Part 1 of this series explained the water difficulties of the oil and gas industry. To accurately measure water use, operators and other industry parties need flowmeters. Decision makers have to consider several factors when choosing a flowmeter type. The goal is to make sure whatever technology is selected works as the best solution for the application. Read it here.
Turbine Meter: Inferential Technology
This technology has been used widely for years in the oil and gas industry for flow measurement. A liquid such as fresh or produced water impinging on the blades of the turbine causes the blades to rotate and with magnets fitted in the rotor assembly, and a flow reading is provided. To calibrate to the liquid being measured, a K-factor of the liquid is entered into the operating parameters of the unit. For the unit to remain repeatable to this liquid, the viscosity must remain constant. For freshwater, the viscosity remains constant. With produced water, a slight change in viscosity occurs causing errors in measurement. Benefits of this technology include:
- Low purchase cost
- Operator familiarity
- Accuracy +/- 0.2 to 0.5 percent and repeatable at +/- 0.5 to 1 percent if maintained properly
- Easily repairable
- Easy setup and calibration
Drawbacks of this choice of flowmeter are:
- Need for preventive maintenance to maintain accuracy and repeatability
- Maintenance may not be needed at the time it is scheduled.
- Keeping an inventory of replacement parts is required.
- An improperly maintained unit will have increased inaccuracies that will go unnoticed, causing lost revenue.
- Mechanical moving parts are subject to wear from particulates in water
- Unsuited for dirty liquids
- Long inlet/outlet sections (10D/5D) required
This technology is suitable for freshwater applications but is not the best option unless the source of freshwater is free of particulates. Because of sand, paraffin and other particulates, the turbine meter is not the best technology for measuring flow of produced water. The biggest benefit of using an inferential technology such as turbine meters is the initial cost of the unit. However, the resources and inventory of repair kits to effectively maintain this technology negate the low cost benefit. If not maintained properly, the loss of accuracy and repeatability negates the benefit of accuracy.
The electromagnetic flowmeter is a proven technology and is the primary flow element for the water and wastewater industry. Its use is growing quickly in the oil and gas industry in frac water and produced water applications. With the addition of battery-operated electromagnetic flowmeters, sensors can now be applied in very remote and harsh locations, while still providing the necessary accuracy. This technology is based on Faraday’s law. The coils in the sensor generate a consistent magnetic field. The liquid flowing through the sensor induces a voltage proportional to the flow velocity. With no moving parts the technology is virtually maintenance-free. At times, build-up may occur and make pigging the line necessary, but the flowmeter does not have to be removed while pigging the line. This technology's advantages are:
- No pressure drop
- Short inlet/outlet sections (5D/2D) or less
- Insensitive to flow profile changes (laminar to turbulent)
- Accuracy of better than ±0.2 percent of actual flow over full range. Battery operated units are slight greater at ±0.6 percent but still remain well below any industrial or regulation requirement.
- No recalibration requirements. The verification of the flow and calibration can be done in the field without interfering with the process
- Bi-directional measurement
- No taps or cavities
- No obstruction to flow
- Not limited to clean fluids
- High temperature and harsh environment conditions, ideal for remote locations
- High pressure capabilities
- Volumetric flow
- Typical battery-operated units can run 6 to 10 years without replacement of batteries
The main disadvantage of this type of flowmeter is that if the coil or electrodes fail or become damaged, the unit cannot be repaired but must be replaced. The long life span and the ability to maintain accuracy at a low cost of operation makes this technology best suited for measuring frac and produced water. However, since not all manufacturers are the same, some of the pros and cons may vary. For example, not all manufacturers provide an integrated battery-operated unit.
The coriolis flowmeter is a proven technology for hydrocarbon liquids and gas measurement, and is critical point of measurement for custody transfer. For water applications, the coriolis effect provides a higher accuracy than most other technologies. The largest advancement is the compactness of the unit, which allows it to be mounted in tight areas and on skids. The largest disadvantage is the cost of unit compared to other technologies. Benefits of coriolis flowmeters include:
- Direct, in-line and accurate mass flow measurement of both liquids and gases
- Accuracies as high as 0.1 percent for liquids and gases
- Mass flow measurement ranges cover from less than five gallons per minute to more than 350 tons per hour
- Measurement is independent of temperature, pressure, viscosity, conductivity and density of the medium
- Direct, in-line and accurate density measurement of both liquids and gases
- Mass flow, density and temperature can be accessed from the one sensor, and can be used for almost any application irrespective of the density of the process
- Expense. The larger the size, the greater the price difference between electromagnetic and turbine meters
- Many models are affected by vibration.
- Current technology limits the upper pipeline diameter to 400 millimeters, and secondary containment can be an area of concern.
In simple, fresh frac water flow applications and flow measurement to and from produced water storage tanks, the coriolis flowmeter is not the most economical choice. Produced water recycling includes new and existing processes where the additives, treatments and conditioners in the water are measured. The coriolis flow sensors have been proven in industries like water, wastewater, petrochemical and pharmaceutical, making them the most logical choice because of the critical nature of accuracy and performance required for these produced water recycling applications.
Ultrasonic Clamp-On Flow Measurement
The ultrasonic clamp-on flowmeter was developed in the 1960s. Both Doppler and transit-time technologies are used, allowing for the measurement of most liquids and gases. It is a non-intrusive device that can be mounted for temporary or permanent measurement. Clamp-on flowmeters are ideal for upgrading any water flow line without having to cut the line or stop the process to install. Transit-time measurement is primarily used on clean liquids that do not contain high percentages of solids or entrained air. Transit-time (or time-of-flight) meters are designed based on the principle that sound travels faster when moving in the same direction as the flow and slower when traveling in the opposite direction. These meters measure the independent travel times of sounds transmitted in each direction between two transducers positioned upstream and downstream. Doppler measurement, on the other hand, is used when gas bubbles or suspended solids are present in the fluid. The required concentration of gas bubbles or solids that dictate Doppler use will vary based on pipe size, particle size and fluid velocity. This metering technique relies on the reflection of sound energy off the bubbles or particles to create a Doppler shift in the fixed-frequency acoustic transmit signal. The frequency shift is proportional to the fluid velocity. For liquid applications that vary from clean to dirty because of process changes and pigging, one flow transmitter with both transit time and Doppler transducers can provide continual flow measurement. The advancement of ultrasonic clamp-on systems with wide beam technology harmonizes the ultrasonic transducers to the pipe producing a strong, stable and coherent signal independent of the flowing medium and velocity, providing improved accuracy and repeatability. Benefits of ultrasonic clamp-on flowmeters are:
- Easy installation; no need to cut pipe or stop flow
- Harsh environment compatibility
- No pressure drop or energy loss
- Minimal maintenance; external transducers that do not require periodic cleaning
- Bidirectional flow
- Detects reverse flow and empty pipe
- Insensitive to outside noise with wide beam technology
- Field installation accuracy: +/- 0.5 – 3 percent of flow at <0.3 meters per second (1 foot per second)
- Custody transfer capability
The biggest drawback of this technology is that not all manufacturers use wide beam technology. The ultrasonic clamp-on flowmeter could easily be used for fresh frac water and produced water applications. A dual channel transmitter including both transit-time and Doppler technology will ensure an uninterrupted flow measurement when particulates are present in the flow.
The oil and gas industry consumes 0.01 to 0.3 percent of freshwater in the United States. Transport, storage and recycling of the water have become big business and is becoming a large sub-industry of oil and gas. The question is what technology to use. A coriolis meter can cost two to three times more than electromagnetic flow measurement, but it can also provide a higher accuracy. On the other hand, some sediment and particulates in produced water can necessitate pigging of a flow line, making the electromagnetic flowmeter the better choice. With no moving parts in the line on an electromagnetic flowmeter, the line can be pigged without shutting down the process to remove and reinstall the unit. Electromagnetic flowmeters meet all the requirements of the industry and its regulations. They provide a lower operational cost and better accuracy (at less than 1 percent), and require less maintenance. It has been an established, proven technology for billing. It is designed for remote locations and harsh environments and can be configured to meet any corrosive conditions of an application. The flow reading can be easily verified in the field and does not require recalibration. For these reasons, electromagnetic flowmeters should be the primary choice when selecting a flow technology for fresh frac water and produced water applications. For a company looking to upgrade or retrofit an existing flow line to gain accuracy and cut down maintenance without shutting down the production of the well, the ultrasonic clamp-on flowmeter is the right technology. Sticking to the old field standard of “if it’s not broke don’t fix it” and not using the best available technologies could actually be costing producers and other support companies millions of dollars. The best way to ensure you are using the best technology to meet all the requirements of the water flow applications and still provide a return on the investment is to evaluate the who, what, where, when and how’s of the flow technologies. Read part 1 of this series, here.
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- “Draft Plan to Study the Potential Impacts of Hydraulic Fracturing on Drinking Water Resources,” download here