Three-way valves

When manipulating a stream whose flow is independently determined, such as flow of a product or a heat-transfer fluid from a fired heater, a three-way valve is used to divert the required flow to the heat exchanger. This does not alter the linearity of the process or its sensitivity to supply variations and even adds the possibility of independent flow variations. The three-way valve shomd have equal-percentage characteristics, and heat-flow control may be even more beneficial. 

In addition to using an arrester element with greater flow capacity, it is common to use two arresters in parallel where frequent cleaning is required, with one arrester in standby. A three-way valve can be used to allow uninterrupted operation during changeover. Where elements have an intrinsically high pressure drop, suA as sintered metal elements used in acetylene sei vice, multiple parallel elements can be used. 

Multiple feeds to Eliminate interconnections single machine,, Interlock feed valves so only one can be open two feeds open at once. Incompati- 1 " three-way valve ble materials come Implement appropriate operating procedures in contact, possi- and training bly leading to runaway reaction. CCPS G-f5 CCPS G-32... 

IX]- T ffo-way valve -C Three-way valve -lx]- Bulterfly valve -tSD- Non-relLirn valve -txj- Valve with control trim T Diaphragm valve actuator (pneumatic)... 

Figure 5.6 Schematic design for recycling cliromatography in wliich the effluent is recycled tlirough the pump 1, eluent 2, pump 3, injector 4, chromatograpliic column 5, detector 6, three way valve 7, waste 8, T-connection.

Figure 2 Reduced flowsheet of the experimental setup. The mass-flow controllers, the three-way valves in the oxidizing and reducing pipes, the multiple-way valve GC and the temperature of the reactor are all computer-controlled.

The direction and flow rate of the test and hydraulic fluids are determined by nine three-way valves and six a1r-dr1ven hydraulic pumps that must be sequenced 1n the proper order. The position of the valves 1s determined by six air-driven actuators. Two of the pumps are miniaturized, air-driven, hydraulic pumps used for sample loading and pressurization. One of the remaining four pumps 1s a high-pressure, constant volume, positive displacement, piston metering pump to provide hydraulic pressure, and the other three are positive displacement syringe pumps for In-line addition of additives. 

Figure 3 System for gas chromatographic measurement of Oz evolution from superconductors dissolved in acid (29) (A) enlarged view of reaction flask, (B) circulating system including flask (G), NaOH U-tube (T), three-way valves (S), 1 mL loop (L), pulse pump (PP) and vacuum system (V), At the right is the gas chromatograph (G.C.). (From Reference 29. With Permission.)...

Vacuum is provided by a continuously running two-stage rotary vacuum pump (P). A three-way valve M) connects the bell jar either to the vacuum line or to the pressure line. Inserted into the vacuum line is a valve (N) which has a permanent bypass consisting of a coil of long, narrow steel tubing. In order to prevent the liquid in the cup from boiling, caused by too sudden fall in pressure, the system is evacuated first through the bypass with valve (IV) in the closed position. This valve is opened after a predetermined delay period. 

The nitrator sketched in Fig. 143 is a cylindrical stoneware vessel of internal diameter 1.1m and depth 0.4 m, strengthened with an iron belt. Its conical bottom is fitted with an outlet pipe. A three-way valve is provided for evacuating and filling the vessel with acid. Inside the vessel there is a perforated stoneware plate to support the cotton charge. Twenty five to twenty nine centimeters above this plate supports carry a perforated aluminium plate 15 mm thick, which acts as a false... 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

The three-way valve is then switched to allow liquid dispensing. When the desired quantity of liquid has been removed from the cylinder, the cylinder is closed and the three-way valve switched back to inert gas, so that the needle is purged by inert gas before it is removed from the reaction flask. 

Figure 14.4 Schematic diagram of the chromatographic system used for the analysis of very low concentrations of sulfur compounds in ethene and propene CP, pressure regulator CF, flow regulator SL, sample loop R, restriction to replace column 2 VI, injection valve V2, three-way valve to direct the effluent of column 1 to either column 2 or the restriction column 1, non-polar capillary column column 2, thick-film capillary column SCD, sulfur chemiluminescence detector FID, flame-ionization detector.

Figure 14.10 Schematic diagram of the aromatics analyser system BP, back-pressure regulator CF, flow controller CP, pressure controller Inj, splitless injector with septum purge V, three-way valve column 1, polar capillary column column 2, non-polar capillary column R, restrictor FID1, and FID2, flame-ionization detectors.

Figure 7.6. Hydride generation/condensation systems interfaced to an ICAP polychromator. D1 and D2 desiccant tubes I, II and III three-way valves. From [115]...

Figure 3.16 Schematic representation of (A) NCE with negative pressure pinched sample injection. SP, syringe pump V, 3-way valve, HV, high voltage power supply. (B) View of the connection from the three-way valve to the sample waste reservoir SW [106].

If changeover valves are required, choose the largest three-way valve or the one with the least pressure drop. There are changeover valves available which have proven pressure drops of maximum 3%. They are usually more expensive than the traditional types, but they are certainly recommended if piping losses run too high. 

Multiport This term refers to any valve or manifold of valves with more than one inlet or outlet. For throttling control, the three-way body is used for blending (two inlets, one outlet) or as a diverter (one inlet, two outlets). A three-way valve is most commonly a special globelike body with special trim that allows flow both over and under the plug. Two rotary valves and a pipe tee can also be used. Special three-, four-, and five-way ball valve designs are used for switching applications. 

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