Circulating pump for high pressure and high temperature applications
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ture-controlled environment in which the pump itself was situated; the mechanical linkage between the two would then restrict the pump mobility during operation. Electromagnetic pump drives avoid the restriction of mobility, but small electromagnets are relatively weak and there may be problems with windings overheating leading to a breakdown of insulation. Bellows designs offer an alternative to magnetically coupled reciprocating pumps, but these suffer from a limited operating pressure, making them unsuitable for the present application.8,9 Another important and desirable feature that most of the previously published pump designs do not exhibit is the all-metal wetted parts free from elastomeric seals or lubricants.
The pump described here is illustrated in Fig. 1. The pump body was fabricated from commercially available high pressure tubing and fittings Sitec-Sieber Engineering AG, Switzerland_ rated for operation to at least 2000 bars and made of type 316Ti nonmagnetic stainless steel. The main pump cylinder was a high pressure nipple of length 200 mm and external diameter 14.3 mm the internal bore of which was honed to _8.00±0.01_ mm diameter. The piston, of length 40 mm and diameter _7.95−0.05 +0.00_ mm, was fabricated from a magnetic stainless steel, type 440C. The clearance was sufficiently small to permit an efficient pumping without the need for any type of piston rings or seals. The piston was magnetically coupled to a ring on the outside of the cylinder. This ring consisted of an annular SmCo magnet _outside diameter of 50 mm, inside diameter of 16 mm, length of 11 mm_ sandwiched between two mild-steel annular pole pieces _outside diameter of 50 mm, inside diameter of 15.00±0.05 mm, length of 14.5 mm_. This arrangement allowed for a closed-loop magnetic system which generates a large driving force. The SmCo magnet was the optimal choice for the present application, although NdFeB could be used for applications not exceeding 150 °C.
The design of the magnetic assembly _including the piston, the external magnet, and the pole pieces_ was aided by the finite element analysis _FEA_ modeling. Various potential designs were characterized in terms of size, shape, location _air gap_, and magnetization direction, and the FEA results were analyzed in terms of flux densities and coupling forces.
Each end of the cylinder was attached through a reducing union to a 1/4 in. tube-size high pressure &ldquo