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



ParameterMinimum Requirements
2TypeVariable capacitance /Piezo resistance/ Resonant silicon sensor
3LocationAcross strainer / nozzle of tank for level measurement
4Accuracy0.065 % of span
5Over range protection150% of range
6Field IndicatorInbuilt, Digital in Engineering Unit (Kg/cm2)
7Output Signal4 –20mA along with HART/ Modbus
8Power24 V DC, 2 Wire
9ProtectionCircuit protected against Lightening & surges (Minimum up to 1 KVA of surges), Reverse Polarity
10Enclosure classEx-proof, IP 65 /NEMA 4 or better and Intrinsically Safe (IS)
11I.S. BarrierRequired
12Area ClassificationZone I & II, Gr. IIA/IIB, T3 as per IS2148
13ExecutionIntrinsic safe with active barrier in control room / Ex-Proof
14MountingAcross Strainer with bottom entry & coplanar style.
15Wetted Parts (M.O.C)SS 316
16Case MaterialPolyurethane coated Die cast Aluminium/ polyester powder coated Aluminium (suitable for use in corrosive environment)
17Range0-2 Kg/cm2 or as per site requirement
18Process Connection½” NPT (F)
19Electrical Connection½” NPT(F)
20ManifoldSS316 – 3 way suitable to process connection.
21Mounting KitNecessary accessories suitable for mounting across strainer
22Tag PlateMetal tag plate to be provided
23ApprovalsCCOE/PESO as per requirements

Differential Pressure transmitter Manufacturer :-

  • Honeywell 
  • Emerson
  • Yokogawa
  • Endress
  • ABB

Differential Pressure transmitter installation procedure in close tank

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