Fluid flow control valves

Although most fluid power motors are capable of providing rotary motion in either direction, some applications require rotation in only one direction. In these applications, one port of the motor is connected to the system pressure line and the other port to the return line. The flow of fluid to the motor is controlled by a flow control valve, a two-way directional control valve or by starting and stopping the power supply. Varying the rate of fluid flow to the motor may control the speed of the motor. 

Flow control valves are used to regulate the flow of fluids. Control of flow in hydraulic systems is critical because the rate of movement of fluid-powered machines or actuators depends on the rate of flow of the pressurized fluid. 

Valve stand — A free-standing frame including several fluid flow runs that is used to physically control the fluid flow to a combustion process. A typical oxy/fuel combustion equipment valve stand will include an instrument airflow run, an oxygen flow run, and a fuel flow run. The equipment on the valve stand typically includes the on/off valves, pressure regulators, flow control valves, and associated instrumentation necessary to operate a combustion process. 

THREE-WAY VALVE - Multi-orifice (opening) flow control valve with three fluid flow openings. 

Alternatively, the pneumatic pressure can be held constant and a valve used to control the fluid flow. Needle valves, spool valves, and rotor valves are commonly used in this type of device. In all devices of this type, the fluid flow rate (0 is directly proportional to the material viscosity. Those factors that change the viscosity should be controlled carefully or the equipment must be able to compensate for the changes in order to achieve consistent results. 

The compressor is a device that increases the pressure of air and pumps the compressed air into a tank. The compressed air tank is called a reservoir. The compressed air is then routed to the desired location through transmission lines. Transmission lines are tubing through which the compressed air flows. Control valves regulate the flow of gases. The pneumatic system uses a cylinder with a movable piston to convert its fluid power into mechanical power in the form of linear motion. 

Selection of Control Valves Depends on the Following Considerations (A typical example is given below for fluid flow control)... 

Process technicians use hydraulic systems (see Figure 9-6) to open or close valves, lift heavy objects, run hydraulic motors, and stop the rotation of a rotary or reciprocating device. A hydraulic system is a collection of equipment designed to apply pressure on a confined liquid in order to perform work. A similar process is used in the brake systems of most cars and trucks. A hydraulic system is composed of a fluid reservoir, strainer, pump, piping, flow control valve, pressure control valve, four-way directional control valve, and actuator (cylinder, piston). 

One-way valves are typically used for flow and pressure control in fluid-power circuits (see Figure 9 9). Flow-control valves regulate the flow of... 

The indicated flow will go up. If the flowmeter is controlling a flow-control valve on a tube inlet to a heater, the valve will then close. Flow to the tube inlet will be lost. The tubes downstream of this flow-control valve will likely be damaged because of overheating, or they will plug and foul as a result of thermal degradation of the process fluid. 

The ease with which the separated products leave the bowl determines the richness of the fat. Fluid whole milk enters the separator under pressure from a positive displacement pump or centrifugal pump with flow control (Fig. 1). The fat (cream) is separated and moves toward the center of the bowl, while the skimmed milk passes to the outer space. There are two spouts or oudets, one for cream and one for skimmed milk. Cream leaves the center of the bowl with the percentage of fat ( 30 40%) controlled by the adjustment of a valve, called a cream or skim milk screw, that controls the flow of the product leaving the field of centrifugal force and thus affects the separation. 

Schemes to control the outlet temperature of a process furnace by adjusting the fuel gas flow are shown in Figure 13. In the scheme without cascade control (Fig. 13a), if a disturbance has occurred in the fuel gas supply pressure, a disturbance occurs in the fuel gas flow rate, hence, in the energy transferred to the process fluid and eventually to the process fluid furnace outlet temperature. At that point, the outlet temperature controller senses the deviation from setpoint and adjusts the valve in the fuel gas line. In the meantime, other disturbances may have occurred in the fuel gas pressure, etc. In the cascade control strategy (Fig. 13b), when the fuel gas pressure is disturbed, it causes the fuel gas flow rate to be disturbed. The secondary controller, ie, the fuel gas flow controller, immediately senses the deviation and adjusts the valve in the fuel gas line to maintain the set fuel gas rate. If the fuel gas flow controller is well tuned, the furnace outlet temperature experiences only a small disturbance owing to a fuel gas supply pressure disturbance.

When manipulating a stream whose flow is independently determined, such as flow of a product or a heat-transfer fluid from a fired heater, a three-way valve is used to divert the required flow to the heat exchanger. This does not alter the linearity of the process or its sensitivity to supply variations and even adds the possibility of independent flow variations. The three-way valve shomd have equal-percentage characteristics, and heat-flow control may be even more beneficial. 

The Cv Concept. From a quantitative standpoint, steam loss can be estimated through the application of the C, concept. A familiar term in control valve technology, C, expresses the flow capability of a fluid-controlling device—in this case, a steam trap. A large C,. means a high flow rate a low C, means a low flow rate. 

When a fire occurs, it is assumed that all fluid flow to and from the fire risk area has been stopped. Therefore, flow loads such as control valve failure or incoming feed streams are not additive to the fire load. Credit is not generally given to flow out through normal channels, since they could also be blocked during the fire emergency. 

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