1. Why can’t a closed tank use a single pressure point for level calculation?
In chemical, petrochemical, power, metallurgy, food and pharmaceutical processes, storage tanks and process vessels are often closed. The hydrostatic pressure generated by the liquid is related to level, but the pressure in the gas space also acts on the liquid surface. If pressure is measured only at the bottom of the tank, the instrument may detect the combined effect of liquid-column pressure and gas pressure. Changes in roof pressure, gas condensation, pressurization or vacuum can therefore shift the calculated level. In vessels with agitation, heating, corrosive media or sanitary requirements, impulse lines may also introduce plugging, condensation, buildup, leakage and maintenance problems.

A closed-tank level solution should first define the measurement objective and pressure boundary. Is the project measuring liquid level, interface or density? Does the gas-space pressure vary? What are the medium temperature and corrosiveness? What is the centre-to-centre distance between the high- and low-pressure flanges? Are remote diaphragms, capillaries or special process connections required? The basic principle of differential-pressure measurement is to sense pressure at the high and low sides and use the differential pressure to represent the liquid column or medium condition. It is not simply a matter of installing one pressure transmitter at the tank bottom. The high- and low-side locations, capillary routing, diaphragm orientation and temperature effects must be designed together.

2. How does HCDP-32 address the field requirements of closed-tank level measurement?
HCDP-32 is a remote double-flange differential pressure transmitter for liquid-level, pressure and density measurement. It has a 0–1 MPa measuring range, 0.1% or 0.2% accuracy, long-term stability better than 0.2% FS per year, a medium-temperature range of −40 to 350°C, two-wire 4–20 mA/HART output and an intelligent LCD display with backlight. In a closed-tank level project, the double-flange structure extends the pressure-sensing points to the vessel connections and can reduce the need for long impulse lines arranged directly at the process. The exact flange specification, diaphragm material, capillary length and mounting orientation still need to be checked against the medium and vessel structure.

When the vessel is hot, the medium can crystallize or impulse lines may face condensation and blockage, a remote-flange arrangement can place the pressure-sensing points at a more suitable process connection while locating the transmitter body where inspection and wiring are easier. In a closed tank with significant gas-space pressure variation, correct high- and low-side pressure connections and impulse transmission are essential for stable level measurement. If the high- and low-side positions, capillary elevation or temperature conditions are mismatched, the level may deviate even when the transmitter accuracy appears adequate. HCDP-32 is also used for differential-pressure measurement of liquids, gases and steam. When paired with a throttling device, it can be used for relevant flow measurement, but the pressure-tapping design for each application must be engineered separately.

3. How should the complete solution be designed and commissioned?
The first step is to prepare a vessel and medium-condition sheet covering tank height and diameter, normal and limiting levels, gas-space pressure, medium density and variation, temperature, corrosiveness, crystallization or buildup, high- and low-side flange spacing, process connections and available mounting space. The second step is to design the remote flanges and capillary routing. Capillaries should be kept away from intense heat, crushing, sharp bends and mechanical tension. The two sides should be arranged as symmetrically as practical so that temperature difference and elevation difference do not introduce avoidable measurement effects. The third step is to match the control system by confirming the two-wire 4–20 mA/HART connection, power supply, display requirement and site protection conditions.

During commissioning, check the zero point, range and 4 mA/20 mA relationship against a known level or reliable process reference. Observe the output during pressurization, depressurization, filling, discharge, heating and cooling. If the level changes abnormally with gas-space pressure, first inspect the high- and low-side pressure connections, capillary routing, diaphragm condition and temperature effects. If the output drifts or responds slowly, also investigate flange-chamber blockage, crystallized medium, wiring and parameter settings. After receiving the vessel drawing, medium information, pressure and temperature, level range, flange spacing, site photographs and control-system requirements, HCCK can further evaluate whether the HCDP-32 remote flanges, range, diaphragms and output configuration fit the project.

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