Presentation on theme: "Mild Steel MG Cylinders By: D K Singhal"— Presentation transcript:
Mild Steel MG Cylinders By: D K Singhal
Mild Steel MG Cylinders 1.Mild Steel MG Cylinders. 2.Mechanical Design Considerations for Mild Steel MG Cylinders. 3.Stress Relieving. 4.Energy Audit of SR Process. 5.First Grinding. 6.Related BIS Specifications. 7.About the Author
Mild Steel MG Cylinders
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Why Mild Steel MG Cylinder? Low initial cost. Short delivery periods. Equipment free from blowholes. Easy to grind whenever required.
Is not CI better? Well, CI has some inherent problems related to casting. With a huge job, in case of problem related to uneven cast material properties, blowholes etc., there is no other option to discard a job. Furthermore, as ultrasonic testing is not possible on CI, determination of such defect is normally difficult.
CI has better thermal conductivity! The thermal conductivity of CI is nearly 15% better than that of MS. But during papermaking, other heat transfer resistances do add up in the system resulting in an overall heat transfer coefficient reduced by nearly 5-7% only. This too can be compensated with reduced shell plate thickness to some extent.
CI is more hard than MS! Yes. From material properties it appears that CI is harder and hence we need frequent grinding for MS MG cylinder. But, being a softer material, grinding takes smaller time. Also, well suited doctor blades available now-a-days ensure longer service life between two grindings. Furthermore, today grades of MS are available that give better hardness also.
Who can make MS MG Cylinder? Your friendly machine manufacturer. Well, some manufacturing facilities are must. As all the operations involved can be done easily by a team of skilled fabricators, welders, machinists etc., it is very easy to maintain quality.
Manufacturer Must have good fabrication facilities. Must have good machining facilities. Must have good welding facilities. Must be capable of carrying SR (Stress Relieving)
Ideal Time of Fabrication A higher temperature is generally advisable for good welding. Hence, a summer season is normally ideal for good welding. In winter, external heating of plates to be weld may be done.
Suitable MS for MG Boiler grade mild steel is most suitable for fabrication of MG. It is extremely important that if more than one plates are being used, all must have same properties as well as chemical composition. This is extremely important to have uniform bending, to ensure uniform shell thickness after final grinding.
Testing of Plate Material To ensure the quality of plates, it is necessary to get the plates checked for chemical composition. This can be done by cutting a small piece of each plate and sending to a laboratory performing such tests. The plates should also be tested ultrasonically for any hidden crack, or abnormality.
Chemical Composition Chemical composition of MS plate used for MG fabrication is important as it provides some useful information e.g. thermal and mechanical properties, wear and corrosion resistance, etc. Presence of different elements affects different properties. Some of these are listed in following slides.
Effect of Carbon A higher amount of carbon means marginally better strength and machinability, but poor wear resistance. Typical values for some different materials are as under- IS: % IS: % IS: %
Effect of Sulphur Presence of sulphur imparts strength, but in some cases it results in poor ductability or cracking.It makes steel harder, stronger, but brittle.
Effect of Nickel & Chromium Presence of Nickel imparts strength, and wear resistance, but it results in very hard steel, that is difficult to machine, bend and hence gives improved wear resistance. In Normal grades, nickel is absent or found in negligible amounts. Similar is the case with chromium.
Chemical Composition Chemical composition plays a significant role in physical and strength properties of material. Presence of Ni and Cr imparts wear resistance, though due to hardness, it becomes difficult to bend the plate. Other elements also affect different properties.
Ultrasonic Testing Plates are made in re-rolling mills by pressing red-hot iron Ingots. Sometimes, due to entrapment of air due to some reason, an air film is left between two layers of plate. This is normally called as lamination in plate. The object of ultrasonic testing is to ensure that the plate should be free from such laminating defects.
Plate Lamination During manufacturing of MS plates, hot ingots are pressed together repeatedly to form a uniform layer plate. There exists a possibility that some air is entrapped between two layers, which is not fully removed during rolling operation of the plates. This is called lamination.
Plate Lamination Plate lamination reduces the strength of material marginally, but greatest disadvantage is significant reduction in heat transfer coefficient at the location of lamination due to a bad conductor (Air film) sandwiched between two layers of metal. Thus, laminated spots have lower heat transfer coefficient and thus colder outside surface of MG cylinder, thus paper drying is not uniform. This result in poor quality paper, reduced production, frequent grinding etc.
Conducting UT It is recommended to mark the plate in 9X9 sections with the help of chalk. Now, each section is carefully examined to with approx. 1 outside all boundaries.This is done to ensure that the the plate is completely tested and no part is left untested.
UT Results Normally, if the plate is perfect, there would not be any defect. In some cases there may be defects, particularly at the edges, In many cases, these are small, to the tune of 1-3 inch wide, and several feet long. With such plate, there may be problems of wet edges on paper. In case there is a defect in between the plate, and is more than the plate thickness in dimension, the plate must be discarded, and not used for fabricating MG.
Ultrasonic Tester It is recommended that the ultrasonic testing is done by a government approved ultrasonic tester. Presence of customers representative during ultrasonic test is preferable.
Ensuring Plate Originality Sometimes, plate is procured by the customer and handed over to MG manufacturer for MG fabrication. If customer is interested, he may get some mark punched on the plates to ensure only the plates checked in his presence have been used in fabrication.
During Plate Bending It is important to ensure that the plate bending is done at a very slow rate. Too high a bending speed may cause minor cracks in the plate, which may create problems later on. The possible problems could be- reduced localized heat transfer from MG, under or over drying of paper, frequent requirement of grinding etc.
During Welding Being plates of sufficient thickness, it is necessary to cut a Vee before welding. Welding must be done with suitable electrodes only, and in no case blow holes must remain in the weld. Of course, this can be checked later on using ultrasonic testing, but then it is too late to rectify the problem.
During Welding It is strongly recommended that the welding is done at suitable temperature, and sudden cooling is not done. In winter season, external preheating of area nearby should be done.
Welding Rods The welding rods used must be of same material as that of plates. Rods of reputed manufactures, having correct specification should be used. Welding should be done at specified current only.
Condensate Removal Particularly for slow speed machines, operating under 400 mpm, rotary siphon can be effectively used. The saveall treys should be of sufficient size.
Position of Rotary Joint Rotary joints can be placed at either location- drive or tender side. In some cases mills prefer to have separate rotary joints for steam and condensate.
Condensate Treys in MG Being MG a slow rpm device, installation of condensate trays with siphon pipes is a good arrangement. The trey must be of sufficient size to collect condensate in one rotation of MG, but should not be overdesigned as it may result in imbalance during operation.
Mechanical Design Considerations for Mild Steel MG Cylinders
Shell Plate Thickness Obviously, a higher shell plate thickness means longer operation life, possibilities of using higher steam pressure, but also reduced heat transfer coefficient, and hence reduced productivity at a particular steam pressure.
Shell Plate Thickness Checking of final shell plate thickness can be done by subtracting thickness lost during grinding from original shell plate thickness. Thickness loss is obtained by dividing the weight of peelings during machining and grinding by density of plate material and by area of shell plate.
Shell Plate A uniform shell plate thickness is must in order to get uniform bending force application during drum fabrication. If more than one plates are being used for shell fabrication, it is extremely important that the all plates are of same chemical composition as well as mechanical properties. To ensure this, use of shell plates from a single manufacturer and with same batch number (or heat number) should be used.
Steam Entry Normally, steam entry and condensate outlet is provided at the same side, but it can be made at different ends. The machine configuration (Right hand drive or Left hand drive) and the operators ease are the most important while deciding these factors. Yet, many papermakers prefer tender side steam entry as well as condensate removal.
Condensate Removal On slow speed MG cylinders, scooping is the most efficient method for condensate collection. This becomes also the only method available as position of tie rods does not allow fixed siphons to be mounted. Since sufficient information is not available on design of MS cylinders for higher speeds, presently, we can use only the scoops for condensate collection and use MG cylinder for slow speed operation only.
Tie Rods Dish ends face a significant load of few hundred tons towards outside during operation. This load tries to deform the dish end towards outside at the center, resulting in increase in cylinder diameter at both ends. This results in camber disturbance locally at the both ends. For the same, tie rods are used to place the dish ends in position.
Diameter and Number of Tie Rods Due to huge loads, tie rods must be adequately designed. There could be 6, 8, 12, or 16 tie rods as per requirement. A normal rule of thumb is that the total cross section area (in square centimeter) of tie rods must be at least 1.5 times the outward force on each dish end(calculated as multiplication of pressure inside MG and area of dish end) in metric tons.
Manhole Usually, one manhole is provided in MG for access to internal parts for inspection or repair. In some cases, two manholes may be provided. Manhole should be designed to provide easy access of maintenance personnel to MG as well as a being totally leak proof.
Inside Surface Inside surface is important due to two reasons- 1.Uneven surface means variations in thickness, and hence in heat transfer coefficient and hence results in uneven drying of paper. 2.In case, there is pitting on inside surface, these pits wear off rapidly, and pose problems related to poor runnability of paper.
Defects on Inside Surface Pitting is first defect, as explained earlier. This results in abnormal and non-uniform drying of paper Thickness variation across circumference is another defect caused by ovality in MG during manufacturing due to which, during machining and grinding, different thickness of material are removed from different circumferential locations. This results in moisture variations in paper across machine direction, relativerly lower machine speeds, improper condensate removal etc.
Drain Plug Drain plug is provided opposite to manhole. To purpose is to easily drain the condensate in case there is water filling due to poor steam trap, rotary joint or siphon pipe operation. Drain plug may be plugged with an ordinary mild steel plug, tested under operating conditions, or with a fusible plug, which will fuse off in case of elevated temperature or a safety valve set to bleed steam in case of increased pressure. However, since pressure inside MG can never exceed the inlet line pressure, putting a safety valve at MG inlet would be sufficient to serve the purpose.
Journals In case journals are to be fitted by shrink fitting/ or press fitting, these can be of EN-8 or EN-9. But, if these are to be welded with dish ends and ribs, mild steel should be used. Due to a large size of MG, it is often not possible to machine the journals to make bearing seats after mounting on MG, these must be pre-machined. Extra care should be taken to ensure that the journals are mounted properly in position as inaccurate fitting would finally result in extra machining requirement for the shell plate during grinding as well as uneven thickness of MG across circumference.
Journal Fitting Being MG a low RPM rotating equipment, mild steel journals are normally used. These provide an additional advantage of being welded with the dish ends, and mounting is very easy. It is strongly recommended that before welding of the journal, position of the same is to be checked with specially designed J hook shaped tool.
Dish Ends Dish ends should have sufficient strength to withstand the load applied to these due to internal pressure. Typically, the force on an end is of the order of 500 Tons.To avoid bending of dish ends due to such pressures, tie rods are used. Use of external ribs on dish ends also helps in preventing bending of dish ends.
Heat Treatment Following are the reasons for heat treatment: –To relieve the effects of strain hardening –To acquire the desired strength and toughness in the finished product –To soften before shaping
Heat Treatment Process –Heat the metal to a temperature (Heating Phase) –Hold at that temperature (Soaking Phase) –Slowly cool (Cooling Phase)
Purpose –Reduce hardness and brittleness –Alter the microstructure for a special property (Better surface smoothness) –Soften the metal for better machinability –Recrystallize cold worked (strain hardened) metals –Relieve induced residual stresses
Effect of Stress Relieving Stress relieving is necessary, as the process of MG fabrication involves bending, which is never completely done by the available techniques. If you cut the shall of a newly formed and welded shell it will split open in following form-
Effect of Stress Relieving In this way, the fabricated job remains under some stress for a long time. This stress is reduced automatically, by a process called CREEP. Under this, when any object is kept under stress for a long time (years or decades), the stress reduces with time; the time reduces with increase in temperature.
Objects under Stress Unfortunately, MG is made of different plates, which may come under different levels of stress during fabrication, and thus stress reduction by creep may not be a uniform process throughout MG surface during routine operation. That is why, SS coating is normally not recommended on new MG cylinders without stress relieving.
When should SR be done? During fabrication. Most common practice is to carry out SR after MG is fabricated but before machining. A better option is a double SR, one of the bare shell duly rounded and welded, to remove any fabrication and welding stresses. Afterwards, rerolling should be done to make the shell more uniform.
When should SR be done? After SR rolling stresses and welding stresses are removed, and during further rolling, minor stresses are developed. Then we may put dish ends in position, fix journals, ribs on dish end, siphon pipes, treys etc., and then finally carry out SR. This SR will result in minor shape variations that can easily be corrected during machining.
From where the energy required for SR comes? For SR temperature of the object is raised to a certain value. This requires a huge amount of heat. This heat is achieved by following sources- Electricity Liquid Fuel (HSD, LDO, FO etc.) Solid Fuel (Wood, Coal Etc.) Gaseous Fuel (LPG, CNG, Methane etc.)
How to check whether SR is being properly done? There are different methods available for the same- Direct method: in this temperature inside the furnace is measured by calibrated instruments. Customer may procure his own testing instruments for the same. Indirect Method: In this an audit of the process is conducted and it is evaluated whether the required amount of fuel has been consumed or not. A fuel consumption lesser than the calculated directly indicated that there has been something wrong.
Direct Method Ensure that the thermocouples or RTDs as well as the recording instruments are calibrated by authorized labs. Use of non contact type (Pyrometer type) thermometer is also recommended. Ensure that the thermocouples or RTDs are not placed near the heat source.
Indirect Method Evaluate the furnace and heating procedure and make a detailed audit of the operation. Collect sample of fuel for calorific value determination, and details of furnace for heat capacity of the same. In case of electric fired furnace, either ensure that the energy meter is calibrated, or use a separate portable energy meter.
Energy Audit of SR Process
Objective To ensure that the SR operation is being done correctly. To explore the possibilities of energy conservation and hence cost reduction during SR.
Heating System Heating may be done by- Electricity Liquid Fuel (HSD/LDO/HCR/FO) Solid Fuel (Wood/Coal) Gas (Propane/Methane/LPG/CNG) Hot Air
Heat Required By the Job For any object, the energy required by the job for heating is calculated by the formula- Energy = m c t, where m is the mass of material c is specific heat and t is temperature rise.
Heat Required by Furnace Some amount of heat is also absorbed by the furnace material. The same can also be calculated as given in previous sheet. In case exact mass of furnace is not available, relevant calculations and approximations may be used.
Convection and Radiation Losses Being a small time operation, losses from furnace walls, and small openings, if any can be calculated. For most practical purposes, these can be taken as 1-2% of the energy required by the job per hour at soaking time. Well, these vary substantially, and should be computed properly for accurate observations. More detailed information on convection and radiation losses can be obtained in chemical engineering books.
Stack Losses In case of electrically fired furnaces, there are no stack losses. But, fuels require amounts of oxygen and hence air for combustion. The air requirement varies from 5-6 kg for coal to around 7 kg for wood, and kg for HSD etc. per kg of fuel. The flue gases, equal to the weight of air and fuel leave furnace at a temperature somewhat higher than the prevailing furnace temperature and thus contribute to stack losses.
Quantification of Stack Losses The stack losses can also be calculated as explained earlier. In case specific heat of flue gas is not available, it can be assumed to Stack temperature can be measured using suitable instrumentation, and flue gas weight can be calculated by multiplying stack area by flue gas velocity by density of flue gases at operating conditions. Alternatively, it can be assumed as sum of fuel weight and theoretical air weight, along with some excess air supplied, which can be suitably assumed.
Total Energy Supplied by Fuel Total energy supplied by the fuel is the sum of all of these- –Energy required by the job –Energy required by the furnace –Convection and radiation losses –Stack Losses
Fuel Consumption From the above, the fuel consumption can be calculated as- Fuel Consumed = Total energy supplied by fuel Calorific Value of Fuel
Efficiency of Furnace Efficiency of furnace can be calculated as- Efficiency = Heat given to job/Total heat consumed
Internal Firing Often, some SR vendors suggest an internal firing arrangement for minimizing time required in SR operation, particularly if SR facility is away from the manufacturer site. They suggest use of mineral wool insulation around MG cylinder, and put firing arrangement inside MG, where the fuel (Conventionally the liquid fuel) is fired.
Internal Firing: Typical Data If we consider a sample case of an MG cylinder weighing say 40 tons, heat required to raise its temperature to 620 deg. From initial 20 deg. At a good 75% efficiency would require nearly 1600 kg of HSD. With this, the flue gas would weight Kg, I.e. approx cu.m. at exhaust conditions. If the temperature has to be raised in hours, we require an exhaust of 4500 cu.m. per hour from the manhole of MG. Furthermore, nearly, 1500 cu.m. per hour air has also to be injected simultaneously for combustion.
Internal Firing: Not Possible As the openings in completely built MG cannot allow such huge volumes of air to be passed; with this information, it is not recommended to conduct an SR with internal firing on completely built MG cylinder. However, if found necessary, it is suggested to evaluate the process before taking SR to get it done properly. However, in case only shell is being subjected to SR, internal firing could be used effectively, with reduced fuel consumption, and thus reduced cost.
First Grinding First grinding is often most important. This is due to the fact that no reference points are usually available before proceeding for grinding. Arrangements should also be made during first grinding for the easy and rapid installation of grinding lathe during subsequent grindings.
Precautions in First Grinding To balance MG statically as well as dynamically. To position MG as per journal positions. Begin with initial tool cuts, with minimal thickness loss. To recheck and correct balancing. To conduct final grinding. To check and correct balancing finally.
Imbalance in MG Imbalance in MG is due to inaccurate mounting of journals, inadequate or improper bending of shell plate, deformations caused during stress relieving etc. Imbalance results in decreased drive load on MG section while the high weight section in moving from up to down and vice versa during rotation of the same during grinding or normal operation.
Problems due to Imbalance Due to imbalance, load on the drive varies during every rotation of MG. Due to such fluctuations, particularly if the drive has not well synchronized with the equipment (MG), there exists minor speed variation. Due to this speed variation, we neither get uniform moisture of paper in machine direction, nor get good runnability of the paper.
How to Check Imbalance? In the initial stage, when there could be a good imbalance, requiring say more than 50Kg weight on one side; imbalance can be checked by rotating MG and allowing to stop by itself. If MG stops at a particular position only in few trials, or makes pendulum like motion before stopping, this means that the top most position after MG has stop needs some extra weight to be put on.
Fine Balancing Checking For further measurements, MG is allowed top rotate with drive arrangement at moderate speed, and the load of the drive is recorded. In case of imbalance, load of drive will increase and decrease alternatively in every rotation of MG. The difference between maximum and minimum load can be used to calculate the amount of weight required for MG balancing.
Why should balancing be repeated during first grinding? The first grinding is done on MG, which has a surface that is not a perfect circle. During initial cuts, a significant part of one side may be lost, resulting in weight loss from that side. Thus, the other side becomes heavier, and hence, MG becomes imbalanced. That is why, frequent balancing is a must during initial grinding.
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D K Singhal Basically an M.E. (Pulp & Paper) from Institute of Paper Technology, (University of Roorkee, now IIT, Roorkee), Saharanpur, Did B.E. (Pulp & Paper) from the same institute in Certified Energy Auditor from Bureau of Energy Efficiency (BEE), India. Having about 15 years of experience and author of more than two dozen technical & research papers covering a vide range of topics including waste paper sorting, pulping, waste paper recycling, energy conservation, optimization, chemical recovery, paper mill electricals, power generation, rewinder optimization for trim loss minimization, paper making, process instrumentation & automation, computer simulation, quality monitoring etc. Contact: D K Singhal, Sargam Theatre, Chandpur , Dist. Bijnor Phone: 01345:220140, (O) Fax: 01345: Mobile: