The measurement of the pumping rate of the various styles of oilfield pumps is done by pump rate sensors. Pump rate sensors are also sometimes called stroke counters. Pump rate sensors can be mechanical switches, inductive proximity sensors, or magnetic pickups. However they all work by sensing the movement of a flange bolt, piston, or sprocket attached to the pump. For positive displacement pumps each rotation of the power end shaft or each stroke of the piston corresponds to a specific volume of fluid pumped.
Mechanical stroke counters usually have a switch with a long steel whisker that is triggered on every stroke of the pump. Magnetic pickups sense the change in the amount of steel (or other ferro-magnetic metal) in front of their face and create an output signal that corresponds to the change. The change in amount of steel in range of the sensor can be in the form of a bolt head or sprocket tooth. Proximity switches also work on a similar principle except that they generally output a binary ON or OFF signal. Rate sensor electrical signals can be fed directly in to the Rapidlogger system and recorded, totalized and displayed.
Inductive Proximity Sensors: These sensors detect ferrous metal targets without physical contact. An oscillating electromagnetic field is generated by the sensor coil. When a metal target approaches, eddy currents are induced in the target, damping the oscillation. This change is detected and triggers the output. Inductive sensors offer reliability, long life, and resistance to contamination from oil, mud, and vibration. Typical sensing distances range from 5mm to 60mm depending on sensor size and target dimensions.
Magnetic Pickup Sensors (Variable Reluctance): Generate an AC voltage proportional to the rate of change of magnetic flux. As gear teeth or bolt heads pass the sensor pole piece, the magnetic reluctance changes, inducing voltage in the pickup coil. Output voltage amplitude increases with speed. Magnetic pickups are passive devices requiring no power supply, making them intrinsically safe. They are ideal for high-speed applications and harsh environments.
Hall Effect Sensors: Use semiconductor technology to detect magnetic fields. Permanent magnets mounted on rotating or reciprocating components are sensed by the Hall element. Hall sensors provide digital switching or analog output proportional to field strength. They require power supply but offer compact size, fast response, and operation down to zero speed. Commonly used on triplex and quintuplex pump driveshafts.
Mechanical Limit Switches: Simple contact switches actuated by levers, whiskers, or rollers triggered by pump movement. While inexpensive and easy to install, they suffer from contact wear, require periodic adjustment, and are susceptible to vibration-induced false counts. Still used in some legacy installations and low-speed applications.
Optical Sensors: Use infrared LED and photodetector to sense reflective targets or interruption of the light beam. Offer fast response and high accuracy but require clean environments. Less common in oilfield applications due to sensitivity to contamination.
Counting Frequency: Sensors must handle maximum pump stroke rates without missing counts. Fracturing pumps operate at 60-120 SPM (strokes per minute). Triple plunger pumps at 120 SPM generate 360 pulses per minute (6 Hz) with 3 targets. Sensors should handle at least 10x the maximum operating frequency for reliability.
Sensor Response Time: Inductive proximity sensors: 0.1 to 3 ms; Magnetic pickups: <1 ms (frequency dependent); Hall effect sensors: <0.1 ms. Fast response is essential at high stroke rates to avoid count errors and ensure accurate totalization.
Operating Temperature: Oilfield pumps generate significant heat. Sensors must operate from -40°F to +185°F or higher. Special high-temperature sensors rated to +250°F are available for exhaust manifold mounting or other high-heat applications.
Vibration Resistance: Pumps generate extreme vibration (10-20 G acceleration). Sensors must be rated for shock and vibration per IEC 60068-2-6. Use vibration-resistant cable glands and secure mounting to prevent sensor displacement.
Output Signals: Proximity sensors: NPN or PNP transistor output, typically sinking or sourcing 100-200 mA; Magnetic pickups: AC voltage output, 1-100 Vrms depending on speed; Hall sensors: Digital square wave or analog 0-5V/4-20mA output. Data acquisition systems must be compatible with sensor output type.
Fracturing Operations: Each frac pump is monitored with stroke counters to track individual pump rates and total treating rate. Stroke data combined with pump displacement calculates volumetric rate in BPM (barrels per minute). Accurate rate measurement ensures proper proppant concentration, prevents screenouts, and documents job execution for quality control.
Cementing Services: Pump stroke counting tracks cement volume pumped and displacement fluid volume. Precise stroke counts verify complete cement placement and proper displacement to prevent contamination. Cementing computers use stroke data to control blending density and additives automatically.
Acid Pumping and Stimulation: Pump rate monitoring during matrix acidizing ensures proper acid placement and treatment effectiveness. Rate control prevents formation damage from excessive injection pressure while maintaining adequate penetration depth.
Mud Pumps (Drilling): Stroke counters on mud pumps measure drilling fluid circulation rate. Pump rate data calculates equivalent circulating density (ECD), annular velocity, and hydraulics parameters. Monitoring pump output helps detect washout, liner failure, and valve problems.
Transfer and Boost Pumps: Stroke sensors on transfer pumps track fluid movement between tanks, blenders, and treating equipment. Totalizing stroke counts provides inventory management and batch tracking for fracturing fluids, drilling mud, and chemicals.
Sensor Positioning: Mount sensors to detect crankshaft rotation (targets on flange bolts or sprocket teeth) or crosshead movement (targets on reciprocating components). Position sensors perpendicular to target movement for maximum signal strength. Maintain proper sensing distance (typically 50-75% of rated range for inductive sensors).
Target Selection and Preparation: For inductive sensors, use ferrous metal targets (steel bolts, brackets, or tabs). Ensure targets are large enough (minimum 3x sensor face diameter) and properly spaced. Clean and paint-free surfaces improve sensing reliability. For magnetic pickups, use gear teeth or ferrous bolts as targets.
Mounting Hardware: Use heavy-duty mounting brackets with anti-vibration damping. Lock-wire or use thread-locking compound on all fasteners to prevent loosening from vibration. Provide adjustment capability to compensate for thermal expansion and wear. Install safety cables on sensors to prevent loss if mounting fails.
Cable Routing and Protection: Route sensor cables away from ignition systems, AC power lines, and radio transmitters to minimize electrical noise. Use metal conduit or armored cable for mechanical protection. Provide drip loops and cable stress relief at connections. Use proper cable glands rated for vibration and environmental sealing.
Calibration Procedure: Determine pump displacement per stroke (typically 0.125 to 0.25 barrels for triplex fracturing pumps). Program data acquisition system with displacement value and number of targets per stroke. Verify calibration by comparing totalizer reading to measured tank level change or flow meter reading.
Multiplier Calculation: For crankshaft-mounted sensors, calculate pulses per stroke based on number of targets. Triple plunger pump with 3 bolt targets = 3 pulses per shaft revolution = 1 pulse per stroke. Program appropriate divider to convert pulse count to stroke count.
Functional Testing: Manually rotate pump or operate at low speed to verify sensor output. Use oscilloscope or multimeter to check signal quality. Confirm no missed or extra counts across full operating speed range. Test at maximum pump speed to ensure adequate response time.
API RP 10B-2: Recommended Practice for Testing Well Cements includes requirements for pump calibration and stroke counting accuracy. Specifies procedures for verifying pump output and maintaining calibration records.
API RP 39: Recommended Practice for Blowout Prevention Equipment Systems for Drilling Wells includes requirements for monitoring pump rates during well control operations.
SPE 13423: Society of Petroleum Engineers technical paper "Recommended Procedures for Field Testing of Hydraulic Fracturing Fluids" specifies rate measurement accuracy requirements of ±2% for quality control.
IEC 60947-5-2: International standard for proximity sensors and switches specifying performance requirements, testing methods, and safety specifications.
NEMA Standards: Proximity sensors should comply with NEMA electrical ratings. Common ratings include general purpose (Type 1), watertight (Type 4), and hazardous locations (Type 7/9).
NEC Article 501: National Electrical Code requirements for electrical installations in Class I, Division 1 and 2 hazardous locations. Sensors must be rated for appropriate explosion-proof or intrinsically safe applications.
Intrinsic Safety: Magnetic pickup sensors are inherently intrinsically safe (passive devices). Active sensors (proximity, Hall effect) require IS barriers or certified IS designs for hazardous area installation. Comply with FM, CSA, ATEX, or IECEx standards.
Calibration Documentation: Maintain pump displacement specifications, stroke sensor calibration factors, and verification test results. Document any changes to targets, sensors, or configuration. Annual verification recommended for critical applications.
