# Level measurement using Differential Pressure transmitter close tank

Level measurement using Differential Pressure transmitter closed tank and a gas or vapour exists on top of the liquid, the gas pressure must be compensated for. A change in the gas pressure will cause a change in transmitter output. Moreover, the pressure exerted by the gas phase may be so high that the hydrostatic pressure of the liquid column becomes insignificant. For example, the measured hydrostatic head in a CANDU boiler may be only three meters (30 kPa) or so, whereas the steam pressure is typically 5 MPa. Compensation can be achieved by applying the gas pressure to both the high and low-pressure sides of the level transmitter. This cover gas pressure is thus used as a back pressure or reference pressure on the LP side of the DP cell. One can also immediately see the need for the three-valve manifold to protect the DP cell against these pressures.

Typical Closed Tank Level Measurement System

The different arrangement of the sensing lines to the DP cell is indicated a typical closed tank application

We have:

P(high) = P(gas) + S . H

P(low)= P(gas)

DP = P(high) – P(low) = S . H

The effect of the gas pressure is cancelled and only the pressure due to the hydro static head of the liquid is sensed. When the low-pressure impulse line is connected directly to the gas phase above the liquid level, it is called a dry leg.

Design Pressure :- To 10,000 PSIG (69 MPa)

Design Temperature:-  To 350 ° F (175 ° C) for differential-pressure (d/p) cell and to 1200 ° F (650 ° C) for filled systems; others to 200 ° F (93 ° C); standard electronics generally limited to 140 ° F(60 ° C)

Range :- Differential-pressure cells and indicators are available with full-scale ranges as low as 0 to 5 in. (0 to 12 cm) H2O. The higher ranges are limited only by physical tank size, since d/p cells are available with ranges over 433 ft H 2 O (7 MPa or 134 m H 2 O).

Materials of Construction:-  Plastics, brass, steel, stainless steel, Monel  , and special alloys for the wetted parts.Enclosures and housings are available in aluminum, steel, stainless steel, and fiber-glass composites, with aluminum and fiberglass being most readily available.

Price :-  \$200 to \$1500 for transmitters in standard construction; \$50 to \$500

# Technical Specification of Differential Pressure transmitter

 Sr.No Parameter Minimum Requirements 1 Service Oil 2 Type Variable capacitance /Piezo resistance/ Resonant silicon sensor 3 Location Across strainer / nozzle of tank for level measurement 4 Accuracy 0.065 % of span 5 Over range protection 150% of range 6 Field Indicator Inbuilt, Digital in Engineering Unit (Kg/cm2) 7 Output Signal 4 –20mA along with HART/ Modbus 8 Power 24 V DC, 2 Wire 9 Protection Circuit protected against Lightening & surges (Minimum up to 1 KVA of surges), Reverse Polarity 10 Enclosure class Ex-proof, IP 65 /NEMA 4 or better and Intrinsically Safe (IS) 11 I.S. Barrier Required 12 Area Classification Zone I & II, Gr. IIA/IIB, T3 as per IS2148 13 Execution Intrinsic safe with active barrier in control room / Ex-Proof 14 Mounting Across Strainer with bottom entry & coplanar style. 15 Wetted Parts (M.O.C) SS 316 16 Case Material Polyurethane coated Die cast Aluminium/ polyester powder coated Aluminium (suitable for use in corrosive environment) 17 Range 0-2 Kg/cm2 or as per site requirement Connection 18 Process Connection ½” NPT (F) 19 Electrical Connection ½” NPT(F) 20 Manifold SS316 – 3 way suitable to process connection. 21 Mounting Kit Necessary accessories suitable for mounting across strainer 22 Tag Plate Metal tag plate to be provided 23 Approvals CCOE/PESO as per requirements

• Honeywell
• Emerson
• Yokogawa
• Endress
• ABB