Showing posts with label Pores. Show all posts
Showing posts with label Pores. Show all posts

June 18, 2009

Permeability of Refractory Bricks and Monolithics

Permeability of any refractory material is defined as the volume of a gas or air which will pass through a cubic centimeter of the material under a pressure of 1 cm of water per second. Permeability is calculated by the following formula:

Permeability = (Vol. of gas/air x thickness) ÷ (Area x Time of flow x Pressure difference)

It is determined by forcing a known volume air or gas through a cube. Time of flow, pressure difference and dimensions of specimens are noted. However, there are ready-made apparatuses and systems available in the market for testing permeability of refractory bricks and monolithics. There is no direct dependence permeability on porosity however, permeability depends upon the existence of closed pores or channel pores and is a measure of these, whereas porosity measures the total pore volume including closed pores.

Permeability for refractory bricks or monolithics is important, wherever molten liquid like metal, slag, glass etc। are in contact with the refractory or when gas under pressure is present. Due to the anisotropic nature of refractories, the result will depend upon on factors such as the direction of flow and presence or absence of the original skin on the test specimen. Low permeability is more important than low porosity from the point of view of slag resistance of the refractory. Uniform permeability is an indication of absence of cracks in the refractory.

Related Articles:

Types of Testing of Refractories

Apparent Porosity and True Porosity of Refractory Samples

May 25, 2009

Pore Size Distribution in Refractories

It is highly essential to measure pore size and its distribution to get the desired optimum properties in any refractory brick. Actually ‘pore size’ and ‘pore size distribution’ are two different things. First, let us discuss the difference between these two terms. The ‘pore size’ is a measure of the diameter of the largest pore whereas; the ‘pore size distribution’ is a measure of the range of pore sizes. The range of pore sizes can be normally distributed, and the spread can be quite narrow (e.g. the ratio of largest to smallest may be less than 2). On the other hand, pore size distribution can be very heterogeneous. In the case of large spreads and heterogeneity, which is not desired, the pore size will be far less predictive.

It is measured by analytical instrument known as Porosimeter based on mercury intrusion and capillary flow methods. The volume of mercury penetrating the pores measured directly as function of applied pressure. Pore Size information is best described graphically, rather than with a single number. An alternative method of describing the distribution curve is to use a collection of values that better describe the distribution curve in more detail, i.e., upper limit, lower limit, mean, standard distribution, etc. This P-V information serves as unique characterization of pore structure. This data together with Microstructural analysis and few other test reports of the refractory sample can be of great help in optimizing the properties as well as for any failure analysis.

Related Articles:

Types of Testing of Refractories [Read]

Apparent Porosity and True Porosity of Refractory Samples [Read]

Manipulating the Test Results of Apparent Porosity (AP) During Testing of Refractory Bricks [Read]