Part 1 - Calculating LOF - Flow Induced Vibration

The necessary question.

Author

Syed Mohd Idrus
December 02, 2023

Imagine you are a Piping Engineer and for some reason your process department is requesting you to evaluate the likelihood of failure (LOF) of a certain piping arrangement due to re-sizing of an orifice.

Your Notes - What is an Orifice?

  1. Orifice Plates: These are the most common type of orifices used in the oil and gas industry. An orifice plate is a thin plate with a hole in the middle. It is installed in a pipeline and acts as a flow restrictor. The size of the hole (orifice) determines the flow rate.

  2. Functionality: When fluid passes through the orifice, its velocity increases, and there's a drop in pressure across the orifice. This principle is based on Bernoulli's equation. The differential pressure (the pressure drop before and after the orifice) is used to calculate the flow rate.

  3. Applications: Orifice plates are used in flow metering, especially for measuring the flow rates of gases and liquids in pipelines. They are crucial in the oil and gas industry for controlling and monitoring the flow of fluids in pipelines.

  4. Design Considerations: The design of an orifice plate (like its diameter and the size of the orifice) depends on factors such as the type of fluid, its density, flow rate, temperature, and the pressure in the pipeline.

  5. Advantages and Limitations: Orifice plates are popular due to their simplicity, low cost, and ease of installation and maintenance. However, they can cause significant pressure loss in the system and may not be suitable for measuring flow rates of fluids with high levels of particulates or viscous fluids, as these can clog or erode the orifice.

  6. Standards and Calculations: The calculation of flow through an orifice is standardized, often following guidelines set by organizations like the International Organization for Standardization (ISO) or the American Petroleum Institute (API).

  7. Other Types of Flow Restrictors: Besides orifice plates, there are other flow restrictors like flow nozzles and venturi tubes, which are used in various applications depending on the specific requirements of the fluid flow system.

What is a Likelihood of Failure (LOF)? Well it is a screening process. Its a measure of probability. It is not absolute and it is based on simplified model. Which is good.

If the score is low, the probability of failure due to FIV is low. If the score is high, the probability of failure due to FIV is high and counter measure is necessary, i.e to add supports so that the span between two supports is closer than originally designed.

So back to the question, will the mainline fail [high LOF] due to the re-sizing?

These are the information necessary to calculate LOF.

  1. Density - this information is from PED (Process department)

  2. Velocity - this information is from PED (Process department)

  3. Size of pipe - from ISO metric drawing, mind you the External Diameter.

  4. Thickness of pipe - ISO metric drawing, the sch number.

  5. Length between two supports - from ISO metric drawing.

For ease of data retrieving, download the apps Pipedata. Its available on IOS and Android.

Lets continue at Part 2 of this series.

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