The following information must be taken into account when choosing a diaphragm seal assembly to guarantee secure and satisfactory operation.
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Process composition
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Temperature
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Pressure range
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Process connection
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System fill fluid
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Mounting position
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Response time
Process composition:
The diaphragm and lower diaphragm seal housing will be subjected to the process medium and it is critical to select materials compatible with this medium for these parts. Tables are accessible to help in selecting these materials, but the client is the ultimate source for selecting the appropriate materials.
Temperature:
The diaphragm seal measuring device is filled with filling fluid at an ambient temperature of approximately 70°F. This temperature is called the temperature filling system.
Depending on temperature changes, the fill fluid will expand or contract. This in turn leads the stress to increase or drop in the sensing component, adding zero changing impacts to the output of the tool.
Pressure range:
The displacement volume required to “drive” each diaphragm seal measuring system on the diaphragm seal must be greater than the displacement volume required to move the pressure sensing element. Normally, to “drive” the system, the reduced the pressure range, the bigger the diaphragm.
Process connection:
The process connection is specified by the customer. Most process connections are threaded, flanged, or clamped; however, additional connections are available.
System fill fluid:
There are wide range of fluid filling systems allowing temperatures between -130°F and 752°F. In the event of a leak, care must be taken to consider the chemical compatibility of the system fill fluid with the process fluid. A non-toxic fluid should be selected in food processing applications.
Mounting position:
For diaphragm seal systems that include a capillary, mounting position is essential. The distinction in level between the diaphragm seal and the pressure tool leads the sensing element to have a hydrostatic pressure:
The pointer on the dial of the gauge will be smaller than the zero point for gages installed above the diaphragm seal level.
For gages that are mounted below the diaphragm seal level, the pointer on the gage dial is higher than the zero point.
Response time:
Response time, i.e., the time it takes the pressure instrument to indicate 90% of the value of a sudden pressure variation, is especially important for instrument/diaphragm seal assemblies which include a capillary. Response time increases significantly in systems with long capillaries. In applications requiring long capillaries, response times can be reduced by using larger diameter capillary tubing and reducing the viscosity of the system fill fluid.