Flooding avoidance

Water management is one of the most critical and widely studied issues in PEFC. Water management is referred to as balancing membrane hydration with flooding avoidance. These are two conflicting needs to hydrate the polymer electrolyte and to avoid flooding in porous electrodes and GDL for reactant/ product transport. 

Refinery Presumably a fixtctionator A restrictive design of a transition tray converting sin e-pass flow to two-pass flow caused premature flooding. Avoid restrictive transition tray designs. 

Figure 6.54 Plot of minimum stoichiometry requirements for flooding avoidance as function of operating temperature and pressure for DMFC. (Adapted from Ref. [44].)...

At high speeds, the pond of the flooded nip size press becomes turbulent. RoU maintenance is a problem with the gate-roU size press. To avoid these problems, the blade/rod metering size press was developed. Short-dweU coater heads are used to apply a precisely controUed quantity of chemicals to the size press roUs. This quantity is controUed with either a metering blade or a metering rod. Blade or rod metering eliminates the pond, and does not increase the number of roUs required for surface chemical appHcation. 

T. A. Royal andJ. W. Carson, "How to Avoid Flooding in Powder Handling Systems," Powder Handl Proc. 1 (Mar. 1993). 

Because dimethyl sulfate looks like water, operations are preferably not performed when water is present, eg, wet floors or rain. Any spills or leaks should not be left unattended they should be contained, and mnoff to sewers should be avoided. Minor spills should be flooded with water to dilute and hydroly2e the dimethyl sulfate. The area should then be covered with a dilute (2—5 wt %) caustic solution or a dilute (2—5 wt %) ammonia solution, or soda ash may be sprinkled over the neat liquid and the mix wetted with a gende spray of water. The neutrafi2ing agent should remain on the affected area for 24 h and then should be washed away. Only personnel wearing protective equipment should perform these operations. The product bulletins should be consulted for procedures to be followed for more severe spills. Concentrated ammonia should not be used with neat dimethyl sulfate because explosions have resulted after their contact (128). 

Pnettmatically Actuated Diaphragm Pumps (Fig. 10-53) These pumps require no power source other than plant compressed air. They must have a flooded suction, and the pressure is, of course, limited to the available air pressure. Because of their slow speed and large valves, they are well suited to the gentle handling of liquids for which degradation of suspended solids should be avoided. 

Downflow Flooding Columns can flood because of their inability to handle large quantities of liqmd. For crossbow plates this hmit on liquid rate Is evidenced by downcomer backup to the plate above. To avoid downflow flooding one must size the column downcomers such that excessive backup does not occur. 

Current designs for venturi scrubbers generally use the vertical downflow of gas through the venturi contactor and incorporate three features (I) a wet-approach or flooded-waU entry sec tion, to avoid dust buildup at a wet-dry pmction (2) an adjustable throat for the venturi (or orifice), to provide for adjustment of the pressure drop and (3) a flooded elbow located below the venturi and ahead of the entrainment separator, to reduce wear by abrasive particles. The venturi throat is sometimes fitted with a refractoiy fining to resist abrasion by dust particles. The entrainment separator is commonly, but not invariably, of the cyclone type. An example of the standard form of venturi scrubber is shown in Fig. 17-48. The wet-approach entiy section has made practical the recirculation of slurries. Various forms of adjustable throats, which may be under manual or automatic control. 

Mechanical agitation is needed to break up the gas bubbles but must avoid rupturing the cells. The disk turbine with radial action is most suitable. It can tolerate a superficial gas velocity up to 120 m/h. (394 ft/h) without flooding, whereas the propeller is limited to about 20 i7i/h (66 ft/h). When flooding occurs, the impeller is working in a gas phase and cannot assist the transfer of gas to the liquid phase. Power input by agitation and air sparger is 1 to 4 W/L (97 to 387 Btu/[fF-h]) of liquid. 

It is veiy important that sealless pumps be flooded with hquid before starting, to avoid damage to bearings from imbalance or overheating. Entrained gases in the suction can cause immediate imbalance problems and lead to internal bearing damage. Some type of liquia sensor is recommended. Sealless pumps must not be operated deadheaded (pump hquid full with inlet and/or outlet valves closed). 

For distillation under reduced pressure there must be careful control of the pressure to avoid flooding or cessation of reflux. 

Lieberman gives two rules of thumb for troubleshooting fractionators that could also be used as checks on a design. First, the pressure drops across a section of trays must not exceed 22% of the space between the tray decks, to avoid incipient flood. Mathematical , hold... 

If a tower does become flooded in the bottom section, a common operator error is to try to pump the level out too quickly. This can easily damage trays by imposing a downward acting differential pressure produced by a large weight of liquid on top of the tray and a vapor space immediately below the tray. To eliminate the flooding, it is better to lower feed rate and heat to the reboiler. It is important to be patient and avoid sudden changes. 

Because polymers are typically non-conductive, sample charging can occur and has to be compensated carefully, e. g. by use of a low-energy electron-flood gun, to avoid line-shape distortion and misinterpretation of the measurements. 

These scrubbers have had limited use as part of flue gas desulfurization (FGD) systems, but the scrubbing solution flow rate must be carefully controlled to avoid flooding. When absorption is used for VOC control, packed towers are usually more cost effective than impingement plate towers (discussed later). 

Milk-of-lime transfer pumps should be of the open impeller centrifugal type. Pumps having an iron body and impeller with bronze trim are suitable for this purpose. Rubber-lined pumps with rubber-covered impellers are also frequently used. Makeup tanks are usually provided ahead of centrifugal pumps to ensure a flooded suction at all times. Plating out of lime is minimized by the use of soft water in the makeup tank and slurry recirculation. Turbine pumps and eductors should be avoided in transferring milk of lime because of scaling problems. 

The column diameter is normally determined by selecting a superficial velocity for one (or both) of the phases. This velocity is intended to ensure proper mixing while avoiding hydrodynamic problems such as flooding, weeping, or entrainment. Once a superficial velocity is determined, the cross-sectional area of the column is obtained by dividing the volumetric flowrate by the velocity. 

The Secondary LOCA sequences are related to large breaks (23/24) with small breaks and expansion joint (< 1%). Response to Secondary LOCA involves shutdown, leak control and/or isolation, water removal from the building to avoid flooding of pumps, and direct core cooling in the event of pump flooding. 

The importance of the downcomer seal is to prevent vapor from the tray from bubbling into the downcomer (see Figure 8-63), whether the trays are bubble cap, valve or sieve types. If a seal weir is not included in the tray design, then operation problems to avoid flooding, weeping and unstable performance, including pressure drop, are increased, particularly during the start-up phase. 

Referring to Figure 8-63, the weir height, h, must always be greater than the clearance under the downcomer, i.e., between bottom of downcomer and tray floor, hdci-Always avoid too low clearance as this can cause flooding of liquid in the downcomer. There are flow conditions... 

For center downcomers as in a two-pass design, the throw must be conservatively less than a distance that would cause the opposing streams entering the same downcomer to interfere with each other. Sometimes the installation of a splash baffle will help avoid conditions leading to flooding and loss of tray efficiency. 

To avoid the possibility of local flooding at the support, it would be well to place a heavy hardwtire cloth... 

In the physical arrangement, make certain that the pressure balance level, plus an allowance for froth, establishes a height that is below the bottom tray of the column to avoid flooding the column. In addition, the estimated froth height on top of the liquid should still be below the level of the vapor return from the reboiler. 

I. Baudelot coolers (see Figures 7.7 and 7.8a). The evaporator stands above a collection tank, and the water runs down the outside surface in a thin layer. Evaporator construction can be pipe coils or embossed plates. The latter are now usually of stainless steel, to avoid corrosion troubles. Evaporators maybe flooded or dry expansion. During operation, a Baudelot cooler... 

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