Differential Pressure (DP) Transmitters

What Is a Differential Pressure Transmitter?

A differential pressure transmitter is a field device that measures the difference between the pressures arriving at two separate process connections and converts that difference into a standard electrical signal. What distinguishes it from gauge and absolute instruments is that its reference is a second point in the process rather than something fixed. This property allows a good many quantities that cannot be measured directly to be derived from a pressure difference instead.

The most widespread use is calculating flow from the pressure drop across a primary flow element. Liquid level in closed and open vessels, the position of the interface between two liquids and density are likewise derived from a pressure difference. Across filters, strainers and heat exchangers, the difference between inlet and outlet is the key indicator for following fouling and plugging.

In differential measurement both sides of the instrument see line pressure. Alongside the measuring range, therefore, the static line pressure rating, the influence of line pressure on zero and span, the impulse line layout and the manifold choice all shape the quality of the reading. A well designed measuring point also lets maintenance staff take the transmitter out of service safely.

How Differential Pressure Measurement Works

A differential pressure transmitter applies the pressures from its two process connections to opposite faces of the same sensing element and measures what separates them.

1.      Taking pressure from both sides: The high pressure side and the low pressure side reach the two isolating diaphragms of the instrument through impulse lines or through a direct connection.

2.      Carrying the difference to the sensor: Pressure acting on each diaphragm travels through fill fluid to opposing faces of the sensing element, which responds only to what separates the two.

3.      Digital processing: The electronics refresh sensor data 48 times each second, or 167 times with the high-speed option, and correct it with calibration and characterization data.

4.      Producing the output: The measurement becomes a two-wire 4–20 mA signal with either a linear or a square-root characteristic, and HART digital data rides on top of that current.

High and low pressure sides

Which way round the two sides are piped decides the sign of the reading, so during installation the high and low connections must follow the direction of process flow. If mistaken valve operation leaves one side exposed to the whole line pressure, the output shifts and the instrument has to be zeroed again.

Flow measurement with differential pressure

The pressure difference across a primary flow element is proportional to the square of flow. A square-root output turns that relationship into a linear flow signal. With primary elements such as an orifice plate, an averaging pitot tube or an integral orifice, the transmitter can also be ordered as a factory-assembled flow meter. The flow-optimized performance class gives reference accuracy of ±0.04% of reading from the upper range limit down to an 8:1 turndown.

Level and density measurement with differential pressure

In a closed vessel, the pressure difference between the lower and upper connections is a function of liquid height and density. Where the fluid is hot, abrasive or prone to plugging, diaphragm seal assemblies replace impulse lines. Configured for level duty, reference accuracy is ±0.055% or ±0.065% of span depending on the performance class.

Static line pressure and zero trim

Besides the small difference it measures, a differential sensor may be exposed to a high static line pressure. The instrument meets its specifications at static line pressures between 0.03 bar and 250 bar depending on the range code, and the high static pressure option lifts that ceiling to 425.4 bar. Line pressure introduces a small shift at zero: for every 68.95 bar, ±0.025% of the upper range limit in the high-performance class and ±0.05% in the standard class. A zero trim performed under line pressure removes it.

Impulse line plugging detection follows changes in process noise and gives warning of a filled or frozen impulse line before the reading degrades; statistical process monitoring tracks variability in the measurement and helps equipment problems come to light early.

Key Features

·         Broad differential coverage: Range codes reaching from ±7.47 mbar to ±137.89 bar; the universal range option spans –2.49 to 2.49 bar with an 800:1 rangedown.

·         High static pressure capability: Static line pressure up to 250 bar, or 425.4 bar with the high static pressure option.

·         Flow-optimized performance class: Reference accuracy of ±0.04% of reading from the upper range limit down to an 8:1 turndown; a class developed for flow duty.

·         Square-root output: Linearization of the differential-to-flow relationship inside the instrument, plus application-driven configuration for level, volume and totalized flow.

·         Impulse line plugging detection: Early identification of a filled or frozen impulse line from changes in process noise.

·         Integrated manifold options: Two-valve, three-valve and five-valve integrated manifolds, along with traditional and level flanges, can be ordered as one assembly.

·         Diaphragm seal compatibility: Use with diaphragm seal assemblies for level and density measurement on abrasive, viscous and hot fluids.

·         Fast dynamic response: Total response of 80 ms, or 40 ms with the high-speed sensor option; dead time of 40 ms or 20 ms.

·         Integrated relay switches: Two double-pole changeover relays on the terminal block for local alarms and switching duty.

·         Graphical display and remote interface: A 128 × 128 pixel backlit graphical LCD with a remote display option, plus local configuration over Bluetooth.

Technical Specifications

The figures below apply to differential pressure service; exact values follow from the range code, the performance class and the connection arrangement that are ordered.

Configured for level duty, reference accuracy is ±0.055% or ±0.065% of span; on diaphragm seal systems the seal type, fill fluid and temperature influences have to be calculated separately.

Key Advantages

·         Many quantities from one measurement: Flow, level, interface, density and filter fouling can all be followed with the same measuring principle.

·         Working under high line pressure: Small differences remain measurable under a static line pressure of 250 bar, or 425.4 bar in the optional build.

·         Consistency in flow duty: The flow-optimized performance class and the square-root output hold measurement quality across a wide flow range.

·         Early warning: Impulse line plugging detection and statistical process monitoring let maintenance be planned before the reading degrades.

·         Straightforward maintenance: Integrated manifold options gather isolation and zero trim work into a single mounting assembly.

·         Suited to difficult fluids: Diaphragm seal assemblies and six wetted material choices give the instrument a wide application envelope.

Application Areas

Differential pressure transmitters are used wherever the pressure difference between two points stands for a process quantity.

Flow measurement with a primary element

Measuring liquid, gas and steam flow with primary elements such as an orifice plate, an averaging pitot tube or an integral orifice; factory-assembled flow meter solutions.

Vessel and tank level

Level, interface position between two liquids and density measurement in closed and open vessels, with diaphragm seal assemblies where these are needed.

Filter, strainer and exchanger monitoring

Following the pressure difference between inlet and outlet to establish fouling, plugging and the right moment for cleaning.

Pump, fan and compressor

Watching the differential across equipment inlet and outlet so that loss of performance and blockages surface early.

Oil, gas and refining

Differential measurement under high static line pressure; wetted materials meeting NACE MR0175/ISO 15156 and MR0103 requirements in sour service.

Power and utility plant

Flow measurement on boiler feed water, steam and cooling water lines together with vessel level monitoring.

How to Select a Differential Pressure Transmitter

In differential measurement a sound result depends as much on a well designed installation as on the instrument itself.

Measuring range and static pressure

·         The minimum and maximum differential expected, and the calibrated span this calls for

·         Maximum static line pressure and whether the high static pressure option is needed

·         The chance of one-sided overpressure and the overpressure limit

·         Turndown in flow duty and the performance class this requires

Connection, manifold and impulse lines

·         Choosing a two-valve, three-valve or five-valve manifold and the maintenance scenario behind it

·         Impulse line length, slope and the risk of freezing or plugging

·         Whether diaphragm seals are needed, and which seal type

·         Assembly with the primary flow element, or separate mounting

Performance class and diagnostics

·         The total measurement uncertainty required and the calibration period

·         The need for impulse line plugging detection and statistical monitoring

·         Hazardous-area and functional safety requirements

·         Expectations for ambient temperature, vibration and ingress protection

Where watching the pressure at a single point is enough, gauge or absolute pressure transmitters offer a simpler and more economical answer.

Certificates and Compliance

Subject to the option codes selected, the following approvals and compliance items are available for the transmitter:

Approval coverage varies with the option code ordered; the hazardous-area and functional safety certificates valid for a given project should be confirmed at the quotation stage.

Differential Pressure Measurement Support from TLY Enerji

In differential pressure measurement the source of error is usually not the instrument but the way the measuring point has been designed. Working from process data, line pressure and fluid properties, TLY Enerji fixes the measuring range and the performance class, weighs up the impulse line and manifold arrangement, and proposes a diaphragm seal solution where one is called for.

In flow duty the primary element and the transmitter are sized together, and turndown and expected uncertainty are calculated. At the quotation stage, device data, hazardous-area approvals and material certificates are compared against the project specification, while a zero trim under line pressure and a loop test are planned for commissioning. For points in existing plants that read erratically or plug frequently, we prepare improvement proposals. Manifold operation and zero trim steps are written into the documentation for maintenance teams.

Frequently Asked Questions

What is a differential pressure transmitter used for?

A differential pressure transmitter measures the pressure difference between two process points. That difference may stand for flow across a primary flow element, for liquid level or density in a vessel, or for the degree of fouling in a filter. Quantities that are awkward to measure directly are therefore followed with one single measuring principle.

Why is square-root output needed?

The pressure difference across a primary flow element is proportional to the square of flow. The square-root characteristic of the transmitter turns that relationship into a linear flow signal, so flow can be followed without any extra calculation in the control system. In linear duties such as level measurement, a linear output is used instead.

How does static line pressure affect the measurement?

Both sides of a differential sensor see line pressure. The instrument works at static line pressures up to 250 bar depending on the range code, and the high static pressure option raises that ceiling to 425.4 bar. Line pressure causes a small shift at zero, and that shift is removed by a zero trim carried out while the instrument is under line pressure.

How is a plugged impulse line noticed?

An impulse line that plugs or freezes slows the measurement down and produces false values. Impulse line plugging detection in the instrument follows changes in process noise and reports the condition before the reading is completely spoiled. Using it alongside installation measures such as heat tracing, adequate slope and a suitable valve arrangement is recommended.

What should be watched in differential pressure level measurement?

In closed vessels the headspace pressure has to be included in the measurement, which is why the low side is connected to the headspace. Density variation, temperature and condensate formed by vaporizing fluids all influence the reading. On abrasive, viscous or plugging-prone fluids, diaphragm seal assemblies are the preferred answer.

How many manifold valves should be chosen?

A manifold lets the instrument be isolated from the process and allows the zero to be trimmed. Differential applications generally use a three-valve manifold; where calibration and draining needs grow, a five-valve arrangement is preferred. The manifold can be ordered as one assembly with the transmitter.