Economiser, some FAQs

Technical, ,

Guide for better economiser design

Is it essential to pre heat feed water?

How one preheats they boiler feed water? We have two options,

  • Via economiser, mounted on exhaust of boiler, and heating incoming feed water by absorbing lost heat in flue gases
  • Via Deareator tank, adding steam to deareator and heating the water temperature. Though the purpose of deareator is as name suggest remove the “oxygen” from the water, by heating it. Nevertheless it also increases the temperature of water in this process.

Let’s talk about economiser

Now that’s the question, one should not ask, if possible, one should always use economiser. It generally speaking will increase boiler efficiency by 3-5 percentage points.

It also helps reduce the thermal stress on boiler as a whole.

Then why this question?

Challenges with economisers are few, apart from how much heat we are recovering vs how much expenditure we are doing in is installation, maintenance and qualification.

Assuming the economics works well, the next question is if you absorb too much heat, and condensation happens, it will lead to sulphur corrosion, distorting not only boiler but also chimney in long run.

Criteria for designing economiser from user perspective

  • Define maximum hot water temperature we expect out of economiser, this has to be above sulphur due point
  • Define maximum pressure drop allowed. As more pressure drop will add more duty to the Blower, adding to running energy cost
Acid Dewpoint Corrosion

Criteria for design from Designer’s perspective

  • Configuration of path, with multiple branches to make water flow, it helps get maximum efficiency but also adds to pressure drop and duty on feed pump
  • Profile of fins for heat exchanger, some profiles of fins, for example serrated fins, has huge heat transfer to area ratio, and are highly efficient, but also adds to pressure drop on air side, and blower duty increases accordingly.
  • Number of passes of hot flue gases, this ensures more residential time for the flue gas, so that we can extract more heat, however, this will also cost us more pressure drop and added duty to the blower.
  • Minimum feed water temperature and maximum outlet temperature, both factors are important to ensure we get better efficiency but also to ensure we have less issues with corrosion (oxygen pitting

Typical failure reasons

Typically in boiler, the flue gas is mostly utilised to super heat the steam, and then passed to economiser, in such cases, The temperature differentials between the flue gas and water are quite low. To maximize heat transfer, water temperatures at the end of the economizer run should be very close to the saturation temperature. If the temperature difference is very low, then it some time can lead to steaming in economiser. Steaming not only reduces the efficiency drastically but also lead to knocking and failure of weld during operation due to thermal impact.

Second reason for failure could be quenching effect. This happens, when boiler is stand by mode, that is steam is not consumed. Water in economiser reduces drastically, and sudden surge in demand, make feed pump, pump cold water to economiser, leading to thermal stresses, even if the delta of water differential is low, this will lead to failure after some time. This issue can be over come by losing some water in boiler itself via intermediate blow down, small water circulation in economiser helps avoid such quenching issues.

Orbital Welding for Sanitary Piping

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Orbital Welding for Sanitary Piping

Orbital welding in Sanitary application is extension to tungsten inert gas (TIG/TGAW) welding. This type of welding is default in piping, which the application demands Sanitary or super clean application, where cleaning is done with CIP/SIP.
The pharmaceutical industry currently uses orbital GTAW/TIG welding almost exclusively. This produces welds of high quality with very low rejection percentages; these joints possess high strength, high purity meta, and good surface finish.
Orbital welding is the controlled rotation of components within a fixed support, while an adjustable, non-consumable tungsten electrode attached to a guide moves (or “orbits”) the joint. The electrode, the arc, the area surrounding the weld, and tube interior are protected by a shield of inert gas—usually argon—with a purity of 99.995/99.999% 
Virtually all the metal alloys employed in the pipeline fabrication sector can be welded and since the process is carried out in an inert atmosphere it produces results that are extremely clean, oxide free and without spatter

Its completely automated process and hence needed precision when preparing the face and edges before welding.

Following mind map will help you understand variable in Orbital welding.
Please Explore. Will add more information shortly.

Non Destructive Testing of Welding

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So, we have covered Destructive testing in my last post, now something on Non-destructive testing.

There are Numerous Non-Destructive tests used to evaluate the base metal to be joined as well as completed welds. However these all NDT shares several common elements, these essential elements are summarized below:

o A Source of Probing energy or Medium
o A Discontinuity must cause change or alteration of probing energy
o A means of detecting this change
o A means of indicating this change
o A means of observing or recording this indication so that an interpretation can made.
Over the years Numerous Non-Destructive Testing Methods have been developed, each one has associated with its various advantage & Limitations.

Followings are the Noted NDT Methods
o Penetrant Test (PT)

o Magnetic Particle Test (MT)

o Radiographic Test (RT)
o Ultrasonic Test (UT)
o Eddy Current Test (ET)   1. Penetrant Testing (PT)

Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a “clean” surface breaking flaw by capillary action. After a period of time called the “dwell,” excess surface

penetrant is removed and a developer applied.This acts as a “blotter.” It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet “black light”.

Detection of Defect using Black-light



Table for Dwell time

2. Magnetic Testing (MT)

Magnetic particle inspection is a nondestructive testing method used for defect detection. MPI is a fast and relatively easy to apply and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.  

      MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective.
     The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component’s fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines


Electromagnetic Yoke Detail Diagram



Electromagnetic Yoke Application



Application of Dry Powder



The Magnetic Field Intensity Measure



Defect Detection in Weld Using MPI (Dry Powder)




Before and after Inspection MPI Detection



3. Radiographic Testing
Covered in detail in my older post

4. Ultrasonic Testing (UT)

Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. To illustrate the general inspection principle, a typical pulse/echo inspection configuration as illustrated below will be used.

 

A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulse. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected signal strength is displayed versus the time from signal generation to when a echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.
cross-section of the Probe


Beam spread occurs because the vibrating particle of the material (through which the wave is traveling) do not always transfer all of their energy in the direction of wave propagation. Recall that waves propagate through that transfer of energy from one particle to another in the medium. If the particles are not directly aligned in the direction of wave propagation, some of the energy will get transferred off at an angle. (Picture what happens when one ball hits another second ball slightly off center). In the near field constructive and destructive wave interference fill the sound field with fluctuation. At the start of the far field, however, the beam strength is always greatest at the center of the beam and diminishes as it spreads outward.

Testing of Weldments!

7 CommentsTechnical,
I’ve covered, welding and weld defect in past posts, what I missed completely is the testing of Weldments.

One of the purpose of effective quality control program is to determine the suitability of a given base metal or a weld to perform its intended service.

When It comes to testing, we have two type of testing.
1. Destructive/Mechanical Testing &
2. Non Destructive Testing
Destructive Testing/Mechanical Testing :
The term mechanical testing is used to describe a group of test methods for establishing or confirming the mechanical properties of a material or a completed weld. Most of these tests involve sectioning or otherwise destroying some part of the object being tested and thus they are sometimes called destructive tests. The tests are generally classified by the property they are intended to define. Each follows a well-established procedure, which is part of a published standard, allowing individual test results to be compared to other results or statistical norms. This section describes the following mechanical tests, some of which are destructive, that are carried out on welds:

– macroscopic & microscopic examinations
– bend test
– tension test
– hardness test
– charpy vee notch test
– Izod test
– crack tip open displacement test
– nick break test
– chemical test

o Macro Test:
To see the penetration inside the parent material, and to qualify welder & welding procedure, generally a chemical etching is done on the weld specimen, see the image below.

Macro Specimen

o Bend Test :

The bend test is a popular test method that is found in many welding standards and specifications throughout the world due to the simplicity of the test method and equipment required. The history of the bend test dates back to the early years of wrought iron and steel testing before the advent of modern testing equipment. Bend specimens are prepared typically from a test plate rather than from an expensive finished product and are used to evaluate the ductility and soundness of welded joints.

There are two different bend testing methods:

  • Guided bend test
  • Free bend test

 o. Guided Bend Test

The guided bend test is commonly used in welder and procedure qualification tests to determine the ability of the welder to make sound welds. The test is performed by bending prepared specimens of a specific dimension (usually specified in the relevant code) in a special jig. The dimensions of the jig will vary with specimen thickness and material.
It is important to note that the strain applied to the test specimen depends on the spacing of the rollers and the radius of the male member. The strain on the outside fibre of the bend specimen can be approximated from the following formula : e = 100 t / ( 2R + t )
When performing qualification tests the specimen thickness and bend radius are chosen according to the ductility of the metal being tested. An elongation in the outside fibre of 20 percent can be easily achieved on sound mild steel welds. Bend tests will consistently fail if the specimens contain weld discontinuities that are on are near the surface of the material.

After bending, the welds are examined for the presence of discontinuities. Many welding standards and specifications consider that a bend specimen has failed if on examination of the convex surface after bending there is a crack or open defect exceeding 3mm (1/8 in.).  


Passed Bend Specimen

 

Failed Bend Specimen



  

 

 

 

 

 

 

 

 

 

 

There are three types of guided bend tests:
  1. Root bend tests
  2. Face bend tests
  3. Side bend tests

Bend Test Limitations
The same weakness that tensile tests suffer from also affects bend tests. Nonuniform properties along the length of the specimen can cause nonuniform bending. Bend testing is sensitive to the relative strengths of the weld metal, the heat-affected zone, and the base metal.

Many problems can develop in transverse bend tests such as an overmatching weld strength may prevent the weld zone from conforming exactly to the bending die radius, and thus may force the base metal to deform to a smaller radius. This will not produce the desired elongation in the weld. Alternatively, with an under matching weld strength, the specimen may bend in the weld to a radius smaller than the bending die. In this case failure may result when the weld metal ductility is exceeded, and not because the weld metal contained a defect.

These problems with weld strength mismatch can be avoided by using longitudinal bend specimens which have the bend axis perpendicular to the weld axis. In this case all zones of the welded joint (weld, heat affected zone, and base metal ) are strained equally and simultaneously. This test is usually used for the evaluation of joints in dissimilar metals.

Weld discontinuities in longitudinal bend tests that are oriented parallel to the weld axis such as incomplete fusion, inadequate joint penetration, or undercut are only moderately strained and may not cause failure.

o TENSION TEST
Tension tests are performed for the following reasons:

– Test results are used in selecting materials for engineering applications
– Tensile properties are frequently included in the material specifications to ensure quality
– often tensile properties are measured during the development of new materials and processes so that different materials and processes can be compared.
– Tensile properties are often used to predict the behavior of a material under different forms of loading, other than uniaxial tension.   The strength and ductility of metals are generally obtained from a simple uniaxial tension test in which a machined specimen is subjected to an increasing load while simultaneous observations of extension are made. If the loading is continued the specimen will eventually break. A typical stress-strain curve that is produced from a tension test is shown in the diagram.

In a welding application, tension tests involve applying a load to the ends of a standard test specimen and recording the point at which the specimen fails by permanent shape change (yielding) and by fracture. A number of mechanical properties can be determined from a tension test, including the following which are of particular significance in welding:
– yield strength ( the stress at which permanent deformation occurs)
– ultimate strength (the highest stress the material is able to withstand)
– breaking or fracture strength(the stress at which the material fails by breaking)
– ductility (the percentage of elongation or reduction of area of a defined segment of the specimen)

Two specific types of tension test specimens are used extensively in testing welding materials and welded joints. One of these uses specimens taken from the weld material only (all weld metal tests), and the other uses specimens taken across the weld(reduced section tension specimens). The latter specimens are machined so that the smallest dimension of width is in the weld area (reduced section tension test).
– All Weld Metal Test
This test is used to determine the tensile properties of a specimen that consists entirely of weld metal. The test specimen is oriented parallel to the weld axis, and is machined entirely from the weld metal. There are two reasons for performing an all weld metal test:
– to qualify a filler metal or
– determine the properties of the weld metal in a particular weldment.
To qualify a filler metal the melting of the base metal is minimized when making a test weld. This procedure is described in the various filler metal standards. If the purpose of the test is to determine weld metal properties in a particular weldment, then the welding process and procedure used in the actual fabrication should be employed to make the test weld. The following are typically properties that are measured and reported in an all weld metal tension test.
– tensile strength
– yield strength
– elongation

Transverse weld tension test specimen
– reduction of area



All weld metal test specimen



Longitudinal weld tension test
o HARDNESS TESTING

Hardness can be described as the ability of a material to resist permanent or plastic deformation, and is usually measured by its resistance to indentation by an indenter of a standard shape and size.

The hardness test is by far one of the most valuable and the most widely used mechanical test for evaluating the properties of metals as well as certain other materials. In general, an indenter is pressed into the surface of the metal to be tested under a specific load for a definite time interval, and a measurement is made of the size or depth of the indentation.

The main purpose of the hardness test is to determine the suitability of a material, or the particular treatment to which the material has been subjected to.

Hardness testing may be used alone or to complement other test methods. This is what makes the hardness method so popular because of the relationship that exists between hardness and other properties of the material. For instance, both the hardness test and the tension test measure the resistance of a metal to plastic flow. Such correlations are approximate and must be used with caution when applied to welded joints or any metal with a heterogeneous structure.

It should be noted that hardness is not a fundamental property of a material and a hardness value is an arbitrary number. There are no absolute standards of hardness and it has no quantitative value, except in terms of a given load applied in a specified manner for a specified duration and a specified penetrator shape.

Measurements of hardness can provide information about the metallurgical changes caused by welding. For example, in alloy steels a high hardness could indicate the presence of untempered martensite in the weld heat-affected zone, while a low hardness may indicate an over-tempered condition. In cold-worked or age-hardened metal, welding may result in significantly lower heat-affected zone hardness due to recrystallization or over aging.

Hardness testing is divided into two categories: macrohardness and microhardness



Hardness scan – fillet welds



Hardness scan – butt weld



The hardness testing methods in use today for testing metals are: – Brinell
– Rockwell
– Vickers
– Knoop   o CHARPY IMPACT TEST
The Charpy vee-notch impact test is the most common fracture toughness test used by industry. A notched specimen is broken by a swinging pendulum and the amount of energy required to break the specimen is recorded in foot-pounds or joules. This is determined by measuring how far the pendulum swings upwards after it fractures the specimen. If the specimen is tough, the pendulum will only swing up a small distance since part of its energy has been absorbed by the specimen. If the specimen is brittle it will absorb little energy thus allowing the pendulum to swing up to almost its original height


Charpy Impact Testing Machine

The amount of energy absorbed can be read directly off of the dial indicator that is located on the machine.
The specimen is supported in place as shown and the pendulum strikes it from behind the notch.  


Charpy vee-notch specimen holder

This puts the notch in tension, causing the specimen to fracture. The dimensions of the specimen are shown in the next diagram. In some cases sub size specimens may be used when the material thickness is to small to accommodate the full size specimens. It is extremely important that the specimen is machined to the tolerances and finishes specified (eg ASTM E23 Standard Methods For Notched Bar Impact Testing Of Metallic Materials).


Charpy vee-notch specimen dimensions

 Metals such as carbon and low alloy steels, exhibit a change in failure mode with decreasing temperature. For this reason, it is common to conduct impact tests over a range of specimen temperatures. The performance of the material at different temperatures can be observed and a conclusion made regarding the temperature below which the material can no longer be used without a risk of brittle fracture. The graph shows the relationship between test specimen temperature and absorbed energy

Impact energy vs temperature

The absorbed energy is the most common value reported, however, the percent shear and the lateral expansion may also be noted. Metals that exhibit a high Charpy vee notch value are typically those that are more resistant to brittle fracture. It is important to remember that these tests are comparative only and are no guarantee of ductile behaviour in actual service.
The fractured ends of a specimen often reveal the manner in which it fractured. If the specimen has fractured in a brittle manner with low energy the faces will have a flat, crystalline and shiny surface. A tough specimen will exhibit more deformation and will have a dull and fibrous surface.

Fractured charpy vee-notch specimen

o IZOD IMPACT TEST

The Izod test is another form of impact testing. It also involves the use of a vee notched specimen and a machine to deliver an impact blow to the specimen. Testing is generally carried out with the specimens at room temperature since the time required to accurately place it in the machine allows its temperature to increase. This can introduce a significant error when conducting tests at various temperatures.

The positioning of the specimen within the testing machine is critical. Unlike the Charpy specimen, the Izod specimen is held rigidly in a vice type fixture with the notched side facing the direction of impact. The centerline of the notch must be in the plane of the vice top within .125 mm. Once the specimen is in place the hammer is released from a preset height and allowed to strike the specimen thus fracturing it at the vee notch

Izod specimen set up

Izod impact specimen dimensions