Various plant applications, such as drying, distillation and filtration, require vacuum pumps. Unfortunately, a number of common errors can lead to incorrect sizing of a pump. Failing to manage temperature and pressure within the pump to prevent condensation and polymerization is one prime example. Not considering how gases and vapors are handled at the exhaust of the pump and the impact of that is another.
Before selecting and installing a vacuum pump in your chemical plant, you should watch out for five additional common pitfalls.
1. Concentrating on the steady-state operating point of the process and ignoring what the pump must do to get to those conditions. Many users size vacuum pumps based on pumping speed and pressure for a specific operating point. In various continuous chemical applications, maintaining that operating pressure, flow and temperature is key to success. Nonetheless, in some cases, the pump must reach those conditions from much different ones. Incorrect sizing may prevent the pump from getting to the desired operating point.
Degassing, outgassing and chemical reactions in the chamber are important issues to consider, as these potentially can add substantial unexpected gas loads to the process.
Gas trapped in the material or the chamber itself can degas large amounts of mass; the degassing will increase when the pressure goes down and the temperature increases. That gas will expand under vacuum, so the pumping system must be sized accordingly to handle it. Degassing affects pump-down times and/or process conditions. An oversized pump is needed if any degassing from the process takes place.
Outgassing occurs when constituents in the process change phase. Material will turn from liquid or solid to gas, potentially generating a large gas load inside the chamber. Small volumes of solids and liquids can generate large volumes of vapor that, in turn, will expand with vacuum and temperature. The effect mirrors that of degassing and requires sizing the pump to handle the new vapor load.
Outgassing, given the dramatic changes in volume, often poses a larger challenge than degassing.
Chemical reactions in the chamber will cause variations in gas load. Breakdown reactions generally generate a larger number of mols, each with a smaller molecular weight. This means if a compound breaks down into two constituents, each having half the original molecular weight, the pumping speed requirement potentially can double. Reactions also introduce new gas and vapor species to the mix that require consideration from a chemical compatibility standpoint.
2. Sizing motors incorrectly. Motor sizing issues can arise with some specific types of vacuum pumps, for example, liquid ring units. Most vacuum technologies come with a motor installed that is sized to provide the pump with the power needed under all or most of its operating capability. The power requirements of liquid ring pumps change depending on the seal liquid being used. Published power values often are based on water as the seal fluid. However, if another seal liquid is required (e.g., because of chemical compatibility, vapor pressure, etc.), the viscosity and, hence, the power requirement can change considerably.
Speed variation is another factor that can pose pitfalls when sizing vacuum equipment for chemical applications. Understanding how the power consumption and performance changes with speed for different types of vacuum pumps is important to a successful application.
The power consumption of most vacuum pumps varies linearly with speed when operating far from their volumetric efficiency limits. However, others, like liquid ring pumps, behave differently — the relationship between power and speed is squared.
Different types of pumps can handle different speed ranges. Vane and liquid ring units have restrictions related to the centrifugal force required to snap vanes out or maintain the liquid ring; booster, claw and screw pumps have limitations based mostly on lubrication and volumetric efficiency. Assuming a pump can operate at any speed can lead to problems.
3. Not accounting for pipe restrictions. Calculating performance losses due to piping for vacuum applications using pressure drop guidelines for compressed air can cause issues. Pressure drops in compressed air directly relate to air speed and friction. They often are modeled as resistances when using electrical-to-pneumatic-analogy analysis. Losses due to pipe length, restrictions, filters, valves, etc., under vacuum instead should be modeled as capacitors, so the components accumulate flow that can be accessed later. The calculations and results differ. Therefore, compensating for “pressure drops” calculated with compressed air guidelines could result in a grossly over- or under-sized vacuum system.
4. Ignoring chemical compatibility. Applications in the chemical industry involve a lot more than just pumping air. Important considerations may include the chemical compatibility of what’s being pumped with the components of the pumps, filtration units, plumbing and accessories such as valves and transmitters. Material specification goes hand-in-hand with sizing, and can greatly affect pump reliability.
Most pump hard parts are made of carbon steel, cast iron, aluminum and stainless steel; most elastomers are fluoroelastomers, Buna, ethylene-propylene-diene monomer, and perfluoroelastomers. Compatibility of these components with the gas stream at process pressure/temperature conditions is critical.
While there’s a lot of published information about chemical compatibility, it may not suffice for your application. Researching, asking around and getting advice from suppliers, colleagues, etc., sometimes can give you a fuller understanding what materials will work. Not paying adequate attention to chemical compatibility may result in anything from performance and reliability issues to serious environmental, health and safety problems.
5. Preventing condensation and polymerization inside the pump. Vacuum pumps are intended to pump gases and vapors, so most users put a great amount of effort into removing liquids and solids before they can reach the pump. This certainly helps protect the pump and system — but only is the first step toward meeting process reliability expectations. Many constituents entering the pump in the vapor phase can have a hard time staying that way as they make their way to the exhaust. Some can condense, solidify or polymerize, any of which can cause premature pump failure. So, to mitigate such risks, you should take appropriate steps, which may include:
• using vapor traps to prevent vapors from entering the pumps;
• selecting specific pump technologies that are more tolerant of certain deposits, polymerization/condensation;
• creating internal and exhaust conditions that allow the pump to better handle specific constituents;
• integrating flushing and purging procedures to remove these constituents during operation or between batches; and
• providing redundancy to allow for pump maintenance without impacting the process.
Always remember that vacuum pumps and systems are much more than the sum of their components. They represent the combined efforts and experience of the companies and people that designed, built, sized and sold them. Take advantage of their knowhow and learnings to ensure the success of your application.
ANTONIO MANTILLA is a product marketing manager for Atlas Copco Compressors, Rock Hill, S.C. Email him at [email protected].
