Proportional valves

Chase [2.32] presents an alternative method to monitor and control the freeze drying process by measuring the flow of nitrogen to keep the operation control pressure, pc, constant. The Mass Flow Controller (FMC) consists of a proportional valve, an integral flow meter and a capacitance manometer (CA). The CA measures the total pressure in the plant, the valve opens, if the pressure gets below the preset value and vice versa. The flow of... 

Proportional valve 2 open Purge valve open... 

We have to make a distinction here between electric controllers (e.g. PID controllers) with a proportional valve as actuator and mechanical diaphragm controllers. In a regulation system w/ith electric controllers the coordination between controller and actuator (piezoelectric gas inlet valve, inlet valve A/ith motor drive, butterfly control valve, throttle valve) is difficult because of the very different boundary conditions (volume of the vessel, effective pumping speed at the vessel, pressure control range). Such control circuits tend to vibrate easily when process malfunctions occur. It is virtually impossible to specify generally valid standard values. 

Tests shall be performed to check the transmission of analog information sent by the system controller to analog peripheral systems (proportional valves, recorders, etc.). These tests check the integrity of the analog transmission chain equally between the control system and the peripheral system... 

The activity of the silica is an important parameter. When silica is wet by several layers of water, pores are filled and solutes cannot undergo adsorption. As a consequence these are retained only slightly. They can even undergo partition mechanism. Figure 7 demonstrates the dramatic influence of water on retention. Reproducible retention can be achieved by Engelhardt s method (17) One tank is filled with dry solvent and another tank is filled with fully wet solvent. A proportioning valve allows for the exact mixing, and the water content is controlled. A moisture... 

Electronically Controlled Proportional Valves Selection and Application, Michael J. Tonyan, edited by Tobi Goldoftas... 

Improperly prepared mobile phase contaminated mobile phase. —> Prepare fresh mobile phase. Check pump-proportioning valve for malfunction. 

Incomplete mixing. Malfunctioning proportioning valves —> Ensure complete mixing. Clean or replace the proportioning valve partially remix solvents. 

Proportioning valve failure —> Check diaphragms, replace if leaky check for fitting damage, replace. 

The dwell volume is the volume within the HPLC system from the point where the solvents are mixed to the entrance of the column. In a low-pressure system it consists of the volumes of the proportioning valve, the mixer, the pump head, the injector and the connecting capillaries. In a high-pressure system it is smaller because only the mixer, injector and the capillaries add their volumes. 

A more popular and less expensive method of forming gradients is by using a single pump and three or four micro-proportioning valves at the inlet of the pump. At low... 

Figure 4.4. Schematic diagrams of a low-pressure mixing system (a) using a single pump and a four-port proportioning valve, and a high-pressure mixing system (b) using two separate pumps and a controller to blend solvents under high pressure.

Case study 4 shows several examples of problems caused by equipment malfunctions and their subsequent diagnosis and solution. The first one involved a situation of poor retention time reproducibility of a gradient assay. It involved the analysis of a complex natural product, using a narrowbore column (2-mm i.d.) at 0.5 mL/min. System suitability test showed retention times to be erratic and could vary by 1-2 minutes without any obvious trends. Flow rate accuracy was found to be acceptable, however, the compositional accuracy test failed (see Chapter 9 on HPLC calibration). The tentative diagnosis was that of a malfunctioning of the proportioning valve. After its replacement, the retention time precision performance was re-established. 

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