Overhead systems

Various types of overhead and underground distribution systems may be used depending on local conditions. Generally, an overhead system will incur only about 30 percent of the cost of an underground distribution system. 

Railway electric traction systems use either ac or direct current (dc). Third rail systems operate on dc at 600 to 700 V. Overhead systems can be found with either 1,500 or 3,000 V dc, or using ac at a range of voltages (from 11,000 to 50,000) and frequencies (16 2/3 to 60 Hz). The modern standard for overhead catenai y systems is usually 25,000 V and cither 50 nr 60 Hz depending on the industrial standard for the country (50 Hz for most of Europe, 60 Hz for the United States and Japan). 

Improving overhead cooling will increase the wet gas compressor capacity. Excessive pressure drop or limited cooling in the overhead system decreases the capacity. This can result from ... 

Fig. 9.4 Corrosion control in the overheads system of a crude distillation unit...

In this approach accident cases and design recommendations can be analysed level by level. In the database the knowledge of known processes is divided into categories of process, subprocess, system, subsystem, equipment and detail (Fig. 6). Process is an independent processing unit (e.g. hydrogenation unit). Subprocess is an independent part of a process such as reactor or separation section. System is an independent part of a subprocess such as a distillation column with its all auxiliary systems. Subsystem is a functional part of a system such as a reactor heat recovery system or a column overhead system including their control systems. Equipment is an unit operation or an unit process such as a heat exchanger, a reactor or a distillation column. Detail is an item in a pipe or a piece of equipment (e.g. a tray in a column, a control valve in a pipe). 

The pressure balance should be examined to determine the normal pressure readings in the reactor, regenerator, air system, flue gas system, and main fractionator and overhead system. These need to be followed on a time basis and plotted against variables such as feed rate, wet gas rate, and dry gas rate to see if and where problems may occur. Adjustments may be possible if the spent or regenerated catalyst slide valve delta P is at a minimum to provide more operating room. 

Fractionation Section. A typical fractionation section includes the coker fractionator and attendant heat exchange equipment, the light gas oil side stream stripper and the overhead system. The coke drum overhead vapors enter the fractionator under shed trays which are located below conventional wash trays. Hot induced gas oil reflux is pumped to the wash trays to condense recycle and to wash the product vapors. The light and heavy gas oil products are condensed as sidestream products. The light gas oil product is usually steam stripped in a sidestream stripper. The overhead vapors from the fractionator are partially condensed and the gas and gasoline products are directed to the vapor recovery unit. 

When inorganic chlorides (e.g., ammonium chloride) and organic chlorides collect (usually in the tops of columns and equipment connected to the tops of columns), mild to severe corrosion occurs. When organic nitrogen compounds in the feed exceed 0.05 wt% (500 ppm), cyanides and ammonia form. These compounds collect in the aqueous phases and cause corrosion of certain materials. The corrosion problems in the aqueous phases are discussed in the following section on overhead systems, in the Chapter Two section on characteristics of sour water, and in the Chapter Three section on coolers. 

Figure 1.3 shows a single drum overhead system. Double drum systems are also used. The difference between the two systems is the reflux temperature at the top of the tower. In the single drum system, total liquid condensation occurs in the overhead condensers. The reflux will be cool and will keep the tower top cool. It is advisable to check the hydrochloric acid dew point vs partial pressure to determine the anticipated location of corrosion. For example, tower top temperatures above 250° F (120°C) can transfer corrosion to the cold reflux. Where dew point conditions exist in the tower, It may be desirable to add ammonia to the reflux to neutralize the acid. 

Removal of salts from the crude is important to avoid corrosion and plugging of the overhead system. Any plugging is normally caused by ammonium chloride. Salt wateV in crude is usually similar to ocean water. Sodium chloride, which is quite soluble in hot water, is easily removed. Magnesium chloride,... 

The main corrosion problems in a nonacidified condensing sour water stripper occur in the overhead system. Exchanger tubes in the overhead condenser are often made of commercially pure titanium. The reflux pump is often alloy 20 (CN7M). In very corrosive waters, such as those containing phenols or large quantities of salts, Hastelloy1141 C276 is used. Two API surveys (API Standards 944 and 950) concluded that the location and severity of corrosion vary with the type of unit as follows ... 

The latent heat of the polymer solution leaving the reactor is used to flash off the solvent, unconverted ethylene and light comonomers in flash vessel (2). Vapors from this vessel are condensed in an overhead-system (3) and recycled back to the reactor feed without purification. 

Cooling water failure. The loss of cooling water is one of the more commonly encountered causes of overpressurization. Two examples of the critical consequences of this event are the loss of condensing duty in column overhead systems and the loss of cooling for compressor seals and lube oil systems. Different scenarios should be considered for this event, depending on whether the failure affects a single piece of equipment (or process unit) or is plantwide. 

The refiner s position on inorganic solids and salt removal is that as much of these contaminants (as is cost-effective) should be removed from the incoming crude oil into the wash water by the desalting process. Excess chloride salts become catalyst poisons that promote excessive catalyst consumption or reduce conversion in the cracking and treating processes. Chloride salts also compromise the reliability of refinery overheads where, because of hydrolysis upon heating, they form highly corrosive hydrochloric acid in the overhead system. (The refinery overhead is the equipment, such... 

What happens if water in the drum starts to go over with the hydrocarbon vapors into the plant vapor recovery system Add two motor operated valves to the overhead system and a water detector sensor. When the drum starts to fill, the detector will sense water vapor trying to go overhead with the hydrocarbon vapors. It will close the hydrocarbon overhead line to vapor recovery and open the other motor operated valve discharging hydrocarbon vapors and water to a blowdown drum. 

The overhead system for a distillation column is as shown below. The composition of the total distillates is indicated, with I0mole% of it being taken as vapor. Determine the pressure in the reflux drum, if the temperature is 100°F. Use the fC-values given below at any other pressure by assuming that K is inversely proportional to pressure. 

Relief valves, bursting disks, and major vents are best located at the top of superatmospheric columns (9, 45) (or in their overhead system), upstream of the condenser (location A, Fig. 9.3). The converse applies to vacuum services, where the vacuum-breaking device should normally he installed at the bottom of the column (192, 207). 

LPG Lean oil stripper Column was pressured up throu a connection in the overhead system while liquid circulated throu its valve tr. The gas could not travel downward, causing mechanical damage to top 12 trays. This later resulted in premature flooding. Always pressure columns fn>m the bottom up, espedaUy when column contains valve trays. 

Dqnxipanizer overhead went to an HF strapper. Strqiper bottoms was the propane product, while stripper overhead was recycled to the depropanizer overhead. When ethane entered the depropanizer due to an upstream imit upset, it entnqtped in the overhead system and could not get out. Dqtrqnnizer pressure climbed and excessive venting was needed. 

Additional studies in progress include a survey of water gas shift and cracking catalysts, one pass steam effect (no pyrolysis recycle) and a survey of various feedstocks. Commercial water gas shift catalysts are limited to a maximum temperature of about 900°F and thus are not appropriate for fluidization at the temperatures under investigation. A fixed bed in the overhead system however with steam feed significantly altered the product composition (Table IV) as predicted ... 

Pluggng of overhead system top temperature not within the operating window for sour water strippers temperature < 82 °C at which ammonium polysulfides form but temperatures too high give excessive water in overhead vapor causing problems for downstream operation/overhead lines not insulated/insuflicient steam tracing on overhead vapor lines. 

One example of a single variable control strategy is applied to a stripper in a hydrocracking unit. The main purpose of the stripper is to remove H2S and noncondensable components from the bottom product. One of the key indicators identified is the water dew point at the top of the stripper column. As a matter of fact, the dew point is a function of column overhead vapor composition and the amount of water. There was no monitoring capability available for the dew point temperature. If the column top temperature is lower than the dew point, the hydrogen sulfide will dissolve in the condensed water and cause corrosion to the column overhead system. 

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