Valves self regulated

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Pictures of a typical sieve tray and valve tray are shown in Figs. 12-1 and 12-2, respectively. Sieve trays consist of metal plates with small circular perforations. The valve of a valve tray consists of a self-regulating variable orifice (Fig. 12-2) which adjusts its opening in proportion to the total flow rate of the vapor. Most of the equations for sieve trays are also applicable for valve trays. A treatment of sieve trays is presented in Sec. 12-1 and the modifications of these equations as well as additional equations needed to describe valve trays are presented in Sec. 12-2. 

Step 6. Fix recycle flow rates and vapor and liquid inventories. The liquid inventories in the flash vessel and reactor are non-self-regulating, and therefore, need to be controlled (Guideline 1). Since the liquid product valve from the flash vessel has been assigned to control the product flow rate, the inventory control must be in the reverse direction to the process flow. Thus, the reactor effluent valve, V-4, controls the flash vessel liquid level, and the feed valve, V-1, controls the reactor liquid level. Both of these valves have rapid, direct effects on the liquid holdups (Guidelines 6, 7, and 8). The vapor product valve, V-5, which has been assigned to control the pressure in V-100, thereby controls the vapor inventory. 

The process dynamics of most process variables can be characterized as a self-regulating process response. One example is the response of a liquid flow rate when a valve position is opened. The liquid flow rate will increase from the initial flow rate to a new steady-state flow rate. Another example is the response of the temperature of a liquid flowing through a heat exchanger that is heated with steam. When the steam valve position is increased, the temperature of the liquid outlet will increase to a new steady-state temperature. 

The self-regulating process gain, Kp, can be measured from the setpoint change and controller output (valve position) change (Equation 9.2). 

As discussed in Chapter 9, the process dynamics of most process variables can be characterized as a self-regulating process response. When the controller output changes the automatic valve position of the manipulated variable, the process variable moves to a new steady-state value. 

With the mechanization that accompanied the Industrial Revolution, other means of automatically controlling machinery were developed, including self-regulating pressure valves on steam engines. Modern... 

The fired heater that we have worked with is an example of a self-regulating process. Following the disturbance to the fuel valve the temperature will reach a new steady state without any manual intervention. Not all processes behave this way. For example, if we trying to obtain the dynamics for a future level controller we would make a step change to the manipulated flow. The level would not reach a new steady state unless some intervention is made. This non-self-regulating process can also be described as an integrating process. 

The level in the tank could be controlled by manually adjusting the valve position, thereby setting inflow. But if inflow varied in the slightest from outflow, the tank would eventually flood or run dry. This characteristic is called non-self-regulation. It, means that the integrating process cannot balance itself-it has no natural equilibrium or steady state. The non-self-regulating process cannot be left unattended for long periods of time without automatic control. 

Replace the metering pump in Fig. 1.14 with a valve. Then an increase in liquid level would inherently increase the outflow. This action works toward the restoration of equilibrium and is called self-regulation. It is as if a proportional controller were at work within the process. This is a natural form of negative feedback. 

Just as level control was used to close a liquid material balance around a tank, pressure control is used to close a gas material balance. The gas-pressure process is ordinarily self-regulating, except at zero flow, because pressure always influences inflow and outflow. The process is fundamentally single-capacity, although the pressure transmitter and valve... 

If the manipulated valve has a linear installed flow characteristic (preferred), and if there is no level self-regulation (if Ap, does not change appreciably with change in level), then the dynamic response of the proportional-onlyt level control system may be defined by a first-order time constant ... 

We conclude that the liquid storage system is open-loop unstable (or non-self-regulating) because a bounded input has produced an unbounded response. However, if the pump in Fig. 11.24 were replaced by a valve, then the storage system would be self-regulating (cf. Example 4.4). 

Various modifications have been made to improve the operation of perforated plates. One of these modifications involves adding distribution slots or louvers across the plate to minimize the hydraulic gradient of the liquid flow for large diameter columns. Another modification which extends the operating range of perforated plates is to install self-regulating valves or caps over larger... 

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