|If not discovered in a timely manner, the environmental and economic consequences of an undetected oil leak can be tremendous.|
Environmental and personal safety issues have created the need for more effective leak detection and location on oil pipelines. For companies operating these pipelines, a leak, no matter how insignificant it might seem, can result in major liabilities. As such, pipeline operators are increasingly concerned with monitoring the performance of pipes that are becoming fatigued and outdated.
According to figures published by the U.S. Department of Transportation’s Pipeline and Hazardous Materials Safety Administration (PHMSA, www.phmsa.dot.gov), as much as 25 percent of all pipeline failure incidents reported in the United States from 2002-2003 were caused by corrosion.¹ In the United States, there are thousands of miles of pipelines that are required to transport oil to support the consumption requirements of the American public, and as pipelines get older and the population grows on what was once vacant land, the public impact of a significant leak becomes more and more likely.
According to figures published by the PHMSA, there are a substantial number of pipeline incidents that cause significant damage to the environment, property, and in some cases life itself.² In addition, the financial loss to the industry due to these incidents is hundreds of millions of dollars annually. This article details how clamp-on ultrasonic technology can be used to provide pipeline operators with timely, accurate, and repeatable leak detection and location.
|Nonintrusive clamp-on transducers measuring fluid flow velocity.|
There are two basic forms of ultrasonic flow measurement, transit time and Doppler. For the purposes of this article, we will examine transit-time technology, as it is the primary ultrasonic method in use in the oil industry for measurement of both refined products and crude oil in pipelines.
All transit-time ultrasonic meters are designed based on the principle that sound travels faster when moving in the same direction as flow and slower when traveling in the opposite direction as flow. By measuring the independent travel times of the sound transmission in each direction (upstream and downstream), the fluid flow velocity can be computed.
Clamp-on ultrasonic flowmeters are nonintrusive devices — which means they do not require penetration of the pipe in a physical sense to arrive at a measurement — and are inherently bidirectional. Clamp-on systems are advantageous because they have no moving parts and do not require the process to be shut down for installation or maintenance. Meanwhile, bidirectional flow capability eliminates the need for additional instrumentation, valves, and expensive piping configurations. Technologies such as orifice-plate measurement and turbine meters require flow conditioning and in some cases multiple metering runs to meet the turndown ratio of the application to accommodate bi-directional flow requirements.
|Pipelines in ecologically sensitive areas pose a particular threat to the environment. Therefore, special attention is required when monitoring such installations.|
Most ultrasonic flowmeters are extremely sensitive at and around zero flow. This permits them to detect and integrate extremely small leaks, as well as recognize the opening of valves along the pipeline. Many pipelines remain at no-flow conditions for long periods of time, and instruments with limited turndown ratio, particularly in the zero-flow region, have proven to be ineffective for leak detection during static-flow conditions.
In addition to measuring fluid velocity, clamp-on ultrasonic technology can measure the speed of sound (sonic velocity) of the liquid in the pipeline. By measuring the speed of sound of a liquid, the flowmeter can identify the type of liquids flowing at any given time. The flowmeter measures the speed that sound travels from one transducer to the other and displays this in either feet or meters per second. This sound speed measurement is a sonic characteristic of the liquid being measured and all liquids have a signature sound speed. As such, the ultrasonic flowmeter, in effect, functions as both flowmeter and density meter. The speed of sound measurement is also very important in determining leak location, as pipelines may have several products between two measuring points, all having a different speed of sound that a pressure transient will travel through. If all the products between two site stations are known, the master station can calculate arrival time more precisely.
A nonintrusive mass flow measurement system is based on actual measurement of pipeline mass input, versus actual measured pipeline mass output. This functionality provides very sensitive mass flow measurement at various points along the pipeline, usually creating segments bounded by two mass flow site stations.
The distance between measurement points can be as much as 100 miles. The usual distance is determined by the need for environmental protection. For example, protecting a river crossing can involve two site-stations only one mile apart. The location and distance of each site station is based on the topology of the land.
Nonintrusive leak detection systems employ mass balance of a compressible liquid. The flowmeter infers the liquid density by the relationship between density and the measured sonic propagation velocity. Pressure is involved, as it has a direct effect on density and this can be seen by the ultrasonic flowmeter. Since pressure has a direct impact on the measured sonic velocity it is already known that the effects of pressure are seen and compensated for without the need for a direct pressure measurement. Thus, the ultrasonic flowmeter can determine density without directly measuring pressure by the change in the speed of sound measurement.
A complete nonintrusive, clamp-on leak detection system consists of multiple site stations. Each site station includes a clamp-on ultrasonic flowmeter, a clamp-on RTD temperature sensor, and a means of data communication compatible with the operator’s communication system. The leak detection system has a master station, which is a computer that runs the leak detection software and polls data once a minute from all site stations. The master station collects flowrate, temperature, liquid density, liquid viscosity, and important diagnostic information, such as transducer signal strength and aeration. These additional diagnostic variables can help determine the health of the meter and the quality of the liquid. All of this information is measured by each site station at least 10 times per second, and the data average over the last minute is sent to the master station.
The master station compiles the mass flowrates for the entire pipeline. Once per minute, the leak detection system alarms if any segment has a mass input/output unbalance that exceeds its leak alarm threshold (less than 1 percent in many cases). Establishing leak alarm thresholds for the last five, 15, and 60 minutes provides even greater sensitivity. At the master station, the entire pipeline and the site stations can be controlled and monitored.
|A complete transit-time leak detection system consists of at least two site stations and a master station. The master station receives data from each site station, ensuring the timely detection of any leaks that occur.|
Leak location is accomplished by sensing the amount of time the low-pressure wave (caused by a leak) takes to travel from its source to each of the segment’s site stations. The site stations can sense the low pressure wave’s arrival by its effect on the density of the fluid, which is being measured many times each second. If the leak is in the center of a segment, the pressure wave will arrive at each of the segment’s site stations at the same time.
The speed in which the low-pressure wave (pressure transient) travels through the pipeline is determined by the speed of sound for the product being transported. For example, if a pipeline was flowing diesel and gasoline, both of these liquids have a different sonic velocity, thus affecting the time that the low-pressure wave propagates through the liquid and arrives at each site station.
One of the key benefits nonintrusive flowmeters offer when monitoring for leak location is the ability to know all the products in a pipeline and the different speeds of sound each has. By measuring the different products as they pass each site station the master station can model the various products between segments to determine the arrival time more accurately. This greatly improves the accuracy of leak location, as compared to many other leak-location devices available, particularly those technologies that utilize the pressure transient caused by a leak to determine location. In such systems, if all the products in the pipeline, starting from where the leak originated, are not known and accounted for, location accuracy may be greatly reduced.
When evaluating leak detection systems, pipeline operators would be well served to not only consider the initial cost of the technology, but also functionality, operation cost, precision, and security. Nonintrusive ultrasonic flowmeter-based leak detection systems provide solid protection against the environmental and economical consequences of an undetected leak. The long-term expenses saved by using this type of leak detection system could ultimately be worth millions of dollars to a pipeline operating company.
Martin Dingman is a product manager for Siemens Energy & Automation’s U.S. clamp-on ultrasonic flowmeter line. Mr. Dingman has more than 15 years of experience in the flow instrumentation industry, serving in a variety of manufacturing, engineering project management, and sales roles. Mr. Dingman has completed numerous engineering-related flow measurement short-courses throughout his career and has conducted training sessions on ultrasonic flow measurement. He is knowledgeable on flow applications in various industries including oil & gas, HVAC, water & wastewater, and power. Mr. Dingman can be reached at firstname.lastname@example.org or 631 231-3600, ext. 215.
1. “Fact Sheet: Corrosion,” U.S. Pipeline and Hazardous Materials Safety Administration, 2004, primis.phmsa.dot.gov/comm/fact sheets/fscorrosion.htm.
2. “Pipeline Incident and Mileage Report,” U.S. Pipeline and Hazardous Materials Safety Administration, 2007, primis.phmsa.dot.gov/comm/reports/safety/PSI.html.