Control components in a hydraulic system:Pressure and temperature switches.

Pressure and temperature switches
Pressure switches

A pressure switch is an instrument that automatically senses a change in pressure and opens or closes an electrical switching element, when a predetermined pressure point is reached. A pressure-sensing element is that part of a pressure switch that moves due to the change in pressure. There are oasically three types of sensing elements commonly used in pressure switches:

1. Diaphragm: This model (Figure 6.44) can operate from vacuum pressure up to a pressure of 10.5 kg/cm2 (150 psi). It consists of a weld-sealed metal diaphragm acting directly on a snap action switch.

Control components in a hydraulic system-0167

2. Bourdon tube-type sensing element: This model (Figure 6.45) can operate with pressures ranging from 3.5 kg/cm2 (50 psi) to 1265 kg/cm2 (18 000 psi). It has a weld­ sealed bourdon tube acting on a snap action switch.

clip_image004

Figure 6.45

Bourdon tube pressure switch

3. Sealed piston-type sensing element: This type of sensing element can operate with pressures ranging from 1 kg/cm2 (15 psi) to 844 kg/cm2 (12 000 psi). It consists of an 0-ring-type-sealed piston direct acting on a snap action switch (Figure 6.46).

clip_image006

Figure 6.46

Sealed piston pressure switch

The electrical switching element in a pressure switch, opens and closes an electrical circuit in response to the actuating force received from the pressure-sensing element.

There are two types of switching elements:

1. Normally open

2. Normally closed.

A normally open switching element is one in which the current can flow through the switching element only when it is actuated. The plunger pin is held down by a snap action leaf spring and force must be applied to the plunger pin to close the circuit. This is done by an electrical coil which generates an electromagnetic field, when current flows through it. In a normally closed switch, current flows through the switching element until the element is actuated, at which point it opens and breaks the current flow.

Temperature switch

A temperature switch is an instrument that automatically senses a change in temperature and opens or closes an electrical switching element when a predetermined temperature

level is reached. Figure 6.47 is an illustration of a common type of temperature switch which has an accuracy of ±1 °F maximum.

clip_image008

Figure 6.47

Temperature switch

This temperature switch is provided with an adjustment screw at the top end in order to change the actuation point. In order to facilitate its mounting on the hydraulic system whose temperature is to be measured, the bottom end of the switch is provided with threads. As in the case of pressure switches, temperature switches can also be wired either normally open or normally closed.

Related posts:

Control principles for variable speed pumping:Speed variation for positive displacement pumps
Vacuum and Low Pressure:Air blowers
Energy and Efficiency:Theory of two level systems
System selection considerations:System selection considerations
Operating problems:Existing plant and Positive pressure systems
Hydraulic accessories:Types of filters.
Introduction to hydraulics:Introduction and background
Sequence valves.
Pump types:Gear pumps ,Vane pumps,Piston pumps and Combination pumps.
Fundamental Principles:temperature measurement
Control Valves:Pilot-operated valves
Process Control Pneumatics:the air relay and the force balance Principle
ENERGY TRANSFER BY HEAT,WORK,AND MASS:FLOW WORK AND THE ENERGY OF A FLOWING FLUID
GAS MIXTURES AND PSYCHROMETRICS:COMPOSITION OF A GAS MIXTURE: MASS AND MOLE FRACTIONS
FORCED CONVECTION:PARALLEL FLOW OVER FLAT PLATES

Leave a comment

Your email address will not be published. Required fields are marked *