As process complexity has increased and new conditions have been added, screw pump technology has attracted increased interest from plant designers, process engineers and plant operators. whole industry.
The need to improve energy efficiency and operating flexibility while reducing operating costs is the primary focus. It’s time to revisit conventional approaches to pump selection and examine the evolution of screw pumps, to determine how they could improve economy and keep critical processes alive in process plants and transfer systems. .
Functioning principles
This article focuses on the types of multi-screw pumps. All screw pumps are part of the positive displacement family and as such are designed to move the flow rate in direct proportion to the rotational speed of the pump. This runs counter to hydrodynamic pumps, which rely on kinetic energy.
Seal cavities form when the screw profiles mesh as the pump rotates, transporting fluids from suction to discharge as it builds pressure to overcome back pressure downstream of the system. Pump screws are the main pumping elements, where the driven screw or power rotor mechanically and hydraulically transfers torque to one or more idler screws. The smooth opening and closing of the pump cavities results in pump flow with low pulsation and airborne noise.
Most screw pumps are designed to eliminate axial hydraulic thrust either by using balance pistons or by having the screws in an opposing flow arrangement. The absence of thrust bearings simplifies the design of the pump and eliminates potential areas of wear and maintenance.
Apps
Most multi-screw pump designs are self-priming and can operate with low suction pressure. They are also gas tolerant and able to handle free and entrained gases without vapor lock. The low internal fluid velocity and soft mesh of the rotors also contribute to low shear rates, which is particularly important for shear-sensitive non-Newtonian fluids as well as different types of emulsions.
The flexibility of operation of the multiscrew pump is manifested by its ability to work over a wide range of viscosity, from light hydrocarbons to residues and emulsions. Screws are normally case hardened for better wear resistance, and custom coatings are sometimes used to protect the rotor bores, rotor sleeve and pump casing.
The current family of screw pumps includes designs that have traditionally been used in hydrocarbon processing. Applications are increasingly found in the chemical, petrochemical, food and biofuel industries. Each pump design has its specific set of advantages and operational possibilities. Finding the right type of pump for specific applications is not only important for the process, but also an opportunity for cost optimization, including total cost of ownership.
Drawings
Twin-screw pumps without timing gears:
A pump design relies on two screws, a lead screw and a driven idler screw, which are radially supported by bushings and lubricated by the fluid being pumped. Bushings are also part of the axial thrust balance configuration where discharge pressure on one side of the journal and suction pressure on the other side create a pressure balance while supplying fluid for lubrication and the recooling.
Torque transfer from screw to screw occurs by means of rolling contact (as opposed to sliding) on the profile of the screw, providing good wear resistance. These screws operate with radial clearance to the bores, making this pump design resistant to abrasive wear and suitable for low lubricity fluids.
Typical applications for this pump design are with fluids like asphalt, bitumen, pitch, emulsions and oily residues as well as a variety of process fluids like methylenedianiline (MDA) and methylene diphenyl diisocyanate (MDI), biofuels and vegetable oils.
In order to eliminate vapors escaping from the mechanical seal, this can be replaced by a vapour-tight magnetic drive to eliminate greenhouse gas (GHG) emissions. It is often a good alternative to the time flow twin-screw pump.
Simplicity and robustness with no timing gears and a single shaft seal make this type of pump easier to maintain and operate.
Three screw pumps:
The three screw pump is used in a number of applications where fluids range from lubricating oil in lubrication systems to process fluids such as pitch, asphalt and light end products, such as condensate and vacuum gas oil (VGO). The most common pump execution used in process applications has a pumping cartridge (separate from the housing) to allow for different installation options. This design uses a driven screw called a power rotor, which does the main work of the pump and is surrounded by two idler rotors. They simply serve as rotating joints for the power rotor and support the radial forces borne by long journals formed by the rotor bores. Similar to other screw pump designs, the thrust load is hydraulically balanced without the use of thrust plates or bearings. The design also allows for higher velocity, which results in high volumetric efficiency, even at higher pressures and low viscosity fluids. Typical three-screw pumps can deliver flow rates up to 2,000 gallons per minute (gpm) with a maximum discharge pressure of 2,500 pounds per square inch (psi).
Many pump manufacturers use a cartridge design, which works for custom installations. The outer casing can be designed to match the casing of an existing pump and can be a direct replacement without changing the pipe fittings or pilot. By taking advantage of the flexibility of a cartridge design, a significant process improvement is possible, as shown in Figure 4.
New process requirements are met by having the pump inlet directly bolted to a suction box. The screw inlet section is cut out and can act as an auger “pulling” fluid into the pump.
For light ends, the three-screw pump offers several advantages. The compact and short design, compared to the commonly used horizontal electric submersible pump (ESP), makes it significantly smaller and less heavy for the same differential pressure. Internal hydraulic thrust balancing eliminates the need for the thrust bearing arrangement and its lubrication system, which is required for ESP. Additionally, the pump is gas tolerant and can handle gas entrained liquids without vapor lock or loss of suction. It also works smoothly and quietly outdoors.
and is not affected by any critical speed range. It is also not prone to surges or pressure pulsations.
Double-flow timed twin-screw pumps:
The most versatile of all types of screw pumps is the self-timer twin screw pump. Although some manufacturers offer programs with standardized twin screw pumps, they are often customized for a particular fluid, installation and service. They are designed for larger flow capacities than most other screw pumps and can also be designed for high head pressure. They are used with a large number of different fluids, from low viscosity non-lubricating fluids to high viscosity heavy oils, bitumen and molasses. Typical twin screw pumps can cover flow rates up to 6500 gpm with pressures up to 1500 psi.
Because the screw profiles rotate without axial and radial contact, the pump is independent of the lubricity of the fluid being pumped. However, at lower viscosity, the internal clearances increase the internal slip, which is a disadvantage for the volumetric efficiency. The reverse occurs as viscosity increases, slip decreases, and the pump becomes volumetrically more efficient. The double suction flow neutralizes the axial hydraulic force and the rotors are positioned axially in a bearing arrangement where the radial load is the dominant load.
The twin screw pump design is widely used in oil fields, refineries, tank terminals, pipelines and chemical plants. As the screws rotate without contact, the flexibility is almost endless in terms of the nature of the fluids it can handle. With variable speed control, a wide flow range can be covered, allowing flow and pressure to remain at a preset level with low inlet pressure and entrained gas. Polymer emulsion transfer in chemical plants, mixing and charging applications in refineries are good examples of processes. Another of them is the knockout drum pump.
The pump’s variable speed controls the level of liquid in the drum, and the vapor transport does not block vapor from the pump as it would with a conventional vertical pump. The low amount of net positive suction lift (NPSH) required by the pump eliminates the pit needed with a centrifugal pump. Precise flow control is an important aspect in pipeline service where variations in flow can be easily managed by varying the speed of the pump.
As the types of screw pumps shown show, there is almost always a good fit for a particular pumping setup. The increasing use of screw pumps testifies to the versatility and appreciation that this type of pump finds in the market. The development never stops, and recently new designs for sanitary applications have come to market. The fundamental characteristics of a screw pump lend themselves to new fluid handling challenges, and we will continue to see new designs introduced to meet demanding new pumping requirements.
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