Liquid through the valve

Valve systems. These systems are used especially in the dairy industry. The narrow gap is created here with a valve, which is pressed shut with a disk and a strong spring. Pressing the liquid through the valve lifts the disk somewhat the force of the spring ensures that the gap width will remain very small for a lab-scale system the gap width will be of the order of 1 pm for industrial-scale systems it will be 10-40 pm (Smulders 2000). 

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

With the exception of simple, manually controlled shutoff valves, process-control valves are generally used to control the volume and pressure of gases or liquids within a process system. In most applications, valves are controlled from a remote location through the use of pneumatic, hydraulic, or electronic actuators. Actuators are used to position the gate, ball, or disk that starts, stops, directs, or proportions the flow of gas or liquid through the valve. Therefore, the response characteristics of a valve are determined, in part, by the actuator. Three factors critical to proper valve operation are response time, length of travel, and repeatability. 

The key components in the fuel vapor control system include the fuel tank, vapor vent valves, vapor control valve, vapor tubing, the activated carbon canister, and the engine vapor management valve (VMV) [25,26], During normal vehicle operation, fuel tank vapor pressure is relieved through the use of vapor vent valves installed in the vapor dome of the fuel tank. The vent valves are designed to allow for the flow of fuel vapor from the tank, and to assure that liquid fuel does not pass through the valve. 

The critical part of the valve consists of a synthetic sapphire ball resting on a seat. The seat may be of stainless steel, PTFE or, more usually, also of sapphire. When the flow is directed against the ball the ball moves forward allowing the liquid to flow past it. When the direction of pressure changes resulting in potential flow-back through the valve, the ball falls back on its seat and arrests the flow. 

Rotary pumps forcibly transfer liquid through the action of rotating gears, lobes, vanes, screws etc, which operate inside a rigid container. Normally, pumping rates are varied by changing the rotational speed of the rotor. Rotary pumps do not require valves in order to operate. 

Are containers placed in the 6 o clock/12 o clock position for storage to reduce the chance of a liquid leak through the valve ... 

In plate columns the two phases are intensively mixed on each plate and separated between each plate (Fig. 6.7-5). For the distribution of the light phase through the liquid a lot of devices were developed. The simplest one is a perforated sieve tray, where the supercritical phase can pass through. To avoid weeping of the liquid through the holes different devices like bubble caps or valves (Fig. 6.7-6) were developed. 

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. 

The absolute tower pressure (in psia) increased by 17 percent, and hence the volume (as well as the velocity of vapor through the valve tray caps) declined by 17 percent. The reduced vapor velocity reduced the dry tray pressure drop, thus reducing both the spray height above the tray deck and the liquid backup in the downcomers. 

Figure 3.9. Steam heaters, (a) Flow of steam is controlled off the PF outlet temperature, and condensate is removed with a steam trap or under liquid level control. Subject to difficulties when condensation pressure is below atmospheric, (b) Temperature control on the condensate removal has the effect of varying the amount of flooding of the heat transfer surface and hence the rate of condensation. Because the flow of condensate through the valve is relatively slow, this mode of control is sluggish compared with (a). However, the liquid valve is cheaper than the vapor one. (c) Bypass of process fluid around the exchanger. The condensing pressure is maintained above atmospheric so that the trap can discharge freely, (d) Cascade control. The steam pressure responds quickly to upsets in steam supply conditions. The more sluggish PF temperature is used to adjust the pressure so as to maintain the proper rate of heat transfer.

Pressure drop. Good control requires a substantial pressure drop through the valve. For pumped systems, the drop through the valve should be at least 1/3 of the pressure drop in the system, with a minimum of IS psi. When the expected variation in flow is small, this rule can be relaxed. In long liquid transportation lines, for instance, a fully open control valve may absorb less than 1% of the system pressure drop. In systems with centrifugal pumps, the variation of head with capacity must be taken into account when sizing the valve. Example 7.2, for instance, illustrates how the valve drop may vary with flow in such a system. 

Although it has been common practice to specify the pressure loss in ordinary valves in terms of either equivalent length of straight pipe of the same size or velocity head loss, it is becoming more common to specify flow rate and pressure drop characteristics in the same terms as has been the practice for valves designed specifically for control service, namely, in terms of the valve coefficient, C. The flow coefficient of a valve is defined as the volume of liquid at a specified density that flows through the fully opened valve with a unit pressure drop, eg, Cv = 1 when 3.79 L/min (1 gal/min) pass through the valve... 

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. 

Diffusion Pump System. After the pump line and trap have been shut off, a large valve is opened slowly enough that the mass flow of gas from the chamber through the valve into the oil-diffusion pump system does not disrupt the top jet of the diffusion pump (DP) (Fig. 4). When the liquid nitrogen is replenished after the trap has been operated for some time, release of previously trapped gas must be avoided. The so-called ionization-gauge response pips at the start of the liquid-nitrogen replenishment are an indication of trap ineffectiveness. 

To size control valves for liquids, use a similar procedure and the relation C = V (G/Ap), where V is flow rate through the valve, in gal/min, Ap is pressure drop across the valve at maximum flow rate, in lb/in2, and G is specific gravity of the liquid. When a liquid has a specific gravity of 100 SSU or less, the effect of viscosity on the control action is negligible. 

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