Hi all,
Welcome to steam boiler ezines. The aim of this blog is to be a resource center for engineers looking for some good ezines and articles on all aspects of steam boilers.
I’ll be updating this blog frequently, so don’t forget to bookmark and visit often.
I wish you all the best!
Hisham Hashim
Steam Engineer’s Certificate of Competency (1st Grade)
Since my area of expertise is in steam boilers, I will not discuss the hydrostatic test (HT) in unfired pressure vessel or power piping.
Hydrostatic test is a form of NDT (nondestructive test) and is the most preferred method to detect leakage and cracks in boiler under water pressure. Other NDT’s such as magnetic particle test (MPT) or penetrant test (PT) are normally done to confirm the extent of damage after the leak point has been discovered. Only water pressure is allowed for HT. Steam or air is not allowed. “Hydro” itself means water. Water for all practical purposes is incompressible, and the pressure developed by the hydrostatic pump can be relieved instantaneously by releasing very small quantity of water, which would happen on leak occurring. Air is compressible and could be dangerous under pressure. The same goes with steam. Under steaming conditions, any failure of the structure would result in the boiler water flashing into steam, or steam leaks, subjected personnel to a danger of burn.
In Malaysia, the local jurisdiction, the Factories & Machinery Act 1970, stipulates that a boiler shall be hydrostatically tested every 7 years in which the authorized inspector will issue a Certificate of Hydrostatic Test in 6th Schedule, stating the date, test pressure, holding time, result, and authorized safe working pressure. The Certificate is valid for 7 years until the next HT. However, this is only a time frame, and your local jurisdictions may require different interval.
As a preparation for the HT, clean boiler waterside and fireside, nozzle flanges, manhole surfaces, and welding joints. Observe all safety precautions (procedures can be obtained from the eBook). The safety valve must be removed. However, if for some reason the safety valves cannot be removed, a test gag may be used following the safety valves’ manufacturer guidelines. Remove electrical connections such as high pressure limit switches and sensors. Note that when a HT is being applied to a boiler, a calibrated test pressure gage must be connected to the hydrostatic pump. This precision test gage shall have the accuracy of 0.5%. Countercheck it with the calibrated pressure gage mounted on the boiler. Blind the nozzles by installing blind flanges or blind plates to fittings connection such as safety valves, main stop valves, feed nozzles, mombrey connections, and gage glasses. Rubber or normal gaskets will do just fine.
Fill up with treated water (at ambient temperature of 26oC) for the entire volume of the boiler and vent air from the system. Use water hose and fill up through one of the top nozzles until water overflows from the nozzle. Because the boiler may remain standby after the test, it is important that the water in the boiler is treated to prevent corrosion pitting. Consult your local chemical vendor for the treating method. The metal temperature for HT shall not be less than 60oF (16oC) and not more than 120oF (50oC). This is as per requirement in ASME Code AT-352. I have heard an engineer used boiler makeup water from the feed tank at 90oC for the HT just because he wanted to use the feed pump. Don’t do this! If the temperature of the water is above the recommended temperature (exceeds 50oC), the boiler metal is subjected to brittle fracture. This brittle fracture is a phenomenon where rapid crack propagation occurs when temperature decreases under extreme stress condition. Moreover, there is a direct relationship between pressure and temperature. As the temperature drops, the pressure also drops, so you will notice a pressure drop from the pressure gage despite there is no leakage.
Make sure all air is removed before connecting the pressure hose from the hydrostatic-test pump to the boiler venting line. If the water overflows from the vent, shut off the valve tightly. Do not allow air pockets from forming inside the boiler. Do not use feed pump during HT although it is the fastest way to build up pressure. You have to standby an operator at the pump and he must be alert enough to stop the feed pump immediately once the test pressure is attained. Any negligence can be catastrophic, so why take chances? Hydrostatic-test pump or hand pump is preferred as the pressure and throughput can be easily controlled whereas the pumping rate of feed pump is high and could subject the boiler to overpressure if there is no precaution. Operate the hydrostatic pump and increase the pressure at the rate of 1 bar per minute and look at the gage carefully. Raise the pressure up to 1.5 times the maximum allowable working pressure (MWAP). MAWP is another term for design pressure. If the MAWP of the boiler is 2,000 kPa (20 bars), the required test pressure is 2,000 X 1.5 = 3,000 kPa (30 bars). Stop the pump operation at the test pressure and close the ball cock of the pump.
Wipe dry or spray with compressed air at all flanges and manholes to get rid of water residue. Hold the water under static pressure for 30 minutes. Visually examine all areas for leaks and cracks, giving particular attention to welds. I recommend you to not spend longer because besides exerting extra stress to the pressure parts, there shall be no leakage if there is no pressure drop within 30 minutes. In addition, 30 minutes is long enough to satisfy the code requirement. Some engineers maintained HT for only 15 to 20 minutes, which is ok IMO. If there is a leak, mark the location, snap photos showing water shooting out from the crack (important for documentation), and if no pressure drop, the HT is conducted successfully. Release the water via the bottom blowdown connection. As a standard procedure, after completion of any repair involving welding at pressure parts of the boiler, carry out a HT at 1.5 X MAWP again.
After the HT is successful, fill up the HT report, stating testing conditions, accessories, and procedures. Include pressure increase and holding time data in the report. This record must be kept for future reference.
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This article is an answer to the question “Outline the procedures to carry out hydrostatic test” in the 101 Q & A: Practical Knowledge of Power Boilers eBook. If you find this ezine article useful, you can learn more by visiting http://www.boiler-ebook.com. There are many more useful articles than this, and if you are concerned in improving yourself, I can guarantee you will acquire new knowledge, which would be very beneficial to your career. Don’t forget to visit my best collection of ezines at http://steamboiler.wordpress.com Hashim, Hisham. earned a B.S. (Hons) in Mechanical Engineering from Mississippi State University (1993) and holds a 1st Grade Steam Engineer’s Certificate of Competency from Department of Safety & Health (Malaysia) since 1998. Mr. Hisham is a Member of the Institution of Engineers (Malaysia) and had spent 8 years in Palm Oil Mills and 6 years in chemical industries. |
Hashim, Hisham “Nondestructive Examination (NDT) – Hydrostatic Test On Steam Boilers (Fired Pressure Vessels).” Nondestructive Examination (NDT) – Hydrostatic Test On Steam Boilers (Fired Pressure Vessels) EzineArticles.com. http://ezinearticles.com/?id=3648238
With the rising cost of fuel prices, industries that use steam boilers for heating or power generation are hard pressed to operate at peak efficiencies.
While steam consumption, leakages, and other heat transmission losses can contribute to the overall energy bill, this article focuses on the heart of the steam generator – the boiler.
Controlling the boiler is of utmost importance in any steam generation energy saving program. Below are some ways to improve boiler efficiencies:
Reducing Excess Air
By far the most common reason for energy inefficiencies in a boiler can be attributed to the use of excess air during combustion at the burners. When there is more air than is required for combustion, the extra air becomes heated up and is finally discharged out to the atmosphere. However, there are reasons for putting in some extra air for combustion – to compensate for imperfect burner fuel-air mixing conditions, air density changes, control system “slop”, burner maintenance, fuel composition and viscosity variation, and imperfect atomizing steam or air controls for burners.
Adjusting the fuel-air ratio for combustion can be quite tricky. If the fuel is too much as compared to the air, incomplete combustion occurs. This will give rise to carbon soot deposits inside the combustion chamber or even over the boiler tubes.
The consequences of having soot deposits over the heat transfer surfaces and the potential of having explosive flue gases inside the boiler are much worst than losing a slight amount of energy through the exhaust stack. Therefore, many boiler operators choose to adjust their burners to be slightly on excess air.
Installing Economizer
This is only appropriate if there are insufficient heat transfer surfaces in the boiler. The economizer tubes may contain either circulating boiler water or circulating feed water. Because the temperature of the exhaust gases can be quite high, the economizer tubes may be fitted with safety valves to avoid over-pressure damage. Also temperature control of feed water is required to prevent pump airlock. To avoid corrosion, careful design is needed to ensure that the exhaust flue gas temperature does not drop below the dew point.
Reducing Scale and Deposits
For any boiler operation, this is a must. The safety of the boiler is at stake. Any scale or deposits will lead to reduced heat transfer that will eventually lead to overheating, reduction of mechanical strength of the steel and finally to bursting.
This should already be in the normal daily procedure of boiler operation.
Reducing Blow down
Blow down of boiler water is discharging hot water into the drains. However, blow down is necessary to maintain the boiler water concentration of dissolved solids that are necessary for conditioning the boiler water. The dissolved solids are necessary for preventing boiler corrosion and scaling.
As steam is generated from the evaporation of water, the remaining water in the boiler becomes more and more concentrated. This must be drained away during blow down.
The challenge is to control the draining to the minimum.
Recovering Waste Heat from Blow down
Since it is necessary to blow down to control the total dissolved solids in the boiler water, methods can be adopted to recover back some of the heat from the drained hot water.
Blow down tanks, heat exchanger tubes and pumping arrangements can be fabricated to recover some of the heat back into the boiler.
Stopping Dynamic Operation
Whenever a boiler starts or stops, a few minutes are spent running the forced draft fan for purging the combustion chamber of unburnt gases. This is a necessary step for the safe operation of a boiler.
During this time the heat from the boiler water in the shell or tubes will be lost to the purging air.
To avoid this type of losses, it is better to maintain a steady firing condition in the boilers.
Reducing Boiler Pressure
By reducing the boiler pressure, some of the heat losses through leakages or transmission may be reduced slightly. However there can be problems with the boiler with reduced pressure. The boiler circulation may be upset and the steam lines may have insufficient capacity and flow to transport the low pressure steam.
Operating at Peak Efficiency
When operating two or more boilers, improved efficiency can sometimes be obtained by unequal sharing of the load so that the combined load operates at peak efficiency.
Preheating Combustion Air
Any heat loss from the skin of the boiler to the boiler room can be utilized back for combustion. By preheating the intake air the combustion in the furnace becomes more efficient.
Switching from Steam to Air Atomization
For burners with steam atomization, switching to air atomization will naturally result in less steam consumption overall and better boiler efficiencies. This is only applicable for heavy fuel oil burners.
Switching to Lower Cost Fuel
When comparing natural gas and fuel oil, if the cost is the same or more per BTU delivered, switch over to fuel oil.
The reason for this is that in the combustion process, hydrogen combines with oxygen to form water. The latent heat of vaporization is lost when water vapor leaves the boiler stack.
Fuels like natural gas with higher hydrogen to carbon ratio will lose this heat more than those with lower hydrogen-carbon ratio like fuel oil.
However one must also recognize that there will be increased maintenance, operating costs and greater need for more excess air in order to achieve complete combustion for fuel oil. In addition, soot deposits and incomplete combustion might also affect the overall costs.
Some of the ways mentioned above may not be feasible at all for your plant. Each of them may result in only a few percentage points of boiler efficiency improvement. However, if carried out carefully and with the proper tools and instruments, they do add up to huge savings.
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Many years of working experience in Marine, Facilities, Construction has given the author material for writing e-books and articles related to engineering, and management. Subscribe to facworld ezine More information at Marine Engineer and M & E Engineer Article Source: http://EzineArticles.com/?expert=Thomas_Yoon |
Yoon, Thomas “Methods of Improving Boiler Efficiency.” Methods of Improving Boiler Efficiency EzineArticles.com. http://ezinearticles.com/?Methods-of-Improving-Boiler-Efficiency&id=51896
A very common welding certification test for boilermakers is a 2 inch schedule 120 tube welded in a 6G position using Tig for the root and second pass, and stick welding for the filler passes and cover passes.
Sound Simple? If you think it’s simple, you are wrong.
A 2″ pipe done in a 6G position means that the pipe is placed with the axis of the pipe running 45 degrees from parallel. Once the test begins, it cannot be rolled, raised, or lowered.
That means you have to be able to position yourself in a way that allows you to move as you weld to move from the bottom, up the side, and to the top. That sometimes means starting out in the kneeling position, moving to a halfway standing bent over position, and then standing. All while trying to maintain a consistent arc length while moving at a steady even speed. This is the kind of welding that separates the men from the boys.
Whoever selected the 6g position for welding certification tests was either totally incompetent or an evil genius.
6G welding certification tests for boilermakers are used because they test the welders skill and because a 6g weld test qualifies for all other positions.
Its impractical to test a welder in the field doing the exact job that will be done once the welder is certified.
The 6G test is often considered the best way to cull the not so great welders and hire the best ones.
(although a 2G along with a 5G might be better)
Another reason 2 inch pipe is used for a weld test is that boiler tubes are often the same diameter as the test and they are also often welded while installed and next to other boiler tubes. Welding a 2 inch tube is hard enough with nothing in your way. Put another boiler tube just inches away on each side and the weld is much harder to do.
Testing using a 6G test gives some indication that a welder can weld from both sides of a joint.
I took a 6g test once and then welded a whole bunch of 2g and 5g tube joints in a fab shop. Never did a 6G for the whole year I worked there. So in my case, a 2g and a 5g test would have been more representative of the work done.
Jody Collier’s web articles on subjects like Welding Certification are full of Down and Dirty welding tips. For TIG, MIG, STICK welding along with a buttload of other welding information, visit http://www.weldingtipsandtricks.com/
Collier, Jody “Welding Certification Tests For Boilermakers.” Welding Certification Tests For BoilermakersEzineArticles.com. http://ezinearticles.com/?Welding- Certification- Tests- For- Boilermakers&id=2841782
Having eight years of experience working with biomass-fired boilers in palm oil mills, I can say that boiler water treatment in palm oil mills is a challenging task if compared with industrial boilers. Unlike industrial boilers which rely heavily on condensate return and industrial water for boiler feed water makeup, the source of raw water for boiler feed water in palm oil mills comes from lake, river, or well. Relatively not much condensate can be recovered in palm oil mills due to the high amount of unrecoverable sterilizer condensate and the application of live steam in some of the process heating. In palm oil mills, low cycle of concentration is not uncommon which indicates more chemicals are wasted through blowdowns.
The raw water from natural resources is high in impurities, turbidity, dissolved gases, and mineral content. Sometimes algae, mud and oil could be present and silica is particularly a problem in water from lakes, and this scenario complicates treatment further. Lack or inadequate water treatment is dangerous as it may cause scaling and corrosion which affect the reliability, efficiency, and safe operation of the boiler.
In general, boiler water treatment consists of external and internal water treatment. External water treatment refers to conditioning boiler feedwater by removing impurities, hardness, oil, oxygen, dissolved and suspended solids, outside the boiler and usually accomplished by mechanical means such as continuous clarifier, pressure sand filter, water softener, and deaerator. Because it is not possible to obtain a perfect boiler feed water by external water treatment, an internal water treatment, which mainly by chemical means, shall be employed. All boiler engineers must be aware that the external treatment of boiler feedwater must be emphasized since more chemical addition in the boiler is undesirable.
In industrial boilers, the removal of turbidity and suspended matters is already done by the municipal water treatment system. This is however a different story in palm oil mills. The external treatment for boiler makeup (feedwater) consists of pH adjustment, coagulation, flocculation, sedimentation, filtration, water softening, and finally, deaeration. Soda ash is added if the pH of the water is low. Soda ash acts as pH adjustment because coagulant functions at pH ranges over 5.5 to 8.0. The coagulation chemicals are dosed into the inlet pipe leading to the bottom of a continuous water clarifier. Coagulation and flocculation are the basic steps in boiler water treatment to reduce turbidity, organic substances, and color of raw water. Coagulation neutralizing the negative charges on colloid surfaces, allowing the particles to agglomerate to form floc, which is slow settling. The most common coagulants are aluminum sulfate (alum), sodium aluminate, and polyaluminum chloride (PAC). Synthetic polymers called polyelectrolites have been developed for coagulation process. Flocculation is further agglomeration of slowly-settling coagulated particles into large rapidly-settling floc with the addition flocculant such as organic polymer (starch) or synthetic polymer such as polyacrylamide (PAM) to attach and bridge between particles to form larger agglomerates.
From the natural resources, the water is pumped by booster pumps into the continuous clarifier to allow flocs to settle down. The water rises with decreasing velocity and the resultant floc forms a sludge blanket at the upper part of the conical section. The clarifier is blowdown periodically to remove the heavy sludge, which settled at the bottom while clear water is transported to a clear water tank before passing through a pressure sand filter where solids escaped from the clarifier and fine particles are filtered and removed. Normally, the sand filter removes fine particles up to 10 μm. Sand filters normally contain sands and gravels, 100% anthracite, or combination of sand, anthracite, and gravel. Sufficient water reservoir is maintained in the overhead tank and if the level of water in the tank becomes low, the operation of boilers could be affected. In water softener, sodium zeolite ion-exchange process is used to remove hardness. The function of a softener is to remove water hardness (calcium and magnesium) using ion exchange process. Normally sodium zeolite is used and a softener would contain 30~36″ of synthetic ion exchange resin. For example, calcium bicarbonate, Ca(HCO3)2 dissolves in water and split up to Ca2+ and (HCO3)-. In ion-exchange process, Ca2+ is removed by replacing sodium ion using resin ion-exchange bed which is covered with Na+ ions. Here in palm oil mills, the softeners are prone to bio and iron fouling. Soft water is then pumped into the deaerator for deaeration. The pressurized deaerator operates by allowing steam into the feed water through a pressure control valve to maintain the desired operating pressure, and hence temperature at a minimum of 105°C. The steam raises the water temperature causing the release of O2 and CO2 gases that are then vented from the system. This type can reduce the oxygen content to 0.005 ppm. The water produced is oxygen-free therefore oxygen corrosion in the boiler can be avoided.
The internal treatment involves the addition of reactant chemicals such as sulfite, sodium phosphate, chelates, polymers, and caustic, which each has different function in treating boiler water. To prevent oxygen corrosion, the chemical known as oxygen scavenger is added. The common oxygen scavengers are sulfite, hydrazine, and DEHA. Hydrazine however should be avoided because it is carcinogenic.. Phosphate prevents scaling by precipitating calcium as calcium triphosphate or hydroxylaptite, which can be removed via blowdown.
For more detail Q & A’s on boiler water treatment, visit http://www.boiler-ebook.com/WaterTreatment.html
Hashim, Hisham “Boiler Water Treatment in Palm Oil Mills.” Boiler Water Treatment in Palm Oil MillsEzineArticles.com. http://ezinearticles.com/?Boiler- Water- Treatment- in- Palm- Oil- Mills&id=3630127
All repairs must be acceptable to the governing code jurisdiction (such as NBIC), therefore all methods of repair must have written approval from the Chief Inspector or Authorized Inspector. The guideline to flush patch can be obtained from RD-2060a Flush Patches of NBIC.
Carry out hardness test (DT) on the bulge area to determine extent of defect. Mark out the boundary line of the defective section to be cut out based on acceptable tensile strength obtained. Use Vickers Hardness test (HV) to test for the metal strength of carbon steel. The method of repair is called “flush patch”, which means the shell is cutout around the failure in elliptical or rectangular, and a piece of new boilerplate rolled to correct curvature is inserted with the edge of original shell and patch butt-welded. For rectangular shape, provide accurate radius as square corners shall be avoided.
Refer the manufacturer’s blueprint on the material specifications of the shell and the patches shall be made from the material that is equal in quality and thickness to the original material. The edge shall align without overlap. Weld preparation to be double “vee” with an included angle of 60o to 70o. The gas tungsten arc welding (GTAW) shall be used for the initial pass joining the patch to shell. The balance of the weld shall be manual shielded metallic arc welding using low hydrogen electrodes confirmed to ASME Code Sect IX.
Weld the inside first, then back gouge on the outside. Grind the sound metal before continuing the welding on the outside. Grind flush the accessible surface, and after completion of welding, visually check for weld undercut and repair if necessary. Refer the manufacturer’s blueprint on the NDT used during construction, and normally the weld on flush patch of shell is 100% radiographed. Acceptance of weld is in accordance with ASME Code or BS 2970 code. Post weld heat treatment (PWHT) is to be done to relieve the residual stress on the heat affected zone (HAZ). Carryout hydrostatic test on the boiler. Test pressure shall be 1.5 times the design pressure. The holding time is 10 minutes.
You can find many other procedures on various aspects in operation, repairs, safety, etc. in the 101 Q&A: Practical Knowledge of Power Boilers e-book. The questions and answers presented in the ebook are concise and clear. The eBook provides you with over 1,000 sample licensing examination questions and answers along with very helpful illustrations; all the important materials necessary to prepare you as a knowledgeable and competent boiler operator or steam engineer.
Visit http://www.boiler-ebook.com and you are welcome to view the contents of the e-book.
Hashim, H. (2009, December 11). Detailed Procedures to Repair the Bulge on the Shell of a HRT Steam Boiler. Retrieved January 28, 2010, from http://ezinearticles.com/?Detailed- Procedures- to- Repair- the- Bulge- on- the-Shell- of- a- HRT- Steam- Boiler&id=3409835
