Liquid-level control valve

To prevent flooding or emptying of the reactor, requires a liquid-level controller (LC). In this case, the pressure exerted by the liquid in the reactor measures the liquid level. The operation and installation of the liquid-level control valve is the same as the flow and temperature control valves. 

For a liquid-level control valve when the liquid is discharged to a lower pressure, the contingency shall be estimated for maximum possible liquid flow or maximiun gas breakthrough rate. 

A vapor poeket on the exchanger s low-pressure side can create a cushion that may greatly diminish the pressure transient s intensity. A transient analysis may not be required if sufficient low-pressure side vapor exists (although tube rupture should still be considered as a viable relief scenario). However, if the low-pressure fluid is liquid from a separator that has a small amount of vapor from flashing across a level control valve, the vapor pocket may collapse after the pressure has exceeded the fluid s bubble point. The bubble point will be at the separator pressure. Transient analysis will prediet a gradually inereasing pressure until the pressure reaches the bubble point. Then, the pressure will increase rapidly. For this ease, a transient analysis should be considered. 

In a typical closed-loop system, liquid from the separator is cooled in a heat exchanger then recirculated. Any excess liquid added by mist and vapor flows out through a level-control valve. ... 

Parameters such as feed rate, catalyst bed temperature, and reaction pressure were optimized by use of the temporary on-line LC installation. Reactor upsets could also be monitored. Figure 10 demonstrates how continuous monitoring can aid in detection of an upset. Due to a problem with a level control valve, the reactor filled with liquid, preventing the reaction... 

We use a simple liquid level controller to illustrate the concept of a classic feedback control system.1 In this example (Fig. 5.1), we monitor the liquid level in a vessel and use the information to adjust the opening of an effluent valve to keep the liquid level at some user-specified value (the set point or reference). In this case, the liquid level is both the measured variable and the controlled variable—they are the same in a single-input single-output (SISO) system. In this respect, the controlled variable is also the output variable of the SISO system. A system refers to the process which we need to control plus the controller and accompanying accessories such as sensors and actuators.2... 

For all commercial devices, the proportional gain is a positive quantity. Because we use negative feedback (see Fig. 5.2), the controller output moves in the reverse direction of the controlled variable.1 In the liquid level control example, if the inlet flow is disturbed such that h rises above hs, then e < 0, and that leads to p < ps, i.e., the controller output is decreased. In this case, we of course will have to select or purchase a valve such that a lowered signal means opening the valve (decreasing flow resistance). Mathematically, this valve has a negative steady state gain (-Kv)2... 

The reactor effluent passed through porous stainless steel filters at the top of D-1 to a water-cooled condenser and then to the high pressure separator, F-4. The liquid from the high pressure separator was removed through a level control valve at the bottom of FA to a low-pressure receiver, F-7, which was surrounded by ice water. 

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. 

Note that in a kettle reboiler, the bottoms product level control valve does not control the level in the tower it controls the level on the product side of the reboiler only. The liquid level on the boiling or heat-exchanger side of the kettle is controlled by the internal overflow baffle. But what controls the tower-bottom liquid level ... 

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.

The product-acid solution is withdrawn from the column using a level control valve on this line. The liquid level in the base of the... 

WARNING There is no valve immediately downstream o( the liquid outlet stream. A liquid level control will not be added automatically as Its output can not be connected You should check whether a level controller is needed for a stable dynamic simulation. 

Step 1. We are assuming in this section that the product stream from the bottom of the stripper is set on the demand of a downstream user. The bottoms stream from the stripper is flow-controlled and so we set the position of the control valve, XMV(8), on this stream (B). The rest of the liquid level controls must be chosen to accommodate this first-priority choice. Note that we could put a flow controller on this stream if necessary, but this was not done in the simulations described later. The quality specification is that component G in the product should not vary more than 5 mol %. 

Liquid sub-cooler level control valve shall "fail open" on Instrument air failure. 

During normal operation, all stop valves around the A-unit are open. The liquid refrigerant inlet solenoid valve (A) is open, allowing liquid to pass through the level control valve (B) and into the bottom of the refrigerant jacket surrounding the... 

Figure 16 pictures a high-pressure autoclave set up as a CSTR. In this arrangement the introduction of the substrate is done using an HPLC pump. The liquid level in the reactor is kept constant using a liquid level controller (A) which is monitored by the computer. When the liquid input reaches a set level an exit valve (B) is opened to drain some of the reaction liquid to maintain the liquid level in the reactor. With such a system there is no need for a second pump to remove the product stream. 

IV.2 Consider the liquid level control loop of Figure 13.2d. The differential pressure transducer cell exhibits second-order dynamics, the controller is proportional, and the control valve is linear with flow characteristic curve given by... 

Exit gas leaves the top of the reactor and passes through a heated line (to avoid condensation of liquids) to a cooler-condenser. Liquid products are collected in a knockout pot and are drained through a solenoid valve actuated by a sonic liquid-level controller. A dome-loaded, back-pressure regulator controls the reactor pressure and reduces the pressure of the exit gas. The exit gas rate is measured by a wet-test meter. Carbon monoxide and carbon dioxide concentrations in the feed and exit gases are measured by two continuous infrared analyzers and are recorded. Aliquot samples of feed and exit gases are taken during each test period for analysis of all components by mass spectrometer (Consolidated Engineering Model 21-103). 

A-FEED GAS CYLINDER B-ORIFICE C-AP TRANSMITTER D-P.T.AP RECORDER E-MANOMETER F-SOLENOID VALVE G-N2 PURGE SYSTEM H-REACTOR BY-PASS LINE J-PREHEATER K-REACTOR L-MAGNETIC DRIVE M-SPEED CONTROL AND TACHOMETER N-CONDENSER P-KNOCKOUT POT Q-LIQUID COLLECTING TANK R-LIQUID LEVEL CONTROL... 

HIGH LIQUID AMMONIA LEVEL N EVAPORATOR CHILLER), WILL THROTTLE LIQUID INLET CONTROL VALVE,... 

More positive control over the position of the interface to ensure satisfactory phase separations requires that a control valve operated by a liquid-level controller, in turn actuated by the position of the interface, be installed on the heavy-liquid exit pipe, with elimination of the siphon break. Devices of the sort described have been the subject of many patents [see, for example (19)]. 

The feedforward system imposes an external material balance as well as a an internal material balance on the process. The internal balance is maintained by liquid level control on the discharge of each effect. Analysis of a level loop indicates that a narrow proportional band (less than 10%) can achieve stable control. However, because of the resonant nature of the level loop which causes the process to oscillate at its natural frequency, a much lower controller gain must be used (proportional bands 50-100%). A valve positioner is recommended to overcome the nonlinear nature of valve hysteresis. 

The CSB s first focused on loss of containment, the prime fundamental. The level control valve was closed by an operator and that the column was filled for over 3 hours without liquid be routed to storage. Overfilling the tower was influenced by ... 

Figure 14-1 shows a typical feed cleanup system. The system s components are a settling drum, circulation pump, mix valve, interface level control valve, and wash liquid. The wash liquid, usually caustic or water, is contacted with a hydrocarbon stream. The mix valve provides the turbulence required for good contacting. The settling drum separates the two phases. The circulating pump returns the wash liquid to the mix valve. Finally, the interface level control valve sets the relative volumes of the wash liquid and hydrocarbon in the settler. 

Note that in Figure 10-3 the evaporator level control valve is being used as a throttle valve. If you see this valve wide open, there may not be sufficient inventory of refrigerant in the system. Check the accumulator drum level and add refrigerant to establish a normal accumulator liquid level. If there are two evaporators in parallel, pinch back on the control valve to the evaporator that you are not having trouble with. 

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