Dual Flange Differential Pressure Transmitter

A Dual Flange Differential Pressure Level Transmitter is an industrial instrument specially designed for continuous level measurement in closed vessels, pressurized storage tanks, reactors, and other complex process containers. It is typically composed of a differential pressure transmitter body, upper and lower flange diaphragm seal assemblies, dual-side capillary remote seal systems, and internal fill fluid. It can provide stable and reliable level monitoring and remote output under demanding conditions such as high temperature, pressure, corrosion, viscosity, and crystallizing media.

Compared with a conventional differential pressure transmitter, the dual flange structure is not simply a matter of connecting two pressure tapping points to the transmitter. Instead, it uses upper and lower diaphragm seal flanges to isolate the process medium from the sensitive internal elements of the instrument, and then transmits pressure to the differential pressure sensor through capillaries and fill fluid. This design effectively helps prevent problems such as blocked impulse lines, leakage, liquid accumulation, and direct heat transfer, making it better suited for long-term continuous industrial operation.

For level measurement applications involving vapor pressure at the top of the vessel, steam space, nitrogen blanketing, corrosive media, or sticky products, a dual flange differential pressure level transmitter is usually one of the more reliable and mature solutions.

Pressure Type: Gauge Pressure, Differential Pressure, Level
Pressure Range:
-1 bar to 30 bar, 0~300mH2O
Accuracy: ±0.075%, ±0.2%
Output Signal: 4–20 mA / HART
Process Connection: DN20 / DN25 / DN40 / DN50 / DN65 / DN80 / DN100 / DN150 (customized available)
Diaphragm Material: SS316L, HC-276, Tantalum, Gold-plated
Capillary length: 1-10 meters (length can be customized upon request)
Power Supply: 12–36 VDC
Protection Rating: IP65 / IP67

OEM Service: Available. The transmitter can be supplied with your company name and logo on the nameplate.
Packaging: Individual carton box with protective foam.
Weight: Approx. 7 kg per unit.
Port of Shipment: Shanghai or Ningbo Port, China.
Delivery Time: 3-7 working days.
Transportation: Air freight, sea freight, or express delivery.
Documentation: Certificate of Conformity (COC); Inspection / Test Report; Packing List; Commercial Invoice

Accessories

Basic Introduction to Dual Flange Differential Pressure Transmitter

1. What Is a Dual Flange Differential Pressure Transmitter?

A dual flange differential pressure level transmitter is essentially a diaphragm seal differential pressure transmitter used for level measurement.

“Dual flange” means that the instrument is equipped with flange diaphragm seal assemblies on both the high-pressure side and the low-pressure side. “Differential pressure level” means that it does not measure liquid level directly as a height, but instead measures the pressure difference between two points in the vessel and then converts that differential pressure into liquid level.

In many applications, especially in closed tanks and pressurized vessels, level measurement cannot be accurately achieved by simply installing a pressure transmitter at the bottom. This is because the bottom pressure includes not only the hydrostatic pressure of the liquid column, but also the vapor pressure at the top of the vessel. If the top pressure is not compensated or eliminated, the measured value will not represent true liquid level alone.

A dual flange differential pressure level transmitter solves this problem by sensing the bottom liquid pressure and the top vapor pressure separately through two process connections, then calculating the difference inside the transmitter to obtain the true differential pressure corresponding to the liquid level.

Compared with an ordinary differential pressure transmitter, it is more suitable for high-temperature, corrosive, viscous, crystallizing, and clogging media.
Compared with a single flange differential pressure level transmitter, it is more suitable for closed tanks, pressurized vessels, and applications with obvious fluctuations in top pressure.
Compared with conventional direct-mount level measurement solutions, its structure is more complex, but under demanding service conditions it is usually more stable and more durable.

2. Structure and Diagram of a dual flange differential pressure level transmitter

A dual flange differential pressure level transmitter typically consists of the following components:

Differential Pressure Transmitter Body
This is the core signal processing unit of the instrument. It receives the pressure signals from both sides and converts them into standard outputs such as 4–20 mA and HART. The common housing style is similar to a 3051-type industrial transmitter and is suitable for long-term field service.

High-Pressure Side Flange Diaphragm Seal Assembly
Installed at the lower part of the vessel or the lower process connection, it senses the combined pressure of the liquid column and top vapor pressure.

Low-Pressure Side Flange Diaphragm Seal Assembly
Installed at the upper part of the vessel or in the vapor space connection, it senses the top vapor pressure. This pressure is canceled out in the differential pressure calculation.

Dual-Side Capillary Remote Seal System
The pressures from both the high-pressure side and low-pressure side are transmitted to the transmitter body through fill fluid inside the capillaries. This is why dual flange products place particular emphasis on capillary routing, capillary length, and consistency of thermal conditions.

Internal Fill Fluid
The diaphragm seal system is filled with special fill fluid for stable pressure transmission. The fill fluid selection is closely related to temperature range, response speed, and thermal drift performance.

Flanged Process Connections
These are used to mount the upper and lower diaphragm seal assemblies onto the corresponding vessel connections. Flange size, standard, diameter, and pressure class can be customized according to project requirements.

Mounting Bracket
Used to secure the transmitter body for more stable installation and easier wiring.

Display Head and Electrical Connection
Some models can be equipped with an LCD display for direct on-site viewing of level, percentage, current, or differential pressure values. Electrical connection options usually include M20×1.5 and 1/2-14 NPT.

3. Inline Pressure Transmitters Working Principle

The working principle of a dual flange differential pressure level transmitter is still based on hydrostatic pressure. In a vessel, the higher the liquid level, the greater the hydrostatic pressure at the bottom. If the vessel is an open tank, the bottom pressure mainly comes from the liquid column itself. If the vessel is a closed tank, the bottom pressure consists of two parts:

Hydrostatic pressure from the liquid column
Vapor pressure at the top of the vessel

In this case, if only the bottom pressure is measured, the reading will vary with top vapor pressure and will not accurately reflect the true liquid level. The solution provided by the dual flange design is:

The high-pressure side flange is installed at the lower part of the vessel to sense the total bottom pressure
The low-pressure side flange is installed at the upper part of the vessel to sense the top vapor pressure

After the transmitter calculates the difference between the two pressures, the top pressure is canceled out, and the remaining signal is the differential pressure corresponding to liquid level. In simple terms: Bottom pressure − top pressure = net differential pressure generated by the liquid column This net differential pressure can then be converted into liquid level when combined with medium density. Therefore, when selecting a dual flange differential pressure level transmitter, it is not enough to know only the liquid level height. The following parameters must also be confirmed:

Medium density
Operating temperature
Vessel pressure
High-pressure and low-pressure side installation positions
Capillary length and structure

All of these will affect final range setting and measurement stability.

4. Applications of Dual Flange Differential Pressure Transmitter

A dual flange differential pressure level transmitter is commonly used in the following applications:

Closed Tank Level Measurement
For tanks with vapor pressure at the top, the dual flange structure can effectively eliminate the influence of top pressure fluctuations on the level output.

Pressurized Vessel Level Measurement
For vessels with nitrogen blanketing, steam protection, or pressure control, the dual flange solution is a more common industrial choice.

High-Temperature Reactor Level Measurement
Hot media should not act directly on the transmitter body. The capillary remote seal structure provides effective thermal isolation.

Steam-Covered Vessel Level Measurement
When there are changes in temperature and pressure in the vapor space, the dual flange structure provides more stable level compensation.

Viscous Media Level Measurement
For slurry, resin, syrup, sludge, and similar media, ordinary impulse lines are prone to clogging, while diaphragm seal structures are more suitable.

Crystallizing Media Level Measurement
When media tend to accumulate, deposit, or crystallize in impulse lines, the dual flange design is usually more reliable.

Corrosive Media Level Measurement
With properly selected diaphragm materials such as 316L, Hastelloy, or tantalum, it can adapt to different corrosive applications.

Volatile Media Level Measurement
When the vapor space at the top is unstable, the dual flange structure can more effectively deal with vapor pressure influence.

5. Why Many Applications Require a Dual Flange Structure

In many projects, a dual flange solution is not just a more advanced option, but a necessary one.

This is because ordinary level measurement methods often cannot operate reliably for the long term when the following conditions exist:

Pressure at the top of the vessel
Media on both the high-pressure side and low-pressure side are not suitable for direct impulse piping
Ordinary impulse lines are prone to clogging, crystallization, liquid accumulation, or leakage
Medium temperature is too high for direct mounting of the transmitter at the connection
The medium is highly corrosive and requires isolation measurement
The medium is viscous or sticky, making ordinary pressure tapping methods maintenance-intensive
The project requires long-term stable level measurement and easy integration with automation systems

For these reasons, in level measurement for closed vessels and complex process containers, a dual flange differential pressure level transmitter is a very mature and reliable type of solution.

Dual Flange Differential Pressure Level Transmitter Product Technology

1. Technical Specifications of Dual Flange Differential Pressure Level Transmitter

SIY3051C Diaphragm Seal Differential Pressure Level Transmitter Selection Table
10	Transmitter type
	D	Differential pressure transmitter
	G	Gauge pressure transmitter
	A	Absolute pressure transmitter
20	Accuracy
	-H	Basic error ±0.075%
	-N	Basic Error ±0.2%
30	Pressure Range
	3051CD Differential pressure
	3	-6kPa～6kPa（-60mbar～60mbar）
	4	-40kPa～40kPa（-400mbar～400mbar）
	5	-250kPa～250kPa（-2.5bar～2.5bar）
	6	-1MPa～1MPa（-10bar～10bar）
	7	-3MPa～3MPa（-30bar～30bar）
	9	Other agreed pressure ranges
	3051CG Gauge pressure
	3	-6kPa～6kPa（-60mbar～60mbar）
	4	-40kPa～40kPa（-400mbar～400mbar）
	5	-100kPa～250kPa（-1bar～2.5bar）
	6	-0.1MPa～1MPa（-1bar～10bar）
	7	-0.1MPa～3MPa（-1bar～30bar）
	8	-0.1MPa～10MPa（-1bar～100bar）
	9	-0.1MPa～21MPa（-1bar～210bar）
	10	-0.1MPa～40MPa（-1bar～400bar）
	3051CA Absolute pressure
	4	0～40kPa（0～0.4bar）
	5	0～250kPa（0～2.5bar）
	6	0～3MPa（0～30bar）
	9	Other agreed pressure ranges
	Note: 3051CD static pressure: 16 MPa / 25 MPa / 40 MPa
40	Communication Protocol
	H	4–20 mA + HART5/HART6/HART7 communication protocol
	M	RS485 Modbus communication protocol
50	Diaphragm material
	2	316 stainless steel
	3	Hastelloy C-276
	4	Monel
	5	Tantalum (only for 3051CD & CG, range 4-9, not for 3051CA)
	6	Gold-plated
	0	Customer-specified diaphragm material
60	Process connection
	S1	One remote flange
	S2	Two remote flanges
70	Wetted seal material
	N	Nitrile rubber (NBR)
	F	Fluoroelastomer (FKM)
	P	PTFE (Polytetrafluoroethylene)
80	Fill fluid
	1	Silicone oil
	2	Fluorinated oil
90	Display
	N	None
	L	LCD display (-20°C)
	O	OLED display (-40°C)
100	Housing material & electrical connection
	B	Aluminum alloy housing, electrical connection M20×1.5
	S	Aluminum alloy housing, electrical connection 1/2 NPT
	C	Stainless steel housing, electrical connection M20×1.5
	T	Stainless steel housing, electrical connection 1/2 NPT
110	Connection accessories
	3M	3-valve manifold
	5M	5-valve manifold
	2F	1/2 NPT oval flange
120	Mounting bracket option
	N	None
	1	Galvanized carbon steel bracket
	2	Stainless steel bracket
130	Hazardous area certification
	E5	Intrinsic safety ia IIC T4-T6
	K5	Explosion-proof d IIC T4-T6
140	Other options
	Q4	Factory calibration certificate
	Q3	Third-party calibration certificate
	V6	Suitable for full vacuum service
	J1	NAMUR NE43 standard
	J3	Built-in lightning protection module
	T1	Oil-free degreasing
	C1	SIL3 certification (for HART communication type only)
150	Remote seal assembly options for diaphragm pressure transmitter (Note: -1199 option)
	RFW	Remote flange type seal
	EFW	Extended flange type seal
	FFW	Flush flange type seal
	PFW	Flat type seal
	SCW	Sanitary type seal
	RTW	Remote threaded type seal
160	Flange rating & material
	A21
	B21
	A31
	B31
	A41
	B41
	A22
	B22
	A32
	B32
	A42
	B42
	A24
	B24
	A34
	B34
	A44
	B44
	YY
170	Diaphragm material
	D4	316L
	D6	Hastelloy
	E0	Tantalum
	F0	Monel
	P	With PTFE/PFA coating
	G	Gold-plated coating
	Z	Other agreed special requirements
180	Insertion tube (for EFW extended flange type seal only)

	EC0
	ES1
	ES2
	ES3
	EP1
	EP2
	EP3
	EH1
	EH2
	EH3
190	Pressure connection port (for RTW remote threaded type seal only)
	A	1/4–18 NPT
	C	3/8–18 NPT
	D	1/2–14 NPT
	H	1–111/2 NPT
	H	11/2–111/2 NPT
200	Seal fill fluid
	A
	C
	D
	H
210	Capillary length
	□□	Agreed capillary length, from 1–15 m (e.g. 2 m: 02)
220	Capillary armor material
	P	304+PVC
230	Other options
	T1	Oil and grease removal
	V6	Vacuum-resistant treatment
	Z	Other agreed special requirements
Note: When selecting a dual-flange DP level transmitter, if the high-pressure and low-pressure side seals are different, select from the high-pressure side first.

2. Dual Flange Differential Pressure Transmitter Selection Guide

1. Determine Whether a Dual Flange Structure Is Really Required

If there is pressure at the top of the vessel and this pressure affects the bottom measurement value, a dual flange solution should usually be considered.
The purpose of a dual flange design is to take pressure from both the upper and lower points, so that the influence of top pressure on level measurement can be reduced.

2. Confirm Whether There Is High Vacuum or High Negative Pressure

If high negative pressure may occur inside the vessel, a standard diaphragm seal structure may face the risk of diaphragm damage, oil leakage, or seal failure.
In such cases, a fully sealed diaphragm seal system is usually required, and this point is very important.

3. Provide the Liquid Level Height or the Center-to-Center Distance Between the Two Flanges

A dual flange differential pressure level transmitter must be ranged before delivery.
The range calculation is usually based on the actual liquid level height on site, or on the center-to-center distance between the upper and lower mounting flanges.

4. Select the Capillary and Fill Fluid According to Temperature

Under normal temperature conditions, silicone oil is commonly used as the fill fluid.
For high-temperature or low-temperature service, the fill fluid and structure must be confirmed again. Otherwise, temperature drift and slow response may occur.

5. Select the Diaphragm Material According to the Corrosiveness of the Medium

The standard configuration is usually a 316L diaphragm, but 316L is not suitable for every medium.
If the medium is highly corrosive, higher-grade materials such as Hastelloy, tantalum, Monel, gold-plated diaphragm, or a PTFE anti-corrosion structure should be considered.

6. Determine the Capillary Length According to the Installation Position

A longer capillary is not always better.
As the length increases, temperature influence becomes greater, and response speed and measurement stability may decrease.
In most cases, the capillary lengths on both sides of a dual flange transmitter should be kept as equal as possible.

7. Confirm the Flange Standard and Specification

Different countries and projects use different flange standards. Even if the nominal size looks similar, the actual dimensions may still be different.
Common flange standards include:

HG / GB flanges: commonly used in China
ASME B16.5: commonly used for American standard projects
EN 1092-1: commonly used for European projects
GOST: commonly used in Russia and CIS countries

In addition to the standard, the flange size, pressure rating, and facing type must also be confirmed. Otherwise, installation problems may occur on site.

8. Confirm Explosion-Proof and Display Requirements

If the project is in a hazardous area, installed outdoors, or requires convenient on-site inspection, it is necessary to confirm in advance whether explosion-proof approval, a local display, or other related options are required.

3. Common Diaphragm Materials for Dual Flange Differential Pressure Level Transmitter

316L Stainless Steel

316L is the most common standard wetted material used in single flange differential pressure level transmitters. It is suitable for most conventional liquid media and general industrial operating conditions. For non-strongly corrosive media such as water, oil, and general chemical liquids, 316L is usually sufficient, while also offering good cost-effectiveness and shorter lead times.

Hastelloy

Hastelloy is suitable for more corrosive chemical media. In particular, it provides better corrosion resistance than 316L in certain acidic, chloride-containing, or otherwise complex corrosive environments. When 316L cannot provide long-term compatibility, a Hastelloy diaphragm should be considered first, although the final selection still needs to be confirmed according to the medium composition, concentration, and temperature.

Tantalum Diaphragm

Tantalum offers excellent corrosion resistance and is commonly used in highly corrosive service conditions, especially where very high diaphragm corrosion resistance is required. However, tantalum is not suitable for all media. It is generally not recommended for strong alkalis and some special corrosive systems, so the final selection should always be based on the specific process conditions.

Monel

Monel provides good corrosion resistance in certain special chemical media, especially in some fluorine-containing media, hydrofluoric acid, and other demanding service conditions. It is not usually the first choice for conventional media, but is typically used in applications with clearly defined corrosion-resistance requirements.

PTFE Corrosion-Resistant Structure

A PTFE corrosion-resistant structure is suitable for a wide range of acidic, alkaline, and highly corrosive media. It offers clear advantages in applications where conventional metal diaphragms cannot provide sufficient corrosion resistance. For strongly corrosive media, a PTFE-based protection solution is often a more reliable choice, although temperature, pressure, and installation design must also be taken into account.

Gold-Plated Diaphragm

Gold-plated diaphragms are commonly used for special media such as hydrogen service. They can effectively reduce hydrogen permeation and the risk of hydrogen embrittlement, thereby improving long-term operating reliability. This type of solution is generally used as a special option for specific operating conditions rather than as a standard configuration.

Diamond Diaphragm

Diamond diaphragms are typically used in service conditions involving strong corrosion, solid particles, or severe wear in the process medium. They have already been proven suitable for black water applications.

Material Selection Recommendation

Diaphragm material selection should not be based on price alone. More importantly, the material must be compatible with the actual process medium and operating conditions. A proper selection should take into account the medium composition, concentration, temperature, pressure, and corrosion characteristics. On this basis, cost, lead time, and long-term operating stability should then be evaluated together.

Installation and Maintenance of Dual Flange Differential Pressure Transmitter

1. How to Install a Pressure Transmitter?

High-Pressure Side Flange Installation
Usually installed at the lower measuring point of the vessel. Keep the flange sealing face clean and tighten bolts evenly in a diagonal sequence.

Low-Pressure Side Flange Installation
Usually installed at the upper vapor space measuring point of the vessel. Sealing and installation direction must also be carefully checked.

Capillary Routing
Both capillaries should be routed naturally, avoiding excessive pulling, compression, sharp bends, or hanging vibration.

Transmitter Body Installation
It is recommended to install the transmitter body in a location convenient for wiring, inspection, and commissioning, while avoiding obvious heat sources and strong vibration areas.

Bracket and Display Head Adjustment
Ensure stable installation and adjust the display head to a convenient viewing direction.

2. Dual Flange Level Transmitter Installation Precautions

With dual flange products, the most common problems during installation usually come from mechanical installation rather than electrical wiring.

High-pressure and low-pressure sides must never be reversed
Capillaries must not be cut
Capillaries must not be excessively bent
Capillaries must not be subjected to external pulling forces
Both capillaries must be securely fixed
Uneven heating between both sides should be minimized
Avoid routing close to steam lines or hot surfaces
The diaphragms must not be damaged by hard objects
Flange sealing surfaces must be kept clean

Under heat-traced high-temperature conditions, heating conditions on both sides should be kept as consistent as possible