Mild Steel MG Cylinders By: D K Singhal Deveshklsinghal@rediffmail.com
Mild Steel MG Cylinders • Mild Steel MG Cylinders. • Mechanical Design Considerations for Mild Steel MG Cylinders. • Stress Relieving. • Energy Audit of SR Process. • First Grinding. • Related BIS Specifications. • About the Author
To Users... • This software is free for use and distribution to anybody, in any quantity provided its contents are not modified, altered, or tempered. In case the user is interested to add his comments to this presentation, another presentation file may be used. • Author accepts no liability arising due to use of this software, or the the information supplied in this software. • Further suggestions in this area are welcome. These can be mailed at firstname.lastname@example.org.
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:2002 0.01-0.025% • IS: 2062 0.01-0.02% • IS:515 0.015-0.025%
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 9”X9” 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 customer’s 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.
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)