The Misunderstood Liquid Ring Vacuum Pump
When selecting the right equipment for an application, the liquid ring vacuum pump can be considered archaic and inefficient. However, labeling this technology that way may be an oversimplification. This article is intended to help judge the reputation of the liquid ring vacuum pump by outlining its common ground and positives (and potential negatives) with technical advice.
There is a reason liquid ring pumps are chosen for critical infrastructure applications such as power generation and offshore oil and gas – when applied correctly, they are reliable. That said, it is important to first summarize the disadvantages of liquid ring vacuum pumps: they consume more energy than alternative technologies and consume too much water.
In applications with little or no condensable vapor loads, liquid ring pumps consume more energy to accomplish the same compression than comparably sized alternative technology such as a dry screw, dry claw, or vacuum pump. rotating paddles. The excess is usually published between 20% and 25%, however, depending on vacuum levels, it can be higher than this.
In applications that have large amounts of condensable vapors, this efficiency gap can be bridged (if not overcome) due to the “condensation effect”. In such applications, liquid ring pumps act as a direct contact condenser, reducing the volumetric gas charge inside the pump and, therefore, increasing the overall intake volumetric capacity. The greater the difference between the temperature of the saturated gas at the inlet of the pump and the temperature of the sealing fluid, the greater the effect. The condensation effect occurs with little or no consequence on the absorbed power. Under the right circumstances, the energy gap can be bridged and an additional generator is not needed.
Liquid ring pumps require a fluid to create the seal between the inlet and discharge pressures, which is fundamental to their operation. In many applications, this fluid is water, which is why liquid ring pumps are commonly referred to as water ring pumps. After fulfilling its function as a seal under pressure, the sealing fluid is expelled to the discharge in combination with the process gas. The liquid and gas phases are quickly separated, and what is done with the fluid establishes the net thirst of the pumps.
There are three potential scenarios.
1. In some applications, this sealing fluid is sent to a floor drain or other treatment system. This is called a once-through configuration and often what users think makes the liquid ring vacuum pump unnecessary.
Applications using this mode of operation are typically those expecting a high particulate load from the application since once-through operation continuously flushes the pump of potential material buildup. Essentially, this design avoids an awkward situation.
2. In other installations, the sealing fluid may be recycled, passed through a heat exchanger to remove the heat of compression and condensation, and then returned to the pump to complete another cycle of its mission. Called “total or complete” recovery, applications implementing this method are generally cleaner in the sense that the process gases should not contain excessive particulates. Additionally, full recovery facilities are often ones where continuous removal of contaminated water or condensed solvents is not desirable.
There will be other trade-offs to consider, depending on the type/design of heat exchanger and the level of vacuum required. In simple terms, a water-cooled heat exchanger will increase the cooling water requirements of the facilities, while an air-cooled heat exchanger will increase the overall power consumption and need to consider the environment of the facility and maximum ambient conditions.
For operation at coarse vacuum levels or for liquid ring pumps with large seal fluid flow rates, a recirculation pump may be required to overcome pressure drops in the seal fluid piping from the separator discharge to the pump.
3. In a partial recovery configuration, some of the hotter discharge seal fluid is recycled and combined with a fresh fluid supply before entering the pump. This effectively minimizes the amount of fresh liquid needed. Pump manufacturers will often quote partial recovery modes that can save up to 50% in fluid usage. The actual amount of savings will depend on the level of vacuum required, the fresh water temperature and the amount of heat added to seal liquid flow.
The sealing fluid setup can seem like an unlimited game of bets and strikes, or worse, a shell game of “hiding the utility requirement”. The good news is that the characteristics of the application (particle entrainment, presence of solvents, degree of condensable vapors) will often make one method more practical than others. Corporate energy and water initiatives can then be addressed by intelligently implementing the design and control scheme.
If there are liquid rings in an installation, users should check how they are configured. If installed in a once-through configuration, some minor piping can save money. Energy and water consumption are the two most cited shortcomings of liquid ring vacuum technology. Hopefully the above helps explain the circumstances under which these shortcomings might be mitigated.
Other benefits of liquid rings include:
In some chemical and pharmaceutical applications, it is desired to recover process gases (solvents) to sell them as a by-product or reuse them in the application. The use of a liquid ring pump with the solvent as the sealing fluid can be a solution to achieve this (see the condensation effect above) while maintaining a small footprint.
When pumping gases with relatively low auto-ignition temperatures, it is desirable to maintain low temperatures. After all, fires are not good for productivity. In a liquid ring pump, the heat of compression and condensation is absorbed by the sealing fluid. Since liquids have higher specific heat capacities than vapors, the temperature rise in a liquid ring pump is much lower than in other dry-running technologies.
Mechanically speaking, liquid ring pumps are much less complex than other technologies (especially dry-running alternatives). The shaft-mounted impeller is enclosed in the cylindrical body, which is capped by orifice plates and end housings, with tie rods holding it together. Tolerances between impellers, casing and end plates are wider than those found in dry-running claw or screw pumps, which require tight tolerances to create the seal between inlet pressures and of repression.
These wider tolerances, in addition to the lower temperature rise (and therefore less thermal expansion), make liquid rings more suitable for construction in varieties of metals. Additionally, liquid rings have seemingly endless design options for rotational speeds, bearing locations/lubrication, and seal types. Whether the application involves air or potentially toxic/corrosive vapors, there is a suitable liquid ring configuration.
Due to mechanical simplicity, liquid ring pumps are available in larger sizes for applications requiring high flow rates. For installations not requiring redundancy, this means that only one piece of equipment needs to be installed and maintained versus the multiple pumps that would otherwise be required.