Overall dimensions and technical information are provided solely for informative purposes and may be modified without notice. A|4 Flow characteristics Each cylinder requires, in order to generate specific forces and operate at the needed speed, specific air flow through the control valve. It is therefore necessary to know and understand the laws that regulate the flow through a valve; and therefore the relation between pressure, pressure drop and flow rate. Only by doing so is it possible to determine whether a valve is capable of supplying the required flow rate to a cylinder at a given inlet pressure and with a reasonable pressure drop. In order to carry out these analyses it is necessary to work with precise functional data; it is not sufficient to know the valve port size. This data is presented in different ways depending on the different applicable, standards and various experimental measurments methods. The figures are mainly coefficients which must be used in specific equations, with which we can estimate the valve flow rate. In order to understand the meaning of these equations it is necessary to examine the flow inside a pneumatic valve. For example, let us consider the following conditions: a valve supplied with an absolute pressure P1 and with a flow regulator connected downstream. On a varying P1 the curves maintain the same form and only shift into a higher or lower flow rate area depending on whether P1 has increased or decreased. The area of interest in pneumatic valve applications is the subsonic zone, just before the critical flow point is reached. This zone is expressed in a number of different ways which average the effective flow pattern enabling simple description of the flow using experimental coefficients. Valve coefficients "C" e "B" CETOP RP50P recommendation (derived from ISO 6358 standard) expresses flow rate in function of two experimental coefficients: - conductance C - critical pressure ratio b. Conductance C = Q*/P1 is the ratio between maximum flow rate Q* and absolute inlet pressure P1 under sonic flow condition at a temperature of 20°C. Critical ratio b = P*2/P1 is the ratio between the output absolute pressure P2 and the inlet absolute pressure P1 at which the flow becomes sonic. The expression that represents an elliptic approximation of the relationship between pressure and flow follows: where: is the flow rate in dm /s at normal condition : 1,013 bar and 20°C; is the valve conductance; is the inlet absolute pressure; is the ratio between downstream and upstream pressure (P2/P1); is the pressures critical ratio; is a corrective factor that consider the absolute inlet temperature T1; is the absolute temperature (t1 is the temperature in °C). [1] Starting condition - flow regulator closed - no flow rate (Q=0) - Upstream and downstream pressure are identical (P2=P1) Intermediate conditions - opening flow regulator By progressively opening the flow regulator the pressure P2 will decrease and the flow rate increase up to a critical point at which the flow rate becomes constant even if the flow regulator is opened further.. This critical point corresponds to the sonic condition of the flow. Final condition - flow regulator completely open - maximum flow rate (constant from critical point) - downstream pressure P2=0 Solutions for pneumatic automation General Catalogue Dimensioning APPENDIX A 2 1 3 P2 P1 Critical point corresponds to the flow sonic condition P2=P1 FLOW CURVE Q P 2 1 3 P2 P1 Critical point corresponds to the flow sonic condition P2=P1 FLOW CURVE Q P
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