Special Section: Flow & Level
Flow measurement common mistakes, corrective actions
By Rajan Batra
In industry, whether it is the chemical, power, or oil and gas industry, accurate flow measurement is of very high importance. Inaccurate flow measurements lead to poor flow and related controls, bad quality products, and incorrect fiscal and allocation measurement, which lead to poor allocation of revenues among partners. In addition, an unsafe process due to wrong quantities could lead to facility damage and create an unsafe environment for facility workers. Inaccurate flow readings can potentially cause serious or disastrous results. The points below discuss some common flow measurement mistakes and corrective actions.
A challenge in industry is how to select the right flowmeter for the specific application. Wrong flowmeter selection is one of the major causes of inaccurate flow measurement. Tangible and intangible factors should be evaluated step by step before going to cost comparison.
First, flowmeter suitability should be defined based on flow range, process parameters, desired accuracy and repeatability, unidirectional or multidirectional flow, upstream and downstream straight run requirements, piping geometry upstream of a flowmeter, filter requirements, continuous or totalized readings, analog or digital signals, possible impact of vibrations on flowmeters accuracy, data update frequency, meter access for possible maintenance, etc.
Next, flowmeter suitability should be defined based on historic failure rates, plant personnel familiarity, and confidence level with flowmeter type, calibration and maintenance ease, spares availability, etc. Finally, tangible and intangible factors should be evaluated along with total installed cost for fair and big picture comparison.
Thorough evaluation should be done in a logical step-by-step approach considering all tangible and intangible factors before making a decision on the type of flowmeter. Because of the importance of flowmeter selection, sufficient time and resources should be allocated for this evaluation.
Gas carryunder conditions
Gas carryunder conditions while measuring liquid flows significantly impacts the accuracy of liquid flow measurement. Improper separation of liquid and gas in separation vessel and shrinkage effect is one of the major reasons for free and entrained gas conditions in the liquids measurement.
Shrinkage is the tendency to lose lighter components of the liquid as the pressure is lowered in the liquid processing train. The percentage of liquid lost to shrinkage depends on the composition, operating pressure, and temperature of the liquid when metered.
API MPMS 20.1 may be followed for shrinkage corrections. The procedure involves obtaining a known volume of a representative liquid sample from the flow line using a pressurized sample cylinder. The sample is then allowed to stabilize at atmospheric conditions, and the volume of the remaining liquid is measured. The shrinkage factor is then computed based on the initial and final liquid volumes. Location of the sampling point will be kept as close as practically possible to the liquid flowmeter.
To ensure further the liquid is free from gas, the separation vessel (gas/liquid) separators must be designed accurately and must be operated and maintained properly as per design limits.
Straight run requirements
Straight run requirements of process pipe for flow measurement devices are among the leading causes of flowmeter inaccurate measurements. Most of flowmeters (particularly differential pressure-based) are designed to operate correctly and accurately in process pipeline where the process fluid exhibits a homogeneous and symmetrical velocity profile before entering the flowmeter device. So the performance of many flowmeters can be significantly affected by a non-homogeneous velocity profile, i.e., velocity profile asymmetry or swirls that enter the flowmeter.
Different flowmeters have different sensitivities to the different kinds of flow field distortions. Hence, most flowmeters (particularly differential pressure-based) must have straight pipe runs upstream and downstream of the flowmeter to achieve desired velocity profile for accurate flow measurement.
The rule of thumb for these flowmeters is a straight pipe run requirement of 10 diameters upstream and 5 diameters downstream. This rule may not be valid in all cases and scenarios as some flowmeters may need even more than 25 diameters upstream to ensure the velocity profile is homogeneous and symmetrical. Similarly, some flowmeters may require less than 5 diameters of straight pipe run. Flow conditioners or straightening vanes can also be used as per suitability to eliminate or at least minimize flow distortions (e.g., swirl or velocity profile asymmetry) and to reduce the straight pipe run requirements. This is particularly critical in offshore facilities where space is limited. Further, control valves and other instrument or devices, which can create turbulence and can distort the velocity profile in the pipe should be located downstream of the flowmeter.
When making gas flow measurements, any process or separation vessel (in hydrocarbon or other process production streams) experiencing significant liquid carryover (i.e., liquids in the gas) will adversely impact the accuracy of gas measurement. Free flowing and entrained liquids disturb and distort the flow profile inside the pipe and can lead to measurement errors. Free flowing and entrained liquids can also lead to high or low measurement readings. Hence the key for accurate gas measurement is to make gas free from liquids or to do relevant compensations for accounting the free flowing and entrained liquids in the gas measurement, if possible.
Measurement of single phase fluid can achieve higher degree of precision than what is possible by measuring two-phase or multiphase fluids using the commercially available state-of-the-art two-phase or multiphase meters. While doing gas flow measurements, attention must be given to ensure the gas measurement is dry gas measurement and not dual-phase (gas and liquid) measurements. So gas under consideration of flow measurement must be a "dry gas." To ensure the gas is a dry gas and free from liquids, the separation vessel (gas/liquid) separators must be designed accurately and must be operated and maintained properly.
Further, regular compositional sampling of the produced gas, potentially on a monthly basis, may be conducted in order to calculate the liquids and correct for liquid carryover.
Improper meter installation
Quite often, errors observed in flowmeter readings are because of improper flowmeter installation, which makes flow measurement inaccurate and unreliable. The most critical step often ignored at facilities for achieving the optimum efficiency of a flowmeter is its proper installation.
Care should be taken on meter alignment, direction of flowmeter, upstream and downstream straight run requirements, piping geometry upstream of a flowmeter, filter locations, possible impact of vibrations on flowmeter accuracy, meter access for possible maintenance, etc. It may be a good idea to have a checklist for specific type of flowmeter and review it before installation to prevent improper flowmeter installation.
Proper attention should be paid on flowmeter installation and its surrounding environment since flow measurement errors often originate from incorrect or bad installation or other environment dependent factors. Flowmeters perform well and contribute to smooth running processes if applied properly.
ABOUT THE AUTHOR
Rajan Batra (Certified Functional Safety Professional) is Instrumentation and Electrical Lead at Chevron Corporation, Houston, Major Capital Project. Batra is a senior member of ISA, a member of the ISA12 working committee, and a senior member of IEEE.