As the price of raw materials fluctuates, industries must minimize costs and maximize profit margins. One way to control costs is to become leaner and more efficient. Every time companies remove waste in their processes, they save money. That includes maintenance and efficiency of pumps that move materials through a facility.
Ensuring that the operating conditions, temperatures and pressures are right for each pump application requires fine-tuning of instrumentation. Also understanding the condition of motors, bearings and couplings that drive pumps requires attention for efficient operations.
Process manufacturing plants require proper operation of hundreds or thousands of sophisticated devices to perform critical operations accurately and reliably. These include instruments, pumps, valves, pipes and tubes. These devices perform best when they undergo regular inspection, testing, calibration and repair. Calibrating measurement equipment enables optimization and can improve quality—whether in the food, chemical, power, pulp and paper, or oil industry.
Centuries of industrial experience have established the value of carefully recording the details of those inspections, tests, calibrations and repairs. Businesses and governments often require specific record keeping to ensure that full value is delivered to customers and that the health and safety of the public is protected.
However, traditional testing, calibration and documentation practices are also labor-intensive. With senior operators in scarce supply, downsized teams sometimes choose to defer regular calibration and testing. The latest industry findings suggest that smaller teams can feasibly conduct and document device calibrations at a lower overall cost than the larger teams they replaced, with additional productivity and operational reliability benefits.
Many maintenance activities were often performed in an instrument shop. But for pumps, motors, control valves and the instruments measuring process flows—because they have many related connections—testing is more cost effective when performed in-situ.
To verify proper operation of a pump system, personnel must be able to collect data about the material moving through the pump. This usually involves a field instrument that has two parts: a primary element and a transmitter.
- Primary elements include flow tubes, orifice plates, pressure sensors, wet chemistry sensors—such as pH, oxidation reduction potential (ORP) and conductivity probes—level gauges and temperature probes. Primary elements typically produce a signal—usually voltage, current or resistance—that is proportional to the variable they are designed to measure, such as level, flow, temperature, pressure or chemistry. Primary elements are connected to the input of field transmitters.
- Field transmitters include pressure, temperature, analytical and flow devices. They process the signal generated by the primary element, first characterizing it in linear format and applying engineering unit coefficients to it. Then they transmit the signal in analog (usually 4-20 milliamp [mA] direct current) or digital format (usually some variety of fieldbus).
When a field instrument is manufactured, both the primary element and the transmitter (or the actuator if it is a control valve) are calibrated at the factory. The calibration data is supplied with the unit and is often lost. Entering this information into centralized calibration records when the device is put into service should be a standard practice—and not just for efficiency's sake. Centralizing calibration information ensures that knowledge stays with the facility even as teams change.
Analog devices—often called 4 to 20 mA loop devices—transmit a signal that is an electrical analog representation of a measured physical quantity. They transmit an electric current that is proportional (analogous) to the magnitude of a measured physical quantity, with 4 mA of current representing the minimum scaled value and 20 mA representing the maximum scaled value. Although many systems that include pumps are now digital, analog devices are still in active use.
Digital devices convert a measured physical value into a digital signal. Many digital encoding methods are used in the process industry, including Foundation Fieldbus, Profibus and Highway Addressable Remote Transducer (HART). Many in the industry believe that fieldbus (digital) field devices do not require calibration. This is not true. Although a fieldbus signal—whether Foundation Fieldbus, Profibus or connected HART—provides diagnostic information, it does not provide information about the accuracy of the device. It also does not verify that the device is reporting the process accurately.
Control valves have actuators that also require calibration to adjust for wear, valve repacking for leakage remediation and the effects of sticking or "stiction." These valves must be given a full or partial stroke test if they have not been actuated regularly to ensure dependable operation.