orifice – What Is Piping

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 Orifice: A plate with the orifice bore of required size for intended pressure loss is known as single stage restriction orifice. Multi-hole Single Stage Restriction Orifice: To reduce the noise generated, single stage multi-hole restriction orifice plates are used. As the high velocity flow at the RO inlet is distributed through several holes, the noise is reduced. To avoid the cavitation problem, multi-hole restriction orifices are used. The flow distribution through multiple holes improves the cavitation factor which in turn reduces the overall noise. Multistage Restriction Orifice Assembly: Multistage restriction orifices are widely used for very high pressure reduction when a single stage RO is not capable. It consists of a number of single stage RO devices. The design can be single-hole or multi-hole. The restriction orifices in a multistage RO are usually arranged in an eccentric manner. The minimum distance between each stage is usually the internal diameter of pipe. Types of Orifice Plates: The plates used in restriction orifice design can be of the following types: Square edge bore or standard bore orifice plate.Quadrant edge boreEccentric bore orifice plateSegmental bore orifice plateRTJ type orifice plate with an integral gasketPaddle type orifice plates 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 Based on the service and requirements, RO needs to be sized for critical or pre-critical condition. During sizing, the pressure control RO plates consider the maximum pressure drop lesser than the critical pressure and flow control RO plates consider the critical pressure drop. ISO 5167 is generally followed for sizing restriction orifices. 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 are: 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 restriction 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 OrificeOrificePurposeRestriction Orifice is used for killing the high pressure or reducing pressure; a pressure reducing device.An orifice is 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 DropRestriction Orifice causes High Pressure dropThe pressure drop is low.Fluid VelocitySonic Velocity to ensure choked flow.Subsonic flow.Acoustic Induced VibrationRestriction Orifice is highly Susceptible to AIV and high noise can generateNot Susceptible to AIV.Table: Difference between Orifice and Restriction orifice Factors for Sizing RO device There are various factors that must be considered while sizing restriction orifice. Pressure Drop: For sizing and selecting the restriction orifice, pressure drop is a critical parameter. The required minimum thickness of a RO device is dependent on the pressure drop across the device. Flow Rate: As the pressure drop is dependent on the flow rate changes, the restriction orifice need to be sized for normal flow rate. For critical RO, the downstream flow rate should be considered. Sonic Flow: Choked or sonic flow condition may arise due to decrease in density and increase in velocity when a gas accelerates through a restriction. A Sonic flow in the pipeline generates high noise and vibration in the pipeline that may cause mechanical failure. To avoid this, the maximum pressure drop across a single-stage restriction orifice plate must be limited to critical pressure drop. Cavitation: In liquid flow restrictions with very large pressure drop, cavitation may occur. While passing through the restriction orifice, the velocity of liquid drops and pressure increases. Due to these, vapor bubbles can collapse and flashing can occur. this phenomenon is known as cavitation. To avoid cavitation, the restriction orifice should be sized to maintain the cavitation index less than the incipient cavitation index of the RO plate. Inlet pressure, outlet pressure, and vapor pressure are the parameters for cavitation index, and the incipient cavitation factor will be dependent on the beta ratio of the plate. Noise Level: Noise levels in RO can be predicted by calculating sound power generated due to pressure reduction. Next, the transmission losses can be subtracted to find the sound level at any pre-decided location. To reduce the noise in restriction orifice, the following options can be selected when sizing. Reduction of the pressure drop.Increase of the number of the stages of reduction.Using multi-hole RO plates.Optimizing the pressure drop across each stage.Increasing the margin between the cavitation index and incipient cavitation index. Click here to know the sizing procedure of restriction orifices for single phase fluids 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.