Below are some of the frequently asked questions regarding screw pumps: -
What type of media can be handled by screw pumps?
Progressive cavity type screw pumps are used for highly viscous, heavy, abrasive, and aggressive media like potato pulp, cement slurry, mine water, sugar molasses, sludge, fruit pulp, municipal solid waste, phosphate dosing, milk of lime, slurry, animal feed transfer, high consistency pulp, viscous adhesives, edible oil, resin, solvents and binding agents etc.
What is the working principle of progressive cavity type screw pumps?
The screw pumps work on the principle of rotary positive displacement system. The main component, a metal-single threaded rotor, rotates around its principal axis and also around the axis of the stator section producing cavities at reduced pressures, which move axially from inlet to outlet.
Are screw pumps self-priming?
Yes. Screw pumps are inherently self-priming.
What Capacity and pressure can a single screw pump handle?
A single screw pump can handle capacity up to 300m3/hr and pressure up to 48 Kg/cm2 generally. Each manufacturer has its own range. It does not cover downhole screw pumps.
What are the distinctive features of single screw pumps?
Some of the distinctive features of screw pumps are-
- Single screw pumps are positive displacement type pumps. These pumps can handle solids in suspension or media containing a high percentage of solids.
- Due to the reversible rotation capabilities, progressive cavity pumps can perform with equal efficiency in either of the directions.
- These are silent running pumps which have separate bearing housing. It enables the fluid to flow with no contamination.
- Single screw pumps have high mechanical efficiencies which helps in extra power savings.
- Screw pumps also have low downtime losses.
- Screw pumps are non-clogging. They can handle solids in suspension or media containing a high percentage of solids.
What type of drive systems options are available with screw pumps?
Screw pumps can be run through following types of drive arrangements-
- V-belt/Pulley type drive system
- Geared motor drive system with flange mounted motor with driven coupling
- Gearbox with electric motor with drive and driven coupling
What importance does drive system has in the pumps?
The selection of drive is really important. The starting torque depends upon fluid being pumped. Slurries and viscous fluids shall have higher starting torque values. The selection also depends upon compression fit between rotor and stator. Viscosity correction factor should be taken into account while calculating drive power required. The drive should have torque values higher than that required by the pump at all times; during start as well as during operation. Drive manufacturer's published data should be consulted to select proper drive.
Can these pumps be customized as per requirement?
Yes. Screw pumps are versatile in nature. Each pump can be customized on the basis of the duty parameters. Refer to the enquiry format as shared in pump specification sheet.
- Media to be handled-
- Presence and size of solid particles-
- Flow rate-
- Discharge pressure required-
- Viscosity of fluid-
- Duty Conditions (Continuous/ Non-continuous)
What precautions should a buyer take before installing and running the screw pumps?
Screw pumps, if installed with all precautions and instructions given by the supplier, prove to last for 25- 30 years in terms of life. The most important precautions to be taken care of are-
- Pump should NEVER run dry.
- Flushing line provision to be given in the pumps. Flushing the pump with normal tap water is required before starting and after stopping the pump as the liquid/media has the tendency to jam/solidify.
- Foundation and grouting of the pump should be strong to avoid any vibrations in pumps.
- Timely replacement of small/fast moving parts due to operational wear and tear to ensure that life of bigger parts is not effected.
Should be as per pump type
While sending us single screw pumps enquiries many buyers share with us the standard technical enquiry sheet of other type of pumps like centrifugal, gear pumps and various other technology pumps while asking for screw pump. We have been discussing with the buyers time to time that there are multiple parameters which are applicable for one pump type but not for other pump type. We suggest the below while enquiring for screw pumps-
The client must keep in view following points while sharing the enquiry-
- Single screw pumps are not cost- effective and efficient on water.
- If a pump designed for viscous fluids is run on water, then -
- At slow speeds the slip will be more,
- Volumetric efficiency will be low,
- Installed motor for viscous product will run inefficiently,
- Mechanical efficiency will be low,
- This clarifies why absorbed kW on water is more than the liquid power.
- On water or water like fluids the same model can be run at much higher rpm; implying that for a rated capacity we can select a lower model at higher rpm; entire characteristics of the two pumps on two different liquids water and viscous fluids will be different.
- Impeller is not applicable in screw pumps as it is a part of centrifugal pump.
- Pump shut-off Head, suction specific speed or critical speed is not applicable for single screw pumps.
Therefore it’s important to restrict the data sheet to the actual fluid /pump type ONLY.The client must specify following:
- Sealing pattern- whether the requirement is for pump with gland packing or mechanical seal
- Drive system- whether the preferred drive system is V-Belt pulley driven, geared motors with driven coupling or motor and gear box with drive and driven coupling.
- VFD- Whether the pump will be operated with VFD or without VFD.
- Material of Construction- Preferences for MOC of pump parts like casing, rotor, stator etc. NOTE: If any data is not available with the client, it is always advisable to discuss the same with pump supplier and vice versa.
It is important to note-
- Capacity cannot vary once the rpm is fixed.
- NPSHA cannot be negative. If it is negative, liquid will not reach the pump and the pump will run dry.
- NPSHA should always be greater than NPSHR.
- Mechanical efficiency of the pump is dependent on capacity, differential pressure, measured absorbed kW and design of pump.
- There is nothing such as cold efficiency/hot efficiency in progressive cavity type pump.
Courtesy: Risansi Industries Limited Email: @risansi.com email@example.com Web: www.risansi.com