Control valves pressure drop across

Rather than assuming a pressure drop across the control as 25%, 33%, or 40% of the other friction losses in the system, a logical approach [9] is summarized here. The control valve pressure drop has nothing to do with the valve size, but is determined by the pressure balance (See Equation 2-59 [9]). 

For adequate process control, the pressure drop across the valve for a linear valve is... 

When pumping a liquid from an operation at one pressure, /, to a subsequent operation at a higher pressure, P2, the pressure increase across the pump must be higher than P2 — P[ in order to overcome pipeline pressure drop, control valve pressure drop, and possible increases in elevation (potential energy). This additional pressure increase may be estimated by the following heuristic. 

Liquid flow through a control valve will be increased, when the pressure drop (dp) across the valve is increased. But this flow increase will be stopped, when choke flowthrough a control valve is reached. Further increase control valve pressure drop above this minimum choke flow control valve pressure drop (dp choke), flow through the control valve will not be increased. A liquid control valve flow capacity curve (flow versus square root of control valve pressure drop) is shown in Figure 10 [2]. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

APc = pressure drop across control valve F i = friction pressure drop at maximum flow rate, psi... 

For good control by the valve, the pressure drop across (or through) the valve must always be greater than the friction losses of the system by perhaps 10% to 20% (see [9]). 

If the pressure drop across the valve is to be more than 42 per cent of the inlet absolute pressure the valve selection is the same as if the pressure drop were only 42 per cent. With this pressure ratio the steam flow through the valve reaches a critical limit, with the steam flowing at sonic velocity, and lowering the downstream pressure below 58 per cent of the inlet absolute pressure gives no increase in flow rate. When the heater needs a higher pressure, or when the pressure required in the heater is not known, it is safer to allow a smaller pressure drop across the control valve. If the necessary heater pressure is not known, a pressure drop across the control valve of 10-25 per cent of the absolute inlet pressure usually ensures sufficient pressure within the heater. Of course, in the case of pressure-reducing valves the downstream pressure will be specified. 

The pressure (PSI) for calculating heat generation in a flow control valve, for example, is the inlet minus the outlet pressure, or the pressure drop across the valve. 

The purpose of the regenerated catalyst slide valve is threefold to regulate the flow of regenerated catalyst to the riser, to maintain pressure head in the standpipe, and to protect the regenerator from a flow reversal. Associated with this control and protection is usually a 1 psi to 8 psi (7 Kp to 55 Kp) pressure drop across the valve. 

Determine the operating point on the pump characteristic curve when the flow is such that the pressure drop across the control valve is 35 kN/m2. 

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

The higher flow rate might also reduce the head that the centrifugal pump produces if we are out on the pump curve where head is dropping rapidly with throughput. For simplicity, let us assume that the pump curve is flat. This means that the total pressure drop across the heat exchanger and the control valve is constant. Therefore, the pressure drop over the control valve must decrease as the the pressure drop over the heat exchanger increases. 

Liquid (sp gr = 1) is pumped from a tank at atmospheric pressure through a heat exchanger and a control valve into a process vessel held at 100 psig pressure. The system is designed for a maximum flow rate of 400 gpm. At this maximum flow rate the pressure drop across the heat exchanger is 50 psi. 

Figure 11.7 shows two almost identical draw-off arrangements. The only difference is the elevation of the control valve in the draw-offline. Control valve A is at the same elevation as the draw-off nozzle. The pressure drop across the control valve is 2 psi, or 56 in of water. Let s assume that... 

Even after we have made these two unlikely assumptions, the height of hot water in the draw-off sump must still be 56 in above the center-line of the draw-off nozzle. If not, the water would begin flashing to steam, as it experienced a pressure drop of 2 psi, flowing across the control valve. The evolved steam would then choke the water flow, reducing the pressure drop across the control valve until the pressure drop equaled the depth (or head) of water in the draw-off sump. 

The net effect of this exercise will be to save not 20 percent of the motive steam, but 10 percent. The 20 percent reduction in nozzle area is partially offset by the opening of the governor valve. The inefficient, irreversible, isoenthalpic expansion and pressure drop across the governor speed control valve are reduced. The efficient, reversible, isoen-tropic expansion and pressure drop across the nozzles are increased. 

When the control valve downstream of a pump is operating in a mostly closed position, the upstream pump is a good candidate to have its impeller trimmed. Sometimes, the pressure drop across a control valve is so huge (>100 psi) that it makes a roaring sound. The energy represented by this wasteful AP is coming from the electricity supplied to the pump s motor. Hence, trimming the impeller also reduces wear because of erosion in downstream control valves. 

A. Flow Control without Feedback. Plow can be controlled by means of a needle valve if the pressure drop across the valve is constant. The pressure on the upstream side often can be held constant with a single- or two-stage mechanical diaphragm regulator (Section 10.1. B). If the stream of gas does not experience a variable constriction after the needle valve, the above combination provides a simple and convenient means of providing a steady flow. Often an arrangement such as this is used in conjunction with a rotameter or electronic mass flow meter (Fig. 7.14). 

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