Backpressure, increased

For a conventional spring-operated relief, the valve opens based on the pressure drop across the valve seat that is, the set pressure is proportional to the pressure drop across the seat. Thus, if the backpressure downstream of the valve increases, the set pressure will increase and the valve may not open at the correct pressure. In addition, the flow through the conventional relief is proportional to the difference in pressure across the seat. The flow through the relief, therefore, is reduced as the backpressure increases. 

Although HPLC column technology is considered to be a mature field now, improvements and new developments are being made continuously in the stationary phases. One of the improvements has been the reduction in particle sizes. Smaller particles help to improve mass transfer and provide better efficiency. Manufacturers are producing particles down to 1.5 J,m in diameter, although 3- and 5- J,m particles are still the most popular. Because of the smaller particle sizes, the backpressure increases proportionally to the inverse of the square of the particle size. Most commercially available HPLC systems cannot accommodate the pressures required to operate these columns at optimum flow rates. This has led to the introduction of systems that run at high pressures. 

Equation 1.3 explains one reason for the slow increase in backpressure associated with column aging. Silica particles liacture under the continual pressure swings from high pressure when in use to no pressure when not used (i.e., compression and decompression). The small fragments that are framed, called fines, collect in the interstitial column flow paths. This process blocks the available flow paths and, just as importantly, the presence of fines in the column decreases the average both processes lead to increased operating backpressure. Increased colunm backpressure also occurs when components precipitate on the packing matmal or in the pores in the inlet frit. 

The backpressure is represented by the straight lines labeled minimum, normal and maximum. Only one capacity curve is shown since the increase in capacity resuldng from the lower steam pressure is negligible [4]. 

First and foremost these venting ducts should be as straight as possible, with few, if any elbows, and even these should be sw eeping bends. There should be no valves of any type to keep flow resistance as low as possible, as this creates friction that creates backpressure on the relief de ice and raises burst conditions, which can be terribly dangerous. Figure 7-66 and 7-67 are used to assess the increased pressure due to ducts on relief discharge as affected by duct length. (This data is limited. See Ref. [56 and 53].)... 

The problem is eliminated by restricting the discharge air flow from the blowers. By increasing the backpressure, the blowers are able to operate within their normal envelope and the shaft deflection disappears. 

Sometimes insufficient differential across the regenerated catalyst slide valve is not due to inadequate pressure buildup upstream of the valve, but rather due to an increase in pressure downstream of the slide valve. Possible causes of this increased backpressure are an excessive pressure drop in the Y or J-bend section, riser, reactor cyclones, reactor overhead vapor line, main fractionator, and/or the main fractionator overhead condensing/cooling system. 

Some engineers believe that hydraulic conductivity tests on compacted clay soil should be performed on fully saturated soils in an attempt to measure the highest possible hydraulic conductivity. Most, if not all, of the gas can be eliminated from laboratory hydraulic conductivity tests by backpressure saturation of the soil. This technique pressurizes the water inside the soil, compressing the gas and dissolving it in the water. Increasing the backpressure will increase the degree of water saturation and reduce the amount of air, thereby increasing hydraulic conductivity.5,21... 

Several commercial HPLC manufacturers addressed this issue by increasing the operation ranges of their equipment either by increasing the maximum allowed backpressure and/or increasing the... 

Both LC modules and columns must withstand higher generated backpressures. Table 3.3 presents an overview of commercially available columns for use with particle sizes below 2.0 /.an. Wu17 cites additional manufacturers and the numbers increase continuously. According to Kofmann et al.,18... 

Monolithic column — The trend to use shorter columns in liquid chromatography means that the resultant lower separation efficiency is of concern. One way to improve HPLC separation efficiency on a shorter column is to reduce the size of the packing material, but at the cost of increased backpressure. Another approach to improve performance is increasing permeability with a monolithic column. Such a column consists of one solid piece with interconnected skeletons and flow paths. The single silica rod has abimodal pore structure with macropores for through-pore flow and mesopores for nanopores within a silica rod8182 (Figure 12.1). 

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