Hot:19082013-07-06 12:37:45 From: FTD Automation Instruments Limited - Field Instruments
High-viscosity liquids Flow Measurement
Measuring custody transfer of high-viscosity fluids has always presented challenges, and as the world relies increasingly on heavy crudes with various amounts of wax, sand and bitumen, the challenge increases. Crude oils typically flow at rates up to 1,500 BPH at ship, up to 50,000 barrels/hour (BPH) at export terminals- and truck-loading facilities and to 7,000 BPH in pipelines.
In addition to heavy crude, other challenging high-viscosity products include heavy fuel oils, lubricating oils, bunker fuels, grease components, and asphalts. In most cases, flow requires elevated temperatures--up to 400°F (205°C)--and flowmeters must be reliable in this environment.
This article will recap measurement problems related to viscosity and flow and discuss the choice of equipment available to meet the challenges, plus special design, operational and maintenance requirements.
Viscosity, Specific Gravity And Temperature
We consider a viscous hydrocarbon to be any liquid hydrocarbon that requires special treatment or equipment in order to be handled or stored.
Viscosity (resistance to flow) is related to specific gravity (density), although the relationship between the two is not direct, as the table below shows.
In addition, each hydrocarbon has a characteristic viscosity vs. temperature curve, making it possible to infer viscosity for known stable hydrocarbon mixtures using a curve developed in the laboratory. In cases where mixtures of hydrocarbons vary over time, viscosity can be measured using one of the many on-line viscometers available.
Measuring flow in fluids with viscosities above 100 cp requires special consideration by those charged with designing, operating and maintaining the equipment. The sliding vane positive displacement meter has been the first choice for measurement since the 1930s; however, in the last 10-15 years other technologies have emerged that can offer improved cost/benefit ratios, depending on the viscosity and temperature of the fluid and the required accuracy of the meter. Most custody transfer flowmeters fall into one of four categories:
1. Positive displacement (PD) meters
2. Helical turbine meters
3. Coriolis meters
4. Liquid ultrasonic meters
The positive displacement meter is the only direct measuring alternative, in that every molecule of fluid passes through the meter. The other technologies infer total flow by measuring velocity within a separate flow conduit, where changes in the physical properties of the fluid or of the conduit itself increase the uncertainty of the measurement.
This disadvantage can be overcome by proving the meters on a regular basis and monitoring fluid characteristics to be sure the meter readout has not gone beyond acceptable tolerances. Inferential meters can also be influenced by upstream (and to a lesser extent downstream) flow conditions, such as elbows, strainer debris or partially open valves.
In defining appropriate flowmeters for given conditions, the total turndown range is key. In general, turndown range is the range of flow rates over winch a meter will perform within a specified linearity, usually ± 0.15%.
There are two components of the total turndown range: the viscosity turndown range (which depends on the viscosity/flow relationship), and the meter turndown range (which depends largely on the type of meter). Each of the four technologies noted above has a characteristic turndown range, defined as the ratio of the maximum flow rate divided by the minimum flow rate. The higher the turndown range, the greater the range of flow rates over which the meter will be linear.
The total turndown range is equal to the flow turndown range multiplied by the viscosity turndown range. If meters are operated within a narrow band of flow rate and viscosity and proved at flow rates and viscosities close to these, linearity becomes less of a factor, and meter performance becomes more repeatable. Meter linearity is less of an issue with large parcels such as pipeline and ship loading/unloading applications but becomes more important in truck loading, rail-car loading, bunkering, and other small-parcel applications.
Most of the very high-viscosity refined products are handled at temperatures above ambient to facilitate pumping, transportation and metering, so it is important to specify measuring component materials that can withstand the elevated temperatures provided by steam, hot oil, or electric heat-traced and recirculation lines. In cases where it is uneconomical to heat large quantities of high-viscosity crudes, they are treated with diluents to enable transportation and metering at near-ambient temperatures.
Meter Selection And Sizing
This section lists the advantages and disadvantages of the four meter types used for high-viscosity fluids and shows graphs of suitability depending on viscosity and flow rate.
Positive Displacement Meters
High accuracy over a wide range of viscosities and flow rates up to 2,000 cST with proper clearances
Extremely good repeatability on high-viscosity fluids, very low slippage, long life if there is little or no abrasive material in the fluid
Low pressure drop
Functions without external power
Special construction available for high viscosities and temperatures
Can register near zero flow rate
Flow conditioning not required
Measures directly, not an inferential device, for more consistent results
More moving parts leads to increased maintenance compared to other meters
May become damaged by flow surges and gas slugs
Chance of corrosion and erosion from chemicals and abrasive materials
Derated flow rate capacity for high viscosities and temperatures
Relatively high cost for large meter sizes, since all fluid must pass through the meter
Helical Turbine Meters
Higher turndown range on high-viscosity crudes than conventional turbine meters
Very good repeatability
Reduced susceptibility to fouling, abrasives and deposits
Less sensitive to viscosity changes
Lower pressure drop than conventional turbine meters
Available in large sizes, providing good value for high flow rates
Requires flow conditioning
Back pressure required
Requires pulse interpolation due to low-resolution pulses
An inferential device
Low maintenance, minimally affected by abrasives and corrosives
Not susceptible to damage by gas slugging
Registers near zero flow rate
Minimally affected by viscosity changes
Direct mass and density measurements
Flow conditioning not normally required
Sensitivity to installation conditions, including shock, vibration, pulsations and effects of adjacent parallel runs
Deposits can affect accuracy
Difficult to prove due to time lag of the pulse output
Requires periodic re-zeroing
Needs back-pressure control
High pressure drop that increases drastically with viscosity
An inferential device
Liquid Ultrasonic Meters
Wide dynamic flow range
Negligible pressure drop
Non intrusive and no moving parts, making this the least influenced by abrasive materials
No need for upstream strainer
Self diagnostic capabilities
Flow conditioning recommended
Susceptible to fouling or deposits
Sensitive to installed conditions
Sampling and microprocessor-based output contributes to difficulty in proving
High cost for small sizes
Maximum size limited by proving capability
Back pressure required