Common Pressure Switch Designs and Applications ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Common Pressure Switch Designs and Applications

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The pressure switch is a very common device in any plant. Being an extremely important piece of control and monitoring equipment, its technology has evolved over the years. Pressure switches are used to interface pneumatic or hydraulic systems with electrical control systems by opening or closing electrical contacts in response to
pressure changes in the system. They have outstanding repeatability and drift performance. Their efficient design uses durable, low mass components for excellent performance under heavy duty vibration and shock conditions.

There are various designs for the pressure switch but the most common of these designs are diaphragm and piston actuated pressure switches. In this article, we shall discuss diaphragm and piston actuated pressures switches and their application criteria.

Areas of Application
Pressure switches can generally be used in any application where electrical contacts must open or close in response to a system pressure change, within the electrical and pressure ratings of the switch. Pressure switches are used in a wide variety of applications such as the following:
•    Compressed air systems
•    HVAC equipment
•    Pumping systems
•    Process equipment etc.

Basic functions of the Pressure Switch
Pressure switches typically perform one of the following two basic functions:

(1) Monitoring the Pressure in a System:
The pressure switch can be used either as an interlock that sequences operations in an automatic system, or to give an audio or visual signal, typically an alarm of an undesired condition, at predetermined pressures. A pressure switch with a fixed differential is generally used in these types of applications.

(2) Controlling the Pressure in a System:
The pressure switch can be used to start a pump or compressor at predetermined pressures. A pressure switch with an adjustable differential is usually needed in these kinds of applications.

Pressure Switch Application Tips:

Surges
One of the most destructive application conditions for a pressure switch is hydraulic surge. A surge is a high rate of rise in pressure, normally of short duration, caused by starting a pump or by opening and closing a valve. Extremely high rates of rise in pressure can be damaging even if they are within the limits of the maximum allowable pressure.

To minimize the effect of surges, the switch should be mounted as close to an accumulator and as far from pumps or quick acting valves as possible.  A restrictor with a small orifice placed in the line between the switch and the pump or valve can help to protect the switch.

Vibration
Excessive vibration can cause contact bounce, chatter, or premature contact transfer, especially when system pressure is near the operating point of the pressure switch. Remote mounting of the switch is the best way to avoid problems

Use on Steam Lines
Pressure Switches should not be applied directly on steam exceeding 15 psig. However, if steam capillary tubing is installed between the pressure connection and the switch, steam pressure of up to 250 psig can be applied, provided this does not exceed the maximum allowable pressure rating of the switch or the maximum temperature rating at the switch actuator.

Designs of Pressure Switches 
Various designs of pressure switches are in circulation. These designs are typically a variant of the two most popular designs namely:

Diaphragm actuated pressure switches
The diaphragm actuated pressure switch uses an elastic diaphragm. The diaphragm life is affected by excessive pressure cycling. The diaphragm can withstand wide pressure changes on each operating cycle of the switch. However, the pressure applied to the diaphragm during the normal operating cycle should never exceed the maximum value for the switch. Diaphragm actuated pressure switches are typically used in low pressure applications.

Piston actuated pressure switches
Piston based pressure switches are very popular in high pressure applications. Piston actuated pressure switches are commonly used in hydraulic applications. Hydraulic systems experience higher pressures, have wider pressure variations, and produce more surges, since the medium does not compress.

Comparison between Piston and Diaphragm Actuated Pressure Switches:
Selecting between piston and diaphragm devices depends on several factors:
(1) Maximum allowable pressure
(2) Range and differential
(3) Surges
(4) Medium (Hydraulic or pneumatic)

Let us take a look at each of these factors:

Maximum Allowable Pressure 
The maximum allowable pressures for piston devices are much higher than for diaphragm devices. Most diaphragm devices have a maximum allowable pressure of 850 psi or less, whereas all piston devices have a maximum allowable pressure of 10,000 psi or more.

Range and Differential
The range and differential for diaphragm devices are lower than for piston devices. Many applications call for a low differential, such as 20 psi. This may exclude piston devices, which have a minimum differential of 60 psi or more.

Surges
Surges are a part of every hydraulic system. While many are small and have only a small effect on the pressure switch, some are significant and can potentially destroy a pressure switch. Diaphragm devices are the most sensitive to surges and are most easily damaged. Piston devices are more tolerant of surges and last longer in the same application.

Medium
Hydraulic systems, which typically use oil-based media, are more demanding applications than pneumatic systems. Pressure switches used in hydraulic applications typically experience higher pressures, have wider pressure variations, and produce more surges, since the medium does not compress. Pneumatic systems, which typically use air, place fewer demands on a system, since these applications typically experience lower pressures and the medium can compress, cushioning the effects of surges.

Difference between Piston and Diaphragm Actuated Pressure Switches                                        

Selection Criteria Piston Diaphragm
Maximum Allowable Pressure High Lower
Pressures High pressures Lower differentials or pressures
Surges Constant Minimal
Media Hydraulic Systems Pneumatic systems