Gas cushion (with reverse acting rupture discs) – When using reverse acting rupture discs it must be ensured that there is always a gas cushion between the bursting disc and the medium to be secured. Gases are compressible when pressurised whereby they store a certain energy. This energy is required in order to ensure a perfect opening of the bursting disc. Reverse acting rupture discs are torn open by the impulse generated when the burst membrane turns. The energy of the impulse is taken from the compressed gas. For pure liquid applications this results in the burst membrane being turned (or deformed); however, this does not ensure opening. If the burst membrane of a reverse acting rupture disc turns but does not open the bursting pressure of this (damaged) reverse acting rupture disc will increase significantly. As a consequence, the safety function of the bursting disc device is definitively no longer available. Therefore it must be accurately checked during each single installation whether there is a gas cushion – or not.
Gas density – The density of a gas in the prevailing operating conditions (temperature, pressure). It is of decisive importance when designing/dimensioning a demister as this parameter crucially influences the result of the calculations.
Gas exchange area – Gas exchange is a physical process whereby gases (sometimes separated by a permeable membrane, sometimes by openings or pores) are spatially redistributed between two compartments, ideally until the same concentration is everywhere. Gas exchange is also referred to for gases which are physically dissolved in liquids. As large a membrane surface as possible favours gas exchange. For static mixers from STRIKO process technology this means that the bubble size of gases has a not insignificant influence on the solubility in liquids. The bubble size of a gas to be dissolved in a liquid affects the solubility of the same. The gas exchange area increases with decreasing bubble size which, in turn, increases the potential of dissolving a gas in a liquid.
Goodness of mixture – The goodness of mixture is a measure of the homogeneity or uniformity of a mix and is calculated from statistic base items. The coefficient of variation is the most commonly used measure. The closer this value approximates 0 the more uniform the mix. For visualisation, it is subtracted from 1 and specified in percent. Thus, 100 percent goodness of mixture (or coefficient of variation = 0) refers to the best goodness of mixture possible which, however, is practically not achievable.
From a mathematical perspective, the coefficient of variation is the quotient from the standard deviation of the chemical composition of samples from the mixing chamber, the arithmetic mean of the samples. With static mixers, the mixing chamber is the cross-section of the mixing tube with an infinitesimally small length. The value can thus be interpreted through the mixer cross-section as relative error of the target composition. At a goodness of mixture of 95 percent (coefficient of variation = 0.05; often referred to as technical homogeneity) – as known from stochastics – approximately 68 percent of all samples would lie in a range of +/- 5 percent of the target composition. Already 96 percent would lie in the range of +/- 10 percent. This is generally applicable for all normally distributed random experiments.
The chief task in designing a static mixer appropriate for the requirements consists in determining the number of mixing elements of a particular type that must be arranged in sequence to achieve the desired goodness of mixture at an acceptable pressure loss. The goodness of mixture to be aimed at for a particular application may greatly vary. Only a few elements are often already sufficient to reach a very good homogeneity for simple mixing applications where, for example, low-viscosity components, such as water, readily dissolve. Other scenarios require 20 or more elements to achieve acceptable results.
Graphite bursting disc – The advantages of bursting discs made of graphite are their high resistance to corrosion, a good price-performance ratio and an easy assembly. Furthermore, they are available from stock in the common nominal sizes and burst pressures. STRIKO safety bursting discs of the G2 and LPG2 (LP = low pressure) ranges are flat graphite bursting discs for low to medium reaction pressures. The G3A range which is also fitted directly between the flanges is made up of a graphite bursting disc integrated into a steel ring made of stainless steel. The steel ring absorbs increased axial forces during fitting which may occur due to misalignment of the flanges. A special holder is not required. STRIKO graphite bursting discs always open with fragmentation.