Pressure measurement is a critical activity in industries involving fluid processing operations and systems. These fluids are not always non-reactive and, therefore, can damage the material of pressure sensing equipment (such as pressure gauges and pressure transmitters) through corrosion.
This results in erroneous readings and sensor malfunctions. Diaphragm seals are widely used in such pressure measurement processes in controlled environments to protect pressure sensors from being affected by the process fluid. BCST is one of the best manufacturers of diagram seals and supplies a large amount of material.
What is the diaphragm seal?
A diaphragm seal is a flexible membrane that covers the sensor and its adjacent path to prevent process fluid from coming into contact with it. The pressure of the controlled system can be easily measured through this membrane while holding back the liquid. Isolation diaphragm seals are typically made from elastomers and can be customized to the fluid’s requirements.
To accurately measure fluid pressure, the space created between the chemical seal and the sensing kit is filled with the appropriate fluid. The pressure of the process fluid is transferred to this fluid, which in turn is read by the pressure sensor.
What is the structure of the diaphragm seal?
The diaphragm seal is assembled with the help of open flanged, threaded, sanitary, or other similar connections. They are also known as gauge protectors or chemical seals.
For systems operating in high-pressure environments, diaphragms are made from materials such as Hastelloy, Inconel, Monel, stainless steel, or titanium. Some materials are more suitable for certain fluids than others.
What is the role of the Diaphragm Seal in pressure gauge?
This isolation is achieved through the use of a pressure-sensitive diaphragm made of a material resistant to the process medium and enclosed in a system installed at the base of the instrument.
An appropriate fill fluid is injected, the empty space between the diaphragm and the metering element is filled, and the space is sealed. The process pressure exerts a force on the exposed face of the diaphragm, and when the diaphragm flexes, it pushes in and compresses the fluid, which is channeled directly into the metering element.
What are the possible problems when selecting a diaphragm seal?
Your chances of choosing and installing a diaphragm seal properly will increase if you stay away from typical blunders. A diaphragm with a diameter that is too narrow or one that isn’t flexible enough to accommodate the fill fluid’s normal thermal expansion can provide misleading pressure readings and zero shifts.
- Process or ambient temperature exceeds the acceptable service range of the fill fluid.
- In differential pressure applications, very small differential readings may not be possible due to the amount of force required to actuate the diaphragm and metering element.
- Due to resistance created by the capillary’s length, internal diameter, and viscosity of the fill fluid, the system’s response time is increased to intolerable levels.
- The temperature of the process can influence the ambient temperature near the process.
- Care must be taken when specifying a sealing system to measure a vacuum or high vacuum pressure. While they normally work for most standard vacuum applications, as the pressure approaches a perfect vacuum, acceptable levels of accuracy become more difficult to achieve.
What are the functional characteristics of the diaphragm seal?
Diaphragm seal accuracy at 20°C can vary depending on chemical properties, and these values should be added to the accuracy class of the instrument. However, vacuum accuracy cannot be guaranteed beyond -0.85 bar in standard executions.
This is because most fill fluids contain microscopic amounts of trapped air or gases, which tend to expand significantly as absolute zero pressure is approached. This expansion affects the measurement element in the instrument.
What is the temperature influence?
A suitable process fill fluid is used to fill the whole pressure sensor system, and it is heated to the reference temperature. An increase or decrease in ambient or process fluid temperature causes a proportional variation in the volume of the fill fluid.
Consequently, this has an impact on the internal pressure of the closed detection system, which could lead to a zero error in the instrument.
To minimize this, it is necessary to compensate for the volume variation caused by temperature. Small diameter diaphragms can compensate for only a small variation in volume. It is advised to utilize chemical seals with the biggest effective diaphragms feasible in accordance with the process parameters.
The fill fluid used is very important to the temperature application range of the chemical seal, so the minimum and maximum temperatures of the process medium must be considered.
Additionally, the fill fluid must be appropriate for the medium, particularly when using oxygen or for applications in the food sector. This is important in case the diaphragm ruptures and, as a result, the fill fluid can mix with the process medium.
What are the features of the diaphragm seal?
The diaphragm valve uses the movement of the diaphragm to make adjustments and replaces the valve core assembly with a corrosion-resistant lined valve body.
The body material of the diaphragm valve is cast iron, cast steel, or cast stainless steel and is lined with various corrosion-resistant or wear-resistant materials, including diaphragm material rubber. Strong corrosive media, such as strong acids and strong alkalis, can be adjusted using the lining’s diaphragm due to its high corrosion resistance.
The main characteristic is that the diaphragm separates the inner cavities of the lower valve body and the upper valve cover, preventing the medium from corroding the valve stem, valve disc, and other parts above the diaphragm, negating the need for a packing sealing structure and preventing medium leakage.
- The diaphragm made of soft seals such as rubber or plastic has better sealing performance. Since the diaphragm is a wearing part, it should be replaced regularly based on the condition of the medium.
- Limited by the diaphragm material, the diaphragm seal can be used for high or low temperatures.
What are the uses of a diaphragm seal?
A diaphragm seal is a piece of equipment with a diaphragm that has been appropriately gasketed and welded (or clamped) between two suitable flanges to stop the leaking of liquid or gas.
The diaphragm is a thin dividing membrane or partition that acts as a barrier to a liquid or gas that can clog or corrode the sensing element of a pressure instrument. Gauges, pressure transmitters, or pressure switches can all be utilized with it.
How to select a diaphragm seal
Diaphragm seals are available in several styles: one-piece construction, two-piece construction, flanged, and saddle types are the most common. Excluding the one-piece construction unit, the materials for the upper casing, diaphragm, and lower casing can be chosen.
Particular attention should be paid to the diaphragm seal for the maximum/minimum process pressure and the pressure instrument to which the diaphragm is being adjusted. Other options, including cooling towers and liquid make-up, can be used to accommodate high-temperature applications.
The most critical factor for diaphragm seal performance is the integrity of the filled system. Regardless of proper diaphragm seal size and proper fill fluid selection, performance will tend to be inaccurate and unpredictable if the filled system contains trapped gases or air pockets.