factor in the determination of the valve (type andorifice). The smaller the orifice of the valve or the stronger the coil,the greater will be the switchable maximum pressure.The k v value can be determined using the following formulae. Onthe basis of the calculated k v value and the pressure range of theapplication, the valve type can be read off in the Type SelectionDiagram. The valve types of range 28xx are only listed in the fullcatalogue.Calculation formulae for the k v value determination:Pressure dropp 1Liquids,k v in m 3 /hover-critical, p 2 < = Q · ρ=Q N2∆p · 1000 257 · ·p1k v = Flow rate in m 3 /hQ = Flow rate of the application in m 3 /hQ N = Standard flow in m N 3 /h(Q Nn at ∆p=1 bar, p 1=6 bar and T 1=(273+20) K)p 1 = Inlet pressure in bar (a)p 2 = Outlet pressure in bar (a)∆p = Pressure differential in barρ = Density in kg/m 3ρ N = Standard density in kg/m 3T 1 = Media temperature in (273+t) KGases,k v in m 3 /hp 1ρ Qunder-critical, p 2 > = Q ·= N2∆p · 1000 514 ·Conversion of standard or operationalconditions to normal conditions for gases:ρ N · T 1∆p · p 2ρ N · T 1Q N = Q ST N · p ST S · p NQ S = Flow at standard conditions (1.013 bar and 20 °C) or atoperating conditionsp S = Absolute pressure at standard conditions (=1.013 bar) orat operating conditionsT S = Standard temperature- (= 293 K) or operating conditions(= (273+t)K)Q N = Normal flow ratep N = Normal pressure (=1.013 bar)T N = Normal temperature (= 273 K)Subject to modifications