Sulfuric acid is one of the most important industrial chemicals. In the contact process, circulation pumps are required for the pumping of very corrosive high-temperature and high-concentration sulfuric 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 the case of the fertilizer producer, the design featured a vertically mounted end suction pump with the volute casing installed on a suspension column with a separate discharge pipe. The volute casing geometry generates radial thrust on the pump line shaft. The thrust leads to deflection and vibrations and causes wear of the pump bush bearing and the roller bearings. The highly corrosive conditions, where the clearance increases over time, further deteriorate the mechanical conditions of the pump. This effect is exponential, and as a consequence, the equipment lifetime is rapidly decreased.
Equally distributed thrust
The vertically mounted pumps were replaced by Sulzer’s VAS vertical turbine pump specially designed to handle sulfuric acid applications. The symmetric diffusor casing of the pump distributes the thrust equally. 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. VAS pumps handle liquid sulfuric acid concentration s of 92 to 99.9 % at temperatures of 60 to 150 °C (200 to 250°C for heat recovery system). The flow rates are up to 2800 m³/h at a pumping head of 20 to 30 m. The lifetime of Sulzer’s vertical acid pumps is more than the double compared to the previous equipment at the customer’s site, and they have increased the MTBM (mean time between maintenance) remarkably. 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. Differential thermal expansion does not occur thanks to discharge through a suspension column, without a discharge branch. The fact that there is only one acid/gas interface in the discharge column instead of four (two on the support column, one on the shaft and one on the discharge pipe) reduces corrosion significantly. These two factors are very important because stress corrosion and local temperature/concentration are two very damaging mechanical side effects.
Sulzer AG, Winterthur, Switzerland
Author: Benoît Martin
Product Portfolio-Manager, Sulzer