How Screw Pumps Handle Harsh Gases in Chemical and Pharmaceutical Industries
Vacuum technology is crucial in chemical and pharmaceutical industries, enhancing the safety, speed, and cost-effectiveness of various processes. Vacuum is indispensable in operations like conveying, distillation, drying, and concentration. For decades, liquid ring vacuum pumps and steam ejectors dominated these processes. However, these pumps have a major drawback: they rely on working fluids that come into contact with process gases, leading to contamination and operational inefficiencies. In contrast, dry screw vacuum pumps have become the preferred choice due to their ability to operate without any working fluid.
Principle of Operation
Dry screw vacuum pumps operate using two screw-shaped rotors that rotate in opposite directions. As the process gas is drawn in, it is sealed between the pump housing and the screws, then transported to the discharge port. During this process, the screws do not touch each other or the pump casing, thanks to precise manufacturing that maintains minimal clearance between moving parts. This design allows dry screw vacuum pumps to achieve an ultimate vacuum of up to 0.01 mbar.
Cooling water is essential for these pumps to dissipate the heat generated during gas compression and maintain the pump's temperature stability. Controlling the cooling system allows for setting an optimal operating temperature—high enough to prevent process gas condensation and low enough to avoid problems like polymerization or spontaneous combustion.
Vacuum Systems for Process Gas Management
The chemical and pharmaceutical industries often involve handling corrosive, condensable, and dust-laden gases. Effective vacuum systems must be designed with these challenges in mind, incorporating the necessary components to prevent damage to the vacuum pump. Below are several strategies for addressing the risks associated with such harsh process environments:
1. Corrosion Protection
Corrosion is a significant concern in vacuum systems exposed to aggressive chemicals. The first line of defense is preventing corrosive substances from entering the pump. This can be achieved by installing upstream condensers or gas scrubbers to remove harmful components before they reach the pump.
Another approach is to maintain process fluids in the gaseous phase by controlling the pump’s operating temperature. This ensures that corrosive vapors do not condense within the pump. Additionally, adding dilution gases to reduce the partial pressure of condensable components can prevent condensation.
The third strategy involves using corrosion-resistant materials in the pump construction. The pump’s wetted parts, typically made of cast iron, can be coated with protective layers such as PEEK, Ni+PFA, or Hastelloy coatings, or constructed entirely from special materials capable of withstanding corrosive environments.
2. Dust Management
Screw vacuum pumps are highly sensitive to particles due to the precise tolerances between their rotors. In pharmaceutical applications, dry screw pumps are often used alongside dryers, where fine powder particles can be drawn into the pump, particularly during the end of the process. To mitigate this risk, inlet screens or filters should always be installed to protect the pump.
3. Handling Condensate
Processes like distillation, concentration, and dehydration often produce condensable vapors that can accumulate within the pump or exhaust lines. Condensate build-up increases pump vibration, noise, and can damage seals and lubricants. If the condensate is corrosive, it further heightens the risk of pump degradation.
Solutions include:
- Keeping the drain valve open to prevent condensate build-up in the silencer or separator.
- Ensuring that the discharge pipe is lower than the pump outlet to allow gravity drainage.
- Installing steam/liquid separators and recirculation pumps at the discharge port to manage condensate.
- Adding a check valve on the discharge side to prevent backflow of liquid into the pump.
Operating Tips for Screw Pumps in Harsh Environments
Pre-Operation Nitrogen Purge: Preheating the pump with a nitrogen purge stabilizes the temperature and helps prevent condensation during start-up.
Post-Operation Nitrogen Purge: After completing the process, it is advisable to purge the pump with an inert gas like nitrogen to remove residual process gases before shutting down the pump.
Steam or Solvent Cleaning Before Shutdown: In cases where there is a risk of deposits forming inside the pump, a steam or solvent flush followed by a purge can be performed to clean and dry the pump before it is shut down.
Dry screw vacuum pumps offer significant advantages in chemical and pharmaceutical processes by eliminating the need for process fluids, thus avoiding contamination and operational issues. With proper system design, corrosion protection, dust management, and condensate handling, these pumps can reliably operate even in the harshest environments, making them a robust solution for modern vacuum applications in these industries.
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