New techniques of immediate repairs for road surface damage: progressing cavity pump ensures efficient conveying of cold asphalt
According to the German automobile club ADAC, there were 538 roadworks on German motorways in September 2018, causing traffic jams and delays. But not all road closures have to take long: A Dutch company has developed a technology for quickly clearing roads for traffic after repairing minor surface damage such as scoring, milling or accident damage. A special fast-curing, resin-based repair asphalt is mixed in a special vehicle directly on site and applied to the surface. The great advantage of this method is that the surface is ready for traffic again in less than an hour. To further increase the efficiency of the process, a NEMO® progressing cavity pump from NETZSCH Pumpen & Systeme GmbH in Waldkraiburg/Germany has been used since April 2018 to pump the highly abrasive substance from the vehicle tank through the mixer. It features a special displacement principle which allows especially gentle pumping. Pulsation was reduced by half compared to the previous model – a rotary lobe pump. In addition, rotating stainless steel components and EPDM elastomer elements are used in the new model, which are particularly wear-resistant despite the aggressive medium and therefore ensure a long service life of the pump.
As surface damage can endanger road users, it should ideally be repaired as soon as possible – especially as delays will create further wear on the affected area. The technology from the Dutch specialists in the application of road surface coatings is a way of carrying out these repairs very quickly, efficiently and with lasting effect. The technology is suitable for asphalt seams, rim damage, scoring and surface damage. The process differs from others in that only the damaged area of the asphalt surface is removed, rather than larger areas. The company then applies the compound and levels the treated area. Because the special resin-based mortar cures especially fast, the technology is also used directly after accidents, for example: The repair vehicle then often works with police support to repair any damage immediately so that the road can be cleared for use again within the shortest time possible. Usually this can be within 15 minutes after application of the compound.
Mixing of the repair material using progressing cavity technology
“To get the special mortar ready for use, the cold asphalt mixture is stored in a tank with an agitator and pumped through the mixer, in a special vehicle directly on site,” explains Max Waser, Engineer Internal Sales bei NETZSCH Pompen Nederland. “The same vehicle then spreads gravel from a reservoir over the cold asphalt.” Until April 2018, a rotary lobe pump was used for the first step of pumping the mixture of resin, sand and silver sand. The pump had to be operated at speeds of around 250 to 300 rpm and suffered high levels of wear due to the highly abrasive medium. “This especially affected the elastomer lobes – in the end the company for the application of road surface coatings had to replace them every two to three weeks, so the vehicle was often out of operation,” explains Waser. Those responsible therefore decided to replace the previous rotary lobe pump with a more suitable progressing cavity pump with less pulsation and wear.
A NEMO® progressing cavity pump of type SY from NETZSCH was selected: Due to its special displacement principle allowing smooth pumping with low shearing and pulsation, this pump type is ideal for use on the specialized repair vehicle. The main parts which define the system are a rotating component – the “rotor” – and a fixed component – the “stator” in which the rotor turns. The rotor is designed as a type of round threaded screw with an extremely large pitch, large thread depth and small core diameter. The associated stator has one extra thread and twice the pitch length of the rotor. The precise geometrical mating means that conveying chambers are maintained between the stator and the rotor, which rotates inside it and also moves radially. These chambers continuously move from the intake to the outlet side and transport the medium. Basically, this allows any free-flowing substance to be pumped.
Low speed and double capacity
The volume of the conveying chambers remains constant with this technology and the actual chambers are self-contained. This not only prevents backflow, but also ensures that the conveyed medium is moved at stable volume and pressure, so that – in contrast to rotary lobe pumps – no shear forces and hardly any pulsation occur which could have a negative impact on the medium. In addition, the consistency and viscosity of the medium are insignificant in terms of product flow with this displacement technology. The progressing cavity pump is therefore ideal for the cold asphalt which has a temperature of 20 °C and a viscosity of 7,000 to 10,000 mPas.
The size of the conveying chambers and therefore the theoretical delivery rate depends on the size of the pump. A 360° rotation when the outlet is free produces the volumetric delivery rate per revolution. In the process, the volume conveyed is directly proportional to the speed, enabling accurate dosing. “We selected the largest possible pump model for our customer which makes optimum use of the space on the vehicle and also ensures low rotation,” explains Waser. “This allowed us to achieve values of around 50 to 75 rotations per minute, meaning particularly slow and precise pumping.” At the same time, the capacity of the pump was doubled to 1 m³/h at this speed.
Particularly wear-resistant design
The NEMO® SY model now used by the customer also features a design with free shaft end, grey-cast iron housing, coupling rod with mixing elements and rotating components made of stainless steel 304 or 316. These materials are particularly corrosion resistant and ensure a long service life of the pump. “For the same reason, a version with wear-protection chrome plating was used for the metal rotor and the stator, as well as all other elastomer elements, are made from particularly durable ethylene propylene diene monomer rubber (EPDM)”, explains Waser. This material is highly wear-resistant, chemically resistant to the medium and yet soft enough to prevent premature wear on the contact points with the rotor. The shaft sealing was additionally designed as a single action, direction-of-rotation dependent mechanical seal MG1, which – like the sealing insert – is also wear-free.
The coupling rod is equipped with mixing elements. This construction ensures ideal feeding and mixing of the compound of resin, sand and silver sand. This keeps the individual phases in suspension and do not separate. This successfully prevents sedimentation and the mixture is applied to the road through the discharge distributor nozzle.
The free shaft end of the pump allows attachment of a hydraulic motor. With this motor, the desired speed of the pump can be set easily and allows flexible use of the vehicle.
The NEMO® progressing cavity pump from NETZSCH has been used at the Dutch specialists in the application of road surface coatings since April 2018. With a pressure of 5 bar, it pumps 1 m³ of the resin/sand/silver sand mixture per hour. Thanks to the stainless steel components and the EPDM elastomer components, the pump has been running without problems so far. That it works without malfunctions in contrast to the previous model has a positive effect on the curing time of the repair asphalt: “If the components of the medium are mixed well continuously, which is supported by the coupling rod with mixing elements, the mortar now needs less time to fully cure. So the road can be cleared for traffic even sooner,” explains engineer Max Waser.
NETZSCH Pumpen & Systeme GmbH
Geretsrieder Strasse 1
Phone: +49 8638 63-0
Fax: +49 8638 67981