Pumping flammable vapours with dry vacuum pumps
01 September 2019
Modern chemical, biochemical and pharmaceutical manufacturing plants utilise vacuum systems in many of their processes. In this article, Rudi De Koninck of Edwards Vacuum looks at the advantages of dry vacuum pumps and what to consider as part of a proper safety strategy when pumping potentially explosive atmospheres.
To provide the safe, reliable, capable and efficient systems for the provision of the vacuum conditions essential to their processes, many companies are turning to dry vacuum pump technology to meet their needs.
Proven and rugged non-contacting dry pump designs, both screw and reverse claw mechanisms, include the ability to control pump body and internal gas temperatures allowing a comprehensive range of solvents, flammables and corrosives to be pumped.
Dry pumps can provide the clean and robust vacuum solution needed to successfully drive these processes. A range of features and benefits enables dry vacuum pumps to excel in such environments:
- Excellent performance characteristics:
· Continuous operation from atmospheric to ultimate vacuum pressure
· Excellent temperature control
· Large liquid slug handling capability
· Capable of low-pressure operation without being constrained by sealing liquid properties
- Safe and compliant operation, capable of handling:
· Flammable gases
· Low auto-ignition temperature gases (T4)
· Corrosive gases
· Hydrogen
- Environmentally safe operation:
· No effluent generated
· Dry running mechanism
· Efficient solvent recovery
· Low power consumption
- Reduced maintenance
· On-site seal and bearing change
· Flushable with water, solvent or steam
- Simplicity
· Very good cost of ownership balance
· Configured for hazardous area installations
· No requirement for complex liquid sealing systems
This has led to a situation where dry vacuum pumps are now the preferred technology across a range of industries and market segments, including:
· Bulk Chemical
· Pharmaceutical
· Fine Chemicals
· Oil & Gas
· Specialty Chemicals
· Petrochemicals
· Oleochemicals
Dry pump technology is used over a myriad of applications, from bench scale/lab tests through pilot scale and API contract manufacturing to full scale process plants. Typical applications where dry vacuum pumps are used nowadays concern:
· Crystallisation
· Deodorisation
· Desorption
· Distillation - “normal”, molecular, short path
· Drying, including filter & freeze drying
· Evaporation
· Filtration
· Fluid Handling
· House vacuum/Central Vacuum/General service/Pilot plant
· Pervaporation
· Polymerisation
· Reactor service
· Solvents/ Solvent recovery
· Sterilisation - Ethylene Oxide
System design
In order to settle on a robust solution, the vacuum system designer will consider many factors to ensure an optimally efficient, safe and reliable vacuum system is delivered for use in a specific application. Factors considered in designing the system include but are not limited to:
· Process design
· Equipment specification and selection
· Safety and operating procedures
· Vacuum systems and control integration
· Commissioning and installation
· Installation and Operation Qualification
This will typically include a dry pump system with a series of accessory modules including:
· Mechanical boosters – for increased pumping speed and improved ultimate vacuum.
· Gas purges – to dilute flammables and extend seal life.
· Safety devices; solvent flush, inlet isolation valves, flame arrestors
· Recovery vessels – for solvents or other fluids
· Monitors and controls – from gauges and 4-20 mA transmitters to fully enclosed bespoke control units.
Safety/ATEX considerations
The European ATEX legislation is the most stringent in the field, requiring pump manufacturers to consider “malfunction” and “rare malfunctions”, and products to be tested by external certified houses.
Dry pumps have contact-free pumping mechanisms and no ignition sources in normal operation. However, they are often required to pump potentially explosive atmospheres and it is necessary to consider the possibility of ignition sources resulting from failure conditions. A proper safety strategy should include the following steps:
1. Identify the potential for explosive atmospheres. Understand where an explosive atmosphere may occur and where an ignition might take place. To do so, one should consider the external atmosphere surrounding the pump and the internal atmosphere within the pump, including the process interface to the inlet of the vacuum pump, and the area from the inlet on the vacuum pump to the exhaust interface.
2. Avoid explosive atmospheres. Operation outside the flammable range ensures that even if an ignition source is present there will not be an explosion.
3. Eliminate both spark and auto-ignition sources. Auto-ignition can be avoided by configuring the pump correctly, but it is impossible to eliminate all potential sources of spark ignition in a dry mechanical pump. Like other mechanical, rotating equipment, dry pumps contain bearings that can fail and cause spark ignition. Debris such as built-up solids could also lead to hot spots. Therefore, if pumping in the flammable zone is unavoidable, steps are to be taken to minimise sources through good pump design, operation, and maintenance practice.
4. Limit the effects of a potential ignition. If, despite the above precautions, an ignition does occur, it is important that it does not cause a major incident or any damage.
The following considerations will limit the effect of an explosion:
· Protection by constructional safety. The protection principle of constructional safety is defined in European standard EN13463-5. This specifies the way in which mechanical equipment should be designed to avoid active ignition sources. This is particularly applicable to the internals of dry vacuum pumps that commonly operate with small clearances. It is possible, after careful consideration, to use this alone and safely pump flammable vapours.
· Protection by containment. Containment in a vacuum system requires the prevention of flame propagation to the external atmosphere, the process chamber and the exhaust pipework. Preventing propagation along the pipework requires individual consideration. Flame arrestors have been used effectively for many years in the chemical, pharmaceutical and associated industries to prevent the propagation of internal explosions. However, their performance is dependent on the geometry of the connecting pipework and pump. It is therefore essential that the performance of the flame arrestors is tested with the pumps to prove that as a combination they will contain any explosion.
· Protection by an inert gas. This often leads to bigger pumps because of the continuous flow of nitrogen at the inlet of the vacuum pump. More controls also have to be considered which leads to a higher cost.
Summary
Dry pumps are now well established around the world as an efficient, reliable option for demanding chemical processing industry applications. When looking into an application involving dry pumps, it is important to consider a few points when specifying equipment.
Firstly, a proper strategy for identifying and classifying any hazardous conditions, such as flammability of gases and the potential for an explosive environment (both internally and external to the pump), is essential. Dry pump technology has contact-free pumping mechanisms and the principle of “constructional safety” can provide the required protection omitting the need for additional flame arrestors and inert gas flows.
Secondly, mechanical dry pumps should have a temperature-pressure profile that is not too cold nor too hot in order to reduce the risk of condensation (and subsequent corrosion), polymerisation, auto-ignition, and pump failure.
Lastly, business success depends on maximum equipment uptime and minimum total cost of ownership. The total cost of ownership includes the capital, installation, operating, and maintenance costs. Dry pumps offer the best thermal efficiency of any chemical vacuum producing system. Reduced power consumption results in a lower carbon footprint and environmental impact.
The final choice of a vacuum system should be based on safety, cost of ownership and payback. Despite the higher capital cost of a dry pump system, the lower cost of ownership often means these pumps pay for themselves particularly quickly.
About the author
Rudi De Koninck has a long career and vast experience in the realm of vacuum, particularly in the chemical market. He started with Edwards in 1990 as Sales Specialist Chemical Industry. Later, he was Sales Manager for Benelux and Scandinavia. From 2000 till 2006, he held the role of Market Manager and from 2007 he took the lead for the CPI Market in Central Europe until 2017. Since 2018, he has been in his current position of Business Development Manager CPI.