Experience in on-site diagnosis of abnormal data collection of pressure transmitters

In industrial automation control systems,Pressure transmitter as a key signal acquisition device,Its stability directly affects the reliability of the entire control circuit. However, in the actual operation and maintenance process,We found that the failure modes of such devices often exhibit a certain regularity. Based on recent on-site maintenance experience in the petrochemical, pharmaceutical, and water treatment industries,This article summarizes several typical fault phenomena and their diagnostic approaches.

1、 Pressure rises but output shows no response phenomenon

This issue is particularly common during the debugging phase of newly installed equipment. Last year at a chemical plant site,I have encountered this situation with three newly installed transmitters at the same time.

Preliminary investigation pathIt should be like this:Firstly, check the sealing condition of the pressure interface. Complex on-site environment,Welding slag, oxide scale, and polymer residue may all block the pressure hole. Once we discovered on a polypropylene production line,A small piece of solidified material actually condensed in the pressure pipe,Blocked the passage.

After excluding mechanical factors,The wiring issue cannot be ignored. Especially in strong electromagnetic interference environments,Is the grounding of the shielding layer correct, and are the signal lines and power lines routed separately,These details will affect signal transmission. I have seen engineering teams working to save time,turn4-20mASignal lines and380VThe power cables are bundled together and run on the cable tray,The output signal of the transmitter is distorted.

Power quality inspectionIt is also a crucial link. nominal24VDCThe power supply,The measured voltage may be reduced due to the voltage drop in the circuit20Vstand out,Some transmitters cannot function properly in this undervoltage state. At this point, the output signal can be directly measured at the sensor end——If there is a significant change in millivolt level output after applying pressure,Indicating that the sensor body is good,The problem lies in the subsequent circuit or instrument;If there is no response at all,It can be basically determined that the strain gauges or capacitive elements inside the sensor have been damaged.

It is worth mentioning that,Sometimes problems hide in unexpected places. Once inDCSAfter system upgrade,It has happened before because the configuration parameters were not migrated correctly,Causing abnormal display of all pressure circuits. So when conducting investigations, it is necessary to have systematic thinking,You can't just focus on the transmitter itself.

2、 Delayed pressure response and abnormal return to zero

This fault phenomenon is quite confusing:At the beginning of pressurization, the transmitter output remained unchanged,Continue to apply pressure to a critical point,Sudden output jump;Even more bizarre is that the reading cannot return to zero after unloading,Always hanging there.

The root cause often lies in the sealing ringThis conclusion is a lesson learned from using several scrapped sensors. standardOThe specifications of the ring are particular——There are corresponding selection standards for hardness, cross-sectional diameter, and inner diameter. But in actual installation,Maintenance personnel often collect materials nearby,Using low hardness(Such as Shore hardness60Rather than standard70)Or a sealing ring with a thick cross-section is acceptable.

The specific failure mechanism is as follows:When tightening the sensor,A sealing ring that is too soft will undergo plastic deformation under axial pressure,Squeezed into the pressure hole to form a variable'valve core'In its initial state, it blocks the channel,The medium pressure cannot be transmitted in;When the pressure accumulates to a sufficient level,Instantly break through this rubber seal,The sensor senses a pressure step,The output suddenly jumps;And when releasing pressure,The sealing ring has re blocked the hole due to elastic recovery,Internal residual pressure is sealed and cannot be released,Therefore, the output signal cannot return to zero.

The verification method is very simple:Remove the sensor from the process pipeline,Use a handheld pressure pump or calibrator to conduct a separate pressure test. If the response is normal and the linearity is good at this time,That's the real problem——Replace with a sealing ring that meets the specifications,Pay attention to using fluororubber or EPDM material(According to the compatibility of the medium),Hardness control within70-80between,Do not tighten the torque wrench too tightly when reinstalling.

I have also encountered a special case:Intermittent slow return to zero of a certain pressure transmitter,Finally, it was discovered that there was a design issue with the pressure pipeline——a segment10A meter long thin tube in winter environment,The incomplete discharge of condensed water inside the tube resulted in the influence of liquid column gravity on pressure transmission. This situation needs to be addressed from the perspective of process design,Add drainage points or switch to diaphragm sealing method.

3、 Analysis of Output Signal Volatility

Signal instability is actually a problem'universal remedy'Type of fault description,There may be five or six different causes behind it.

The pulsation characteristics of the pressure source itselfIt is the primary consideration factor. Location of reciprocating compressor outlet, centrifugal pump head detection point, steam pressure reducing valve, etc,The process pressure itself has periodic or random fluctuations. In this case, the transmitter accurately reflects the real process variables,It cannot be considered as a device malfunction. The solution is to install buffer tanks or damping devices on the process side,Or enable damping filtering function on the instrument side——But be careful,Excessive damping will sacrifice response speed,A balance needs to be struck between stability and dynamic performance.

Electromagnetic compatibility issuesIt is very common in industrial sites. High power frequency converters, high-frequency welding machines, and thyristor voltage regulating equipment all emit strong electromagnetic interference into space. I once tested it in an electroplating workshop,At the moment of starting the frequency converter,Near the instrument4-20mAThe signal will have an amplitude exceeding2mAThe burrs. Response measures include:Adopting twisted pair shielded cables, single point grounding of shielding layer, and parallel connection on the instrument side0.1μFFilter capacitors and install signal isolators if necessary. Some advanced transmitters have built-in digital filtering algorithms andEMCProtective circuit,The anti-interference ability is significantly stronger.

The impact of mechanical vibrationEasy to be overlooked. Piezoelectric or strain sensors are sensitive to vibration,If the transmitter is directly rigidly connected to reciprocating machinery or pipelines,The vibration acceleration will be converted into parasitic signals and superimposed on the measured values. Catalytic cracking unit of a certain refining unit,Because of the vibration of the compressor foundation, all nearby pressure gauge readings are in the same range±0.05MPaSwing within the range. Later, measures such as installing with soft connections, adding shock-absorbing pads, and shortening the length of rigid pressure pipes were adopted,Just solved the problem.

Besides,Oxidation and loosening of wiring terminalsandAging of internal components of sensorsIt is also a common reason,These are relatively easy to diagnose,Just investigate one by one.

4、 Systematic troubleshooting of no output when powered on

This type of malfunction may seem simple,But if lacking experience,Often wastes a lot of time.

My personal habit is to use**'From outside to inside, from easy to difficult'The segmented investigation method**Firstly, inDCSorPLCMeasure the circuit voltage and current at the control cabinet end——If connected24VThe power supply hasn't been added yet,Or the load current is zero,Explain that the problem lies in the peripheral circuit;If the power supply voltage is normal but the current is abnormal(For example, exceeding25mAOr lower than3.5mA),Prompt that there is a short circuit or open circuit on the on-site equipment side.

Wire malfunctionThe investigation requires the cooperation of a multimeter. Long distance cables are worn out during the threading process, insulation decreases due to water ingress into the cable well, poor contact of the terminal block, and dust and moisture ingress into the intermediate junction box,These can all cause open circuits or short circuits. Once I encountered a bizarre case:a strip500Meter long signal cable,A thread was bitten off by a mouse somewhere in the middle,Because the outer sheath is intact and undamaged,No problem visible to the naked eye,Finally, useTDR(Time domain reflectometer)Locate the breakpoint.

Power matching issueIt is also worth paying attention to. The transmitter is divided into two-wire system, three wire system, and four wire system,The corresponding power supply and wiring methods are different. Two wire transmitter circuit power supply,The negative pole of the power supply should be shared with the negative pole of the signal;If mistakenly connected to a three wire system,There will be a phenomenon where there is power but no output. Additionally, it is important to pay attention to the load capacity of the power supply,Some switching power supplies have nominal specifications24V/1A,But in reality, only three or four transmitters can be carried,After overload, the voltage will drop.

Matching between instruments and sensorsEqually important. Range and output signal type(mVThemADigital signal)The range of power supply voltage and explosion-proof level should correspond one-to-one. I have seen it before4-20mAThe output transmitter can only be connected to0-10VOn the input acquisition card,Naturally, there will be no reaction.

If all of the above are confirmed to be correct,That's highly likelyEquipment body damageThat's it. The sensor may have a broken strain bridge or a short circuit in the capacitor plate insideASICChip breakdown;The instrument side may beA/DThe converter, signal conditioning circuit, or output driver stage is damaged. In this case, it is necessary to return to the factory for repair or replace it directly.

5、 Comparison and verification with pointer pressure gauge

Operators often question on site:'whyDCSThe pressure displayed on the screen does not match the on-site gauge?'This issue requires rational analysis.

Firstly, it is necessary to clarify,The existence of deviation is an objective lawAny measuring instrument has limitations on accuracy levels,It is impossible to achieve absolute accuracy. The key is to determine whether the deviation is within the allowable range.

Error composition of pointer pressure gaugeIncluding fundamental errors and reading errors. The basic error is determined by the accuracy level,for example1.5Grade pressure gauge,full scale30MPa,The basic error is30×1.5%=0.45MPaThe reading error comes from human eye disparity and scale resolution,Usually take half of the minimum scale value,Assuming the minimum scale0.2MPa,The reading error is0.1MPaThe total error is obtained by superimposing the two items（0.55MPa).

Accuracy analysis of electronic pressure transmitterRelatively more complex. A complete measurement circuit includes sensors, transmitter circuits, signal transmissionDCSCollecting cards and other multiple links,Each stage contributes to errors. Assuming sensor accuracy0.5%FS,Transmitter circuit0.2%FS,A/Dtransformation0.1%FS,According to the theory of error propagation(Root mean square of each independent error),The overall accuracy of the system is approximately√(0.5²+0.2²+0.1²)≈0.55%FSFor20MParange,Corresponding absolute error（0.11MPa).

Principles to be followed in actual comparisonyes:The error of equipment with lower accuracy shall prevail. The pointer table in the above example allows for deviation±0.55MPa,The transmitter is±0.11MPa,So what is the difference in readings between the two0.55MPaEverything inside is considered normal. If the deviation far exceeds this range,For example, the difference2-3MPa,We must investigate the cause.

Recommended verification processThat's how it is.:Prepare a high-precision digital pressure gauge(Accuracy at least one level higher,as0.25Grade or0.1level),Connect the process pipeline or apply pressure to three meters simultaneously using a pressure pump,At zero25%、50%、75%Read at each of the five points on the full scale,Draw an error curve. This way, we can determine which watch has a problem,Can also analyze the nature of errors——Is it zero offset, range drift, or nonlinear error.

There is a detail worth noting:Pointer table after long-term use,The spring tube will undergo irreversible deformation,Causing zero negative drift or range contraction. In this case, even if the on-site meter displays normally,Actual measured values may also deviate from the true values. Therefore, we cannot blindly question electronic transmitters based on pointer meters,Instead, a standard should be used for arbitration.

6、 Gravity effect compensation of micro differential pressure transmitter

Micro differential pressure measurement is a particularly delicate task,Often used for cleanroom pressure difference monitoring, filter blockage detection, duct static pressure control, and other occasions,The measurement range may only be0-500Paeven0-100Pa.

The physical essence of the problemlie in:When the measurement range is extremely small,Sensor sensitive components(Silicon film, differential capacitor, etc)The weight itself will have a significant impact on the output. One piece thickness0.05mmDiameter10mmSilicon film sheet,How many milligrams is the quality,The additional pressure generated in the gravitational field may reach10-20Pamagnitude——For100PaFull range transmitter,This is equivalent to10-20%Additional errors!

The manifestation of gravity effectyes:Same transmitter,When installed horizontally, the zero point is a value,When installed vertically, the zero point changes to another value,The two may differ by several percentage points. This can be ignored in macroscopic pressure measurement,But it cannot be accepted in a micro differential pressure field.

Correct installation postureThe sensor diaphragm plane should be perpendicular to the direction of gravity,That is to say, to make the membrane'stand'rather than'lie flat'In this way, the direction of gravity is parallel to the plane of the membrane,No normal force will be generated,Thus eliminating zero point shift caused by gravity.

However, in actual engineering,The installation angle of the transmitter is often limited by factors such as pipeline layout and equipment location. Encountering this situation,just needPerform zero point calibration on site:First, fix the transmitter in its actual working position,Then in the absence of pressure difference(Both positive and negative pressure chambers are open to the atmosphere)In state,Adjust the zero point knob or modify the zero point parameter through a manual controller,Corresponding output0This zero point value actually includes compensation for gravity effects,As long as the installation angle remains unchanged in the future,Measurement is accurate.

Some advanced differential pressure transmitters are built-inMulti axis accelerometer and digital compensation algorithm,Can automatically recognize installation posture and perform software compensation,Eliminating the hassle of manual zeroing. But these products are expensive,The cost-effectiveness requires specific project evaluation.

There is also an easy to step on pit micro differential pressure transmitterThe layout requirements for the pressure pipeline are very strictPositive and negative pressure conduits must be of equal length, height, and symmetrically arranged,There should be no bubbles or condensation inside the pipe,Otherwise, additional static pressure difference will be introduced. Once in a cleanroom project,Just because the positive pressure side pressure pipe took an extra bend,Resulting in a height difference of approximately30cm,Corresponding liquid column pressure(Water column approximately3000Pa)Concealing the true wind pressure difference signal,It took several days of troubleshooting to locate the problem.

Summarize,Fault diagnosis of pressure transmitter is a systematic engineering,It is necessary to comprehensively consider various factors such as process conditions, installation quality, electrical environment, and equipment performance. Avoid on-site maintenance'treat the symptoms',To have the analytical ability to trace the essence from the surface. At the same time, we should also attach importance to preventive maintenance,Regular calibration and timely replacement of aging components,Many faults can actually be avoided in advance.

These experiences have been developed through extensive practical experience,Different industries and working conditions may encounter new types of problems. Maintain a learning mindset,Communicate more with peers,To continuously improve the efficiency and accuracy of fault handling.