General back pressure

In the case of a conventional relief valve, general back pressure shall not exceed 10% of the relief valve set pressure. 

In the case of a balanced-bellows relief valve, the impact of general back pressure depends on the percentage of overpressure and is known as the back-pressure correction factor. For 10% overpressure, the back-pressure correction factor is 1 up to 30% general back pressure. For 16% overpressure, the correction factor does not change (value = 1) up to 37% back pressure, and for 21% overpressure, fhe correction factor does not change (value = 1) up to 50% back pressure. 

The liquid-phase rate coefficient is strongly affected by fast chemical reactions and generally increases with increasing reac tion rate. Indeed, the condition for zero hquid-phase resistance m/k-d) imphes that either the equilibrium back pressure is negligible, or that... 

The set pressure of a conventional valve is affected by back pressure. The spring setting can be adjusted to compensate for constant back pressure. For a variable back pressure of greater than 10% of the set pressure, it is customary to go to the balanced bellows type which can generally tolerate variable back pressure of up to 40% of set pressure. Table 2 gives standard orifice sizes. 

The guidelines are quite general, but will at least act as introduction to the types of turbines available to the process designer. Sometimes the decision on which type turbine to select is not obvious. The back pressure turbine is most frequently selected. It has lower capital cost, simple construction, is the most suitable turbine for high speeds, and is generally more reliable. 

In general, the total back pressure on a balanced bellows pressure relief valve (superimposed plus built-up) should be limited to 50% of set pressure, because of the marked effect of higher back pressures on valve capacity, even when appropriate correction factors are used in sizing. In exceptional cases, such as a balanced bellows PR valve discharging into another vessel, total pressure up to 70% of set pressure may be used. 

If tv o-phase flow situations are not recognized, pressure drop problems may develop which can prevent systems from operating. It requires very little percentage of vapor, generally above 7% to 8%, to establish volumes and flow velocities that must be solved by two-phase flow analysis. The discharge flow through a pressure relief valve on a process reactor is often an important example where two-phase flow exists, and must be recognized for its back pressure impact. 

In general, only the reciprocating compressor allows for reliable flexibility in applying variable volumetric flowrate and variable pressure ratio in an operation. The rotary compressor does not allow for variation in either (except that of pressure through the decompression of the air or gas if the system back pressure is below the design pressure of the machine). The dynamic compressors are designed for specific volumetric flowrates and pressure ratios and are not very useful when these design limits are altered. 

Most pumps are considered constant speed, but the true speed changes with variations in suction pressure and back-pressure caused by restrictions in the discharge piping. The narrowband should have lower and upper limits sufficient to compensate for these speed variations. Generally, the limits should be set at speeds equal to the full-load and no-load ratings of the driver. 

Noncondensing turbines (back-pressure turbines) are widely used by industry to fulfill any number of process requirements. As a general mle, turbine generator design efficiency is reduced by the absence of a... 

The back-pressure drive flame propagation theory has been extended to a general case of variable burning... 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

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