A look at vacuum pumps 14-9
The idea is simple enough. The pump will be designed to pump liquid as well as gas or vapors. If you need to create a vacuum you can connect your piping to the suction side of the pump, but if you need a positive pressure you can connect the piping to the discharge side of the same pump.
We can use the vacuum we create to prime a centrifugal pump, evacuate a condenser, lower the pressure in a chemical vessel or do any thing else we can think of where a vacuum might be needed. The pressure side could be used to create a head or pressure in a nozzle application or a hundred other applications you might consider. The capacity needed, along with how much vacuum, or how much pressure, will dictate the type of pump you will need.
Vacuum can be measured in millimeters of mercury (Hg) or inches of mercury. Atmospheric pressure at seal level is 760 mm. or 29.9 inches. Any less than this amount is considered to be a vacuum. Hard vacuum starts at one millimeter of mercury. The industry calls this amount of vacuum “one Torr.” At one Torr and harder vacuums any elastomer (rubber part) in the system will out-gas, shrink in volume and then allow air to leak into the system you are trying to evacuate.
As we learned from our discussions about conventional centrifugal pumps, these centrifugal do not work very well pumping gas or vapors, so we will be looking at a positive displacement type to do this job and there are a variety of designs available to us.
It is important to remember that packing does not do a very good job of sealing vacuum so you are going to have to install a good mechanical seal on the rotating pump shaft or rotor to prevent the intrusion of atmosphere into the system. A hydraulically balanced, O-ring design would be desirable.
Liquid ring pumps
In these very popular designs that are sometimes called “wet vacuum pumps”, you start with a round multi-finned rotor spinning in an elliptical casing. The casing is full of water or some compatible liquid. The rotor throws liquid outward to the stationary casing where the solid ring of liquid rotates at the same speed as the rotor. The elliptical shaped casing causes the rotating liquid to recede from and enter buckets in the rotor. As the liquid is thrown away from the rotor it draws in gas and vapor through the inlet ports and discharges the gases through the outlet ports as the liquid isforced back to the rotor because of the elliptical shape of the casing.
- Use this design to 100 mm of mercury vacuum. Some manufacturers claim that they can operate as low as 4mm of mercury, but are limited by the vapor pressure of the pump’s sealing liquid.
- This design does not cause problems caused by a pulsating flow of liquid.
- These pumps are sometimes used with lobe pumps or gas ejectors to obtain lower pressures.
- Liquid ring pumps are available in a variety of alloys
- These pumps resemble multistage centrifugal pumps
- Gas is trapped between rings of liquid
- A liquid flow rate of 9 to 10 gpm is needed for a pump capable of pumping 400 cfm saturated air at 250 mm Hg.
- The end vacuum is determined by the vapor pressure of the liquid.
- At too low a vacuum the pump will cavitate.
- Most of the problems you will encounter are associated with temperature of the liquid rather than the flow rate.
- Filters are required in gas/vapor lines at the inlet if powders or solids are in the gas stream.
- Non soluble powders can cause vane wear.
- Because the discharge is at the center of the pump, heavy particles are forced away from the discharge ports by centrifugal force and collect at the periphery. “Clean out pockets and flushing valves” are sometimes provided to remove any accumulated solids.
- If a steady flow of liquid enters the pump it will not cause damage, but the power requirement will increase. Many manufacturers recommend installing a separating chamber or trap to prevent a solid liquid stream from entering the pump.
- Most manufacturers encourage the return of the pump for any repairs. This means that you will need a supply of spare pumps in your inventory.
- The sealing liquid flow rate is critical for the pump’s performance. Too little is as bad as too much. The manufacturer has a recommended flow rate, so try to follow it.
Problems with liquid ring pumps:
- Limited end vacuum and heavy hydraulic loads. To get a better vacuum you need either a booster pump, sealing liquid with a lower vapor pressure, or an added heat exchanger.
Rotary lobe pumps
- Often called “Roots” type.These pumps require timing gears.
- Use these designs to about 200 mm Hg. Often used as a booster pump.
- No contact between the rotating lobes reduces wear problems.
- Corrosive gases combined with condensation can cause severe corrosion problems.
- Powders in the gas stream present fewer problems than other vacuum pumps because of the non-contacting parts.
- These pumps are environmentally friendly because they exhaust only what they take in. They do not add water, oil or other liquids to the gas stream
- Can be used in both gas and pressure applications.
- Low cost to operate and reasonably energy efficient.
- Limited by depth of vacuum and corrosion problems.
Dry screw vacuum pumps
- The design looks like a typical twin screw compressor pump, but the design has been modified for vacuum service.
- No contacting parts in the vapor path.
- One manufacturer claims they can tolerate up to one gallon per minute of entrained liquid in the vapor stream. The pump can be fitted with a variable speed motor if you need pressure control.
Single stage oil recirculating, sliding vane pumps
- Like the liquid ring and rotary lobe designs they have limited vacuum capabilities. 15-20 mm Hg absolute
- Capacities of 1100 cfm free air displacement in the larger models makes them ideally suited for vacuum conveying systems.
- They are equipped with a horizontal carbon steel cylinder that is air or water-jacketed. Inside the cylinder is an off center carbon steel rotor with vane slots. Many design use three vanes made of a composite material.
- A thin film of oil (usually 40W non-detergent motor oil) is fed to the rotor and vanes to provide lubrication, cooling and corrosion protection. The oil is exhausted with the gas and passes through a coalescing filter that removes 99.9% of the oil from the gas stream and is then returned to the oil reservoir where it passes through a filter and the process repeats its self.
- Higher maintenance than lobe or vacuum ring designs.
- Oil level must be maintained and filters changed regularly.
- Cannot tolerate pressure higher than 2psig and therefore cannot be used on the pressure side of vacuum/ pressure conveying systems.
Once through oiling sliding vane pumps.
- Used where moisture, acidic or solvent vapors are in the gas stream and make the oil unsuitable for recycling, typically one gallon in 24 hours. Corrosion is still a problem
- Do not know of any designs with stainless steel rotors to lessen corrosion problems.
- The oil has to be separated from the gas stream. The majority of the oil can be removed with a simple mechanical vapor/ liquid separator. The remainder is a bit more difficult.
Dry running rotary sliding vane pumps
- Available in both single and two sage models
- No oil coating on the internals making them highly susceptible to corrosion from the process vapors.
- Vanes made of carbon impregnated composite material that causes such a small amount of friction that oil in not needed.
- Limited application in the chemical industry but a lot cheaper than oil version.
Two stage, oil once through sliding vane pump
- Excellent corrosion resistance
- Vacuum to 0.5 mm Hg. Absolute
- Sensitive to liquids in the vapor stream so they need very efficient vapor/liquid separators.
- Powders in the gas stream must be avoided
- Improper operating temperature is the biggest problem. If the product is too cold liquid from the gas stream can condense.
- Liquid will wash away the protective film and cause the pump to lock up.
- If the system is not void of oxygen, solvent vapors passing through the pump could ignite.
- At shut off the upper rotor is prone to overheating. A small purge of an inert gas such as nitrogen or carbon dioxide through the intake port of the pump will carry enough heat away from the upper rotor to prevent it from overheating, and then expanding into the backplate and locking up the pump. If air is bled in instead, and the solvent vapors are flammable, two of the three elements needed for a fire are present. Add the heat of compression and you could have ignition.
- The oil selection can be a problem. It must be compatible with any process vapors it will contact. Motor oil cannot be used when the gas stream contains chlorinated compounds such as methylene chloride. Chlorinated compounds react with mineral oil and form tar like deposits on the sliding vanes, rotor and the exhaust ports of the vacuum pumps. In these applications you need an alternative.
- Maintenance of these sliding vane pumps requires special tools and training so the manufacturers discourage in house repair.
All of these very popular sliding vane designs have similar maintenance problems. The following troubleshooting hints should help you solve some of these problems:
Make sure the oil rate consumption is not changing suddenly.
- An increase could mean a leaking oil line, a check valve is sticking open or the metering pump is leaking,
- A decreasing rate could mean that the metering pump is not working, a filter is plugged or one of the oil lines is clogged
Listen for an increase in noise level
- The pump could be running too cold. Vapors are condensing inside. In a dual rotor design the lower rotor could lock up.
- Entrained liquids in the product stream.
- Powders are getting into the pump
- The pump is running too hot. 195°F (90°C) is the maximum coolant outlet temperature recommend by some manufacturers. High temperature can cause “coking” of the lubricating oil.
- Lack of lubrication could cause the vanes to delaminate or become sticky
High internal pump temperature or condensate in the lubricating oil can cause the sliding vanes to “skip” over the walls of liquid lubrication causing a “washboard” affect on the discharge side of the cylinder walls and little to no contact of the vanes on the suction side of the walls.
- Be sure the oil is clean. Many operators remove the screen on the filling port to save filling time and forget to replace it allowing solids to enter during the filling process.
- Be sure to inspect the inlet and outlet ports of the vacuum pump. You should be able to see a part of the rotor and vanes through this port.
- Look for evidence of “coke” or tar at the discharge port indicating high temperature or maybe contaminated oil.
- Listen to the pump at startup to hear if the vanes are sticking and “clicking”.
Upper rotor is stuck, but turns freely after the pump has cooled. Over heating is the problem. That is the reason the rotor turned freely after it cooled. Look for:
- The pump was operated at overload conditions too long
- Too much exhaust back pressure
- No oil lubrication to the upper rotor
- You have had a failure of the cooling system
Upper rotor is stuck and cannot be freed.
- Powders ingested into the pump
- Process gases are reacting with the lubricating oil. Making it sticky.
- No lubrication
- Failure of the cooling system. High temperature can expand metal components and make the oil coke.
Lower rotor is stuck
- Pump is running too cold
- Powders ingested into the system
- Liquids ingested into the pump
- Wrong lubricant
- Poor or no lubrication to the lower unit.
The pump ran alright last time it was used, but now it is locked up
- The oil was “gummy”, but now that the oil is cooled down it has solidified.
- Process gas has entered the pump and corroded the internals
- On March 31, 2018