Liquid flow through the valve

The mass flow (kg/s) of liquid through the valve has been found to be represented well by the simple expression  

The valve conductance is a figure supplied for each valve by the manufacturer, often in US rather than SI units. In this case the parameter, C, is given, the valve capacity for water at 60°F in US gall/min/(psi). It is shown in Appendix 3 that this may be related to the valve conductance, C , by the conversion equation  

Equation (7.3) relates the flow to the pressure difference between the valve inlet and outlet, and it is reasonable to assume that a similar equation will relate the flow to the pressure difference between the valve inlet and the valve throat  

Rearranging gives the throat pressure in terms of the valve upstream and downstream pressures as  

Let us define a dimensionless friction coefficient for liquid flow, Cyr, by  

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Liquid flow through the valve in US units is given by the following equation ... 

The internal design of process-control valves has a direct impact on the flow characteristics of the gas or liquid flowing through the valve. A fully open butterfly or gate valve provides a relatively straight, obstruction-free flow path. As a result, the product should not be affected. 

A relief valve is mounted on the top of a large vessel containing hot water. The inlet diameter to the valve is 4 in., and the outlet diameter is 6 in. The valve is set to open when the pressure in the vessel reaches 100 psig, which happens when the water is at 200° F. The liquid flows through the open valve and exits to the atmosphere on the side of the valve, 90° from the entering direction. The loss coefficient for the valve has a value of 5, based on the exit velocity from the valve. 

A number of runs were carried out with the pump stopped but liquid flowing through the wide-open recycle valve w h an upstream pressure of 500 to 600 kPa [73 to 87 psi). This resulted in little change in performance over the pumping mode. This, combined with the Ap observation, suggests that most of the droplet breakup is taking place in the separator-level control valves. 

One of the most frequent causes of flooding is the use of carbon steel trays. Especially when the valve caps are also carbon steel, the valves have a tendency to stick in a partially closed position. This raises the pressure drop of the vapor flowing through the valves, which, in turn, pushes up the liquid level in the downcomer draining the tray. The liquid can then back up onto the tray deck, and promote jet flood, due to entrainment. 

Under some operating conditions, sufficient pressure differential may exist across the valve to cause the vena contracta pressure to drop to the saturation pressure (also known as the vapor pressure) of the liquid. If this occurs, a portion of the liquid will vaporize, forming a two-phase, compressible mixture within the valve. If sufficient vapor forms, the flow may choke. When a flow is choked, any increase in pressure differential across the valve no longer produces an increase in flow through the valve. 

For liquid discharges, the design of the pipework may be complex. If the liquid is subcooled, the discharge pipework is sized by standard liquid pressure formulae. If the liquid in the equipment is at or near its saturation pressure, the relief device and discharge pipework should be sized for two-phase flow. As the liquid flows through the relief valve. 

This type of tray is designed to allow for wide variations in liquid and vapor flow. One typical design of a valve vapor opening consists of a one- to two-inch diameter orifice in the tray plate, an orifice cover, and a travel stop. At low vapor rates, the orifice cover is settled in its lower position. In this position, slots in the orifice cover allow small amounts of vapor to be distributed evenly. At higher vapor rates, the orifice cover is elevated to its upper position set by the travel stop. In this position, large amounts of vapor can flow through the valves. 

Liquid flow through the installed control valve... 

Liquid flow through the irtstalled control valve 73... 

Since the flow through the valve is governed by the area of the throat, it is reasonable to surmise that equation (7.3) will produce its best results when the specific volume term refers to the throat. The incompressible nature of liquids means that the inlet specific volume and the throat specific volume will be identical, so that the surmise is answered by equation (7.3) in its present form for liquids. However, we must make a change to cater for gases. Assuming a perfectly adiabatic expansion through the valve as far as the throat, the specific volume of the gas at the throat, v, will be given by... 

For nonflashing liquids the flow through the valve is given by... 

Effect of Fuel Pressure. Another major problem with a conventioned single-loop control is that if the fuel pressure drops, less fuel will flow through the control valve. Eventually, the liquid flowing through the process heater will have a lower temperature which is finally affected by the thermocouple in the heater outlet. When this happens, the recorder-controller signals the fuel valve to open wider letting more fuel into the process heater. Unfortunately, more time passes before the controller knows precisely how much to open the fuel valve for correct temperature control. 

A gravity-flow condenser uses the hydraulic head of the liquid in the line from the condenser to overcome the pressure drop over the control valve and the difference between the pressure at the top of the distillation column P and the pressure at the bottom of the condenser Pi. The pressure difference is due to the flow of vapor through the vapor line and condenser. When the flow rate of vapor from the top of the column is 14 1.6 Ibm/min, the pressure drop P - Pi is 2 psi. The pressure drop due to the liquid flowing through the liquid return line is negligible. Liquid density is 62.4 lbm/ft ... 

It is important when specifying the control valves to select the correct valid phase. If the stream is aU liquid, select Liquid-Only in the Valid Phases under Flash options on the Operation page tab of the valve block. If the stream is all vapor, select Vapor-Only. Some valves have both phases (particularly when the inlet is liquid at its bubble point temperature and pressure, which means flashing occurs when the pressure decreases as the fluid flows through the valve) and Vapor-Liquid should be selected. Numerical problems can occur in Aspen Dynamics if these valid phases are not correct. 

An example of a microreactor control valve capable of operating at low Re is presented in Fig. 7 where it is shown to be used for adjusting the reactants flow rate [12]. Visualized in Fig. 8 are two extreme situations of the flows through the valve. The visuahzation was made with water flow (gas or liquid flows are indistinguishable if their Re value is the same). The jet of the control fluid opposing the reactor exit can generate an... 

Thin Aims and rolled sheets are the most widely used format in microvalve fabrication. Usually an SMA microvalve consists of an upper and a lower housing integrated with a valve seat and fluidic connections, a membrane, a spacer, and an SMA microactuator. The deformation actuated by the SMA thin film pushes or pulls the spacer to seal or open the valve seat, thus to close or allow the liquid to flow through the valve seat. 

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