Diaphragm Seals

Single Diaphragm Seal

Without a diaphragm seal, process fluids accumulated in the port or dead- ended sensor cavity of pressure gauges may compromise the physical and microbiological integrity of the process fluid. As an example, milk getting into the pressure port of a pressure gauge may spoil and contaminate the milk in a vacuum pan during its in vacuo evaporation and concentration. Furthermore, certain metals in electronic pressure sensors may contaminate the fluid with lead, zinc, copper, cadmium, etc. Therefore, to protect the process, the use of a diaphragm seal is recommended. In addition, the isolating diaphragm between the gauge sensor and process media prevents the pressure gauge port from plugging up with debris and liquid condensate, as well as protects the sensing element of the gauge by prohibiting corrosive, abrasive and noxious process materials from reaching the dead-ended sensor cavity (Fig. 7.88) (Moerman, 2013b).

A single diaphragm seal is composed of three main parts (Moerman, 2013b):

  • • The housing containing the fill fluid and process and sensor connections for the diaphragm seal.
  • • The isolating diaphragm, a flexible membrane that separates the fill fluid and mechanical or electrical sensing element from the process fluid or process aid. Pressure effects are allowed to cross the isolating diaphragm from the process/utility system and are hydraulically transmitted by the fill fluid to the sensor’s measuring element. Isolating diaphragms have a thickness of approximately 3 mm, and are made of food process compatible materials such as plastics, rubbers, or metal plate. Metal diaphragms of stainless steel (several grades), Hastelloy, Monel, Inconel, tantalum, titanium, and other metals should be welded flush to the housing.
Mechanical gauges in particular, such as the bourdon gauge, require a diaphragm seal

FIGURE 7.88 Mechanical gauges in particular, such as the bourdon gauge, require a diaphragm seal. Diaphragm seal gauges can cope with a greater range of process temperatures and aggressive products. Courtesy of WIKA Alexander Wiegand SE & Co. KG.

Capacitive sensors consist of a ceramic diaphragm of Al2O3, separated from the metallic sensor body via an elastomer seal. This design must be considered carefully as there is a risk of pores.

• A fill fluid (in the cavity between the gauge sensing diaphragm and the isolating diaphragm) is application specific, which means that it varies for food, beverage, or industrial applications. In food applications, a stable, food grade, noncorrosive, low thermal expansion, and viscosity fluid should be used (Table 7.3). For high-temperature applications, a sodium- potassium eutectic is commonly used, while a mixture of glycerine and water is recommended for ambient temperatures. Ethyl alcohol or silicon oil are applied for low temperatures. Water-based glycerine is not an appropriate fill fluid in vacuum and high-temperature applications due to its risk of vaporization, which can destroy the diaphragm seal, while at low temperatures it becomes too thick to produce accurate readings due to the extremely slow response time.

With regard to diaphragm seals, besides the common pressure gauge mounting to existing fittings such as T-pieces or welding sockets, there are also pressure measurement device installations using flow-through diaphragm seals (Fig. 7.89). These diaphragm seals consist of a body with an internal cylindrical thin diaphragm, which can be made of a variety of plastics. The diaphragm in-line seals can be installed directly in the pipeline between two flanges. A variety of nominal pipe diameters enables adaptation to the cross-section of the particular pipeline. These diaphragm in-line seals are ideal for use with flowing process media. With the seal being completely

TABLE 7.3 Fill Fluid in Pressure Gauges (Cole-Parmer, 2009)

Fill Fluid Suitable Temperature Range Kinematic Vi:

at Tempera

scosity

ture

Pabs <1 bar (° C)

Pabs >1 bar (°C)

(10-6 m2/s)

(°C)

Glycerine

N/A

+ 15 to +240

1110

+20

Glycerine/water

N/A

-10 to +120

88

+20

NEOBEE M-20

-28 to +160

-25 to +205

9.8

+25

Food grade silicone oil

N/A

-15 to +300

350

+25

Mineral oil

-20 to +170

-20 to +250

56

+20

Vegetable oil

Propylene

glycol/water

N/A 5 not applicable.

-10 to +120

-10 to +200 -18 to +93

Diaphragm in-line seals. Courtesy ofWIKA Alexander Wiegand SE & Co. KG

FIGURE 7.89 Diaphragm in-line seals. Courtesy ofWIKA Alexander Wiegand SE & Co. KG.

integrated in the process line, measurements are not affected by any turbulence, corners, sharp edges, dead-ended cavities (where solids could accumulate), or other obstructions in the flow direction. Moreover, whereas other designs with grooves or noncircular geometry are more critical to clean, diaphragm in-line seals with their perfectly circular form are less likely to plug, are self-cleaning and are easy to drain. All product residues or films can be easily cleaned, even by pigging in certain applications. However, if maintenance is required, the process has to be shut down.

 
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