How To Reduce Production Safety Hazards, With The Help Of Advanced Diagnostic Instruments

- Dec 20, 2017 -

introduction

Intelligent pressure transmitters are the most common and commonly used measuring instruments in today's process industry. It will measure the media pressure, differential pressure and other physical signals into a control system can be identified and processed 4 ~ 20mAHART standard current signal or FF, Profibus and other digital bus signals. After these measurement data are calculated and processed by the control system, some are used as process variable monitoring, some are used as process measurement, and some are used as the control basis of the terminal actuator [1]. Accompanied by scientific and technological progress, intelligent pressure transmitter product upgrades are also constantly evolving. At the same time, the user requirements of the transmitter is also getting higher and higher, mainly in the transmitter accuracy and long-term stability and other basic requirements of the upgrade, as well as the advanced diagnostic functions of the transmitter requirements. This article will focus on the pressure transmitter advanced process diagnostic functions to discuss these functions in the actual production activities to effectively reduce the potential safety hazard be demonstrated.


1 What is the pressure transmitter diagnostic capabilities

The diagnostic functions of pressure transmitters used in the field of industrial measurement today mainly include three aspects:

1) inherent self-diagnosis; 2) power circuit diagnostics; and 3) advanced process diagnostics.

These diagnostic information are monitored by a sensing element built into the pressure transmitter and signal processing in the transmitter's arithmetic processing unit to determine its meaning and finally based on the result of the judgment and the pre-given group State information output or display. These diagnostic functions are described below one by one.


2 inherent self-diagnosis

Most intelligent pressure transmitters have this feature, including the following aspects of the diagnosis:

Sensor diagnosis: through continuous monitoring to determine whether there is a fault sensor, or to determine whether the sensor and the electronic circuit board match.

Measuring range diagnosis: By comparing the actual process pressure / differential pressure readings and configuration range of the size of the relationship between the size of the pressure transmitter to determine whether the output signal accurately reflect the process value.

Electronic diagnostics: When the pressure transmitter's sensor is disconnected from the electronics board or the electronics board is faulty, it alerts the operator of the cause of the problem.

Operating temperature diagnosis: monitoring sensor and electronic circuit board is in the normal working temperature range.

Other diagnostics include LCD display update diagnostics, fixed analog output and analog output saturation diagnostics, sensor parameter warning diagnostics, etc. [2].


3 power circuit diagnosis

 

Under normal circumstances, each pressure transmitter requires a certain supply voltage to work properly. Communication circuit often need resistance value up to 250Ω, and in the case of special safety requirements retrofit lightning protection to ensure safety. In actual application conditions, the power supply / signal transmission cables of single pressure transmitters often reach or exceed 500m in length. Sometimes these cables and their bridges are sometimes arranged directly on the site to be wet, corroded or even physically damaged.

On the other hand, as most of the pressure transmitters operate in the open air, rainwater or condensed water in the air often flows down the conduit into the wiring compartment of the transmitter, causing the terminals to become flooded or corroded. These conditions are likely to lead to abnormal loop resistance, which makes the pressure transmitter output current signal distortion. Figure 1 is a description of this situation.

blob.png

As shown in Figure 1, a leakage current caused by an extra shunt resistor appears in the power transmitter's power circuit. This additional current signal causes the system's receive value (18mA) and transmitter output (15mA) different. In this case, if a control action is taken based on the error information, an accident may be caused.

In addition, an unexpected increase in the electrical load in the circuit can also result in unstable circuit voltage. An increase in resistance of the wiring circuit caused by corrosion, power instability, or human wiring errors may cause the transmitter to not have enough voltage to drive the circuit to the appropriate milliamp value or even an alarm condition. This is the case shown in Figure 2.

blob.png

As the process pressure increases, the transmitter output current signal can not break 16mA. At this point if the process pressure continues to rise to the critical value, but the transmitter can not give the alarm signal, it is most likely to endanger the safety of production.

In view of the above two situations, the diagnosis function of the power circuit will send an alarm message to the host computer when the loop changes, which will be read by the operator. At the same time, the corresponding header will also be displayed on the field header. Under normal circumstances, the alarm mode can be set according to actual needs. For example, the transmitter's analog output can be chosen to maintain a true measurement or to directly output an alarm value of 21.75 mA when the instrument's terminals experience flooding.


Related News