Pressure switches: Often overlooked

When systems fail, engineers tend to focus on the big, the expensive or the exotic. However, it is often a problem with a simple, inexpensive part that shuts an application down or affects performance. Here, David Almond, head of sales and marketing at switch distributor PVL, explains why he believes pressure switches are one of the most overlooked technologies of all

Pressure switches are one of the most common types of component in process plants. Yet they are also one that many engineers know little about.

Despite this, a clear understanding of the basics and a reasoned, methodical approach will make the selection process much easier.

In its simplest form, a pressure switch is a device capable of detecting a pressure change, and, at a predetermined level, opening or closing an electrical contact.

The most common electromechanical pressure switches consist of a sensing element and an electrical snap-action switch. A number of different types of sensing elements can be used but they have one thing in common: they move in response to changes in the system pressure.

Through their movement they directly act on the opening and closing of the snapaction switch's contacts.

The current market provides a wide variety of solid-state pressure switches, with one to four or more switch points, digital displays, analogue and digital outputs, and full programmability.

In many cases they cross the line from simply being a switch to becoming an openloop controller. In addition to opening or closing the pressure switch circuit or circuits, they provide a proportional analogue 4-20 mA signal or digital output.

The analogue signal can interface with PLCs (Programmable Logic Controllers), DCSs (Distributed Control Systems) or stand alone industrial computers.

Solid-state pressure switches provide a number of advantages over electromechanical switches, including a much longer cycle life, improved accuracy to ±0.25%, high resistance to shock and vibration, the ability to handle a wide range of system pressures, broad frequency response and excellent long-term stability.

However, the biggest advantage lies in cycle life. Solid-state switches routinely have an operational span of 100 million cycles.

Speed, range and switch point The frequency with which the switch is activated will have direct impact on switch life, system downtime and the maintenance schedule. Due to their design, electromechanical switches are subject to metal fatigue although solid state switches aren't.

Cycle speed will also affect switch life and preventative maintenance programmes once the design is used in anger. A solid-state switch should be selected whenever the cycle rate exceeds 50 cycles per minute so that metal fatigue is not a problem.

Establishing the right relationship between the switch point and the operating pressure range of a switch is also important.

When a solid-state pressure switch is selected, the switch point should normally be in the upper 25% of the operating range.

For an electromechanical switch, the switch point should be in the middle of the operating range. Thus, a system that requires a switch to activate at 140 psi should use a solid-state pressure switch with an operating range of 150 psi, or an electromechanical switch with an operating range of 300 psi.

Accuracy, pressure points and housing Pressure switch accuracy is defined as the ability of the switch to operate repetitively at its set-point. If the switch is used to trigger an alarm, ±2% accuracy is sufficient.

If one is controlling a process where the error of various devices is cumulative, then ±0.25% accuracy may be absolutely necessary. Accuracy is referenced at the high end of the operating pressure range and decreases at lower pressure.

Once the required accuracy is established, we should decide on the number of switch points need. When sensing pressure at one point, it is normal that only one switch point is required.

Nevertheless, it's not unusual for a system to require two or even four switch points to be monitored, controlled or alarmed. In designing a system, one could select a single switch for each switch point, or a single pressure switch capable of handling as many as three separate switch points If one considers all of the factors that have to be taken into account when designing a pressure switch into an application, the hidden complexity in one of the manufacturing and process industry's simplest components is revealed. However, despite this hidden complexity, this kind of switch remains one of the simplest to integrate and maintain. The job of the design engineer is to make that process even easier.