Differential Pressure Transmitters for Liquid & Gas Level Measurement

Differential pressure (DP) transmitters are core instruments for level measurement, operating on the principle of correlating static pressure difference with liquid/gas level height to achieve precise monitoring. Widely adopted in chemical, petroleum, water treatment,medical, food& ice cream ， and other industrial sectors, they deliver reliable performance even under harsh working conditions.

I. Liquid Level Measurement Cases

Case 1: Acid-Base Solution Level Monitoring in Chemical Storage Tanks

Actual Working Conditions: A chemical plant utilizes 30m³ vertical storage tanks to hold 30% concentration sulfuric acid solution—highly corrosive media. The tanks operate under atmospheric pressure with an ambient temperature range of -10℃~45℃. Requirements include a level measurement accuracy of ≤±0.2%FS, along with corrosion and clogging resistance.

Application Solution: A flange-type DP transmitter is employed. The negative pressure side is connected to the gas phase space at the top of the tank, while the positive pressure side is linked to the liquid phase area at the bottom via a PTFE-lined corrosion-resistant flange, preventing sulfuric acid from corroding the transmitter diaphragm. Condensate pots and isolation fluid devices are also integrated to avoid direct contact between high-temperature or volatile media and the sensor.

Results: 24/7 continuous level monitoring is achieved with a measurement range of 0~8m. Data is transmitted to the DCS system via a 4-20mA signal, and an audible-visual alarm is automatically triggered when the level exceeds limits, effectively eliminating the risk of acid-base solution overflow.

Case 2: Boiler Drum Water Level Measurement in Thermal Power Plants

Actual Working Conditions: Boiler drums operate at 10MPa pressure and 350℃ temperature, containing high-temperature and high-pressure steam-water mixtures. Precise water level control within ±50mm is critical—deviation may lead to dry boiling or flooding accidents, resulting in equipment damage.

Application Solution: A high-temperature and high-pressure DP transmitter is selected, paired with a balance container (condensation chamber). The balance container condenses high-temperature steam from the drum into room-temperature condensate, ensuring both the high and low-pressure sides of the transmitter operate in a normal-temperature environment and eliminating measurement errors caused by temperature and pressure fluctuations.

Results: The measurement accuracy reaches ±0.1%FS, providing real-time feedback on drum water level data to support the stable operation of the boiler's automatic feedwater regulation system. It adapts to complex working conditions such as boiler startup/shutdown and load changes.

II. Gas Level Measurement Case

Case: Coal Gas Level (Height) Monitoring in Coke Oven Gas Holders

Actual Working Conditions: A 50,000m³ dry gas holder stores coke oven gas—flammable and explosive media. The internal pressure ranges from 0.5~2.5kPa, and the device is installed outdoors. Monitoring the lifting height of the gas holder's piston (equivalent to gas level) is required, with a measurement range of 0~30m and an explosion-proof rating of Exd II CT6.

Application Solution: A micro-differential pressure transmitter is used. The positive pressure side is connected to the gas inlet at the bottom of the holder, and the negative pressure side to the vent at the top. The micro-pressure difference generated by the gas column is used to calculate the level height. The transmitter features an explosion-proof housing, and a surge protector is installed at the signal output end to adapt to harsh outdoor environments.

Results: Real-time monitoring of gas storage capacity is realized, with data uploaded to the central control room's SCADA system. When the gas level falls below the lower limit or exceeds the upper limit, the gas inlet and outlet valves are automatically controlled to ensure the safe operation of the gas holder and prevent overpressure collapse or negative pressure collapse accidents.

III. General Working Condition Precautions

1. For viscous liquids (e.g., crude oil, syrup), select a sanitary clamp-type DP transmitter and install a purging device to prevent media from clogging the impulse line.
2. For volatile liquids (e.g., methanol, gasoline), implement heat tracing and thermal insulation on the negative pressure side to avoid false pressure differences caused by media vaporization, which affect measurement accuracy.
3. For gas level measurement, pay attention to medium cleanliness and install a filter at the front end of the transmitter to prevent dust and impurities from damaging the sensor diaphragm.