Showing posts with label Inspection. Show all posts
Showing posts with label Inspection. Show all posts

May 6, 2009

Apparent Porosity and True Porosity of Refractory Samples

Porosity is the percentage relationship between the volume of the pore space and the total volume of the refractory sample. Apparent Porosity does not include the volume of the sealed pores. The True Porosity includes the volume of the sealed pores also. The usual difference between the apparent porosity and the true porosity is of the order of 1 to 2 percent unless the proportion of the sealed pores is high. The true porosity figure is the higher than the apparent porosity figure as the true porosity includes the volume of the sealed or closed pores also. The difference between the two values represents the percent volume of closed pores. Porosity can be controlled by the following:

=> By controlling the texture of the brick i.e. by controlling the size distribution of the particles.

=> By the methods of green manufacturing and composition.

=> By controlling the firing temperature, soaking time etc.

=> Quality of raw materials i.e. the inherent grain porosity of the raw materials used.

Higher the porosity, lower will be the strength of the brick. Bricks with lower porosity will have greater resistance to slag attack and more sensitiveness to fluctuations in temperature. Their thermal conductivity will be more.

Hence, apparent porosity is the percentage ratio of the void space in the refractory specimen to the total bulk volume of the same. There are two methods used for the determination of apparent porosity of refractory materials which are:

=> Boiling point method, and

=> Evacuation method.

Samples (Refractory Specimen) measuring 6.5 cm x 6.5 cm x 4 cm is cut from burnt refractory bricks by a cut off wheel from within its core and cleaned any dust or loose particles adhering to its surface and are dried in an oven at 110OC to a constant weight. For graded materials take 3 to 5 mm size grains and dry at 110OC. It can be determined by following the steps given in any of the Standard Methods for Refractory Testing like - ASTM, Indian Standards (IS), Ghost, DIN etc.

True Porosity (%) = {1 - (Apparent Sp. Gr. ÷ True Sp. Gr.)} x 100

Apparent Porosity (%) = {(Soaked Wt - Dry Wt) ÷ (Soaked Wt - Suspended Wt)} x 100

>> Types of Testing of Refractories [Read]

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

April 22, 2009

Types of Testing of Refractories

The durability of refractories is a matter which vitally concerns all manufacturers and users since it is intimately connected with economics and efficiency of the process in they are employed. Refractories are characteristically anisotropic in nature which makes it all the more difficult to judge exactly the durability while it is in use. The furnace designer has to choose the refractories according to the data from different tests available to him. That means in order to fulfill the users or various application requirements and to achieve long service lives, refractory products must be assessed by testing.

The main functions of refractories testing may be classified into three types:
1. Evaluation of new materials before use.
2. Quality control by the manufacturer or user.
3. Post-mortem examination of refractory bricks that behaved differently than the normal.

The testing of refractories can be categorized as:
  1. Non-destructive type.
  2. Destructive type.

A list including both these types of testing is given below:

=> Particle size analysis
=> Refractoriness Under Load (R.U.L)
=> Modulus of Rupture (M.O.R)
=> Modulus of Elasticity (M.O.E)
=> Permanent Linear Change (P.L.C)
=> Reversible Thermal Expansion (R.T.E)
=> Thermal Conductivity
=> Differential Thermal Analysis (DTA)
=> Thermo-gravimetric Analysis (TGA)
=> Spalling Resistance (water quenching and air spalling)
=> Hydration Resistance Test
=> Creep in Compression Test
=> Abrasion Resistance Test
=> Microstructural analysis under Optical Microscope
=> Mineral Phase identification by X-ray Diffractometer (XRD pattern)
=> Chemical Analysis

There are several Standard Methods to carry out the above testing which have been accepted and performed globally. Here one must remember that the sampling of the refractories specimen for testing become very important because of several reasons. Methods of carrying out all the tests given in the above list will be discussed individually in separate posts at this site. To know more about Standard Methods and Sampling read the following -

Standard Methods for Testing of Refractories [Read]
Sampling of the Refractory Specimen for Inspection [Read]

October 31, 2008

Sampling of the Refractory Specimen for Inspection

The sample selection or what is know as Sampling, is an important function of inspection. The inspection procedure starts by selecting, generally a minimum number of units, called a ‘Sample’, from the lot. A ‘Lot’ is meant for the quantity of refractory materials from which sample specimens are drawn for inspection. The test results of these samples make the basis for acceptance or otherwise of the refractory lot. Since Refractories are characteristically anisotropic in nature so instead of drawing only one sample, a predefined number of sample specimens are collected from the lot while sampling. Although there are provisions for ‘Hundred percent inspection’ and ‘Sampling inspection’ but, hundred percent inspections is not preferred generally since most of the tests are destructive type besides, consideration of factors like time consumption and the costs involved.

Sampling inspection of both refractory bricks and monolithic refractories can be done by two methods - (1) Inspection by Attributes, (2) Inspection by Variables. The first method is performed visually by gauging or counting the number of defects and so, is non-destructive and cost effective. The second method is performed by all tests including the destructive ones on Refractories.

Sample size or the number of samples to be drawn from the Refractory lots to be inspected depends on the type of Standard Method being followed for testing. Sampling and their testing are done strictly according to the specified procedures already mentioned in that Method, the results of which should be binding for both the manufacturer and the customer.

What is Standard Method for Testing of Refractories?