Load sensors are used to measure the weight or load of various things in the oilfield. Load sensors can be installed in various ways. Load cells can be of the compression type, tension type, double acting (compression and tension), and shear type.
Tension load cells come in the form of tension links for use for measuring chain and cable tension. They can come in the form of flat cylinders (pancake style) for use under tanks, bins, and pieces of equipment to measure their weight. They can come in the form of load pin (shear type) used as a shaft or axle to measure the weight being carried by a pulley or sprocket.
Compression Load Cells: Designed to measure pushing forces, these sensors use strain gages bonded to a metal column that compresses under load. The strain gages are arranged in a full Wheatstone bridge configuration to provide temperature compensation and maximum sensitivity. Commonly used under tanks, hoppers, and structural supports.
Tension Load Cells: Engineered to measure pulling forces, tension load cells feature strain gages mounted on a tension member that elongates under load. These are essential for measuring cable tension, hook loads, and suspended weights in lifting operations.
Shear Beam Load Cells: These sensors measure force applied perpendicular to the sensing element. The strain gages detect shear stress in a specially designed beam section. Shear beam cells offer excellent side-load rejection and are commonly used in platform scales and conveyor belt systems.
Load Pins: A specialized form of shear sensor, load pins replace conventional pins or shafts in mechanical assemblies. Strain gages are embedded inside the pin to measure shear forces as the pin flexes under load. Ideal for measuring forces in sheaves, pulleys, and pivoting mechanisms.
Capacity Ranges: Oilfield load cells are available from 1,000 lbs to over 500,000 lbs capacity. Common ranges include 10,000 lbs, 50,000 lbs, 100,000 lbs, and 200,000 lbs for drilling and workover applications.
Output Signals: Standard outputs are 2-3 mV/V (millivolts per volt of excitation) for strain gage load cells. With a 10 VDC excitation, this provides 20-30 mV full-scale output. Signal conditioners or amplifiers convert this to standard 4-20 mA or 0-10 VDC signals for data acquisition systems.
Accuracy: High-quality load cells achieve accuracies of ±0.03% to ±0.25% of full scale. Combined error, including non-linearity, hysteresis, and repeatability, typically ranges from ±0.05% to ±0.5% of rated capacity.
Overload Protection: Load cells should be rated with safe overload capacity of 150% to 300% of rated capacity without damage. Ultimate overload rating is typically 300% to 500% of rated capacity before permanent deformation occurs.
Drilling Operations: Hook load measurement using tension load cells on the traveling block provides critical information about string weight, overpull, and stuck pipe conditions. Crown load cells monitor total suspended weight to prevent derrick overload.
Workover and Completion: Load cells on sheave blocks measure tubing and casing weights during running and pulling operations. Accurate load monitoring helps identify tight spots, ensure proper makeup torque, and prevent equipment overload.
Wireline Operations: Tension measurement on the wireline cable using load pins in the sheave wheels allows operators to monitor tool weight, detect obstructions, and calculate downhole tool positions accurately. Real-time weight data prevents cable breaks and equipment damage.
Proppant and Liquid Additive Monitoring: Pancake load cells installed under proppant silos and chemical tanks provide continuous weight monitoring. This enables accurate inventory tracking, automated blending control, and real-time consumption calculations during fracturing operations.
Coiled Tubing: Load cells measure injector grip force and tubing tension to optimize running speeds and prevent tubing buckling or failure. Weight monitoring during coiled tubing operations ensures safe working loads are maintained.
Mounting Requirements: Load cells must be installed on rigid, level surfaces to ensure accurate measurements. Use hardened steel mounting plates and proper grade bolts (typically Grade 8 or higher). Ensure loading is concentric and axial to prevent side-loading errors.
Mechanical Alignment: Tension load cells require proper alignment to prevent bending moments. Use swivel eyes or spherical bearings to accommodate misalignment. For compression cells, ensure load is applied through the center axis using ball seats or self-leveling caps.
Environmental Protection: Use NEMA 4X or IP67 rated load cells for outdoor oilfield applications. Apply protective boots or bellows over compression cells to prevent debris accumulation. For submersible applications, use hermetically sealed load cells rated IP68.
Wiring and Signal Conditioning: Use 6-conductor shielded cable for load cell connections (excitation +/-, signal +/-, sense +/-). Keep cable runs as short as practical. For long distances, use remote signal conditioners or transmitters to convert to 4-20 mA near the load cell.
Calibration Methods: Calibrate load cells using certified test weights or master load cells traceable to NIST standards. Perform multi-point calibration at 0%, 25%, 50%, 75%, and 100% of capacity. Record both ascending and descending load points to verify hysteresis.
Zero Balance: With no load applied, adjust zero balance to account for dead load of hardware. Most load cells allow mechanical zero adjustment via a potentiometer or digital trimming. Perform zero balance at operating temperature after thermal stabilization.
Calibration Frequency: Recalibrate load cells annually or after exposure to overload conditions exceeding 150% of rated capacity. Critical safety applications may require quarterly verification against known weights.
NIST Handbook 44: For commercial weighing applications, load cells must comply with NIST Handbook 44 requirements for accuracy class and metrological specifications.
API RP 2D and 2A: Drilling and production operations should follow API RP 2D for safe operation of lifting equipment and API RP 2A for platform design requirements including load monitoring systems.
ASME B30 Series: Cranes and lifting equipment using load cells must comply with ASME B30 standards for construction, installation, operation, inspection, testing, and maintenance.
OIML R76: International Legal Metrology Organization recommendations for non-automatic weighing instruments specify accuracy classes (III, IIIL, IV) based on the number of verification scale divisions and environmental conditions.
ISO 376: Standard for calibration and classification of force-proving instruments used in materials testing applications. Defines accuracy classes 00, 0.5, 1, and 2 based on relative deviation from applied force.
Material Specifications: Load cells for sour service environments must use materials compliant with NACE MR0175/ISO 15156. Stainless steel 17-4 PH or 15-5 PH are commonly specified for corrosion resistance.
