How to calculate pressure drop across orifice plate (liquid/steam application)? How to calculate LRV and URV?

While calculating an orifice there are three variables of which two can be selected: flow, bore diameter en pressure drop. If you need to know the pressure drop, you must not only have selected a required (maximum) flow, but also the bore diameter in respect to the line size. Typically the ratio d(bore) / Di (line) would be 0,65 - 0,75. Rule of thumb is that the calculated dP at max flow is 70 to 80 % of (URV - LRV). Usually LRV is taken as 0 and URV is a multiple of 500 mbar

ISO standard for measurement of fluid flow - ISO 5167

Tool to calculate flow rate through an orifice

http://www.tlv.com/global/TI/calculator/water-flow-rate-through-orifice.html