Читать книгу Hydraulic Fluid Power - Andrea Vacca - Страница 86

Point P is located at the junction of the three main elements of the system (the pump, the relief valve, the variable orifice). Therefore, the operating pressure at point P can be found by intersecting the characteristics curves of these components, while also satisfying the constraints of maximum allowed pressure and available flow.

To clarify this statement, the characteristic curves of the three components in the (Δp, Q) chart are shown in the figure below. In particular,

 the orifice characteristic curves are plotted according to the orifice equation 4.5 for decreasing values of the area Ω (Ω1 > Ω2 > Ω3…). This trend is as also shown in the plot of Figure 4.4. It is important to observe that, in this case, the pressure drop across the orifice equals pP, since the pressure downstream the orifice is pT = 0 bar.

 the pump curve represented by a horizontal line (constant flow rate). In fact, for this problem the pump provides a constant flow independent on the system pressure.

 the relief valve curve, represented by a vertical line. The relief valve, which will be explained more in detail in Chapter 8, limits the maximum pressure at the junction point P to p*.

The behavior of the system can be analyzed for different openings of the variable orifice O.

In case of a large orifice area (Ω = Ω₁), the intersection between the pump characteristic and the orifice curve is at X1. This point is located at a pressure lower than p*: the entire pump flow rates Q_P goes to the orifice (Q_P = Q_O); and the relief valve is closed (Q_RV = 0). In this case, the orifice Eq. (4.5) can be used to find the pressure at the point P:

In this condition, the orifice O behaves as a compensator, since it establishes a pressure drop for a given flow rate.

The situation is similar for the smaller opening area Ω₂ < Ω₁: the operating point shifts from X1 to X2 at a higher pressure. The relief valve remains closed and the orifice equation can be used again to find p₂.

The orifice maintains the compensator function until it reaches the opening area Ω*. This area value is critical, as it corresponds to a pressure at point P equal to p*. When the opening area is Ω*, the full pump flow is still passing through the orifice.

For area values smaller than Ω*, as in the case of Ω₃, the intersection between the orifice curve and the pump curve would result in the operating point X3′, which corresponds to a system pressure , greater than p*. As a consequence, when Ω = Ω₃ the operating point is defined by the intersection of the orifice curve with the relief characteristic (point X3). In other words, the relief valve opens, relieving a portion of the pump flow to tank, so that the pressure is limited to p_P = p*. The flow passing through the orifice is Q₃, while the relief valve discharges the flow Q_{RV, 3} = Q_P − Q₃.

The orifice equation in this case can be used to find the flow rate Q₃, knowing that the pressure drop across the orifice is defined as Δp₃ = p* − p_T = p*

A further reduction of the opening area of the orifice (Ω₄ < Ω₃) results in a reduction of the orifice flow, as visible by the location of the intersection point X4 in the figure.

In these conditions, the orifice behaves as metering, since it establishes a flow rate as a consequence of a given pressure drop.

This example shows how a metering element can be used to regulate flow in a hydraulic system. In this simple case, the orifice flow is directed to tank at null pressure. In general, an actuator can be located downstream of the metering element, which is ultimately used to adjust its speed.