This guest blog post is part of a series written by Edward J. Farmer, PE, ISA Fellow and author of the ISA book Detecting Leaks in Pipelines. To download a free excerpt from Detecting Leaks in Pipelines, click here. If you would like more information on how to purchase the book, click this link. To read all posts on pipeline leak detection in this series, scroll to the bottom of this post for the link archive.
Leak detection performance requirements are usually the result of regulation or of risk analysis studies. Let’s wander in this forest for a few minutes.
Basic goals are broad and common. We don’t want the problems with pollution and public hazard that are endemic in parts of the oil-producing world. Reasonable people recognize a need to balance performance objectives and the economics of development. Societal needs for energy are balanced with the cost involved in producing it.
Developers are faced with regulations and a usually keen awareness of the impact of public outrage over a developer or producer making a mess. Probably the best example in the world of this process and its outcome is Alaska where ambitious regulations were developed and competently enforced. The regulations could be met but required a developer’s very best effort. Alaska benefited from development and the developers benefited from their production producing decades of safe and stable operation in a largely beneficial way.
Over those years I did projects “up there” for several operators and the instructions to us always stated or implied that meeting the regulations in a demonstrable way was, of course, essential but if we saw an opportunity to do a little better they wanted us to develop it and add it to their project. They saw a future in Alaska and wanted to care for their place in it.
Elsewhere in the world we did a project in a place that will remain nameless that had much the opposite attitude. If energy was required by the public then the government’s mission was to provide it for them. The result was largely uncoordinated development and obvious transgressions on the environment. You have probably checked into a seacoast hotel and been asked if you would like an ocean view room at a higher cost. In this place you were offered a room facing away from the ocean at a substantially higher price. We could go on and on about conditions there, but today’s point is that societal objectives and demands influence outcomes, for the good or bad.
One of the important objectives of a leak detection plan is specifying the required sensitivity. The two usual ways of doing that begin with either a leak hole size or a percent of mainline flow criterion. This percent of mainline flow (or some other relative flow rate or time-change of volume) is understandable, scalable, and testable in a fully transparent way. Since most leaks result from abnormal happenings the path the fluid takes getting out really isn’t relevant, but how much gets out will influence jobs and behavior for months to years yet to come.
The hole size specification provides a certain test condition. One regulator specified an 18mm hole. Pipeline behavior certainly changes with a new 18mm diameter hole, but how much? It depends on the fluid in the line and to a lesser extent on the conditions along the escape path. When this hole opens, some of the flow upstream of it will go into it, and that which does will not show up at the downstream end. Typically, observability is at the ends so the question becomes, can the instruments at both (or either) end see a change in flow and/or pressure of the size involved? We, of course, can use sensitivity analysis to assess the impact of this leak on things we can observe. After a lot of experience, we can also learn how to empirically relate the percentage of leak flow to line flow, from which we can estimate performance from statistics involving various detection methodologies. This gets analysis back to the same situation involved with the leak rate specification in the previous discussion. Well, almost but not quite.
ISA has a wonderful collection of standards and explanations regarding flow through orifices, e.g., round holes with 18mm diameters. Scanning through all the factors that can be involved in flow through such an “orifice” we run into things such as a “choked flow factor” which worries about gas formation in an expanding liquid stream effectively reducing the area of flow through the orifice.
Other issues might involve whether the edges of the hole were sharp or beveled. There is also discussion about the effective diameter of the fastest flowing portion of the escaping fluid – the “vena contracta.” There are ways calculate all these things and when we make reasonable assumptions about configurations and values and the confidence, we have in what we know it’s easy to wonder where in the range between 0 and 1 that coefficient might reasonably be.
A fellow I used to work with, a guy with a lot of experience, would almost instinctively plug in 0.5 in this loosely understood situation, meaning about half the area of the subject orifice was actually carrying flow. I always thought that we could do better but another two decades of experience has caused me to wonder. So, estimating the flow through an 18mm orifice would involve assuming equivalence with flow in a tube about 12.7mm in diameter.
The point we need to consider is, does it make any sense to have a standard that few really understand and has no common or unique meaning in practice? When whether a standard has been met cannot be evaluated and demonstrated transparently one must wonder why, and what process would be applied to assessing if it has been met.
The purpose of such a specification should be clear, obvious, and understandable to all involved. That doesn’t mean it should be done a specific way. It does require that the way it was done make sense to all, or at least most, practitioners.
Learn more about pipeline leak detection and related industry topics
Book Excerpt + Author Q&A: Detecting Leaks in Pipelines
How to Optimize Pipeline Leak Detection: Focus on Design, Equipment and Insightful Operating Practices
What You Can Learn About Pipeline Leaks From Government Statistics
Is Theft the New Frontier for Process Control Equipment?
What Is the Impact of Theft, Accidents, and Natural Losses From Pipelines?
Can Risk Analysis Really Be Reduced to a Simple Procedure?
Do Government Pipeline Regulations Improve Safety?
What Are the Performance Measures for Pipeline Leak Detection?
What Observations Improve Specificity in Pipeline Leak Detection?
Three Decades of Life with Pipeline Leak Detection
How to Test and Validate a Pipeline Leak Detection System
Does Instrument Placement Matter in Dynamic Process Control?
Condition-Dependent Conundrum: How to Obtain Accurate Measurement in the Process Industries
Are Pipeline Leaks Deterministic or Stochastic?
How Differing Conditions Impact the Validity of Industrial Pipeline Monitoring and Leak Detection Assumptions
How Does Heat Transfer Affect Operation of Your Natural Gas or Crude Oil Pipeline?
Why You Must Factor Maintenance Into the Cost of Any Industrial System
Raw Beginnings: The Evolution of Offshore Oil Industry Pipeline Safety
How Long Does It Take to Detect a Leak on an Oil or Gas Pipeline?
Pipeline Leak Size: If We Can’t See It, We Can’t Detect It
An Introduction to Operations Research in the Process Industries
The Enigma of Process Knowledge
Energy in Fluid Mechanics: How to Ensure Physical Line and Operating Data Are Consistent
The Role of Standards and Regulations in a Pipeline Leak Detection Plan