For pressure leakage in thread-mounted differential pressure sensors, first inspect the sealing connection

When a thread-mounted differential pressure sensor develops pressure leakage, first check the sealing condition between the sensor process connection and the equipment mounting port. Most field leaks do not originate inside the sensor, but are caused by mismatched thread specifications, improper sealing materials, misaligned installation, or insufficient tightening. Especially in gas, steam, hydraulic oil, and corrosive media applications, even minor leakage may cause differential pressure display fluctuations, zero drift, or control system malfunction.

Before troubleshooting, depressurize, isolate, and disconnect power from the equipment. Disassemble and inspect only after confirming that no residual pressure remains in the piping. For high-temperature, toxic, flammable, or highly corrosive media, wear protective equipment in accordance with site safety procedures and confirm that discharge and purging measures have been completed. Do not loosen the sensor or blindly apply additional tightening force while under pressure, as this may worsen leakage or damage sensitive elements.

Inspection can be performed in the order of “connection location, thread condition, seal condition, installation torque, piping stress, and sensor body.” This helps eliminate high-probability external issues first, then determine whether the sensor process fitting, isolation diaphragm, or housing seal has failed, reducing unnecessary replacement and downtime.

If leakage occurs at the high-pressure side, low-pressure side, or impulse line connection, inspect the piping on both sides simultaneously. Differential pressure measurement relies on stable pressure transmission at both high- and low-pressure ports. Addressing only visible leaks while ignoring minor leakage on the other side may still cause measurement deviation and affect the reliability of flow, level, filter differential pressure, or air pressure control.

Step 1: Confirm whether the leak is at the threaded connection

Distinguish connection leakage from sensor body leakage

First clean oil, water marks, and dust around the sensor connection, then repeat the test at low pressure. In gas applications, apply leak detection solution to the external thread root, fitting joint surface, and impulse line fitting to observe whether bubbles continue to form. In liquid applications, use absorbent paper, a dry cloth, or visual inspection to identify the seepage location. Test pressure should remain within the equipment's allowable range; do not repeatedly subject the sensor directly to full-scale pressure.

If leakage traces are concentrated at the end of thread engagement, fitting shoulder, or around the gasket, the issue is usually related to the installation connection. If liquid or gas escapes from the electrical wiring chamber, housing joint, or near the pressure sensing element, stop using the device and contact qualified technical personnel to assess whether the sensor is damaged.

For differential pressure transmitters, confirm leakage points separately at the high- and low-pressure ports. Although the two connections may appear identical, their actual pressure load, medium temperature, and pulsation level may differ. For example, when the high-pressure side is exposed to 0.6 MPa while the low-pressure side is close to atmospheric pressure, even a small leak on the high-pressure side will be more apparent and is more likely to cause low differential pressure readings.

After initial localization, record the leak location, medium type, operating pressure, ambient temperature, and time of occurrence. For intermittent pressure leakage, this information helps determine whether the issue is related to thermal expansion and contraction, pressure pulsation, vibration, or frequent start-stop operation, and facilitates selection of a more suitable sealing and installation solution.

Common leak points and priority inspection items

If continuous bubbles form at the thread root after applying leak detection solution, first verify the connection identification rather than immediately increasing tightening force. NPT, G, M, and other threads differ in profile, taper, and sealing method. Connections that appear similar may engage only 2 to 4 turns when mixed, yet fail to form an effective seal.

If equipment vibration is significant, check whether the sensor is directly bearing piping weight or mechanical pulling force. Long-term vibration can gradually loosen threaded connections and may also cause fatigue cracking of impulse lines. For applications near pumps, compressors, fans, and stamping equipment, add support, damping, or flexible connection measures where appropriate.

If paint peeling, white crystallization, corrosion, or medium residue is found near the connection, also be alert to chemical corrosion. When ordinary carbon steel fittings or rubber seals are incompatible with acids, alkalis, solvents, or high-salinity media, seepage may occur after weeks or months even if there was no leakage during initial installation.

Step 2: Check thread specification, engagement condition, and sealing materials

Thread mismatch is an important cause of recurring pressure leakage

Common process connections for differential pressure sensors include G1/4, G1/2, M20×1.5, 1/4NPT, and 1/2NPT. Parallel pipe threads are generally sealed by face gaskets or O-rings, while tapered pipe threads rely on thread taper engagement together with an appropriate amount of sealing material. Before installation, refer to the product nameplate, specification sheet, and equipment connection drawings; do not judge by diameter alone.

After removing the sensor, inspect the thread teeth for flattening, missing teeth, scoring, uneven wear, or metal debris. If the thread front end is already damaged, rewinding PTFE tape will generally not fully resolve the issue. For stainless steel threaded connections, also watch for galling; incorrect lubrication or excessive tightening may cause thread seizure.

During normal installation, first screw in the sensor by hand and confirm that several turns can engage smoothly before tightening with a wrench. If it becomes noticeably tight after only 1 to 2 turns, or the sensor is tilted after installation, remove it immediately for inspection. Forced assembly may damage the equipment's female thread, turning a manageable pressure leakage issue into a scrapped connection.

Sealing materials must suit the medium and temperature conditions. A general sealing method for room-temperature compressed air may not be suitable for 150℃ steam, hydraulic oil, oxygen, or corrosive liquids. Especially for oxygen and strongly oxidizing media, use materials confirmed as compatible and keep connections clean to avoid introducing grease contamination.

Key inspection points for different sealing methods

When using PTFE tape, it is recommended to begin wrapping 1 to 2 thread pitches away from the first thread and apply it smoothly in the tightening direction. This reduces the chance of sealing tape being extruded into the pressure passage. Excessive wrapping can create false tightness: the connection feels tightened externally, while actual effective thread engagement depth is insufficient, increasing the risk of pressure leakage.

If an O-ring is flattened, cracked, cut, hardened, or visibly swollen, replace it directly with a new part of the same specification and compatible material. For sensor connections that are frequently disassembled, establish a replacement interval for wear-prone seals to prevent recurring leakage in the measurement system caused by reusing aged sealing rings.

Sealant should not be applied excessively. If sealant enters the pressure inlet, it may block the damping orifice, causing the differential pressure sensor to respond slowly, delay returning to zero, or even produce distorted measurements. After application, allow sufficient curing time according to product requirements before conducting a pressure test.

Step 3: Verify installation torque and piping stress

Both insufficient and excessive tightening can cause seal failure

A threaded connection is not better simply because it is tightened more. Insufficient tightening provides inadequate contact pressure at the sealing surface and may lead to seepage after pressurization. Excessive tightening may crush the O-ring, crack the gasket, damage threads, or even subject the sensor process fitting to mechanical stress beyond its design range. The correct torque shall be based on the sensor manual and matching fitting requirements.

During installation, use the specified hexagonal wrenching area of the sensor. Do not rotate it by gripping the housing, terminal box, cable fitting, or display head. For smart differential pressure transmitters with displays, buttons, and wiring plugs, improper force may loosen the housing, damage terminals, or pull internal wiring.

When a differential pressure sensor is installed at the end of a long impulse line, rigid metal hose, or valve manifold, observe whether lateral pulling exists after installation. The sensor connection should primarily withstand medium pressure and should not carry piping dead weight, bending moment, or equipment vibration loads. If the piping exerts obvious pushing force on the connection, first adjust the piping route and supports.

For applications with large pressure pulsation, such as compressor outlets, hydraulic stations, pump outlets, and fan ducts, also check whether suitable damping measures are installed. Continuous pulsation repeatedly loads the connection. Even if static pressure holding passes, loosening or fatigue leakage may occur after several hours of operation.

Staged pressure-holding verification is recommended after installation

After reinstallation, first inspect the connection appearance and leak detection solution response at lower pressure, then gradually increase to normal operating pressure after confirming no abnormalities. For a differential pressure sensor with a 1 MPa range, pressure may be increased in stages according to process requirements, for example, first confirming no leakage near 0.2 MPa and then observing stability at higher pressure.

Pressure-holding duration should take both medium properties and system volume into account. General connection checks should not only observe whether liquid leaks instantaneously; observe for 10 to 30 minutes under stable pressure and confirm that the pressure gauge, transmitter output, and connection surface are all normal. For gas systems with a higher risk of minor leakage, extend the observation period as appropriate.

In differential pressure measurement systems with impulse pressure on both sides, confirm that high- and low-pressure valve positions are correct to avoid single-side overpressure caused by incorrect valve manifold switching. After pressure holding, verify the zero point and output signal where process conditions permit, for example whether zero output near 4mA is stable or whether digital communication readings show no abnormal jumps.

If normal operation is restored briefly after retightening but pressure leakage recurs after several days, focus on vibration, temperature cycling, material aging, and piping stress rather than repeatedly increasing torque. Repeated forceful tightening can conceal the root cause and increase the risk of sensor connection damage.

Step 4: Determine whether the sensor should be taken out of service and repaired

Continued operation is not recommended under the following conditions

If leakage is confirmed to originate from the sensor body, process fitting weld, area near the isolation diaphragm, or electrical wiring area, stop using the device immediately. Such issues may indicate internal seal failure, corrosion perforation, overpressure damage, or structural cracking. Continued use may allow the medium to enter electrical parts and even cause more serious site safety risks.

If pressure leakage occurs together with frequent output signal fluctuations, inability to calibrate the zero point, obvious full-scale deviation, failure of displayed values to return to zero, or communication alarms, address sealing and measurement performance issues together. After being exposed to overpressure, reverse pressure shock, or corrosive media, a differential pressure sensor may suffer reduced accuracy even if its external connection temporarily does not leak.

For sensors used in boilers, chemical reactions, gas transmission, environmental emissions, level interlocks, and critical equipment protection, qualified personnel should perform pressure resistance, insulation, zero point, and range verification. The device should not be considered fully restored in the field merely because it no longer leaks; confirm that measurement performance meets actual control requirements.

When replacing a sensor, simultaneously verify range, overload capacity, wetted material, process connection size, output signal, power requirements, and protection rating. For example, if the original system is a 4-20mA two-wire differential pressure application with a range of 0 to 10kPa, the replacement product must match both the control system and the actual differential pressure range to avoid new measurement issues caused by an unsuitable model.

Routine maintenance recommendations for reducing threaded connection pressure leakage

Include differential pressure sensor connection inspection in equipment inspection items. Under regular stable operating conditions, inspect seal condition in conjunction with annual verification or maintenance intervals. For high-temperature, high-pressure, high-vibration, or corrosive media conditions, shorten inspection intervals according to operating load and focus on leakage traces at connections.

Installation and maintenance records should retain thread specification, seal material, installation date, operating pressure, and abnormal conditions. After multiple maintenance cycles, these records can help determine whether a type of connection has insufficient selection and whether it is necessary to use a valve manifold, diaphragm seal, corrosion-resistant fitting, or differential pressure measurement solution with a higher protection rating.

For spare sensors that remain out of service for extended periods, seal process connections with protective caps and store them in a dry environment free of corrosive gases. Moisture corrosion on connection threads and long-term compression deformation of sealing rings both increase the likelihood of pressure leakage during reinstallation.

Xi'an Shenghongchuang Instrumentation Co., Ltd. can provide technical support for differential pressure sensors and matching fittings, valve manifolds, and sealing solutions according to on-site medium, pressure range, connection specification, and installation space. If repeated pressure leakage, unstable readings, or difficulty confirming connection specifications occurs after threaded installation, it is recommended to prepare the product nameplate, connection photos, medium parameters, and on-site pressure data for technical consultation, enabling a faster determination of the appropriate solution.