the effective size of orifice 106 is reduced or enlarged and the pressure drop caused by choke valve 102 is correspondingly increased or decreased in the depicted embodiment port 106 has a 5 8 inch diameter and is used for a relatively low oil flow rate of approximately one to seven gallons per minute the manner by which substitutable plate 90 is held within the gap in pipe section 36 is especially advantageous as it allows quick release and substitution and yet is of simple construction as depicted in fig4 the plate is of such a diameter as to extend somewhat beyond the outer periphery of the pipe section the protruding portion 112 is received within annular groove 114 of annular coupling gasket 116 the coupling gasket is composed of a suitable sealing material such as rubber about the gasket is disposed an annular grooved pipe coupling 118 of standard construction such that the gasket is received within annular recess 120 of the coupling coupling 118 is provided with annular legs 121 which are received within annular grooves 122 the coupling is separated into two semicircular halves and these halves are bolted together with bolts 122 124 in order to.