Pressure Transmitter

Pressure transmitters are a common and well understood technology especially within the petroleum, petrochemical, refrigeration and fuel gas markets and they are extremely economical compared to other technologies.

Pressure transmitters are a common and well understood technology especially within the petroleum, petrochemical, refrigeration and fuel gas markets and they are extremely economical compared to other technologies. If a level is to be made in an open vessel a pressure transmitter (PT) is placed at the bottom and the head pressure caused by the weight of the liquid can be used to calculate the height of the liquid. Differential pressure transmitters can be used on pressurised vessels using the same principles (9).

Level measurement using pressure transmitters cannot be deployed in primary separators as the change in temperature will impact the measurement, the change in density of the oil during operation will impact the measurement and any fluctuation of the oil/gas differential pressure control valve or pressure disturbances would also impact on the reading.

Portable dP gauges are used to monitor the dP across strainers, online densitometers, DB&B valves, prover interchange valves, and other devices. All pressure gauges shall be equipped with calibration stickers indicating the last test or calibration date and who performed the calibration. The calibration sticker should be clearly visible and constructed of a rugged material. All recordings on the sticker should be made with indelible ink. The dial gauge cover should be clear and abrasive resistant. The pressure gauge should be readable with permanent figures and markings and equipped with a blowout disc for safety. The scale shall be in graduations of no greater than 2 psig with longer graduations at each 5 psig and numbered at not greater than 10 psig intervals. The error at any point on the scale should not exceed ±2 psig. The maximum allowable working pressure (MAWP) of the dP gauge should be at least equal to the MAWP for the measurement facility.

A differential pressure transmitter is a device that translates physical force to an electrical signal. The most common type of force transducer uses a diaphragm, piston, bourdon tube, or bellows to sense the physical force and various strain/force-sensing devices to convert the deflection of the physical element to an electrical signal. Traditional strain-sensing devices include the following: capacitive cells, piezoresistive strain gauges, piezoelectric quartz material, and electromagnetic devices.

In the United States, companies such as Rosemount, Cameron/Barton, Foxboro, and Ultrasystems have provided transmitters that specialize in safety system pressure measurements. These transmitters may still function successfully on some iPWR designs, but many will have to be reengineered for different mounting configurations, size constraints, and environments. Many iPWR designers, when faced with a modification program, may choose to go with new technologies rather than modifying the old ones. The new technologies may offer advantages in size, redundancy, accuracy, and environmental resilience. Some of these new technologies include microelectromechanical system (MEMS) sensors, fiber optic sensors, and ultrasonic sensors.

In the optical fiber category, a company called Luna Innovations has developed and successfully tested fiber optic pressure sensors, like the one shown in Fig. 6.1, in a research reactor environment. These fiber optic pressure sensors have been shown to operate in radiation environments with flux levels much higher than those compatible with most electronic pressure sensors. With traditional technology, it is necessary to protect traditional electronic gauge pressure transmitter from harsh radiation conditions near the core; this requires the use of long pressure sensing lines, which limit the response time to pressure transients and increase the number of wall penetrations. Luna’s fiber optic pressure sensors are designed to operate in harsh environments. When these pressure sensors were combined with Etalon-based fiber optic temperature sensors providing temperature compensation, drift effects were minimized. The attractiveness of this technology for iPWRs is obvious, with the elimination of sensing lines, the minimization of penetrations, the small size of the sensor, the rapid response to pressure fluctuations, and the operability in high radiation fields. With these attributes, this technology bears merit for primary and secondary side pressure measurement (Dickerson et al., 2009).
   
Pressure or DPT transmitters have been used to measure liquid levels of open or pressurized tanks, respectively, with proper calibration taking care of the density value of the subject liquid. These types are used for level measurements of boiler drums, all heaters, deaerators, condenser hotwells, condenser surge tanks, and all dosing tanks.

For open tanks, a pressure tapping at a suitable location is connected through the impulse line, to the high-pressure port of the DPT with the low-pressure port open to atmosphere. The pressure (or DP) sensed by the DPT is proportional to the level with a known liquid density (ρ) and force due to (earth’s) gravity (g). With proper calibration, the potential transform can now be used as a level transmitter.

Regarding the pressurized tank or vessel, single-pressure tapping does not serve this purpose as it senses the pressure of the vessel and the pressure of the liquid level. To balance out the vessel pressure, another pressure tapping for sensing it is connected, through the impulse line, to the low-pressure port of a DPT so that the output will only be proportional to the liquid level only with known ρ and g.

  Another new technology for pressure sensing is the polymer-derived ceramic MEMS sensor. At the forefront of this technology, a company, Sporian Microsystems, has developed a pressure/temperature sensor made to survive high temperatures (Fig. 6.2). This technology offers a solution for pressure sensing in iPWRs due to its hardy environment survivability and its small size. The small size allows for the installation of redundant units and the measurement of pressure at many points, possibly with fewer penetrations than traditional sensors.

These new technologies have attributes like small size, heat survivability, radiation hardness, fast response, and low maintenance. These attributes are highly valued in iPWR designs for obvious reasons.
               
The Series-211 differential absolute pressure transmitter can accurately measure positive, negative or differential pressure and send the corresponding 4–20 mA output signal to a recording system. It is not position sensitive and can be mounted in any orientation without compromising accuracy. It also features a power LED, so one always knows when the transmitter is operating. The compact, light-weight design makes installation simple and easy. Two inlets for the pressure connections are located on the front of the unit, labeled “High” and “Low”. For differential pressure measurement, the higher pressure should be connected to the “High” pressure port. For positive pressure, the “Low” pressure port should be left vented to atmospheric pressure. Periodically, it is necessary to recalibrate the gauge to maintain the accuracy. For “zero” gauge, the pressure connection needs to be removed from both pressure ports and to adjust the zero potentiometer until the output is 4 mA. To span the gauge, the full scale pressure to be applied to “High” pressure port and to adjust the span potentiometer until the output is 20 mA. The salient features of the pressure transmitter are compact design, LED power indication, 2-wire design, 4–20 mA output, ± 0.25% accuracy level, 0–90 mmHg pressure range, –25 °C to 70 °C operating temperature ranges, 2-wire output signal and 12–30 VDC power supply.

  Drum level is measured by differential thermowell temperature transmitter installed in line with Figures XII/3.5-1(a,b). Note that for drum-level measurement a temperature-equalizing column has been used to ensure that in both limbs of DPTs temperatures are equal, meaning that the densities are equal. So far, the discussion has been about installation and making the two limbs’ temperatures equal. Now level is measured by because there is an incumbent pressure P1 at the top, which needs to be subtracted to get the level. So, finally, one is left with P1 at the top and P1 + pressure due to water head (P) on the other limb. Therefore DP = pressure due to water head, P = h × ρ × g.
               
For a particular place, g is always constant. So water head P varies with h (level) and density, which in turn depends on temperature. In the drum, incumbent pressure is due to saturated steam pressure, and for saturated steam for each pressure there is a particular temperature. Therefore, saturation pressure is chosen to compensate/correct the density effect. Temperature is a sluggish parameter, so pressure is chosen to compensate.
               
Degradation of the transmitter’s accuracy and response time (two uncorrelated phenomena) are the two most important consequences of ageing. Ageing caused by heat and humidity can cause the transmitter sealing materials to fail, allowing moisture to enter the transmitter housing. This can cause calibration shifts and high-frequency noise at the transmitter’s output, which can render the transmitter inoperable or unreliable. Though NPP I&C failure data indicates that calibration drift accounts for anywhere from 59% to 77% of all age-related failure in pressure transmitters (fow blockage, fatigue, and other factors accounting for the remaining age-related failures), a survey of the nuclear industry in the early 1990s showed that fewer than 10% of NPP pressure transmitters actually drift out of tolerance and that in a typical two-year fuel cycle only about 1–3% of transmitters suffer calibration failure.
               
  The analog signal operates over a 4–20 mA range (0 to 5 Vdc with a 250 Ω high-precision pull-up resistor). For proper analog signal discrimination, the tertiary device should be equipped with at least a 12-bit ADC converter. Digital signals are preferred over analog signals for accuracy. However, the tertiary device must be able to communicate with the secondary devices using a common communications protocol. The status signals typically are used for indicating block valve status (fully open, fully closed, in travel), generating commands to obtain a sample (activation of sample extractor), and alarm status.
               
  The transmitters should be equipped with a display of the current readings at the transmitter housing in appropriate engineering units. The transmitters should be equipped with both analog and digital output to the tertiary device.

For analog communications to the tertiary device, a 4–20 mA signal is preferred and requires a high-precision 250 Ω pull-up resistor (to convert from 0 to 5 Vdc). The dampening parameters should be turned off or set to its lower limit. For digital communications to the tertiary device, the update interval of the transmitter should be less than or equal to 1 second. The transmitters should be equipped with an appropriate communications protocol and the dampening parameters should be turned off or set to its lower limit.
               
The sensing lines that bring the pressure signals from the process to the transmitter can become partially or totally blocked due to sludge, boron solidification (PWRs), and other debris in the reactor coolant, causing sluggish dynamic performance in the transmitter. According to NRC data, blockages, voids, and leaks account for nearly 70% of the age-related problems in sensing lines. Nevertheless, the effects of ageing on response time are even less significant than the effects on calibration. The response times of 84% of transmitters tested in a 1994 study written by the author for Nuclear safety were unaffected by ageing. Of the remainder, only 4% delivered response times that could be considered failing.

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