The enigmatic RDLP Pump
With greater demands being made on the availability of clean water in countries where water is very much a precious commodity, greater resources are being invested in the types and designs of the water supply systems. One of the key issues in designing a water transportation system is hydraulics and this is where the experience and technical capabilities of pump manufacturer is all important. KSB AG with its considerable global experience in providing pumping equipment for water transportation systems has vast experience in this particular area and recognises that lower hydraulic pressure losses result in energy savings.
With its Omega, RDLO and RDLP horizontal split case single and multi-stage pumps, KSB provides a large and varied range of system designs that cover the most diverse of requirements and customer specifications. Water supply on a broad basis is becoming one of the biggest challenges to mankind because of population growth in areas where it is hot, where the ground water sources are scarce or not clean due to pollution. In addition to this is a growing demand for water from agriculture, industrialisation and mining in remote areas.
As a result, there is a fast growing requirement to transfer water over far longer distances, making it necessary in many cases to customise the water supply structure and this will involve project engineering to suit the recognised and defined needs of the project. It also has an impact on pump technology and KSB has responded by developing a pump capable of transferring water over longer distances, namely the RDLP (Fig. 1).
“Within the water industry, no tailored water pump product range exists for water applications to accommodate flows beyond 2.5m³/sec for medium to large heads above approximately 250m,” comments Axel Luedecke, KSB General Manager, Pumps, and Applications Water Transport. “Historically, what has been offered by pump manufacturers to the water industry has been API (American Petroleum Institute) pumps. The problem is that these pumps have shortcomings for water transport applications because they were neither designed nor built specifically for water transport duties. Possibly 30 years old in terms of design, this has an influence on maintenance and wearing parts. “
The API specifies requirements in terms of design that are heavily over engineered for standard water transfer applications. Accordingly, such solutions based on a horizontal split casing design are not cost effective. In addition they are often not efficient in their performance compared with efficiencies of dedicated water transfer solutions. These factors provided the opportunity for pumps to be developed specifically for the application. Historically, the majority of the pump manufacturers have been offering API pumps and customers have had to accept what was on offer. “Here at KSB we are taking a different approach and have developed a water transport ‘concept’ pump suitable for applications in the DN350 – 1000 hydraulic system range. This design now meets the application’s specific requirements in terms of cost and efficiency,” continues Axel Luedecke.”
For applications where the nominal diameter of the discharge (DN) ranges between 80 and 700, KSB has for many years been supplying its Omega and RDLO series pumps, which were developed for the water industry.
The RDLP pump is something of an enigma within the KSB product offering as it is a fully engineered pump. It is a customised pump that grew out of the RDLO axial spilt case single stage pump series developed for water transportation. Pump sizes are developed fully project specific and are based on certain basic design principles.
“The RDLP fits alongside the Omega and RDLO series pumps, which cover the 80 – 700 range, but it was never developed as a range in its own right, only as part of the strategy to respond to the trend of market needs towards large flows to high heads. As part of that strategy, an engineering design tool was developed that would allow KSB to minimise time and resources for engineering such a pump,” says Frank Hafner, Senior Engineer Hydraulic Development KSB AG. “Historically it had taken a long time from the moment where KSB would have a defined requirement of flow, head and NPSH alongside working on hydraulic engineering and determination of the hydraulic shape. From then on it took a lot of time to model and engineer data as required for cost and price estimation.”
Based on dimensionless parameters, this tool generates the main dimensions of the hydraulic shape for given flow, head and rotational speed. From that the pump dimensions are derived including those of the raw material parts, such as casing, impeller, diffuser etc. Further it generates a dimensional drawing of the pump as will be necessary for layout planning by the contractor or customer (owner/operator). Such information can now be generated within less than a day, and this can be presented to procurement so that they can go out to enquire for purchasing. Proposing such a customised solution requires a lean process supported by appropriate tools that generate the appropriate information in a time frame to meet market expectations in a fast moving environment.
Whilst it appears that it is possible to press a button (in this engineering tool for proposing customised solutions) and out comes the pump, this is not completely true. However, it does look very much like the real thing. The approximation of what the pump will look like once it is fully engineered after the order and contract have been received is very close, particularly in respect of dimensions and weight.
Once an order for a proposed solution has been received the detailed design process starts in hydraulic engineering. As an input into hydraulic modelling serve basic design data, such as flow, head and NPSHa, new hydraulics are developed using a combination of methods and tools. CFD (computational fluid dynamics) modelling is the state of the art resource, complemented by the experience from existing and documented hydraulics that may be used for scaling up or down. Finally model testing on a technical scale is a useful step to verify performance for completely new hydraulics.
The result of hydraulic modelling will be a shape that serves as the input into mechanical engineering design (Fig.2). This design stage generates a 3D model, which needs to be analysed via structural mechanics using FEM (finite element mechanics). Via a number of iterations in which reinforcements may be introduced, reallocated or deleted the 3D model is optimised to the best possible compromise of weight on one hand, but sub-critical tensions and stresses on the other.
“The term RDLP refers to the bearing type, so it can still be a single stage pump with plain bearings as opposed to anti-friction bearings,” explains Axel Luedecke. “The latest incarnation of the RDLP is the single stage 600-1300/1, which uses plain bearings and a forced oil lubrication system. However, RDLPs have been produced with antifriction bearings but these are multistage pumps.”
Generally, for all pump design types the choice of bearing is determined by the weight of the rotor. Radial (from rotation of the impeller) and axial (from thrust) forces must be considered too. However for double flooded hydraulics the axial component is usually considered negligible. Radial forces are normally rather low due to the double volute type hydraulic. Antifriction bearings are generally preferred, but they are limited whilst plain bearings can take higher loads (from the rotor weight) as are often present on such large scale pumps. Irrelevant of the bearing type, the double entry radial impeller principle is applied. For multistage pumps, that would require two suction nozzles to divide the flow evenly feeding two single flooded first stage impellers both of which feed the double flooded second stage impeller in the middle via a diffuser. Mating flanges are to DIN, ISO, BS or ANSI with materials complying to DIN in all required material combinations.
The RDLP 600-1300/1 is the latest version of the RDLP and has been designed for water transportation duties in Setif East pipeline in Algeria. Here the requirement is for pumping stations to be equipped with pumps and providing flows up to 1.44m³/sec per pump. Four of the stations are equipped with RDLP600-1300/1 single stage pumps with diffusers and fitted with bearings. The customer specifically requested that these pumps be supplied as single-stage variants with a diffuser. The material used for the manufacture of all impellers was stainless steel as required by the strength properties.
“Our preferred solution for this type of application was a two-stage pump on the grounds of performance and efficiency,” continues Axel Luedecke. “In order to achieve a maximum head of 262m and the minimum efficiency specified, the diffuser became some sort of a safeguard, whereby it was specified anyway. Normally we would supply the standard RDLO with a double flooded impeller and no diffuser. By incorporating the diffuser in the RDLP, KSB applies a known principle to a new application, i.e. single stage, double flooded horizontal split casing design. The diffuser on a single stage pump of this design principle was perceived as too big, too heavy, too expensive up to and until this project was specified. We never felt that we could ever sell such a pump.”
Other locations for which RDLP pumps have been designed include Boussiaba and Setif West in Algeria (both using same pump RDLP 600-1300/1), Hofuf in Saudi Arabia (RDLP 350-550/3), Dandalup (RDLP 350-700/2), Perth in Australia (RDLP 500-950/2), and Caracas in Venezuela (RDLP 400-690/2).
“We have supplied these pumps in a range of discharge nozzle sizes, nominal impeller diameters and stages for these projects, and in doing so we are in effect creating a type series of RDLP pumps project by project,” says Axel Luedecke. “The target is to develop new sizes as and when we have projects of a size where we can tell the customer that it matches our range. Where it doesn’t match, but the project is for a reasonable number of units, we will develop the pump.”
Note for editors
Author: Bryan Orchard
Telephone: +44 (0) 1420 588194
Bryan Orchard is a UK-based independent journalist and media consultant specialising in fluid handling technologies and environmental engineering.