Nitrogen+Syngas 366 Jul-Aug 2020
31 July 2020
Impact of heat recovery arrangement on package reliability
HEAT RECOVERY SYSTEMS
Impact of heat recovery arrangement on package reliability
Dr M. Olbricht and Dr J. Weidenfeller of Schmidtsche Schack | ARVOS (SCS) discuss the impact of the heat recovery equipment arrangement in an ammonia plant downstream of the secondary reformer on reliable boiler operation. A detailed investigation was performed by SCS in response to an operator experiencing difficulties in maintaining water quality in this critical equipment. Water quality has a crucial impact on the lifetime and reliability of the equipment.
The case presented here describes such a parallel arrangement. Schmidtsche Schack | ARVOS (SCS) as designer and fabricator of the process gas coolers was requested for support by a plant operator due to difficulties in maintaining the water quality and water level of each steam drum of both process gas coolers. Since the water quality has a crucial impact on the lifetime and reliability of this critical equipment a detailed investigation was performed by SCS.
Heat recovery package arrangement
SCS recommended arrangement
For a typical waste heat boiler package for two parallel ammonia trains SCS recommends an arrangement consisting of one process gas cooler and one steam super heater per train. Both process gas coolers are connected to a common steam drum as shown in Fig. 1. An arrangement with two individual steam drums, one per process gas cooler, is also possible. In that case, SCS recommends an individual boiler feed water supply for each steam drum. The process gas coolers work as natural circulation boilers which means that the water circulation between process gas cooler and steam drum is driven by gravity only and no additional pump is needed. This proven, robust design saves investment and operational costs and leads to a reduced maintenance effort. The steam superheater is either designed as a U-tube heat exchanger or bayonet tube heat exchanger, depending on the particular process conditions and customer demand.
Customer arrangement
The set-up at the customer site is shown in Fig. 2, whereas SCS provided two process gas coolers and two steam drums. In contrast to the above mentioned recommendations the final set-up was realised based on the plant operator’s philosophy. The focus was on further cost reduction and therefore both steam drums were serially connected to the single boiler feed water (BFW) line. As a consequence, the boiler feed water flows through the first steam drum before it enters the second one. The assumption of this setup was that it behaves similar to a single drum thereby reducing the total instrumentation cost. Only one drum was originally equipped with sensors because equal process conditions were assumed for both drums due to the direct hydraulic connection. However, the operator observed different water levels in the steam drums and reported problems with water quality which was shown by an insufficient pH value. Water quality is a critical issue in boiler operation and has a huge impact on lifetime. The wrong chemical conditions in the boiler water can cause serious corrosion of the equipment even over a short period of operation.
Water quality and composition during operation
The quality of the steam and boiler water is set up in the steam drum. The water steam mixture generated in the process gas cooler is separated into liquid water, which is utilised again in the process gas cooler, and high quality saturated steam, which is provided to the downstream equipment. This process is illustrated in Fig. 3. Both the quality of the water and the steam is maintained by the addition of chemical additives to the boiler water in the steam drum. The pH value of the water is an important indicator for water quality and is adjusted by additives within a narrow range to ensure a long lifetime. Thus, it is crucial to frequently monitor the additive concentration and pH value in the boiler water. This is even more important for non-volatile components in the boiler water because these can be easily enriched in the boiler water by steam production. Besides the amount of chemicals that are added, the concentration of additives in the boiler water can also be controlled by the continuous blowdown (CBD). The continuous blowdown is a small mass flow of liquid boiler water which is drained from the steam drum. A typical range for the continuous blowdown is 1 to 2% of the steam mass flow that is produced by the process gas cooler. Correct continuous control for good water quality is mandatory to ensure long lifetime of the boiler and to protect the entire water steam system from corrosion damage. A high mass flow of the continuous blowdown has a negative effect on overall steam production.
To support the plant operator in this water quality related issue SCS simulated the water steam system of the entire heat recovery package to identify critical operation conditions. This involved individually analysing the water composition in every apparatus. A schematic view of the models used for the simulation is shown in Fig. 4. The boiler arrangement consisting of two process gas coolers with a common steam drum as recommended by SCS is illustrated on the left hand side. The arrangement at plant operator site consisting of two process gas coolers with two serially connected steam drums is shown on the right side. The simulation focuses on the non-volatile constituents in the boiler water owing to their higher potential to get enriched in the boiler water.
Within the simulation the dosing of additives and the continuous blowdown flow rate as well as the heat recovered by the process gas cooler were varied to get an overview of the system behaviour in a wide range of possible operation parameters.
Results
To provide insight into the typical effect of the water composition on different parameters the SCS heat recovery package arrangement was simulated as a baseline case. The results of the investigation of the customer’s arrangement with two serially connected steam drums were then compared with this baseline case. Both simulations were performed with the same boundary conditions regarding the process gas side of the heat recovery package. The results were presented in a normalised form. Critical operational areas were highlighted. In this way, the impact of a change in operating conditions and the flexibility of the individual concepts becomes obvious.
Simulated SSC arrangement
The steam production and the normalised concentration of chemical additives in the steam drum, depending on the continuous blowdown rate for the SCS arrangement, are illustrated in Fig. 5. It is shown that the pH value can be adapted over a wide range by a slight change of the continuous blowdown. The chemical additives remain within a concentration range which is adequate for boiler operation even when the continuous blowdown rate is significantly changed. This configuration also enables easy and safe control of the water chemistry in the system. Consequently, this promotes a long lifetime of the heat recovery equipment. Another positive effect is that the steam production is only very slightly affected by the pH value and additive control due to the good controllability of the water chemistry. Generally the steam production is reduced with increasing continuous blowdown. Thus, the slight change of the continuous blowdown, which is necessary for additive concentration control, leads only to an almost negligible reduction of the steam production. This ensures an efficient use of water and recovered energy.
Simulated customer arrangement
The simulation results of the additive concentration and steam production for the serial drum customer arrangement are shown in Figs 6 and 7. Since the arrangement consists of two serially connected steam drums, in principle, two continuous blowdown rates can be adjusted and every steam drum has its individual steam production and additive concentration. The impact of changing only the continuous blowdown (CBD) rate of the first drum is shown in Fig. 6. The concentration of additives is set up to be optimal for the first drum in case of a low CBD of 0.5% as shown by the green line. It can be observed that the additive concentration in the second drum is significantly higher than in the first drum. Furthermore only 20% of the total steam produced is released from the first drum whilst 80% of the steam comes from the second drum.
Both phenomena can be explained by the fact that the boiler feed water entering the first drum is usually subcooled and needs to be preheated to steam drum temperature. This process occurs mainly within the first process gas cooler and steam drum. The recovered heat from the first process gas cooler is utilised to preheat the total amount of boiler feed water that is needed in both the first and second steam drum. Thus only a minor part of the recovered heat from the first process gas cooler is used for export steam. Consequently less steam is released from the first drum and in this way the chemical additives are only slightly enriched in the first drum. This leads to a higher demand for chemical additives in the first drum to reach the desired pH value for the first process gas cooler and steam drum.
After preheating of the boiler feed water to saturation temperature in the first drum all of the recovered heat from the second process gas cooler is used for export steam. The chemicals in the boiler water are enriched by the higher steam production. This causes a significantly higher concentration of chemicals in the second drum compared to the first drum by a factor of 150, which is shown in Fig. 6. The resulting pH value is too high to ensure a long lifetime of the heat recovery equipment and reliable process gas cooler operation. It is also obvious that no significant reduction of the additive concentration in the second drum is achieved with increasing continuous blowdown rate of the first drum. That means that adjusting the continuous blowdown of the first drum is not a sufficient control measure to achieve sufficient water quality. The only effect is a loss of steam.
The better control strategy is to adjust the additive concentration in the second drum with the continuous blowdown rate of this drum as illustrated in Fig. 7. Although there is also a huge range of operation conditions where adequate water quality cannot be achieved, it is possible to decrease the additive concentration to a level at which the pH value is adequate to achieve an acceptable water quality. However, this is done at the cost of steam production and flexibility. To achieve safe operational conditions the continuous blowdown rate must be increased so drastically (>7.5%) that a further rise has literally no further effect on the water quality.
During this investigation the observations made during operation were discussed with the plant operator, who reported a shift of the pH value between both steam drums. The different pH values are a result of the unequal enrichment of chemical additives in the two steam drums as already explained. It was also reported by the operator that different water levels in the two steam drums were observed during operation which were hard to control. This phenomenon is related to the unequal steam production in both process gas coolers due to the serial connection of the steam drums as predicted by the simulation model. The average density of the water steam mixture is lower by the higher steam production in the second drum than the one in the first drum. This effect enables the water level in the second drum to rise above the level in the first drum despite their hydraulic connection.
The model used for investigating the water steam system accurately describes qualitatively the observations made by the operator. That means that the simulation provides physical plausible information and is able to predict the distribution and enrichment of chemical additives during operation under consideration of different arrangements. The model is a suitable tool for failure analysis in water quality related issues.
Recommendation for action
Even though a possible combination of parameters for reliable process gas cooler operation was identified for the arrangement of two serially connected steam drums, SCS advised against operating the heat recovery package over a longer time under these conditions. The aforementioned drawbacks of the arrangement are too limiting to satisfy the flexibility and reliability demands of an industrial heat recovery application. SCS recommended completely instrumenting the second steam drum and changing the serial connection of the two steam drums to a parallel one. Each steam drum must be equipped with its own individual flow control valve for boiler feed water supply. These measures are of limited cost and complexity and can be done during the next turnaround of the plant. The two parallel steam drums will then essentially act like two individual steam drums and will provide a similar flexibility to the recommended arrangement.
Conclusion
In general, water quality and water circulation have a huge impact on the performance and lifetime of the heat recovery package. The simulation of the water steam system supported the troubleshooting requested by a plant operator. For the installed heat recovery package arrangement the results indicated additive enrichment in one of the two steam drums caused by the arrangement and interconnections. It turned out that operating with the customer arrangement is possible but only over a small and limited operation range. The operation involves the risk of reduced lifetime due to suboptimal water quality. The control of water quality is less flexible compared to the SCS setup. Water and energy are wasted during operation with this arrangement. Thus, SCS advised the plant operator how to change the current arrangement with minimal effort. It was recommended to rebuild the serial drum arrangement to a parallel setup with individual boiler feed water supply for reliable and safe operation.
The arrangement recommended by SCS, which is based on long term experience in process gas cooler design and water quality related questions, ensures easy and safe water quality control over a wide operation range, efficient boiler operation with maximum steam production and a long lifetime of the heat recovery equipment.
