Pumps & Systems, April 2008

Dynamic pressure sensors and accelerometers are frequently used for precision measurements in oil, gas and petrochemical industries, as well as for natural gas supply monitoring applications. The development of piezoelectric sensor technology with integral charge amplifier, as well as the development of high temperature charge output designs, has been especially important for dynamic sensing applications. To truly understand the benefits of this technology, one must first review the evolution of this type of sensing.

Sensing technologies for harsh environmentsPiezoelectric sensing technology emerged in the mid 20th century as a viable means for conducting dynamic acceleration, pressure and force measurements. The piezoelectric principle of squeezing a crystal to generate electrical charge offered a device that was self-generating, repeatable and robust. The technique was-and still is-not without implementation challenges. The high-impedance output signal requires a clean, moisture-free transmission environment and a costly, sophisticated signal-conditioning apparatus is needed to enable interpretation by readout, recording and analysis instrumentation. The advent of the transistor, however, permitted a revolutionary transformation.

In the late 1960s, by utilizing miniature, transistorized, signal-conditioning circuitry, PCB Piezotronics developed Integrated Circuit Piezoelectric (ICP®) sensors, which incorporate necessary impedance conversion circuitry into the sensor housing. Operating over only 2-wires, the output signal would be biased on top of the necessary transistor excitation voltage, permitting use of a simple, constant-current power source and low-cost cabling for signal transmissibility.

The low output impedance would allow sensors to be used in adverse environments and the simplicity of operation would reduce operating costs, leading to more widespread appeal. As implementation of these sensors flourished, recording and analysis instrument manufacturers supported their use by integrating the necessary constant current excitation for direct connection to the sensor, further simplifying the measurement chain.

By the mid 1980s, volume production of these piezoelectric sensors permitted further cost reduction for test engineers. While already employed for critical industrial requirements, there was limited use of piezoelectric sensors for industrial monitoring applications.

By the early 1990s, further cost reductions and performance enhancements of accelerometers contributed to their recognition as a beneficial machinery health-monitoring tool. Industrial vibration monitoring instrumentation, once vital to only oil and gas, power generation and paper manufacturing, flourished for all industries employing rotating equipment for critical processes. Today, tens of thousands of accelerometers are used annually for predictive maintenance, to ensure uninterrupted productivity within the oil and gas industries.

The concept for these dynamic sensors has truly altered the way such measurements are conducted and has permitted proliferation of piezoelectric sensors for dynamic measurements into dirty, industrial and hazardous environments. Enhancements such as embedded memory, or TEDS (Transducer Electronic Data Sheet), integrated transmitters for 4-20 mA signal format, on-board filtering and various self-test circuit schemes have since been developed and integrated within these sensors.

Critical Application Considerations for Applying Dynamic Sensing TechnologiesFor the oil, gas, petrochemical and natural gas environments, three critical application considerations must be made when applying such dynamic sensing technologies. These include intrinsic safety needs, to ensure added protection in hazardous environments; assessment and selection of sensors and cabling rugged enough to reliably perform within a splash zone environment; and overall protection against corrosion in harsh conditions. Two types of sensing technologies typically used in high reliability monitoring within this type of application environment are piezoelectric dynamic pressure sensors and loop-powered 4-20 mA industrial vibration sensors, with designs capable of effectively measurements within hostile environments.  

Quartz dynamic pressure sensors typically offer fast response, ruggedness, high stiffness, extended ranges and the ability to measure dynamic pressures. For oil, gas and petrochemical equipment monitoring of dynamic pressure, pressure sensors, specifically designed with intrinsic safety certifications to ATEX and CSA standards, are widely used on gas and oil well heads; pumps; pipelines; reciprocating engines, supply lines, natural gas power engines; multistage gas compressors, gas turbines and other machinery operating in hazardous environments.

Piezoelectric pressure sensors feature no moving parts, and offer the capability to detect and monitor dynamic pressure spikes, pulsations and surges in gaseous or liquid material. These sensors also detect wellhead acoustics to determine properties of mud and water.

Some intrinsically safe pressure sensors can operate in ranges from to 50- to 5,000-psi (345- to 345,000-kPa), with sensitivities from 1- to 100-mV/psi (0.15- to 14.5-mV/kPa), are ground isolated with a 10-32 connector, and operate at temperatures to 250-deg F (121-deg C), with a 20k g pk measurement range. Other intrinsically safe pressure sensors offer measurement ranges from 50 to 500-psi, with sensitivity of 10- to 100-mV/psi, and can help determine flow and contaminate content. Sensors are case isolated with a robust 2-pi connector, and offer temperature operation to 250-deg F (121-deg C).

Quartz sensing elements and a stainless steel diaphragm are welded hermetically sealed within a stainless steel housing, to prevent against corrosion and contamination, and ensure reliability of measurements in dirty, oily, industrial and submerged environments. Optional DIN-rail mounted intrinsic safety barriers may be used to eliminate risk of an electrical arc, or spark at the hazardous location.

For additional pressure measurements which do not require intrinsic safety approvals, such as compressor monitoring, monolithic strain gage pressure transmitters can provide excellent reliability, repeatability, accuracy, and value for a wide range of industrial process monitoring, depth and level measurement requirements, including withstanding H2S and sour gas environments. Sensors feature a thread, port and 17-4 stainless steel or Inconel® diaphragm as one piece. Traditional uses of adhesives or fluid filling are eliminated, resulting in a sensor with excellent long-term stability.

With this model, the integrated circuit provides either 2 mV/V output signal, or loop-powered, 4-20 mA output signal proportional to the desired input span. This type of pressure sensor features full-scale gauge or absolute pressure ranges from 300- to 10,000-psi and are available in a variety of pressure ports, electrical connectors and submersible, integral cables, which help adapt the stainless steel sensor to any number of process control applications, including tank level, pipeline pressure, HVAC equipment, sump monitoring and pump control. 

Another type of sensing technology for continuous, 24/7 on-line monitoring of pumps and motors used in the oil and gas monitoring may be found in ATEX and CSA certified 4-20 mA industrial vibration sensors. These industrial vibration transmitters adapt to existing PLC, DCS, SCADA alarm and control systems, for continuous vibration monitoring of machinery and equipment-at a fraction of the cost of dedicated on-line monitoring systems. The loop-powered sensors output a 4-20 mA signal, proportional to peak or RMS vibration.

By trending vibration levels, they can help protect pumps and motors from catastrophic failure and downtime, and reduce the need for sophisticated vibration analysis, maintaining an uncomplicated-yet-accurate approach to vibration monitoring. They are available in AC- or DC-coupled versions with top exit or patented 360-deg swivel mount side exit, optional temperature signal and a variety of integral cable and detachable connector configurations.

The temperature output version of this type of sensor is typically from -40-deg F to +185-deg F (-40-deg C to +85-deg C), with the vibration signal offered in peak velocity, RMS velocity and RMS acceleration. This sensor has improved RF filtering and turn-on time, withstands exposure to harsh industrial environments, and is used for monitoring rotating machinery, gears, bearings and motors.

Additionally, high-temperature charge output industrial piezoelectric sensors, with 100-mV/g (10.2-mV/(m/s²)) sensitivities, are designed to reliably perform in environments up to +900-deg F (+482-deg C), with Inconel® housing to ensure added strength in high temperature applications, and are supplied with an integral hardline cable. Integral charge amplifiers allow for use with standard data acquisition equipment. High temperature hardline coaxial dielectric cables, with Inconel® or stainless steel jacket, are available to help support demanding measurement requirements to +1,500-deg F (+816-deg C), with multiple connector types and configurations available.