In a recent project, the operator – a leading minerals processor – was facing difficulties in optimising the operation in one of its sulphuric acid plants because of limited pumping equipment lifetime and increased process downtime.
Sulphuric acid is one of the most important industrial chemicals. In the contact process, circulation pumps are required for pumping very corrosive high-temperature and high-concentration sulphuric acid in drying towers, absorption towers and heat recovery systems. In most cases, vertical pumps are installed at the top of a tank to provide safe operation. In this particular case, the design featured a vertically mounted end suction pump with the volute casing installed on a suspension column with a separate discharge pipe.
Typical process conditions include:
• Liquid sulphuric acid
• Concentration: 92-99.9%
• Temperature: 60-150°C (200-250°C for heat recovery system)
• Flow: up to 2,800m³/h
• Head: 20-30m
Observations and troubleshooting
In the process, some issues were encountered with the vertically mounted volute casing. The two most significant issues were mechanical failure and corrosion.
On the mechanical side, operators were facing abnormal and fast vibration level increase. This could have blocked the pump if it was not shut down in time. After dismantling the pump, erosion and corrosion damages were observed on wear parts.
In most cases, the damages were concentrated in one direction. This was due to the volute casing geometry, which generated radial thrust on the pump line shaft. The thrust led to deflection and vibrations, which caused wear of the pump bush bearing and roller bearings. The highly corrosive conditions, where the clearance increased over time, further deteriorated the mechanical conditions of the pump. This effect was exponential, and consequently the equipment lifetime decreased rapidly.
Another problem encountered was due to corrosion. Monitoring of part thickness, such as support pipe and discharge pipe, revealed that the erosion was not uniform. Both parts were more corroded at operating liquid level in the tank, while the support pipe was corroded along its whole length. The reason was localised concentration and temperature conditions at the liquid interface. As a result, the pump had to be rebuilt with new parts on a regular basis. Moreover, predicting the progress of corrosion is difficult, and a complete replacement of the pump might therefore be required.
Sulphuric acid is highly corrosive, and a variation of concentration or temperature could increase corrosion drastically, resulting in considerable damage to the plant equipment. The challenge for operators is to keep those parameters under control, and this is more difficult to achieve during transient phases (i.e. starting, shut-off, surge, etc.) than other phases. For that reason, equipment lifetime and reliability are some of the most important factors to limit the number of these difficult phases and related risks.
The vertically mounted pumps were replaced by vertical turbine pumps specially designed to handle sulphuric acid applications. The symmetric diffusor casing of the pump distributes the thrust equally – as shown in the graph below. Detrimental thrust on the line shaft does not occur and as a result, the vibrations level and shaft deflection can be kept at a minimum. This advantage does not only apply to the best efficiency point of the pump, but to the entire flow range.
The symmetric construction ensures homogenous distribution of the flow stream and velocity in the discharge pipe. Consequently, local corrosion is avoided and monitoring of the pump conditions is easier.
The lifetime of the new vertical turbine acid pumps is more than double compared to the vertically mounted volute casing pump replaced at site, and they have increased the MTBM (mean time between maintenance) remarkably.
Additionally, differential thermal expansion does not occur in discharge through the suspension column. Combined with the fact that there is no acid/gas interface on the shaft, this makes the pump more reliable, considering that stress corrosion and local temperature/concentration are very damaging side effects.
Vertical turbine pumps with roller bearing and sealing design are also suitable for heat recovery systems, where the conditions are stringent due to high temperatures.
Sulzer’s vertical turbine pump type VAS has been specifically designed for sulphuric acid pumping. Combined with experience and continuous support, it has helped users improve operating performance considerably even in the most stringent conditions.
The pump design extends the lifetime of the equipment and allows the customer to increase the MTBM significantly. The compact design of Sulzer’s vertical turbine pump ensures easy installation and maintenance-friendly operation. The symmetric diffusor case design allows the development of bigger pumps for higher sulpuric acid circulation flows than with existing equipment, opening new paths for process development.