David Brown Pumps Ltd., of Penistone, South Yorkshire, is a supplier of specialist, high technology pumps for the oil, gas and petrochemical industries. The pump cases for the company’s DB21 heavy duty series consisted of an assembly of three or five castings. These castings accounted for approximately 30% of the finished product cost when manufactured in carbon steel and as much as 50% when duplex stainless grades were used. The company implemented a design project to reduce the casting costs and the outstanding results have had a significant impact on the competitiveness of this range of pumps in world markets.
A New Design
The case for the DB21 range is of double suction and double volute design. David Brown Pumps’ new product team considered modifying it from a three or five component assembly to a two-piece construction consisting of a main casing and an end cover. This concept design also incorporated a significant reduction in casting wall thickness – from 25 mm to 20 mm – which took the company into territory where application of the most advanced, computer-aided design and manufacturing techniques would be beneficial, if not essential. Castings Technology International was identified as having the necessary expertise and facilities for the project, which called for finite element analysis and casting simulation of the component design through to CNC machining of the large-scale pattern and core boxes. The key tasks were to assess firstly whether such a substantial reduction in wall section would satisfy the performance requirements within the American Petroleum Institute (API) 610 Standard and, secondly, if the components could be readily manufactured to the quality and cost requirements.
Assessment of the Pump Case Performance
It was necessary first for Castings Technology International to create full 3D solid models, using SDRC’s I-DEAS Master Series software, from the drawings supplied. These models not only provided explicit digital definition of the parts for subsequent analysis of the geometry but they also helped all parties visualise the new design and enabled accurate weights to be determined. The finite element analysis module within SDRC’s I-DEAS software was then used to assess the effects of loading conditions on the new casing. Hydrostatic pressure and API-defined flange moment loads were analysed and the results predicted that the design would comply with API requirements despite the relatively thin walls. The API specification stipulates the maximum allowable deflection at the impeller shaft coupling when subjected to the flange moment loadings. Since the pump is mounted upon a fabricated bed-plate, this was also included in the finite element analysis in order to quantify the cumulative displacement of both pump casing and bed-plate.
Optimisation of the Design for Manufacture
As soon as the 3D solid models were available they were imported into MAGMAsoft’s casting filling and solidification simulation software so that the castability of the new case could be optimised concurrently with the finite element analysis. The software is designed to predict areas where defects such as micro-shrinkage are likely to occur and areas which are likely to prove difficult to feed.
The casting production method was simulated on screen and, due to the reduced wall sections, considerable modifications were needed to produce sound castings. For example, initial simulation runs highlighted the need for additional feeding in the bearing bracket area of the main case component. However, an area sufficient to locate the feeder was not available and so, after consultation, a small amount of material was added to the outside of the case to provide the required surface. The necessity for such an adjustment would normally only come to light after one or more defective trial castings had been produced. As an extra bonus, Castings Technology International’s engineers, who have many years of practical experience in gating and feeding system design and casting production techniques, discovered and solved a problem of gross shrinkage in an area of the casting unrelated to the reduced wall section. The area was found to be inaccessible to a supply of feed metal so the casting design was modified to remove the shrinkage and further increase its integrity.
The time very quickly arrived for a prototype pump to be manufactured and the company again broke new ground by creating the patterns and core boxes using CNC machining. For maximum pump performance it is essential that the patterns and especially the core boxes accurately represent the original hydraulic design. In this instance, the four core box assemblies had complex split line joints and the resulting cores had to align with a high degree of accuracy to obtain close control over the reduced wall section thickness. On this basis, David Brown Pumps felt that CNC machining had the potential to offer improved accuracy compared with traditional, manual techniques.
The casting geometries were transferred to Delcam’s DUCT software which was used to generate CNC machining code. The patterns and core boxes were readily accommodated on Castings Technology International’s large-scale Rambaudi Digi-Mill (4 x 2.5 x 1m working envelope), which guaranteed the precision of form and the accuracy of wall sections in the prototype castings. The castings met the ASTM E446 Level I X-ray standard, and passed dye-penetrant and magnetic particle inspection. Nozzle loading tests were carried out on David Brown Pumps’ facilities to test the structural integrity and the physical deflections were found to be well within API limits.
The project was executed within the target time limits and faster than would have been possible using traditional methods. More importantly, it achieved significant weight and hence cost savings over previous designs of pump cases. The fact that the prototype castings and test results agreed with the software predictions has given David Brown Pumps the confidence to apply concurrent computer-aided engineering techniques on future projects. The company now sees the creation of 3D solid models of key components as the ‘fundamental kernel’ of all design and development activity, helping to achieve further reductions in lead time of better designs at lower cost to the customer.
For further information contact Pete Bond.