During normal operation fertilizer plants operate round the clock; therefore, periodic maintenance is required along with occasional major overhauls.
Frequency of inspection shall be established based on the actual plant operating experience and would be usually dictated by the rate of degradation. Sometimes, the frequencies are dictated by code requirements.
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Technical literature [Ref 2] recommends that maintenance activities during a planned turnaround shall include:
- Routine inspection for corrosion, equipment integrity, deposit formation, integrity of electrical and instrumentation systems, integrity of piping systems, valves strokes and functionality checks, etc.;
- Special inspection (because of anomalies in the prior operating e.g. high pressure drop, low efficiency, etc.) of reactors, syngas convertor, heat exchangers, reboilers, or rotating equipment or pumps to investigate for abnormal situations;
- Installation of replacement equipment for part of or entire pumps or of instruments that have worn out;
- Replacement of catalyst or process materials that have been depleted or degraded during operations.
Areas in an ammonia plant that are susceptible to internal corrosion and should be carefully inspected during turnaround are:
- CO2 removal system
- Steam system
- Cooling water system
- Stress corrosion areas
CO2 Removal System:
The CO2 removal system has a corrosive environment, especially in the areas of high CO2 concentration in the gas phase as well as in the liquid phase. The proprietary hot carbonate systems and the inhibited MEA system use corrosion inhibitors, usually vanadium-based.
The protection of carbon steel equipment is based on maintaining the vanadium in a pentavalent state. The tetravalent vanadium is ineffective for corrosion protection. Sodium nitrite is generally used for maintaining the vanadium in an oxidized state.
It is important that the inhibited solution covers all carbon steel components of the CO2 system. Dry spots, due to maldistribution of the scrubbing solution, that are exposed to wet CO2 are susceptible to corrosion attack by carbonic acid.
The areas within the CO2 removal system that are most susceptible are the bottom of the absorber, piping downstream of the rich solution let-down, the top of the stripper, the overhead condenser and the reboiler. Also, accumulation of chlorides in the scrubbing solution can cause stress corrosion. The sources of chlorides are impurities in potassium carbonate, makeup water, and contamination by cooling water.
Corrosion in the steam system of an ammonia plant is generally related to the following causes:
- Improper water chemistry in steam drum
- Acid breakthrough in the makeup demineralized water
- Acid breakthrough from the condensate polisher
- Cooling water leak from the surface condenser
- Inadequate deaeration in the deaerator
- Synthesis gas leak into the boiler feedwater
Cooling Water System:
All problems relative to cooling water systems generally fall into the three main categories:
- Deposit formation
- Biological fouling
Stress Corrosion areas:
Stress corrosion of stainless steel by chlorides is a well-known problem. The potential for chloride stress corrosion exists in the CO2 removal system. Also, the “U” in the U-tube bundles of the reboilers can sustain stress corrosion, if there are residual stresses from the fabrication of the bundles.
Shutdown inspections are normally scheduled during plant turnarounds. When catalyst is changed in the secondary reformer or shift converters or methanator or ammonia converter, the situation presents an ideal opportunity for the internal inspection of catalytic vessels. When a piece of equipment is shut down for a suspected major problem, a thorough inspection is necessary. The type of inspection is dependent on the nature of problems that can develop for a specific type of equipment.
Primary reformer tubes
Creep rupture is the most predominant mode of failure in steam reformer catalyst tubes. Majority of these failures are longitudinal splits caused by creep rupture. The fissures usually initiate between the inner and mid-wall as a result of stress. These fissures propagate in both inner wall and outer wall directions of the centrifugally cast tubes. Four important variables that influence tube life are:
- Tube wall thickness
- Type of alloy
- Number of cyclic operations (startups and shutdowns)
- Operating severity (temperature)
Temperature is the most important parameter affecting tube life.
The bayonet type of vertical thermosyphon waste-heat boilers need to be monitored carefully due to the severity of the application. Some of the causes of potential failures are:
- Foreign objects and scale creating a blockage in the tubes
- Metal creep due to excessive tube wall temperature
- Steam blanket due to reverse circulation and starvation
- Water treatment chemistry (Dissolved solids and silica)
- Corrosion (acid/alkali breakthrough, hydrogen attack)
Many plants change tube bundles about every 4 years to prevent unexpected failures due to scale buildup, blockage and overheating.
During a shutdown, the deaerator should be internally examined. Some of personnel involved into the non-destructive do not differentiate between original weld defects and service-induced stress corrosion cracking or corrosion fatigue. Inspections have been performed using visual, liquid-penetrant, magnetic particle, and ultrasonic testing methods. All circumferential and longitudinal welds should be examined using these methods. The most effective and sensitive test for finding cracks in deaerators is the wet fluorescent magnetic particle method. Careful examination of the deaerators is especially important if they are older than 10 years.
During a turnaround, plant improvements activities could include:
- Installation of new upgraded equipment or technology to improve the plant processing
- Installation of new major capital equipment or system that may significantly alter the ammonia, urea or nitric acid process and product output
As the plant becomes older, the outage becomes more frequent and hence accepting shutdown / turnaround becomes inevitable with so much revenue at stake. This strategy has associated changes in plant operating philosophy and inspection technique. Fertilizer companies should use their technical expertise for setting intervals for inspection. So, shutdown and turnaround planning and preparation should be carried out more carefully, by assessing plant deterioration and its impact on reliability, planning stocks and logistics impact of supplies to customers, and checking the availability of contractors for plant turnaround activity.
Shutdown / turnaround cost normally comprises over 30% more of annual maintenance budgets and a delay in start-up can cause a loss of operating profit that exceeds the cost of shut down / turnaround. During the turnaround, the event requires many people to be diverted and external resources to be brought in.
Shutdowns can be costly in terms of lost production, and carefully designed plan can reduce cost. Minimizing the duration of the outage can have a major impact on reducing the cost of lost production. Augmenting the resources available to handle shutdown plant scheduling, managing the shutdown activities, and assisting in the start-up of the facilities will minimize the out of service time.
- Trinath Sahoo, “Process Plants Shutdown and Turnaround Management”, CRC Press, ISBN 978-1-4665-1733-2, 2014
- Madhavan, S. Y. Bathe, “Inspection of Ammonia Plants”, AICHE Ammonia Safety Symposium, 1986