Screw Pumps - An Overview
A screw pump is a type of rotary pump which is equipped with screws that mesh together and rotate within a cylindrical cavity or liner. The fluid enters from the suction side of the pump and moves linearly along these intermeshing screws to the discharge side of the pump. The clearances between the screws and the liner are very small hence the fluid gains pressure while moving through the pump.
Types of Screw Pumps
Screw pumps are available in a variety of different designs. Following are short descriptions of the most common types:
- One Screw Pump - One screw pumps are referred to as Progressive Cavity Pumps, sometimes called PC Pumps or Worm Pumps. These are generally not considered in the Screw Pump family because the rotor used in these pumps is not shaped like a typical screw, but rather a twisted round shaft. A Progressive Cavity Pump has one shaft that is slightly twisted in the shape of a screw, and is housed in a pumping chamber that is usually rubber lined.
- Two Screw Pump - The two screw pump, also known as the twin screw pump, is the most common type for high power applications such as heavy oil pipeline transfer. One of the screws is driven from the power source (motor, engine, etc.), and timing gears are usually incorporated to rotate the second screw.
- Three Screw Pump -The three screw pump, also known as the triple screw pumps, is typically used for small applications, such as lubrication systems. One of the screws is driven from the power source, which then rotates the other two screws around it, without the use of timing gears.
- Four Screw Pump - The four screw pump is essentially a Two Screw Pump, but with two screws per rotor, each facing opposite directions. The four screw pump sucks fluid in the suction port, then splits equally and is routed to both ends of the pump. The two fluid streams flow through the pump towards the middle and join together again before exiting the discharge port. Like the twin screw pump, the four screw pump has timing gears to drive the second rotor. Four screw pumps are often used in Multi-phase applications as well as oil transfer pipelines.
- Five-Screw Pump - The five screw pump is primarily the same as a triple screw pump, but with 5 screws instead of 3. Like the three screw pump, the five screw pump has 1 diving rotor that drives all of the other screws. The five screw pump is often used in lube oil or hydraulic applications.
Screw pumps are sometimes designated as “Double Suction” or “Single Suction”. Double Suction Screw Pumps vs Single Suction Screw Pumps is another way of differentiating between Two Screw Pumps and Four Screw Pumps. Two Screw Pumps are a single suction design, having the fluid enter on one end of the pump and discharge from the other end. Four Screw Pumps are a double suction design, having the suction stream split to both ends and moving to a discharge in the middle.
The Working of a Screw Pump
A Screw Pump is a type of Positive Displacement Pump. This means that it moves fluid by continually displacing the area that the fluid occupies. The screws are encased inside of a liner, usually made of some sort of metal. The fluid fits into the screw cavities within this liner and is forced through the pump and out of the discharge as the screws rotate and inter-mesh.
Since there needs to be some clearance between the liner and the screws, it is possible for any fluid that is pumped to slip backwards into the pump to lower pressure zones. For high viscosity fluids like this volumetric slippage is usually a non-issue. As the viscosity decreases, however, this slippage becomes substantial thus; reducing the efficiency of the pump. This has to be taken into consideration when pumping water or similar fluid, and particularly in multi-phase applications where vapor slugs are mixed into the fluid stream. In these cases, all the clearances within the pump must be minimized to reduce slip.
When a screw pump is pumping oil or some other type of viscous fluid, the screws can intermesh closely with very little or no clearance, since the surfaces are being lubricated as the fluid is pumped. When pumping water, water/gas mixture, or some other type of light fluid, these parts can not contact each other, else rapid wear on the parts will occur. For this reason, a three screw pump, (where one screw drives the other two screws without the use of timing gears) should never be used for water service or multi-phase service.
In the case of a triple screw pump, shaft seals are only needed on the driving rotor. The other rotors, including their bearings, are encased inside the pumping chamber and do not protrude out. In the case of a two screw pump or a four screw pump, both rotors generally protrude through the pump case, into a gearcase where the timing gears are housed. For this reason, four shaft seals are needed in a screw pump with two rotors.
Applications for Screw Pumps
There are many uses for screw pumps in a variety of different industries, including Manufacturing, Mining, and Oil & Gas. Most of these applications deal with high viscosity fluid, such as oil or asphalt, or multi-phase fluid, which means there is liquid and vapor mixed together in the fluid stream. Below are some examples of where screw pumps are used:
- Hydraulic and Lubrication Systems - Many lubrication systems and hydraulic systems use a screw pump. These are usually small, triple screw pumps. Lubrication and hydraulic systems like this are used for supplying lube oil to large machinery, supplying hydraulic power to high-pressure hydraulic systems or operating elevators in buildings.
- Heavy Oil Pipeline - When pumping high viscosity crude oil, screw pumps can offer an ideal solution for moving it through pipelines. Able to pump higher viscosities than a centrifugal pump, and able to pump higher flows than a reciprocating pump, screw pumps are often used in these applications. In these scenarios, very large pumps are sometimes used to get to the flow rate required. These pumps are usually driven by large electric motors or diesel engines.
- Multi-phase Pumping - Multi-phase pumping refers to pumping a mixture of fluid and vapor together, and is known to be one of the most challenging pumping applications. Serious damage can occur to compressors when liquid enters, just like serious damage can occur to most pumps when air enters. When built correctly, a screw pump can be a hybrid between a pump and a screw compressor, being able to handle both liquid and gas.
In order to prevent damage to a screw pump when air enters, the screws must not contact each other. For this reason, a four screw pump is usually used. In addition to this, the pump must be accompanied by a liquid boot, which is sometimes built into the pump but can easily be added externally as well. A liquid boot is a pressure vessel that collects fluid on the discharge of the pump. When an air slug comes through the supply pipe, a small pump moves the liquid from the boot and injects it into the pump. This provides slight lubrication to the screws and liner, as well as seals the clearances between the parts so the air/gas that is being pumped does not slip backward in the pump to lower pressure zones. Depending on the capacity of the liquid boot vessel, a multi-phase pump can pump 100% vapor for several minutes.
Multi-phase pumping is a challenge in several different industries. Gas Well Deliquification and Propane Cavern Storage are common challenges in the Oil & Gas industry.
Brands of Screw Pumps
Colfax Fluid Handling is the largest manufacturer of screw pumps, having numerous sub-brands including IMO, Allweiler, Houttuin, Warren and more. Bornemann, Leistritz, and Netzsch are other big players in the screw pump market.
Power Zone’s Screw Pump Experience
Power Zone has first-hand experience with screw pump applications, including Multi-phase Pumping. We designed and built the first Blow Down Unit for Gas Well Deliquification which is currently being used in Northern Colorado. We have sold other screw pump packages all over the world for Oil Pipeline Transfer. We carry used and new screw pumps from all leading manufacturers, and are able to supply refurbishing services and spare parts for each.