Pressure drop 521: surveys

Other causes of excessive coke drum back pressure are badly fouled combination tower overhead condensers, partially plugged trays, or insufficient tower pumparound heat removal. Fouled condensers and lack of pumparound heat removal overload the wet gas compressor. Plugged trays are best identified with a pressure drop survey. 

A careful pressure drop survey indicated that there was an inexplicably high pressure drop in the liquid inlet line. When the tower was opened for inspection, the carcass of a dead rat was discovered lodged in the reboiler liquid inlet nozzle. Inadvertently, this rat caused the tower to flood as the reboiler rate was increased. If the rat had expired in the vapor outlet line, the effect would have been the same. 

A section of the demister pad has become dislodged. A pressure drop survey showing a substantial decrease in AP across the demister is evidence of such a failure. 

A pressure drop survey. The total head developed by the pumps and... 

Tube leaks may also occur in the low-pressure steam condensers. The leaking condenser is identified through a pressure drop survey. Measure the pressure drop across each condenser. The first condenser in the train exhibiting a disproportionately high pressure drop is the leaker. If the leaking tubes are found in the bottom of the condenser, formation of sulfuric acid may be the cause (see following section on start-up tips). 

One day I asked Norman how he would calculate the pressure loss for flow through circular cross-section pipes—the sort of pipes that are used throughout most process plants that we work in. I felt sure he would have some simple method. Here is the method with an example of its use for a pressure drop survey, more or less as he gave them to me. 

Otherwise, bear in mind that very often the range of shear rates to which a process fluid is exposed in pip>e flow is often cjuite limited, and because of this it is often possible to adecjuately represent the flow behavior of a process fluid over a limited range of shear rates by the power law or Ostwald de Waele model, and the Metzner-Reed Reynolds number, which we shall shortly discuss. Meanwhile, in the absence of any lab equipment to provide you with shear-stress versus shear-rate data, we suggest that you vary the flow rate to provide several flow rates, and run pressure drop surveys over the pipeline in question. Tabulate the data and use this to develop a "power law" relationship (see following sections) as a first approximation for your pijjeUne and process fluid. Most likely you will not need to perform any more elaborate study than that... 

Fiqure 40.1 Pressure drop survey in a kerosene pipeiine. 

Parallel-plate hemodialyzers using flat membranes, with several compartments in parallel, separated by plastic plates, are now only available from Hospal Co (Crystal and Hemospal models). Blood circulates between two membranes and the dialysate between the other side of membrane and the plastic plate. These parallel-plate dialyzers have a smaller blood-pressure drop than hollow-fiber ones and require less anticoagulants as flat channels are less exposed to thrombus formation than fibers, but they are heavier and bulkier and thus less popular. A recent survey of the state-of-the-art in hemodialyzers is given in [13]. 

Mersmann s correlation and Madkowlak s correlation. Mersmann (73) postulated that a thin liquid film forms in the flow channel of the packing. The ratio of film thickness to equivalent packing diameter is a function of the liquid load. Mersmann combined this function with a trickle flow model to yield an expression for dry packing pressure drop at flood as a function of liquid rate. Mafikowiak (78a) surveyed sources that followed up and improved on Mersmann s initial model. 

The modeling and design of a three-phase reactor requires the knowledge of several hydrodynamic (e.g., flow regime, pressure drop, holdups of various phases, etc.) and transport (e.g., degree of backmixing in each phase, gas-liquid, liquid-solid mass transfer, fluid-reactor wall heat transfer, etc.) parameters. During the past decade, extensive research efforts have been made in order to improve our know-how in these areas. Chapters 6 to 8 present a unified review of the reported studies on these aspects for a variety of fixed bed columns (i.e., co-current downflow, co-current upflow, and counter-current flow). Chapter 9 presents a similar survey for three-phase fluidized columns. 

D. C. Groeneveld and W. W. Yousef, Spacing Devices for Nuclear Fuel Bundles A Survey of Their Effect on CHF, Post CHF Heat Transfer and Pressure Drop, Proc. ANSIASMEINRC Information Topical Meeting on Nuclear Reactor Themtal-Hydraulics, Nuclear Regulatory Commission/CP-0014 (2) 1111-1130,1980. 

T. J. Rabas and J. Taborek, Survey of Turbulent Forced-Convection Heat Transfer and Pressure Drop Characteristics of Low-Finned Tube Banks in Cross Flow, Heat Transfer Eng., Vol. 8, No. 2, pp. 49-62,1987. 

FIGURE 22.3 Survey of the experimental results for pressure drop as a function of superficial gas velocity. 

Down-hole pressure fluctuations, due to the termination of flow in the annulus, are unquestionably of great concern. When circulation is stopped, during a connection or survey, for example, the foam can break-back to soapy water and its gas phase. The gas can then expand toward the surface. In order to re-establish circulation of foam, the "broken-back water in the bottom of the well must be displaced by newly introduced foam through the drill pipe. The BHP increases until the fluid begins to move up the wellbore. As the fluid is lifted out of the well, the BHP decreases, subjecting the formation to a dramatic pressure drop. In pressure sensitive formations this may have an unfavorable effect on borehole stability. In these cases, special additives can be used to create exceptionally stable foams. These foams are many times referred to as "stiff, or "stable, foams. 

I then met with the day-shift foreman on the unit, and we performed a pressure survey on the unit. The pressure survey revealed the location of the problem. The pressure drop across the lube side of the coker fin... 

The cheapest way to debottleneck a process operation that is limited by a pump is to reduce downstream pressure drop. A detailed hydraulic survey is the key. Do not forget to check the pressure drop across the downstream control valve (see Chapter 9). 

While X-ray pictures of tower internals easily detect most types of tray damage, this can be a cumbersome and expensive troubleshooting procedure. A simpler way to obtain almost the same information is by a pressure survey. Tower pressure drops of less than 1 in. of water per tray typically indicate tray damage, assuming a tray spacing of 24 in. 

The ability to run a complete performance test requires experience and preparation. For a first performance test, you should simply obtain a complete unit pressure profile. This is done with a single pressure gauge, which you move downstream from point to point. Obtain pressure drops across heat exchangers and control valves. Then, obtain a complete temperature survey. Where no thermowells exist, use a glass thermometer inserted under the pipe insulation. Next, record all flow rates. You will probably find many flow inconsistencies. Finally, obtain a complete set of samples. Learn how and where the samples are taken (see Chapter 27). 

Pressure surveys are taken to find pressure drops (AP) due to friction and not to determine an absolute pressure. Therefore, consistency is more important than accuracy. To obtain consistency in liquid-filled systems, the measured pressures must be corrected for differences in elevation. This is done as follows ... 

Continuing the pressure survey, next I measured the pressure drop across the bottom three trays used to steam strip light gas oil from resid. I found that the AP was 0.81 psi. The tray spacing was 18 in. and the resid had a 0.79 sp gr. Therefore ... 

Take a complete pressure survey just after the unit comes onstream after a turnaround. This will give you a base point from which to judge future problems. When comparing pressure drops at different throughputs, normalize the data by ... 

When a seal leg plugs, liquid sulfur backs up in the condenser. This restricts gas flow and results in high pressure drop. Again, the best indication of this problem is a routine pressure survey. Having determined that a condenser has excessive pressure drop ... 

The key tool in troubleshooting flash-zone pressure problems is a vacuum-tower pressure survey. The time to initiate this survey is just after start-up when the trays, demister, and ejector system are clean and in good condition. Pressures are best measured with a portable mercury-filled vacuum manometer. Using a vacuum pressure gauge will reduce the accuracy of observed pressure drops. Relying on permanently installed gauges for pressure drop data will not give reliable results. 

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