Screw pumps are the pumps with the discharged liquid head imparted by displacement of liquid; screw rotor(s) rotating in the stator, designed with the appropriate shape, are used to displace it. In terms of impacts from operating elements, screw pumps are classified as volumetric pumps.
Screw pumps are rotary geared pumps, and they can be made by altering gear pumps; for this purpose, number of gear teeth shall be reduced and their obliquity number shall be increased.
Screw pump types. Single-screw, double-screw and three-screw pumps
Single-screw pumps are horizontal volumetric pumps. The main components in these screws are the static holding ring made of rubber, with double-threaded helical surface, and the single-threaded metal screw rotating in the holding ring. During rotation, cavities appear between the screw and the holder surface; the pumped liquid is suctioned into these cavities and moved along the screw axis towards the discharge cavity.
Double-screw pumps are used predominantly to pump sea water, fresh water and mineral water with petroleum products admixture.
Double-screw pumps for fuel oil are used predominantly to pump fuel oil and other viscous liquids. These pumps include single end sealing and heating jacket; the b part is made of structural steel.
Three-screw pumps are used predominantly to pump non-aggressive liquids having lubricating properties, free of abrasive mechanical admixtures. The minimum viscosity is limited by the liquid lubricity; the maximum viscosity is limited by the electric motor power and the pump suction capacity.
Worm pumps and screw pumps: differences
Sometimes, screw pumps and warm pumps are considered as the equipment of the same group, because both types of pumps are displacement pumps. However, in terms of impacts of their operating elements, warm pumps are dynamic pumps.
The main component in a warm pump is a gerotor pair (screw pair) determining the properties of the pumping unit and its operation procedure. The screw pair consists of a static element (stator) and a movable part (rotor). The stator is an inner spiral with the number of starts m+1. This element is made of elastomeric material and joined with the holding ring made of iron. The rotor is an outer spiral with the number of starts m. This element is made of steel, coated or not. The most common standard model is a pump with a double-threaded stator and a single-threaded rotor.
Eccentric screw worm pumps has many properties making them the only choice for processes with non-uniform viscous substances or substances containing various admixtures as well as for locations where other pumping plants cannot be used due to different substrates. These pumps are self-priming, and they don’t need priming; in other words, the pumped substance is not an actuating medium.
Eccentric worm pumps can pump non-uniform substances with various admixtures. The size of permissible inclusions depends on the size of closed cavities between the rotor and the stator. There is a specific amount of pumped substance per revolution of a rotor; so, fine adjustment of the unit is possible. The pumps of this type are capable to withstand any industrial stresses under a persistent pressure; however, they are not prone to pulsations or flow breaks.
The screw pump structural components are as follows: the drive gear motor; the adapter stand; the stator – rotor pair; the outlet discharge pipe; the chamber; the hinge; the shaft sealing device.
The key operating element in the screw pumping plant is the screw pair. The screw-shaped rotor made of metal is inside the stator made of elastomeric material. Rotor revolutions result in variations of the volume of cavities within the pair; as a result, the pumped liquid moves along the pump axis. Liquid is displaced, and cavity volume variation results in suction effect.
Screw pump operation principle
The screw pump consists of a rotor (the only moving part, with the number of starts m) and the stator (the inner spiral, with the number of starts m+1). For pumping, liquid is moved along the screw axis; it cannot move back due to the closed space formed by the screws when their helical protrusions enter into the grooves of an adjacent screw.
The pump stator type is a factor determining the maximum liquid operating temperature.
Also, the temperature can depend on the liquid nature and the pumping plant operation conditions.
Liquid intake parameters
The screw pumping plant is a self-priming unit (this is also the case for small discharge amounts). For water (20ºC, specific gravity 1, viscosity 1º), the resulting head is 7 m H2O.
In terms of their operation principle, screw pumps are volumetric pumps with the positive liquid displacement. This operation principle contributes to stable smooth liquid discharge free of abrupt variations.
Scope of liquids for operation
Screw pumps have higher level of chemical and mechanical stability because the stator is made of an elastomeric material. These pumps are used for any liquids having abnormal (non-Newtonian) viscosity. Also, screw pumps can pump liquids with solid particles (except for abrasive particles), not resulting in worsening the pump capacity.
Commissioning and adjustment
To protect the pump stator, pour the liquid for pumping into the pump. Then, ensure that the shutoff valves are open on the inlet pipe and the discharge pipe. If the pump is provided with a continuous operation motor for which the RPM is adjustable, set the desirable discharge rate directly on the pump or install a control valve between the pump inlet and outlet to control the substance flow rate.
When you activate a pump, observe several safety rules. Never activate a pump when it is dry; otherwise, a stator can be damaged.
Also, avoid the discharge amount adjustment by closing the supply valve. Otherwise, the driving mechanism and the motor can be damaged if they are not protected by the overload relay.
Major parameters to be considered for the pump choice
To choose the most appropriate pump for your manufacturing process, several key parameters must be taken into consideration. First, decide what type of the pump installation will you use, and how this pump will be operated within the system. Also, the important parameters to be considered are as follows: the pumped liquid flow discharge rate (litres/min or m3/hour), the total discharge head pressure (bar), and the liquid column height over the pump suction pipe (NPSN). Other key parameters that must be considered for the pump choice are the characteristics of the pumped substance, such as chemical composition, density, viscosity, pH, temperature, and the concentration of suspended solid particles and the abrasiveness level. Also, the power grid voltage and frequency (for the motor connection) and the type of the driving gear mechanism used between the motor and the pump shall be considered.
Screw pump advantages
Screw pumps, as compared with other types of piping plants, demonstrate several advantages. Unlike the centrifugal pumps, screw pumps deliver the pumped liquid as a smooth pulsation-free flow; as a result, liquid is not mixed and its structure remains unchanged.
Unlike the impeller pumps, screw self-priming pumps demonstrate better operation with more viscous substances and provide the discharge pressure as high as 4-24 bar.
Screw pumps demonstrate several advantages as compared with piston pumps and plunger pumps, such as smooth liquid discharge and capability to pump the mixtures from liquid and solid phases with solid inclusions remaining undamaged.
As for other advantages of screw pumps, high self-suction capability of these plants and their easy design making them convenient for maintenance shall be noted. The pump consists of three major components: the stainless screw, the holding ring made of rubber, and the shaft seal. Liquid discharge in a screw pump is in proportion to RPM; as a result, pump parameters adjustment is easy, with the frequency drive available. The holding ring design with the self-adjusting gap results in high pump pressure with small sizes. With pressure growing, the gap between the operating surfaces of the screw and the holding ring remains unchanged.
Similar to other volumetric pumps, screw pumps are capable to intake liquid by self-suction from depths up to 10 m. Also, mechanisms of screw pumping plants are optimally balanced, resulting in low noise. These types of pumps also require only minimum service.
Application in industry
The first models of screw pumps arose in 1920th, and they were applied in industry 10 years later. The maximum operating pressure for this type of pumps is 30-35 MPa.
Screw pumps are used to pump liquids having various viscosities, gases, vapors and their mixtures. As a result, screw pumps are widely used in waste treatment, tobacco industry, textile and paper industry, metalworking, food industry and chemical industry.
Since 1980th, screw pumping plants are actively used in petroleum extraction; until now, these pumps are considered as one of the most promising technologies in this industry.
Screw pumps are used to pump petroleum products free of mechanical admixtures, such as fuel oil, petroleum, oil, diesel fuel, paraffin and other liquids demonstrating the lubricity property.
Sanitary single-screw eccentric pumps have been designed specially for food, chemical and pharmaceutical industries. These units consist of standard components with tight hinge joints, resulting in their strength and, also, easy operation and service.
We are convinced that our company ENCE GmbH will become your reliable and efficient distributor (authorized representative) of your screw pumps in the Russian market.
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