Flow – What Is Piping: All about Piping Engineering

What is a Restriction Orifice? Restriction orifice or RO, in short, is a flow control instrument device whose primary function to provide a restriction to the flow so that a controlled or restricted flow is achieved. Due to this restriction by the orifice plate, a pressure head drop from the upstream of the orifice to the downstream is observed. For a specific temperature and pressure condition, the orifice area at the outlet determines the volumetric flow rate of the fluid inside the pipe. Due to the high pressure drop at the restriction orifices, it can create acoustic induced vibration. Hence, studies must be performed by specialists. Orifice plate is basically a thin plate with an orifice in the middle. The plate is inserted between two flanges of a pipe for providing restriction or flow measurement. Purpose of Restriction Orifice Restriction Orifice is used primarily for two main reasons: If there is a requirement of Reduced flow orIf there is a requirement of high pressure-drops. The device is sized depending on the required pressure drop. They are designed to slip between the piping flanges. Restriction orifice comes with single restriction orifices or multiple restriction orifices in Series. Applications of Restriction Orifice Engineers and designers are familiar with a restriction orifice in the following applications for impeding flow or reduce pressure. They are installed downstream of blowdown valves to ensure a controlled flow rate in the blowdown piping or blowdown header.They are installed in the minimum flow bypass lines around centrifugal pumps.They are installed in Wellhead applicationsRestriction Orifice is sometimes used to restrict excess flow in case ruptures.Minimum bypassingInjection, cooling, and flushing of fluid.Sampling.The steam let down.Use as a simple static mixerN2 purge or constant gas sealControllability improvement Restriction Orifice Standard There is no direct standard addressing the design of restriction orifice, but some associated references are available as listed below: ISO 5167 Part 1 and Part 2ISA RP 3.2API -RP 550/551API 2531IEC 60534-8-3API Manual of Petroleum Measurement – Chapter 4AGA Report No.3API MPMS 14.3.2ISO 5024ISO 5168 Working Principle of Restriction Orifice/Orifice Plate Both Restriction Orifice and Orifice plate work following the Bernoulli's principle that states that pressure drop across the restriction orifice is directly proportional to the volumetric flow rate passing through the orifice plate. While fluid flows through the plate, fluid velocity changes that in turn, as per Bernoulli's equation change the pressure. This change in pressures upstream and downstream is measured to calculate the volumetric flow rate. Restriction Orifice Types Restriction Orifice Plates are of three types Single Stage Restriction OrificeMulti-hole Single Stage Restriction OrificeMultistage Restriction Orifice Assembly Inputs for Restriction Orifice Design The following inputs are required for the design of restriction orifice: PFD and P&IDHydraulic Calculation SheetPMSLine operating conditions like upstream-downstream pressure, temperature, flow rate, line size, Density, Viscosity, molecular weight, Cp/Cv, vapor pressure, etc. Restriction Orifice Design Steps The design of Restriction orifice or RO is carried out as per the steps similar to as mentioned below: Determination of ApplicationData Preparation for the orificeRestriction orifice sizing (by design engineer or vendor)Checking of critical design elements like cavitation index (Kc=0.93), pressure drop, minimum orifice diameter, allowable space in piping routing, etc.Filling out the necessary information in P&ID and RO datasheets Installation of Restriction Orifice The restriction orifice is physically a thin plate with one or more holes in it. In piping application, They are normally inserted in between two flanges. It is possible to inadvertently leave out the plate when the piping is assembled, or, more likely, to forget to replace it when the piping is reassembled, following maintenance or cleaning. So leaving out the orifice is difficult. Restriction Orifice Types Safe solution To prevent this, one should make it impossible to assemble or reassemble the piping without including the orifice. A simple, practical and foolproof method is: the restriction orifice has its own spool piece. So in such a situation, there will not be scope for forgetting the RO element during installation or Construction. Orifice upstream and downstream requirements It is a standard engineering practice to keep 10 pipe diameters (10D) upstream (before the orifice) and 5 pipe diameters (5D) downstream of the orifice. Restriction Orifice Symbol The following symbols are used in P&ID to describe restriction orifice and orifice plates. Restriction Orifice Symbol Restriction Orifice vs Orifice Plate The major differences between Restriction orifice and orifice are tabulated below: Restriction OrificeOrificePurposeUsed for killing the high pressure or reducing pressure; a pressure reducing device.Used for flow measurement.Hole Profile The hole of the restriction orifice has a straight profileThe hole profile of the orifice is straight at first but then bevelled (notched) with a 45° slopePressure DropHigh Pressure dropPressure drop is low.Fluid VelocitySonic Velocity to ensure choked flow.Subsonic flow.Acoustic Induced VibrationHighly Susceptible to AIV and high noise can generateNot Susceptible to AIV.Table: Difference between Orifice and Restriction orifice Few more Resources for you.. Piping Design and Layout Articles Related to Piping and Instrumentation Interface Reference: CHEMICAL ENGINEERING/ APRIL 13, 1987 About the Author: Part of this article is written by is Mr. Amir Razmi, an International, dynamic, and multi-functional chemical engineer with 14+ years’ experience in engineering and EPC of oil and energy projects from pre-contract activities to execution, and closeout.