Flow ratio control

Recommended nominal steam rates at 60 m/s exit velocity for a typical flare tip are shown in Figure 2. At lower velocities, higher steam ratios are required. Typical steam control consists of a flow ratio controller with adjustable ratio set point, related to flare gas flow. The ratio adjustment, located in the control house, provides for the higher steam ratios necessary at low flaring rates. 

The feed pump will be re-rated for the new conditions. With higher viscosity and higher gravity, the pump driver may need work. If the system is not adequate, heavier feed can be piped through a separate circuit in parallel with the existing circuit, preferably on flow ratio control. 

F, as the second letter indicates ratio eg. FFC indicates a flow ratio controller. 

Control. The control scheme for a hydrocyclone installation in which wc have identified flow rale as a key parameter would logically be based on total flow rate with a ratio device adjusting a flow-control valve in the reject line to maintain the desired reject ratio. However, problems can be expected with an installation based on flow/ratio control because of gas breakout, which exists preferentially with the reject stream and creates two-phase flow. Normal metering devices will not tolerate two-phase flow and give sufficiently accurate and reproducible results for control purposes. 

Flow ratio control is essential in processes such as fuel-air mixing, blending, and reactor feed systems. In a two-stream process, for example, each stream will have its own controller, but the signal from the primary controller will go to a ratio control device which adjusts the set point of the other controller. Figure 3.17(a) is an example. Construction of the ratioing device may be an adjustable mechanical linkage or may be entirely pneumatic or electronic. In other two-stream operations, the flow rate of the secondary stream may be controlled by some property of the combined stream, temperature in the case of fuel-air systems or composition or some physical property indicative of the proportions of the two streams. 

The distribution of the return water among several cooling tower cells can satisfy the dual goals of sending a preset percentage to each and to do that at a minimum pumping cost. The second goal is served by the valve position controller (VPC), which increases the measurement of all flow ratio controllers (FFICs) until the most open valve opens to 90%. 

The load distribution can be computer-optimized by calculating compressor efficiencies (in units of flow per unit power) and loading the compressors in their order of efficiencies. The pressure controller (PC-22) directly sets the set points of SC-21 and SC-23, whereas the balancing controllers (FFC-22 and FFC-24) slowly bias those settings as they follow the total H2 generation of the electrolyzers (FT-4). The flow ratio controllers (FFCs) are also protected from reset windup, as was explained in Section 2.5.4. 

FC Flow controller or fuel cell FCEV Fuel cellnbased electric vehicle FCP Flash compressor FFC Flow ratio control FT Flow transmitter FW Feedwater... 

I) High temperature endothermic processes may need several reaction vessels with intermediate heat input. For example, the inlet temperature to each stage of a catalytic reformer is about 975°F and the temperature drop ranges from about 100°F in the first stage to about 15°F in the last one. In the two-stage assembly of this figure, the input is on FC, the outlet of the last reactor on PC, and the fuel supply to each furnace is on TC of its effluent, with the air supply on flow ratio control, as shown for example (e). 

There are many techniques for diluting caustic, but perhaps the best is to blend water and a concentrated solution in an in-line mixing device, cool the dilute solution, and take the product to a tank for storage and distribution. To control the concentration of the mix, the two streams are under flow ratio control. Normally the caustic flow is controlled directly and the water flow by a ratio controller. The set point of the latter is reset by a downstream temperature-compensated density instrument. The quality of the water and the material of construction of the mixing device depend on the end use(s) of the dilute caustic. Stainless steel is perhaps the standard material of construction. 

Clean dry air added to the liquefiers along with the chlorine dilutes the hydrogen and eliminates that hazard. The rate of addition of air is governed by a flow-ratio controller, with the chlorine flow to the liquefier acting as the primary flow (Fig. 11.28). The ratio to be used depends on the concentration of hydrogen in the chlorine and on the depth of liquefaction. It should be reset as required, based on the concentration of hydrogen in the tail gas from liquefaction. The control valve should fail open and have an equal percentage characteristic and a positioner. A small bypass flow of air may... 

It is preferable to operate a preliminary pH control stage after ion exchange. Controlling this stage to pH 3 neutralizes any carbonate residual from primary brine treatment Deep acidification (pH < 2) is best practiced by ailding acid to feed brine on an individual electrolyzer basis. Flow ratio control may be used. 

Flow controllers set the rates of both streams, one being under flow-ratio control. In principle, either caustic soda or dilution water can be the master stream, with the other following it to maintain the ratio. Blending is controlled by a feedforward system, ultimately reset by the product concentration or density. Feedback from caustic concentration measurement (usually by density) could be used for final adjustment, but the concentration of the hypochlorite solution is the more important variable. The simple flow-ratio controller mentioned here can be replaced by a multi-stream version that allows use of other streams in addition to the principal 50% NaOH and dilution water. A cooler downstream of the mixing point removes the heat of dilution. The standard design is a titanium plate exchanger, which can also provide turbulence to complete the mixing process. Chlorine joins the diluted caustic in the reactor. Its rate of addition is controlled by an oxidation-reduction potential (ORP) instrument. The reaction mass recirculates from a collection tank around the system to reduce the increase of temperature across the reactor and to promote turbulence. The net production is removed from the tank, normally under level control. 

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