Akber Pasha and Robert Allen,
Vogt-NEM Inc., Louisville, KY, USA
Today’s merchant plants are required to come on line or shut down at short notice which puts a strain on the HRSG. This article describes various mechanisms which affect the integrity of the boilers and discusses monitoring requirements to keep the boiler from developing forced shutdowns.
Existing heat recovery steam generators (HRSGs), which are already in service, can pose problems for operators in deregulated markets. Most of these, although designed for base load operation in combined cycle plants, are now often cycled to satisfy the demand requirements of deregulated markets. Fortunately, it is possible to modify them for cycling operation or at least set up a monitoring programme to assess the effect of cycling.
Figure 1. Operating in competitive markets can place a strain on power plant equipment such as HRSGs. PHOTO COURTESY OF: SIEMENS
Cycling is defined as the reversal of stresses in a component and can happen by the imposition and relaxation of loads and because of reversal of temperatures. When a component is exposed to a rapidly changing high temperature fluid, all parts of the component may not heat up uniformly. Consequently the differential expansion creates high differential stresses and stress reversals.
Cycling can happen for a number of reasons, the main one being the change of plant load requirements and emergencies. Plant load changes include short notice start-ups, gas turbine load changes, part load operations and varying auxiliary burner inputs. The way a boiler is shut down and laid up can also affect the boiler integrity. In an emergency such as a steam turbine trip, the boiler has to shut down very rapidly causing undue or reversing stresses. Sometimes, with a very narrow working window, a boiler may be force-cooled to make repairs. This forced cooling can also create temperature differentials. Failure or abnormal operation of other equipment such as pumps and valves may impose cycling conditions on the HRSG.
Most of the time cycling either creates or enhances the effect of mechanisms which affect the life of the component. Creep damage by definition is caused by a prolonged exposure to high temperature and stress. Creep may be the only mechanism which is not caused or enhanced by cycling. Fatigue and fatigue damage are the most prevalent mechanisms affecting boiler life and are a direct consequence of cycling. Water chemistry upsets which result in corrosion may be due to cycling or because of failure of water chemistry controls.
Figure 2. Schematic multiple pressure HRSG with reheater and supplementary firing
For example, for a rapidly starting HRSG, the superheater is exposed to high temperature on the outside of the tube and headers whereas inside may still be cool. This creates high thermal stress.
When designing a boiler, the most important requirement is to make sure that all the factors which affect the life of HRSG have been accounted for. This means that either the design features must be modified to mitigate the effect of a particular damaging factor or the damaging factor must be eliminated. The most common way to ensure this is to do a life cycle analysis.
A typical life cycle analysis consists of the following steps:
- Define basic operating conditions
- Establish lifetime operating details
- Determine most critical components
- Dynamic analysis of the critical components. This is perhaps the most critical factor in the life cycle analysis. There are various programmes available which indicate the dynamic behaviour of a component. However these programmes need to be custom fitted to a given HRSG and to do this the model has to be very well defined. In addition the operating conditions such as pressure and temperature ramps that cause the changes in the components need to be defined very thoroughly. Most of the life cycle analyses do not give good results because of conservatism employed in the model. This happens because often the details of the component or the operating conditions are not worked out completely and the model is not refined.
- Calculate the cumulative damage factor (CDF): There are various methods available to calculate the CDF and include the methods suggested by ASME, TRD and British Standards etc. One important factor in calculating the CDF is to use a consistent method rather than indiscriminately mixing various methods with different assumptions and data used in deriving the formulas.
- Revise the design and/or operating conditions if the CDF is higher than allowable.
Some of the items which would help in minimizing and monitoring the damage due to cycling may consist of: smaller diameter, tubes, headers and drums; automated vents and drains; HP SHTR bypass to RHTR; wet reheaters; on line water chemistry monitors; stack dampers; leak-proof valves; metal thermocouples; avoidance of bends; flexible connections; motorized blowdowns; economizer recirculation; no cascading; blowdowns; external kettle boilers; ability to clean; spray coatings; higher grade metal alloys; bypass dampers; multiple attemperators; good steam/water side flow distribution at all loads.
Figure 3. HRSG components and their vulnerability to damaging mechanisms
It is possible to design an HRSG without these features and still maintain the integrity of the boiler over the intended life of the boiler. The key is to do a life cycle analysis. If the life cycle requirements are met with only some or none of these features, then the design should be acceptable.
Start-ups: How the boiler is started is a key item affecting boiler life. Rapid starting results in enhanced fatigue and if not avoidable entirely, it should be minimized. One way to do this is to plan the start-ups and shutdowns and slowdown where possible. The manufacturer supplied start-up and shut down procedure may have some contingency built in. These may be analyzed and removed. However this is possible only if all the operating conditions, including the condition of the HRSG when starting, are well defined.
Ramping to load: Generally after a soak period where the HRSG components have minimized temperature differences, the ramping can be done very fast. Again a planned ramping is much better. Ramping a running boiler is often much faster than starting a new boiler. So it is advisable to ramp up the running unit and ramp down as the cold unit comes on line.
Load changes: Similar to ramping, the load changes, if done gradually, should result in minimizing the cycling effects. Manufacturers often suggest load changes in steps rather than continuously; instead of a continuous ten per cent change, the load may be changed in five 2 per cent load changes with a dwell after each change.
Shutdowns: Shutdowns by themselves may not be a bigger problem, unless the purge requirements cause cold air to blow into the HRSG. Limitation on air flow should help in minimizing its effect.
Lay-ups: The chemistry upsets and oxygen intrusion during lay-ups affect the boiler conditions. When the boiler is put on line, the heat will increase the damage.
Modifying units for cycling
Today many HRSGs which were designed for base load conditions are required to run as cyclic units. Obviously the plant owners are interested in knowing what will be the impact and how to improve the life expectancy. To do this a vendor, preferably an HRSG manufacturer, is selected to make a study, suggest modifications and implement and monitor the operations.
The basic programme consists of the following: condition assessment; critical component identification and determination of their condition; checking of instrumentation and controls; review of operational history; establishing a baseline benchmark; develop required modifications for cyclic operations; definition of the proposed operations by the owner; review of the proposed cycling operations; development of complete life cycle; analysis of the critical components; determination of the hardware modifications required; physical modifications (physical modifications are done to the extent possible and permissible. Sometimes a modification may be feasible and cost effective but may not be permitted because of code requirements).
After the modification, a monitoring programme should be developed and installed. The programme should monitor a number of variables: temperatures, gas, water and steam drum levels; attemperator flows; steam and water flows and velocities; water chemistry steam purity and constituents; gas turbine and burner fuel exhaust gas conditions; review and analysis of data.
Once the monitoring programme data is available it is reviewed and checked against the assumptions. If there are any changes between the actual and initially specified operating conditions these are noted and their effect on the life expectancy is assessed. Based on this assessment some further adjustments are made.
After at least a year the unit condition is checked. This includes checking of thicknesses, corrosion and deposits etc. and would involve non-destructive and destructive testing of some suspected components. The values thus obtained are checked against the baseline benchmark. Based on this data, further modifications are suggested or the operating conditions are changed.
Designing and operating the HRSGs for cyclic operation requires detailed definition of the operating conditions and close co-operation between the designer and the operator. The monitoring system should give a thorough picture of the mechanisms affecting the life and integrity of the boiler.
Periodic review of the data is necessary to compare the actual conditions with the expected conditions. For the existing units long term cooperation between the operators and the designers and free exchange of data is essential. Generally, the boilers designed for baseload can run with cyclic loads with minor hardware changes. Sometimes a very minor adjustment in the operating condition may preclude the necessity of a major modification. For this reason if nothing else, thorough understanding of the operations by the designer and good understanding of the design and its impact on the unit by the operators is essential.