How to Select a Pressure Switch ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Select a Pressure Switch

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With the prevalence of pressure switches in most industrial plants, it often becomes a challenge when there is the need to replace a bad pressure switch or to select a pressure switch for a new application.

The deciding factors in the selection of a pressure switch for use in your application depend on the requirements of the application. The following requirements should be considered to help you select the right pressure switch for your application:

(1) Set points
How many set points do you require in your application? Do you want to control/monitor one set point or two?
•    For one set point, use a fixed differential pressure switch
•    For two set points, use an adjustable differential pressure switch

(2) Fluids
What fluids do you want to control? Common fluids used with pressure switches include:
•    Hydraulic oils
•    Air
•    Fresh water
•    Sea water
•    Steam
•    Corrosive fluid
•    Viscous fluid
To use any of the above fluids with a pressure switch, ensure that the wetted parts of the switch are compatible with the system fluid.

(3) Pressure Range
The pressure range of a pressure switch is a critical factor in its application. What pressure range does the system experience? It is always practical to select pressure settings that fall within the middle 80% of the pressure range of the switch. The pressure applied during a normal cycle should never exceed the maximum range value listed for the switch. In the application, pressure surges should be less than the maximum allowable pressure listed for the pressure switch.

(4) Surges
During the service life of a pressure switch, surges are going to occur and the frequency of its occurrence depends on the particular application.
How frequent are surges in your system, and what is their maximum pressure level?
Applications experiencing frequent or high-pressure surges may require a device with a higher pressure range.

(5) Switch Enclosure
What type of enclosure do you need? Common enclosure types include:
•    Open style 1
•    NEMA Type 1
•    NEMA Type 7, 9
•    NEMA Type 4, 4X, 13 / IP66, IP65

(6) Output
Pressure switches comes in different output configuration. What output type do you require? Popular output configurations include:
•    SPDT contacts, 1 N/O, 1 N/C
•    2 SPDT contacts, 1 N/O, 1 N/C
•    Dual stage, 1 SPDT contact each stage, 1 N/O, 1 N/C

(7) Electrical connection
Just as there are different output configurations, there abound different electrical connection configurations. Typically you would ask: What type of electrical connection do you require in your application? Common electrical connection configurations include:
•    ½"- 14 NPTF
•    ¾"-14 NPTF (available only on NEMA 7 & 9)
•    ISO M20 metric threads
•    Type 13 (PG 13.5) metric threads
•    No threaded connection (Open style or NEMA 1 only)

(8) Pressure connection
What type of pressure connection do you require in your application? Common pressure connections include:
•    ¼"- 18 NPTF (female)
•    ½" - 14 NPT
•    PT ¼ (JIS B0203)
•    7/16"-20 UNF-2B
•    G 1/4 BSP (female) metric thread

(9) Special features
Do you require any special features on the pressure switch? If you do then you have to specify it according to the specification guide outlined by the manufacturer of the pressure switch. For example when pressure switches must be factory set and only one setting is identified, specify whether this setting is on rising or falling pressure.

(10) System response time
If the system response time is critical in your application, select a switch with a volumetric displacement that is compatible with the overall system.

With these common requirements for a pressure switch, you will be in a position to select the right switch for your specific application.

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