What makes a pressure switch ideal for a particular application?

Engineers and facility managers commonly ask this question when selecting pressure switches for an application. And the answer to this question goes beyond just choosing a switch that meets the basic application requirements like accuracy, pressure, size and mounting requirements. Engineers must also ensure to choose a rugged pressure switch.


A pressure switch is rugged if its design allows a stable switching response and long lifecycle under several conditions like pressure spikes, leaks, high temperature, chemical exposure, moisture, shock loads and vibration.

A pressure switch is rugged if its design allows a stable switching response and long lifecycle under several conditions. Source: ekkaluck/Adobe StockA pressure switch is rugged if its design allows a stable switching response and long lifecycle under several conditions. Source: ekkaluck/Adobe Stock

Managing pressure spikes

A pressure spike refers to a rapid rise (or drop) in pressure, and it is one of the most common causes of pressure switch failure. It is usually caused by a valve's rapid opening and closing, which sends a shock wave through the fluid. If this pressure shock wave is higher than the measuring range of the switch, it can lead to the switch’s destruction.

Modern-day pressure switch designs can withstand operating pressures of up to 7,000 psig and proof pressures of 11,500 psig. However, it is not the steady-state pressure that causes a pressure switch to fail. Instead, it is the rate at which the pressure spike rises (or falls). For instance, heavy-duty industrial pumping applications can create a pressure spike rate of up to 900,000 psig/second, which will likely cause damage to regular pressure sensors.

Engineers and facility managers should choose piston-type pressure switch designs with robust switch housings, pistons, seals and actuators. In addition, some pressure switches have design features like snubbers that prevent extremely high-pressure spikes from reaching the switch’s internal electrical switching element. These snubbers act as flow restrictors by reducing the size of the orifice in the switch’s pressure chamber, choking off the flow into the switch.

Preventing leaks

Leaks typically develop in piston-based switches around the dynamic piston seals, usually made of elastomer O-rings. While it’s normal for the seals in pressure switches to gradually deform (and cause leakage) at high pressures and cycle counts, some pressure switches have designs and engineering materials that make them more rugged.

For instance, the use of fluoropolymer seals in conjunction with an O-ring can reduce the deformation and wear problem common in conventional switches with elastomeric O-ring seals. This is because of fluoropolymers’ high rigidity and low coefficient of friction, making them capable of withstanding high pressures of up to 29,000 psig.

Use the ideal piston size to minimize hysteresis

The actuation pressure of a piston-based switch isn’t the same as the pressure required to move the piston back to its original position. For example, consider a piston-based switch designed to actuate when the pressure reaches 600 psi. Assuming an application causes the pressure to rise steadily to 620 psi, the switch would actuate at 600 psi. However, when the pressure decreases steadily from 620 psi and reaches 600 psi, engineers would notice that the switch doesn’t typically close at 600 psi. Instead, it will only close when the pressure hits a lower point, say 570 psi.

Hysteresis simply describes the percentage difference between the ON and OFF pressures of the switch. It is caused by the natural reluctance of the pressure sensing material to return to its original position (or shape) after being displaced (or deformed). For example, the switch hysteresis is obtained as 5% in the previous scenario, as shown below.

Piston-based pressure switch's accuracy and reliability depend on the piston's proper design. As such, engineers and facility managers want to choose pressure switches designed to minimize hysteresis to achieve a highly accurate and reliable system. And one way to achieve this is by opting for switches with a large piston size relative to the size of the switch.

Piston-based switches with large piston size (or area) are generally more sensitive to small changes in applied pressure. In addition, this design lessens the impact of friction forces on the system since the force developed from applied pressure increases exponentially with piston area, whereas friction forces only increase linearly.

Use the right materials

The choice of engineering materials greatly affects the performance and ruggedness of pressure switches, especially in harsh environments. For instance, engineers looking to use pressure switches in a moist environment might want to select switches whose exteriors are made from 18-8 stainless steel as it minimizes corrosion.

In addition, engineers also want to choose construction materials capable of withstanding the system’s possible temperature extremes. A rugged switch must have a relatively flat response across a broad operating temperature range. In contrast, poorly designed switches will have a reduced accuracy (and increased hysteresis) at very low or high temperatures since thermal expansion alters the normal movement of switch components relative to one another.

Summary

Piston-based pressure switches are extremely efficient control devices so long as they are rugged and accurately designed to satisfy the application requirements. Therefore, engineers and facility managers are advised to reach out to pressure switch manufacturers to discuss their application needs.

To contact the author of this article, email engineering360editors@globalspec.com