Differential Pressure Level Transmitters and Diaphragm Seal Systems

What Is a Differential Pressure Level Transmitter?

A differential pressure level transmitter is a measuring instrument that determines the difference between the liquid pressure near the bottom of a vessel and a reference pressure, and converts this difference into level information. The pressure a liquid column exerts at its base is directly proportional to the column height, the liquid density and gravitational acceleration. In tanks open to atmosphere, the reference side is vented to atmosphere; in closed or pressurized vessels, the low-pressure side is connected to the vapor space, so that the static pressure in the headspace no longer affects the reading.

A diaphragm seal (remote seal) is a closed system installed between the transmitter and the process, consisting of a flexible metal diaphragm and a fill fluid that has been filled free of air. Process pressure deflects the diaphragm and the fill fluid carries this pressure to the transmitter’s sensor, so the process medium never reaches the transmitter’s own diaphragm. Diaphragm seal systems are the preferred solution where process temperature lies outside the transmitter’s limits, where corrosive media call for a special diaphragm material, where suspended solids or viscous fluids could plug the connections, and where a flush-mounted hygienic connection is mandatory.

Because hydrostatic measurement depends on the weight of the liquid column rather than on the liquid surface, it is comparatively little affected by conditions such as foam, vapor or a turbulent surface. On the other hand, level accuracy depends on the liquid density being known and stable. Differential pressure level measurement is therefore a dependable method in processes with a defined density, and by keeping the distance between two seals constant it also allows interface and density to be measured directly.

How Hydrostatic Level, Interface and Density Measurement Works

The fundamental relationship of hydrostatic measurement is P = ρ · g · h: in a liquid of constant density, the pressure at the bottom rises linearly with liquid height. A differential pressure measuring chain with diaphragm seals consists of four steps:

1.      Process pressure acting on the seal diaphragm: The diaphragm of the high-pressure-side seal, connected to the lower nozzle of the vessel, deflects under the combined pressure of the liquid column and the vapor space. In closed vessels, a second seal or reference connection on the upper nozzle senses only the pressure of the vapor space.

2.      Pressure transfer through the fill fluid: The sealed volume between the seal and the transmitter, filled with an air-free fill fluid, transfers the pressure to the transmitter’s isolating diaphragm. The link is made either by a short assembly in which the seal is mounted directly on the transmitter or by a flexible, armored capillary line.

3.      Measuring the differential pressure: The transmitter measures the difference between its high- and low-pressure sides. In closed vessels, the vapor-space pressure acts on both sides, so the differential pressure reflects only the hydrostatic effect of the liquid column; in atmospheric tanks, a gauge pressure measurement is sufficient.

4.      Conversion into level, interface or density: Based on the seal positions and the liquid density, the transmitter converts the measured differential pressure into level, volume or percentage fill. The value is passed to the control system via a 4–20 mA analog signal with HART digital communication or, depending on the selected model, a digital fieldbus.

Fill Fluid Selection

The fill fluid is the component that most strongly determines the operating temperature, vacuum capability and process compatibility of a diaphragm seal system. General-purpose silicone-based fill fluids cover roughly –45 °C to 205 °C. For low temperatures there are special fill fluids reaching down to –105 °C; for high temperatures, high-temperature silicone fill fluids reach 315 °C and 370 °C in combination with extensions and thermal extension elements. The very-high-temperature fill fluid used with a thermal extension element is suitable for continuous use at 410 °C; its 454 °C design temperature applies only to non-continuous exposure totaling no more than 12 hours.

For reactive media such as oxygen or chlorine, an inert halogenated fill fluid (–45 °C to 160 °C) is preferred, and for food and pharmaceutical processes, food-grade fill fluids are used. The food-grade options include glycerin-water and propylene glycol-water mixtures, which are used from –15 °C to 95 °C and are not suitable for vacuum applications, as well as food-grade fill fluids that reach 225 °C and 300 °C depending on the configuration. Since the specific gravity of fill fluids ranges from about 0.795 to 1.85, the pressure produced by the fluid column in the capillary line must also be included in the system calculation. In vacuum service, the temperature limits of all fill fluids are reduced.

Direct-Mount, Capillary and Asymmetric Seal Systems

In a direct-mount seal, the seal is joined to the transmitter body by a short connection; as the fill fluid volume is kept small, temperature effects and response time are favorable. This arrangement is common for gauge or absolute pressure measurement on tanks that are open to atmosphere or vented. In capillary-type remote mounting, a flexible capillary line runs between seal and transmitter; this solution is preferred for differential pressure measurements that need a connection to the upper nozzle of the vessel and for high-temperature applications where the electronics must be kept away from a hot process connection.

In a conventional balanced system, both seals are connected to the transmitter by capillaries of equal length. In an asymmetric system, a direct-mount seal is used on the high-pressure side and a capillary seal on the low-pressure side. Eliminating unnecessary capillary length and separate transmitter mounting hardware simplifies installation and can reduce response time and temperature-induced errors compared with a balanced system. The capillary is made of 316L stainless steel tubing and protected by stainless steel or PVC-coated stainless armor; the PVC coating must not be exposed to temperatures above 100 °C.

Temperature Effects and System Performance

The total performance of a diaphragm seal system is determined not only by the transmitter’s reference accuracy but also by the thermal expansion of the fill fluid in the seals and capillaries. The seal temperature effect results from the change in fill fluid volume inside the seal; the head temperature effect results from the change in density of the fill fluid in the capillary column between the two seals as ambient temperature varies. Capillary inside diameters of 0.7 mm, 1.1 mm and 1.9 mm are available: a small bore reduces temperature effects but lengthens response time, while a larger bore gives a faster response.

All of these effects can be calculated at project stage and documented in a system performance report covering seal temperature effect, head temperature effect, seal response time and total probable error. With a remote-mounted seal, a zero shift of up to ±2.49 mbar can occur when the seal is mounted in the vertical plane and up to ±12.45 mbar in the horizontal plane (for extended seals, the effect of the extension length is added); these shifts are removed by zeroing and have no effect on span. When process or ambient temperature exceeds 85 °C, the permissible temperature limits are reduced because of heat transfer to the transmitter.

Electronic DP Architecture

In applications that would otherwise require long capillaries or impulse lines, such as tall vessels and distillation columns, an electronic differential pressure architecture can be used, in which two separate pressure sensors are linked by a standard electrical cable. One sensor measures the pressure in the lower part of the vessel and the other the pressure in the upper part; the electronics then derive differential pressure, level and volume and transmit them over a single two-wire 4–20 mA HART loop. The high- and low-side pressures can also be monitored as separate variables.

This architecture eliminates the need for wet or dry legs and for heat tracing and insulation of capillary lines, and it reduces the temperature effects and slow response typical of long capillaries. Cable options between the sensors extend up to 152.4 m; maximum cable distances of 68.58 m for intrinsically safe installations and 60.96 m for safety instrumented system installations must be observed. The sensor range must be selected according to the maximum static pressure rather than the differential pressure, and the ratio of differential to static pressure should be evaluated at project stage.

Seal systems are available in two constructions: a welded-repairable design, in which every joint except the seal between the sensor module and the transmitter flange is welded, so that the transmitter can be reused if required; and an all-welded design, in which every joint including the sensor isolators is welded, recommended for vacuum applications below 400 mbar-a. In both constructions the systems contain no threaded connections and undergo a 100% helium leak test.

Key Features

Diaphragm seal differential pressure systems have a modular structure in which the transmitter, seal, fill fluid and connection architecture are selected together. The main technical features are:

·         Comprehensive range of seals and connections: Flush-mount, extended and low-profile (pancake) flanged seals, flanges with ring type joint (RTJ) faces, remote flanged and threaded seals, hygienic seals, and special seals such as saddle, pipe-mount, chemical tee, in-line wafer and flow-through flanged designs.

·         Application-specific fill fluids: General-purpose, low-temperature, high- and very-high-temperature, inert and food-grade fill fluid options cover process conditions from –105 °C to 410 °C, depending on the configuration.

·         Wide choice of diaphragm materials: Options such as 316L stainless steel, Alloy C-276, tantalum, titanium Gr. 4, Alloy 400, Alloy 600, Alloy C-22, nickel 201, zirconium 702, duplex 2205/2507 stainless steel and 304L match corrosive media.

·         Diaphragm coating and thickness: A 5 μm gold-plated diaphragm against hydrogen permeation, a PTFE-coated diaphragm to reduce sticking, and a 150 μm thick diaphragm for applications with abrasive particles are available.

·         Robust seal design: Support convolutions on the diaphragm preserve seal integrity, and a recessed diaphragm reduces the risk of damage during handling and installation.

·         Welded, leak-tight system construction: A welded design without threaded joints, air-free filling and a 100% helium leak test deliver a measuring system that stays stable over time; the all-welded construction is suitable for operation under full vacuum.

·         Three capillary bore sizes: Capillary inside diameters of 0.7 mm, 1.1 mm and 1.9 mm allow the balance between accuracy and response time to be optimized for the application.

·         Flushing ring option: In plugging-prone processes, a lower housing with ¼–18 NPT or ½–14 NPT flushing connections, placed in front of the seal diaphragm, allows the diaphragm surface to be cleaned without dismantling.

·         Electronic DP option: An architecture in which two pressure sensors are connected by cable eliminates long capillaries and provides differential pressure, level and volume from a single loop on tall vessels.

·         System performance calculation: A system calculation report covering seal temperature effect, head temperature effect, response time and total probable error helps verify the design before ordering.

·         Choice of communication protocols: Depending on the transmitter family, 4–20 mA HART, low-power 1–5 Vdc HART, FOUNDATION Fieldbus, PROFIBUS PA and WirelessHART (IEC 62591) options are available; some families also allow local configuration via Bluetooth.

Technical Specifications

The values below are overall technical data for the diaphragm seal differential pressure level measuring systems described on this page. They vary with transmitter family, performance class, seal type, fill fluid and capillary length; final values should be confirmed at project stage with a system calculation.

For hygienic seals, the standard wetted surface roughness is Ra < 0.81 μm. The overpressure limit is set by the lower of the flange pressure rating and the sensor limit; flange pressure ratings decrease with rising temperature.

Key Advantages

·         Transmitter isolated from the process: Corrosive, hot or contaminating media touch only the seal diaphragm; the transmitter’s sensor and electronics are protected from process conditions.

·         Lower risk of plugging and fouling: Flush-mounted diaphragms and flushing rings support measurement continuity with viscous and solids-laden media that could plug impulse lines.

·         Accurate level in pressurized vessels: Connecting the low-pressure side to the vapor space cancels the effect of headspace static pressure, so the true liquid level is measured in closed tanks and reactors.

·         Measurement at extreme temperatures: With the right fill fluid, extension and capillary, measurement is possible in processes far beyond the transmitter’s own temperature limits.

·         Compatible with hygienic cleaning: Flush-mounted hygienic seals make CIP/SIP procedures easier and help prevent cross-contamination between batches.

·         Simpler installation: Asymmetric seal systems and the electronic DP architecture reduce the need for excess capillary, wet or dry legs and heat tracing, lowering installation and maintenance effort.

·         Predictable measurement performance: A system calculation report reveals seal- and capillary-related errors before ordering and helps select the right combination of seal, fill fluid and capillary.

·         Supplied as a single system: Assembling, filling and testing the transmitter and seal at the factory prevents errors from filling or assembly on site.

Application Areas

Differential pressure level transmitters with diaphragm seals are used for level, interface, density, flow and pressure measurement, especially in processes with extreme temperatures, corrosive or plugging-prone media and hygiene requirements. Where volume and mass must be calculated in storage tanks, the measurement can be designed together with tank gauging systems; magnetic level gauges can complement it for local indication on the same vessel.

Chemicals and Petrochemicals

In reactors and storage tanks holding corrosive chemicals, tantalum, zirconium, titanium or nickel alloy diaphragms isolate the transmitter from the process. In pressurized reactors, the low side is connected to the vapor space to compensate for static pressure.

Refining and Oil and Gas Production

In sour (H₂S-containing) environments, material options meeting NACE MR0175/ISO 15156 and MR0103 requirements can be used. Gold-plated diaphragms are considered where hydrogen permeation is an issue, and flanged seals up to Class 2500 for high-pressure-rated lines.

Distillation Columns and Tall Process Vessels

On tall columns that would need long capillaries or impulse lines, the electronic DP architecture reduces temperature effects and response lag. Asymmetric seal systems can also be chosen to shorten capillary length.

Food, Beverage and Dairy

Seals with hygienic clamp, tank spud and DIN 11851 connections, combined with food-grade fill fluids, provide level measurement in tanks cleaned by CIP/SIP. Seal types compliant with 3-A and EHEDG are available.

Pharmaceuticals and Bioprocessing

A Ra < 0.81 μm surface finish, an optional surface finish certificate and USP Class VI compliant O-ring options create a cleanable, documentable measuring point in sterile processes.

Viscous, Solids-Laden and Vacuum Processes

With suspended solids or viscous media, flush-mount or extended seals and flushing rings reduce the risk of plugging. In vessels under vacuum, all-welded systems and vacuum-rated fill fluids are used.

How to Select a Diaphragm Seal Level Transmitter

The success of a differential pressure level transmitter with diaphragm seals depends on sizing the transmitter, seal, fill fluid and capillary together for the process conditions. For a technical assessment we recommend clarifying the following:

Process and Measurement Conditions

·         Measuring task: level, interface, density or a combination of these

·         Minimum and maximum density of the liquid(s); for interface measurement, the density difference between the two liquids

·         Vessel type: atmospheric, pressurized or operating under vacuum

·         Maximum static pressure and the differential pressure range to be measured

·         Minimum / normal / maximum process temperature and the ambient temperatures along the seal and capillary route

·         Corrosiveness, viscosity, solids content and sticking tendency of the medium

Seal, Diaphragm and Fill Fluid

·         Nozzle type, size and flange standard (ASME, EN, JIS) or threaded / hygienic connection type

·         Need for an extended seal if there is a risk of build-up inside the nozzle

·         Diaphragm material, coating (gold, PTFE) and thickness

·         Fill fluid temperature range, vacuum suitability and food-grade compliance

·         Need for a flushing ring and flushing connection

System Architecture and Installation

·         Direct-mount, balanced capillary, asymmetric or electronic DP architecture

·         Vertical distance between upper and lower nozzles, capillary length and inside diameter

·         Preference for welded-repairable or all-welded construction

·         Exposure of the transmitter and capillary line to sun, heat tracing or hot surfaces

·         Evaluation of alternatives such as guided wave radar level transmitters for processes with variable density, or non-contacting radar level transmitters where process contact is not wanted

Electronics, Communication and Documentation

·         Output type to suit the control system: 4–20 mA HART, FOUNDATION Fieldbus, PROFIBUS PA or WirelessHART

·         Hazardous-area classification and the required Ex approval

·         Functional safety requirement (IEC 61508)

·         Need for NACE compliance, EN 10204 3.1 material traceability and a hygienic surface finish certificate

·         Inclusion of the system performance calculation report in the order scope

In processes where density varies significantly, hydrostatic level measurement may need density compensation; in such applications the choice of measuring technology should be assessed separately on the basis of the process data.

Certificates, Approvals and Connection Standards

Depending on the selected transmitter family and seal type, the following approvals, certificates and standards are available for diaphragm seal differential pressure level measuring systems:

The scope of approvals and certificates varies with transmitter family, output protocol and seal type; the certificate set valid for a specific project should be confirmed together with the TLY Enerji engineering team.

Differential Pressure Level Solutions from TLY Enerji

A good result in diaphragm seal level measurement does not come from picking a single instrument out of a product list; it comes from considering nozzle geometry, process temperature, vacuum conditions, fluid chemistry and capillary routing together. The TLY Enerji engineering team collects your process data and evaluates seal type, diaphragm material, fill fluid and system architecture as a whole; where needed, a system performance calculation makes the expected total error visible before the order is placed.

Throughout the project we support you with measurement calculations for closed-tank, interface and density applications, preparation of P&IDs and instrument data sheets, evaluation of hazardous-area and functional safety requirements, integration of the measurement into PLC, DCS or SCADA systems, and planning of zeroing and calibration steps during commissioning. The result is a level measuring point that remains traceable, with predictable maintenance, even when process conditions change.

Frequently Asked Questions

How does a differential pressure level transmitter work?

The pressure a liquid column exerts at its base is proportional to liquid height and density. The transmitter measures the difference between the pressure near the bottom of the vessel and a reference pressure, and uses the liquid density to convert this difference into level, volume or percentage fill. In diaphragm seal systems, pressure travels from the seal diaphragm to the transmitter through the fill fluid; the measured value is sent to the control system via 4–20 mA HART or digital protocols.

What is the difference between DP level measurement in open and closed tanks?

In tanks open to atmosphere, the pressure acting on the liquid surface is atmospheric pressure, so the reference side of the transmitter is vented and a gauge pressure measurement is sufficient. In closed or pressurized vessels, the pressure in the vapor space affects the reading. By connecting the low-pressure side to the upper part of the vessel with a seal or connection, this static pressure acts on both sides and the differential pressure reflects only the liquid column.

When is a diaphragm seal required?

A diaphragm seal should be used when the process temperature is outside the transmitter’s operating limits, when the medium is corrosive and needs a special diaphragm material, when suspended solids or viscous fluids could plug the connections, or when a flush-mounted hygienic connection suitable for CIP/SIP cleaning is needed. Seal systems are also preferred in processes where connection points must be easy to clean between batches.

How do you select a diaphragm seal fill fluid?

The key factors are the process and ambient temperature range, operation under vacuum and process compatibility. General-purpose silicone-based fluids cover roughly –45 °C to 205 °C; there are fluids reaching down to –105 °C for low temperatures and, depending on the configuration, up to 410 °C for high temperatures. Inert fill fluids are preferred for reactive media, and food-grade fluids for food and pharmaceutical processes. In vacuum service, temperature limits are reduced.

How do capillary length and inside diameter affect the measurement?

A longer capillary holds more fill fluid, which increases both the influence of ambient temperature changes on the reading and the response time. A smaller inside diameter reduces the temperature effect but slows the response, while a larger diameter responds faster. The bore is therefore chosen from 0.7 mm, 1.1 mm and 1.9 mm to suit the application, and capillaries should be no longer than necessary. Asymmetric systems offer an advantage here.

How are interface and density measured with differential pressure?

For interface measurement, two seals are installed at a fixed distance below and above the zone where two liquids of different density meet; as the proportion of each liquid between the seals changes, the differential pressure changes and the interface position is calculated. For density measurement, the vertical distance between the seals is fixed, so as long as the liquid level stays above the upper seal, the measured differential pressure is directly proportional to liquid density.

What can be used instead of capillaries on tall columns?

On tall vessels and distillation columns, an electronic DP architecture can be used, in which two separate pressure sensors installed in the lower and upper parts are linked by an electrical cable. Differential pressure and level are calculated electronically and transmitted from a single 4–20 mA HART loop. Long capillaries, wet or dry legs and heat tracing are no longer needed; however, the sensor range must be selected for the maximum static pressure.

Which diaphragm seals are used in hygienic processes?

Food, beverage, dairy and pharmaceutical processes use seals with hygienic clamp connections, tank spud, thin-wall tank spud, flanged extended tank spud, in-line clamp and DIN 11851 connections. Their wetted surfaces have a standard roughness of Ra < 0.81 μm, and specific types comply with 3-A and EHEDG. Food-grade fill fluids and USP Class VI compliant O-ring options are also available.

What should be considered for vessels under vacuum?

For vacuum applications, an all-welded system construction in which every joint including the sensor isolators is welded should be chosen; this design is recommended below 400 mbar-a. The fill fluid must be suitable for vacuum: some food-grade fluids such as glycerin-water and propylene glycol-water are not. The system calculation must also take into account that fill fluids tolerate lower maximum temperatures under vacuum.