Beitrag

Pressure sensor clogged or responding slowly: What to do with viscous, contaminated or crystallizing media?

Posted on by Lukas Achtelik
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→ Produkt group: Pressure sensors

 

If a pressure sensor suddenly responds slowly, delivers implausible values or only returns with a delay after a pressure change, the first thought is often an electrical problem. In practice, however, the cause is very often not the output signal, the PLC or the power supply, but the process connection itself.

Especially with viscous, contaminated, sticky, fibrous or crystallizing media, pressure channels, threaded connections, bores or small dead spaces can become clogged over time. The sensor then no longer measures the actual process pressure, but a delayed or distorted pressure condition in the connection area. This leads to slow signal response, offset, measurement drift, apparent pressure peaks or permanently incorrect measured values.

Typical applications include dosing systems, wastewater technology, food processes, chemical plants, hydraulic systems, filter monitoring, polymer and plastics processing, coating systems or media containing solids. This article explains why pressure sensors can become clogged, how to correctly interpret the symptoms and which design measures help: flush diaphragm, diaphragm seal, flushing connection, suitable installation position, regular cleaning and as little dead space as possible.

Table of contents

Basics: Why a pressure sensor can respond slowly

A pressure sensor can only measure correctly if the process pressure reaches the measuring cell without distortion. Depending on the design, there is a pressure connection, bore, threaded cavity, channel, diaphragm or diaphragm seal system between the process and the sensor element. This area is particularly critical with difficult media.

With clean, low-viscosity media, this is usually unproblematic. The pressure is transmitted quickly and directly. With viscous, contaminated or crystallizing media, however, the connection can become partially clogged. This creates a kind of restriction point. The pressure still reaches the sensor, but with a delay, damped or only incompletely.

The result is a sluggish pressure indication. The process pressure changes, but the measured value lags behind. In control loops, this can be particularly problematic because the controller reacts to old or delayed values. This can make dosing inaccurate, cause pumps to operate unfavorably or result in limit values being detected too late.

Important: A pressure sensor that responds slowly is not automatically defective. Often, the sensor is still technically measuring correctly, but the pressure no longer reaches the measuring cell cleanly. That is why the process connection must always be considered during troubleshooting.

Typical symptoms of a clogged pressure connection

A clogged or partially blocked pressure connection often does not show up as a complete failure. Much more frequently, gradual changes occur. The measured value responds more slowly than before, no longer jumps cleanly to the new pressure value or remains elevated for some time after pressure relief.

An apparent zero point shift can also be an indication. The process is depressurized, but the sensor still shows a residual value. This does not immediately have to be a zero point error. It may also be that medium is still trapped in the pressure channel, in a line or in a small dead space and is only slowly relieving the pressure.

Another typical sign is measured values that strongly depend on operating condition, temperature or downtime. After system start-up, the sensor initially appears plausible; after several hours of operation, the signal becomes slower or drifts. After shutdown and cooling, residues can become more viscous or crystallize, making the fault more noticeable at the next start-up.

If several comparable measuring points are available, a comparison helps. If one sensor responds significantly more slowly than an identical sensor in a similar position, the process connection should be checked in addition to the electrical system and calibration.

Common causes: Deposits, crystallization and media residues

The most common cause is deposits in the pressure connection. Over time, the medium forms a layer on the diaphragm, in the bore or in the threaded cavity. This layer can be soft, sticky, hard, crystalline or fibrous. Depending on the medium, it can merely dampen the pressure or completely block the connection.

In wastewater and process plants, solids, fibers, sludge, biofilms or particles play a major role. In chemical applications, salts, additives, reaction products or crystallizing components can clog the connection. In food processes, the causes are often sugar, fats, proteins, starch, syrup, doughs, pastes or viscous cleaning residues.

Hydraulic systems can also be affected. Clogging there is less often caused by viscous media, but dirt particles, abrasion, aged oil, seal residues or narrowed adapters can also lead to sluggish measured value transmission.

Small bores, long impulse lines, blind holes, narrow adapters, dead spaces and connection geometries in which medium remains are particularly unfavorable. Anything that is not well flushed, cleaned or flowed through can become a disturbance point in the long term.

Viscous and sticky media: Why small pressure channels are problematic

Viscous media generally transmit pressure, but they move more slowly through narrow channels. If a pressure sensor with a small internal pressure channel is used, this channel can act like damping. The sensor detects pressure changes with a delay.

For media such as syrup, adhesive, resin, paste, sludge, grease, polymer, paint, additive or highly viscous process fluids, the design of the process connection is therefore decisive. A standard sensor with a small pressure bore may work when new, but can become increasingly sluggish over time due to deposits.

System downtime is particularly critical. As long as the medium is warm, moving or fresh, it may remain flowable. If it cools down, it can become significantly more viscous, adhere or harden. At the next system start-up, the pressure connection is then no longer clear.

In such cases, it should be checked whether a flush pressure sensor, a suitable diaphragm seal or a process connection that is easier to clean would be more appropriate. The solution is not always to clean the sensor more frequently. Often, the measuring point must be better adapted to the medium properties by design.

Contaminated media and solids in the process

With contaminated media, the problem is often caused by particles and solids. These can settle in the pressure connection, collect in front of the diaphragm or become trapped in narrow bores. The sensor then not only responds slowly, but may also deliver erratic or implausible values.

In wastewater, cooling circuits, process water, sludge or unfiltered media, particle sizes and contamination load can fluctuate greatly. As a result, a measuring point can work for a long time and then suddenly cause problems when operating conditions change.

Filter or screen points upstream of the measuring point can also have an influence. If a filter becomes clogged, pressure conditions arise that change dynamically. At the same time, maintenance or backflushing can release dirt particles that stress the sensor connection.

In such applications, the sensor should be installed in such a way that particles cannot settle directly in the connection. Dead spaces, downward-facing blind cavities and narrow intermediate pieces are unfavorable. Depending on the medium, a robust flush sensor or a diaphragm seal system may be the more reliable solution.

Crystallizing media and dried residues

Crystallizing media are particularly demanding because they may initially appear unproblematic in the liquid state. However, as soon as temperature, concentration, pressure or humidity conditions change, crystals or solid residues form. These can deposit directly at the pressure connection or on the diaphragm.

This is typical with salt solutions, alkaline solutions, certain chemicals, additives, cleaning media or media that dry out when exposed to air. Even small quantities can be enough to narrow a tight pressure channel or mechanically load a diaphragm.

After cleaning, a sensor may initially work again, but then respond sluggishly again after a short operating time. This is an indication that not only cleaning is required, but that the measuring point should be fundamentally adapted.

With crystallizing media, it is often useful to consistently avoid dead spaces, design the process connection so that it can be flushed well and choose the installation situation so that no stagnant medium residues remain in the connection. In some applications, a heated measuring point or a special diaphragm seal design should also be considered.

Air bubbles, gas pockets and trapped volumes

Not every sluggish response is caused by solid deposits. Air bubbles or gas pockets can also distort pressure measurement. While liquids transmit pressure very directly, trapped gases are compressible. The pressure change is then damped and delayed.

This can occur during commissioning, after maintenance, after cleaning, due to incomplete venting or because of an unfavorable installation position. Especially at low pressures or with fast pressure changes, even a small gas pocket can already become visible.

Impulse lines, hose sections, adapters or vertical line sections can also retain gas bubbles. The sensor itself is then not clogged, but it still does not receive a direct pressure signal.

The solution is to vent the measuring point and connection properly and to install them in such a way that gas bubbles do not remain permanently in front of the sensor. With media that outgas or with changing temperatures, the measuring point should be planned particularly carefully.

Choosing the correct installation position and process connection

The installation position strongly determines whether medium collects in the pressure connection or can drain away again. A sensor mounted in an unfavorable position can become contaminated more quickly than an identical sensor at a better-flushed point.

With contaminated or viscous media, the pressure connection should be designed in such a way that no pockets, blind cavities or areas that are difficult to flush are created. If medium remains in a dead space, the likelihood of deposits, drying or crystallization increases.

The position in the pipeline is also important. Directly downstream of pumps, valves, bends, reducers or dosing points, strong pressure pulsations, deposits or air components can occur. Depending on the application, a calmer measuring point can provide better and more stable values.

There is no universally correct installation position. It depends on medium, temperature, process pressure, cleaning, flow direction and system geometry. The measuring point should therefore be considered already when selecting the pressure sensor, not only when the sensor repeatedly has to be cleaned.

Flush diaphragm: When it is useful

With difficult media, a flush pressure sensor is often the better choice than a sensor with an internal pressure channel. With a flush diaphragm, the measuring diaphragm is directly exposed to the process. There is no deep bore in which medium can collect and clog the pressure channel.

This is particularly useful with viscous, pasty, sticky, fibrous, contaminated or crystallizing media. A flush design is also often advantageous in hygienic applications because it is easier to clean and offers less dead space.

However, flush does not automatically mean maintenance-free. A flush diaphragm can also be affected by coatings, deposits or mechanical damage. In many applications, however, it is significantly less sensitive to classic clogging than a small pressure bore.

It is important not to damage the diaphragm mechanically. A flush diaphragm must not be treated with sharp tools, hard brushes or unsuitable cleaning methods. Cleaning and maintenance must match the sensor design, the medium and the manufacturer’s specifications.

Diaphragm seals and separating diaphragms for difficult media

If the medium properties are particularly critical, a diaphragm seal can be useful. A diaphragm seal separates the measuring instrument from the process medium and transmits the pressure to the sensor or measuring instrument via a diaphragm and a filling fluid. This prevents the process medium from coming into direct contact with the sensitive measuring cell or a narrow sensor connection.

Diaphragm seals are often used with corrosive, hot, highly viscous, contaminated, crystallizing or hygienically demanding media. They can be designed with different process connections, diaphragm materials, filling fluids and constructions.

The advantage lies in better adaptation to the process. A large, easily accessible diaphragm can be less prone to clogging than a small pressure channel. In addition, materials and connections can be better matched to the medium, temperature and cleaning requirements.

At the same time, a diaphragm seal system must be carefully designed. Temperature, measuring range, diaphragm diameter, filling fluid, installation position, capillary line and dynamic behavior influence measurement accuracy and response time. A diaphragm seal solves many media-related problems, but it must fit the measuring task.

Flushing connection, cleaning and maintenance

In applications with regular deposits, a flushing connection can be very helpful. It makes it possible to clean the area in front of the sensor or diaphragm seal in a controlled manner without having to dismantle the entire measuring point. Especially with crystallizing or contaminated media, this can reduce downtime.

However, cleaning must always be suitable for the medium and the system. Not every medium may simply be flushed with water. For chemical media, hazardous substances, hot processes or hygienic applications, cleaning medium, temperature, pressure and disposal must be defined professionally.

The cleaning frequency should also not be determined purely by feel. If a sensor regularly becomes sluggish, a documented maintenance strategy is worthwhile. This records after which operating time deposits occur, which cleaning method is effective and whether a design change to the measuring point would make sense.

It is also important: A sensor should not be “rescued” by mechanically scraping out the pressure channel or diaphragm. This can damage the diaphragm, seal or measuring cell. If media residues are unclear, the diaphragm is damaged or the system is safety-relevant, the sensor should be checked or replaced.

Troubleshooting: Electrical problem or clogged connection?

If a pressure sensor responds slowly, troubleshooting should be systematic. First, check whether supply voltage, output signal, wiring, input card and parameterization are plausible. A 4–20 mA signal can be electrically completely stable and still represent an incorrect process pressure if the pressure connection is blocked.

An important indication is the behavior during pressure changes. If the process pressure changes quickly but the measured value changes slowly, damping, clogging, an air pocket or intentionally configured signal damping may be suspected. It should therefore also be checked whether damping has been parameterized in the sensor or in the control system.

A comparison with a reference measuring instrument at a suitable measuring point is helpful. If the reference device responds quickly but the installed sensor does not, the cause is probably the measuring point, the process connection or the sensor itself.

Caution is required during disassembly. Pressure systems must be safely depressurized, and hazardous media may only be handled by qualified personnel. Residues in the connection can provide clues to the cause: sticky coatings, crystals, particles, dried media or liquid pockets.

Table: Symptom, possible cause and suitable measure

Symptom Possible cause Suitable measure
Pressure sensor responds with a delay to pressure changes Partially clogged pressure channel, viscous medium or air pocket Check connection, vent, clean and evaluate sensor design
Measured value remains elevated after pressure relief Residual pressure, trapped medium or clogged connection Depressurize measuring point in a controlled manner and check for residues
Sensor shows slowly drifting values Deposits, temperature change, media residues or diaphragm coating Check process conditions and sensor connection
Measured value jumps implausibly Particles, air bubbles, unstable flow or electrical contact fault Check electrical system and process connection separately
Sensor works briefly again after cleaning Recurring deposits or crystallization Check flush diaphragm, diaphragm seal or flushing concept
Fault occurs mainly after shutdown Cooling, hardening, drying or crystal formation Evaluate shutdown conditions, cleaning and installation position
Only one of several measuring points responds slowly Local deposit, unfavorable measuring point or clogged connection Carry out comparison measurement and inspect measuring point
Diaphragm is visibly coated Adhesion, product residues or unsuitable cleaning Clean gently, check media compatibility and process connection
Sensor regularly fails with viscous media Unsuitable standard design with small pressure channel Use flush sensor or diaphragm seal system

Practical example: Pressure transmitter in a dosing system responds with a delay

In a dosing system, a pressure transmitter is used to monitor the delivery pressure. The medium is viscous and contains fine solid particles. Initially, the measuring point operates without issues. After a few weeks, however, it becomes apparent that the pressure value responds significantly more slowly than the actual system condition.

When the pump starts, the process pressure rises immediately, but the display in the control system follows only with a delay. After the pump is switched off, the measured value remains elevated for some time. Electrically, everything appears to be in order: The 4–20 mA signal is stable, the supply voltage is correct, and the PLC card shows no anomalies.

After controlled pressure relief, the sensor is dismantled. Viscous media residues are found in the pressure connection. The small pressure bore is not completely blocked, but significantly narrowed. As a result, the pressure was transmitted to the measuring cell only with a delay.

After cleaning, the measuring point initially works again. However, since the fault recurs after a short time, the measuring point is redesigned. Instead of a sensor with an internal pressure bore, a flush pressure sensor is used. In addition, the installation position is adjusted so that less medium remains in the connection. As a result, the measurement responds significantly faster and the maintenance intervals are extended.

Which measuring instruments / products are suitable?

With viscous, contaminated or crystallizing media, the pressure sensor should not be selected only according to measuring range and output signal. The decisive factors are above all process connection, diaphragm design, material, seal, cleanability and media compatibility.

A suitable starting point is the category pressure sensors / differential pressure sensors. It includes different designs for industrial, process, hydraulic and automation applications.

For many standard signals such as 4–20 mA or 0–10 V, the category analog pressure sensors / pressure transmitters is relevant. If difficult media are the main focus, attention should specifically be paid to flush process connections, suitable materials and media-compatible seals.

One example of a pressure sensor with a flush connection is the IMP331 pressure sensor. For dosing systems and industrial applications, the IMP339 pressure transmitter can also be of interest, as a suitable process connection is particularly important in such applications.

For more demanding process measuring points, where diaphragm selection, material, mounting, impulse lines or diaphragm seals play a greater role, the category process transmitters / differential pressure transmitters is also suitable. For particularly critical media, a diaphragm seal system with suitable process connection, appropriate diaphragm and defined cleaning option can be the more reliable solution.

Conclusion: With difficult media, the process connection is decisive

If a pressure sensor is clogged or responds slowly, the cause is often not in the electrical system, but at the process connection. Viscous, contaminated, fibrous, sticky or crystallizing media can narrow pressure channels, coat diaphragms or clog small dead spaces.

The right solution depends on the medium and the measuring point. Sometimes cleaning and venting are sufficient. In many cases, however, a design adjustment is useful: flush diaphragm, less dead space, better installation position, flushing connection or a suitable diaphragm seal system.

For reliable pressure measurement, measuring range, output signal and accuracy should therefore not be considered in isolation. The decisive factor is whether the process pressure is transmitted to the measuring cell permanently, cleanly and without delay. Especially with difficult media, the suitable process connection is often the most important factor for stable measured values.

FAQ: Frequently asked questions about clogged pressure sensors

Why does my pressure sensor suddenly respond slowly?

A slow response can be caused by a partially clogged pressure connection, viscous medium, deposits, air bubbles, residual pressure or configured signal damping. The electrical system and process connection should therefore be checked separately.

Is a sluggish pressure transmitter always defective?

No. Often the sensor itself is still functional, but the pressure is only transmitted to the measuring cell with a delay due to deposits, media residues or gas pockets.

What does a clogged pressure channel mean?

A clogged pressure channel means that media residues, particles, crystals or deposits partially or completely block the connection between the process and the measuring cell.

Which media cause problems particularly often?

Problematic media are viscous, sticky, pasty, contaminated, fibrous, crystallizing or hardening media. These may include sludge, syrups, resins, adhesives, paints, polymer media, salt solutions or wastewater.

Does a flush diaphragm help against clogging?

Yes, in many cases. A flush diaphragm avoids small internal pressure channels and reduces dead spaces. It is therefore often much better suited for viscous, contaminated or crystallizing media.

Can a flush diaphragm also become contaminated?

Yes. Flush diaphragms can also become coated or damaged. However, they are usually easier to clean and less susceptible to classic channel clogging.

When is a diaphragm seal useful?

A diaphragm seal is useful when the medium is corrosive, hot, heavily contaminated, highly viscous, crystallizing or hygienically demanding and the sensor should not come into direct contact with the medium.

What is a flushing connection?

A flushing connection enables cleaning of the area in front of the sensor or diaphragm seal. It can help remove deposits without having to dismantle the entire measuring point.

How can I tell whether residual pressure or clogging is present?

If the measured value falls only slowly after pressure relief or changes significantly during venting, residual pressure or trapped medium may be present. If the fault repeatedly persists, the connection should be checked for deposits.

Can the pressure connection simply be cleaned mechanically?

Only with great care and according to the manufacturer’s specifications. Sharp tools, hard brushes or unsuitable cleaning agents can damage the diaphragm, seal or measuring cell.

Why does the fault often occur after system shutdown?

During shutdown, media can cool down, become more viscous, dry out, harden or crystallize. This can partially block a previously clear pressure connection.

Can an air bubble make the pressure sensor sluggish?

Yes. Trapped air or gas pockets are compressible and can dampen pressure changes. This can be clearly visible, especially with liquids and low measuring ranges.

What role does the installation position play?

An unfavorable installation position can cause medium, particles or gas bubbles to remain in the connection. A better-flushed or better-draining position can make the measurement more stable.

When should the sensor be replaced?

Replacement is useful if the diaphragm is damaged, the sensor does not measure stably after cleaning, the connection repeatedly clogs or the design is fundamentally unsuitable for the medium.

How can clogging be avoided in the long term?

In the long term, a suitable process connection, flush diaphragm, as little dead space as possible, suitable materials, good cleanability, correct installation position and a maintenance concept adapted to the medium will help.

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