Corrosion in Boiler Feed Water Pump
Corrosion is a critical issue for the engineers. Many a proud designer or project engineer has developed a new component or process with outstanding performance only to have it fail prematurely because of corrosion. Corrosion can be defined as the (undesirable) deterioration of materials of construction due to chemical or electrochemical influences, initiating from the interface material or environment . It becomes impossible to calculate the costs of corrosion and corrosion protection when the loss of life is considered, however, from engineering point of view it is estimated that in industrialized countries these costs amount up to 2 to 3.5% of the gross national product (English Committee Corrosion and Protection, chairman Dr. T.P. Hoar) . Furthermore, despite of active research by corrosion engineers, a visit to the local scrap yard shows that large percentages of cars and domestic appliances still fail because of corrosion; this loss pales by comparison when industrial corrosion failures are included. As a result, the annual cost of corrosion and corrosion protection in the United States is over $300 billion, which is more than the annual budgets of some small countries . It has also been estimated that about 25% of the corrosion costs could be saved by the application of existing knowledge of corrosion prevention and control, i.e. by using better protective systems and proper coatings, by improving design and material selection.
Corrosion related problems are of major concern in the fertilizer production and related industries which involve ammonia, urea or nitric acid production and usage. The types of corrosion that can occur in such industries are numerous, ranging from general corrosion to insidious localized corrosion. These corrosion problems can substantially increase operating and maintenance costs. Therefore it is evident that a comprehensive study focusing on the development of alternative materials for improved corrosion resistance in fertilizer production or ammonia based environments, would not only be suitable research project for a young chemical engineer but also a crying need in the path of advancement of our industrialization. The article here focuses corrosion phenomena taken from real time operation experience in ammonia plant in KAFCO fertilizer plant.
Overview of KAFCO:
The Karnaphuli Fertilizer Company limited (KAFCO) is a joint venture project among government of Bangladesh and some international companies mainly from Denmark, Netherlands, UK and Japan. It was established effectively in 1990 with the aim to utilize cheap natural gas as well as to earn foreign currency for the country. It has two production units: 1500 MT/day Topsoe design Ammonia plant and 1725 MT/day Stamicarbon designed Urea plant. The plants production started in December 1994. The KAFCO project is the largest of its kind in Bangladesh. The plant is located in Chittagong and produces high grade granular urea and anhydrous ammonia. The facility consists of an ammonia plant, Urea plant and urea granulation unit. Utility and captive power generation units support it.
KAFCO Ammonia plant was designed 1500 MT /day (100%), however, efficient plant operation and some modification actually enhanced it to some percentage. In 2007 midsummer, the plant was running at 105% load. Suddenly there was a significant decrease in boiler feed water (BFW) header pressure. In ammonia plant boiler feed water (BFW) supply section holds an utmost importance. It ensures plenty supply of water to waste heat boiler in ammonia reforming zone as well as to auxiliary boiler in utility plant. The water used in boilers serves two important purposes- high pressure steam production and process gas temperature reduction. The header pressure is important because it ensures the adequate supply of water to boilers and any decrease in pressure relates to ultimately plant trip logic. There are three pumps used to supply water for boilers: Pump A, B, and C. Pump A is steam driven and other two are motor driven. Two pumps always keep on running as per plant requirements: pump A and either of pumps B or C operate for better efficiency. The specification of the pumps that are used to supply BFW is as below:
Pump Name : Boiler feed water Pump
Type : Horizontal, Centrifugal (multistage)
Capacity : 250 m3/hr
Diff. pressure : 122.75 kg/cm2
Head : 1310 m
Casing : 12Cr
Impeller : 12Cr
Temperature : 1350 C
Suction : 4.29 kg/cm2 (rated)
Discharge : 127.04 kg/cm2 (rated)
Figure 1: Broken wire ring at 11th stage
Figure 2: erosion –corrosion at 11th stage
In the last few days, before the incident has occurred, the BFW data showed a gradual decrease in BFW header pressure. At the time, pump A and B were running. Normally the pressure remains just over 128 kg/cm2. At first to maintain the smooth operation the BFW header pressure low alarm set point was lowered down from 126 kg/cm2. Four days later BFW header pressure low low (set point is lower than normal low) alarm (124.5 kg/cm2) appeared and according to the logic control, auxiliary BFW pump in utility started to maintain the required pressure in BFW line. To understand the root cause of the problem, first the combination was changed and pump C was started along with A instead of B. But the low alarm in header pressure remained. Then the problem was thought to be with pump A and it was to be checked. Since it was steam driven, it was been controlled by speed; at primary level, the speedometer was checked and replaced, however, no improvement observed. Later on, the speed test showed that with significant increase in speed did not increase the pressure as expected. After vigorous testing of one week long, the plant maintenance section confirmed and decided to start maintenance work on BFW pump A. During the maintenance work the 11th stage wire ring was found broken and lower casing surface was found eroded near to the 11th stage impeller.
Figure 3: Close view of the eroded part
In my opinion, the incident was a perfect example of erosion corrosion and corrosion fatigue. The later mechanism took into the first place. At first let us make some discussion on these two kinds of corrosion mechanisms. Erosion corrosion is the acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and metal surface. All types of equipment exposed to moving fluids subject to erosion corrosion: some of these are piping systems, particularly bends, elbows and tees, valves, pumps, blowers, centrifugals, propellers, impellers, agitators, agitated vessels, heat exchanger tubing such as heaters and condensers, measuring devices such as an orifice, turbine blades, nozzles, ducts and vapor lines, scrapers, cutters, wear plates, grinders, mills, baffles, and equipment subject to spray. A special form of erosion corrosion, cavitation damage, is caused by the formation and collapse of vapor bubbles in a liquid near a metal surface. Cavitation damage occurs in hydraulic turbines, ship propellers, pump impellers and other surfaces where high velocity liquid flow and pressure changes are encountered. Before considering cavitation damage, let us discuss the cavitation phenomenon. If the pressure on a liquid such as water is reduced sufficiently, it boils at room temperature. Consider a cylinder full of water that is fitted with a tight piston in contact with water. If the piston is raised away from the water, pressure is reduced and the water vaporizes, forming bubbles. If the piston is pushed toward the water pressure is increased and bubbles condense or collapse. Repeating this process at high speed such as in case of operating a water pump, bubbles of water vapor form and collapse rapidly. Calculations have shown that rapidly collapsing vapor bubbles produce shock waves with pressures as high as 60000 lb/in2. Forces this high can produce plastic deformation in many metals. Evidence of this is indicated by the presence of slip lines in pump parts and other equipment subjected to cavitation.
Fatigue is defined as the tendency of metal to fracture under repeated cyclic stressing. Usually, fatigue failures occur at stress levels below the yield point and after many cyclic applications of this stress. Corrosion fatigue is defined as the reduction of fatigue resistance due to the presence of a corrosive medium. Corrosion fatigue is also influenced by the corrosive to which the metal is exposed; oxygen content, temperature, pH and solution composition influence corrosion fatigue. Corrosion fatigue resistance also can be improved by using coatings such as electroplated Zinc, chromium, nickel, copper and nitride coatings.
In the above mentioned case, the corrosion fatigue was followed by cavitation damage. As it was a rotating equipment and corrosion environment accelerators such as pH, temperature, and pressure were present and there were some unplanned shutdowns also. All these factors led toward the corrosion fatigue. Apparently there was a crack in the wire ring. Therefore, high frequency (cycle per second) rotation favored the transgranular growth of the crack, and finally the ring had been broken. Moreover, as the ring got damaged, there was a decrease in the impeller performance and subsequently the pressure fluctuation occurred at the 11th stage. This was assumed to be the reason as the cavitation damage took place at there and the inspection team had some visual proof relating to that.
Due to this failure, the pump was under maintenance for approximately 10 days. During the maintenance the plant was running with pump B and C without having any stand by pump. At the that time, energy consumption was higher than regular condition; also the capacity and net diff. pressure of the other two pump was lower compared to the A pump (Table 1). As a result, to maintain normal header pressure the other two BFW pump consumed excess electricity which in turns increased the load of Steam Driven Turbine Generators (STG). Therefore, the plant management experienced some economical losses and the plant was operated in relatively vulnerable condition.
Table 1: Comparative review of plant operation data (energy consumption) due to pump maintenance
Condition Plant Load, % Boiler-1, MT Boiler-2, MT STG-1, MW STG-2, MW
Before M/M of Pump A 108 % 90 85 7.18 7.21
During M/M of Pump A 108 % 94 90 8.1 8.25
After M/M & Pump A online 108 % 90 86 7.2 7.25
In conclusion, it can be said that the engineering process industry is a place where these kinds of incidents often take place. It is important for the engineers to monitor the process, machine performance and identify the abnormality. Experience and close observation holds utmost importance relating to these criteria. The article here also emphasizes on tactics used in detecting the problem which is important in plant operation context.
The Author gratefully acknowledges Md. Tareq Iftekhar, Section Manager, Inspection Team, Technical Department, KAFCO for his support.
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To cite this article, please use following information:
(use the given format or any standard citation format)
Mursalin, A.F.M., Corrosion in Boiler Feed Water Pump, ChE Thoughts 2 (2), 26-29, 2011.