10.5 Throttle control valve

Throttle control valve

In the case of throttle regulation, throttle control valve regulation of pump system figure 10.5a, the operating point of the pump changes because the system curve changes. As the through-flow area of the control valve is reduced, the volume flow through the pump reduces from Q₁ to Q₂.

Figure 10.5a Throttle regulation. Block diagram and curves.

Throttle regulation creates in the system an extra, unwanted flow loss h_{L throttle}. To overcome this, power must be delivered to the pump shaft, thus:

In a completely loss-free regulation system, the shaft power supplied should be less by P _throttle. As can be seen from figure 10.5a, h_{L throttle} and therefore P_throttle are greater when the system curve is steeper for the same change of flow from Q₁ to Q₂.

Throttle regulation is the commonest of the continuously variable methods of regulation, but it cannot be used for displacement pumps. In view of rising energy costs, however, the method is becoming uneconomic in spite of the relatively low procurement cost of a control valve.

Necessary head loss for a control valve

The control valve is designed and dimensioned so that the control loop itself has the best possible regulation characteristics. This means that the parameters to be controlled, i.e. the flow or the head, should be as linear as possible in relation to the movement of the valve. A necessary condition is that a loss of head should exist across the valve even at maximum flow Q_O, figure 10.5b. It can also be expressed as the valve having “authority” in the system loop. In order that the control valve can work under reasonable conditions, the differences between pump curve and system curve should not vary too much with flow.

Figure 10.5b Necessary loss of head for a throttle control valve.

When regulation takes place according to a single system curve, as in figure 10.5, the following rule of thumb applies for the least loss of head for the valve at maximum flow:

h_VO = 0,1 * H_stat + 0,3 * H_fo   (m)             (Equ. 10.5b)

The dimensioning of the control valve and its linearity for this case has already been dealt with in Chapter 9, Section 9.4 Valve pressure drop>>>.

When regulating for constant flow, regardless of how the system curve changes, a vertical line in a Q-H diagram is obtained for each required flow, figure 10.5c. The loss of head due to the valve is then obviously the difference between pump curve and the actual system curve.

The loss of head due to the valve must be selected at a considerably greater value than that given by the rule of thumb in equation 10.5b. An  analysis of regulation characteristics should be carried out in the manner shown in the example in Chapter 9, Section 9.4.

Figure 10.5c Throttle regulation with constant flow.

Energy consumption equals the head loss across the control valve

In throttle regulation, the power requirements of the pump are determined entirely by its power curve, regardless of what the system curve looks like (see figure 10.5d), i.e. the power requirement can be read directly from the pump power curve.

Figure 10.5d Pump power requirement for throttle regulation.

Energy consumption P_E (E = electricity since most pumps are driven by electric motors)   is power multiplied by time, or

P_E = P_Mean * t_O           (Equ. 10.5c)

where

P_E = annual energy consumption (kWh/year)
P_Mean = mean power during operational time (kW)
t_O = operational time per year (h/year)

The mean power is most simply determined by the use of Simpson’s integration formula :

where P_Q100, P_Q50, P_QO are the values read off from the pump power curve for flow Q₁₀₀, Q₅₀, Q_O respectively, see figure 10.5d. These flows apply to mean flows at 100, 50 and 10% constancy.

For throttle regulation, the regulation efficiency is the ratio between the delivery head according to the system curve and the pump delivery head.

Figure 10.5e shows the regulation efficiency for the special case of a turbine pump working with water or a liquid with water-like qualities. The loss of head due to the valve has been chosen in accordance with equation 10.5b and the pump curve is assumed to have its zero discharge point (Shut-off head) some 20% higher than the operating point for maximum flow.

In the case of throttle regulation, pump efficiency varies with flow, whereas the motor efficiency remains fairly constant.

Figure 10.5e Regulation efficiency η_r for throttle regulation of a centrifugal pump with a Shut-Off head about 20% higher than the operating point at maximum flow. Delivery head H_O applies exclusively to the loss of head due to the valve, i.e.

H_O = H_stat + h_fo, see also figure 10.5b.