Bypass orifices are used to reduce pressure in fluid flow for a variety of situations, specifically wherein variable flow rates are not available. As compared to studies regarding venturis, square-edged orifices, and similar devices, much less is known about performance of orifices configured similar to pipe nipples or nipple orifices. Thus, in most cases, the system designer chooses a nipple orifice based on assumed losses, which has a discharge coefficient of about 0.6. However, it is important in such situations to have additional information in order to optimally design these systems for minimum energy consumption.

Herein, data and design information are presented for pressure drop as a function of flow rate for nipple orifices ranging from 1/8 to 7/8 in. (3.2 to 22.2 mm) diameter in pipes ranging from nominal 1/2 to 1 1/2 in. (12.7 to 38.1 mm) diameter. Curve fits accuracies for the data given are within plus or minus25% for flow rates above 20 gpm (4.54 m3/h), and range as high as plus or minus 40% for lower flow rates.

Discharge coefficients are also presented as a function of Reynolds Number. It is shown that there appears to be no clear trend in the data as a function of Reynolds Number. However, multiplying Reynolds Number by factors of diameter ratio and orifice diameter yields a trend that could be used in nipple orifice system design. Discharge coefficients can be predicted reasonably well, with the general error for all orifice-line sizes studied herein being plus or minus 5% (with 77% confidence). The results presented herein can be used to develop discharge coefficient predictions with other errors and confidence levels. For future work, more data are needed on large orificeline combinations. Needed then are theoretical studies to understand the basis for modifying pressure drop and/or Reynolds Number by functions of diameter ratio and/or orifice and line diameters. This and future work will then be able to provide basic data to develop modeling and software tools to allow the optimum orifice to be selected to minimize energy waste. In addition, future effort should be focused on the comparison of variable speed pump system design, that does not need flow restrictors like nipple orifices, with constant speed pump system design with some type of flow restrictor.