Simple pressure switches can be an ideal basic control solution

In today’s modern process automation and control landscape, sensor technologies are becoming more sophisticated and widely applied. To make meaningful use of these technologies, process control systems have also become more advanced, requiring expert specialists to architect and implement. Some applications, however, are not better served with the overhead and complexity of these types of integrated control systems. In these cases, something as ordinary as a simple pressure switch can be a perfect solution for a basic control application.

In certain applications, a pressure switch can achieve equal control without all the overheads of an intelligent process controller (like a PLC) and additional control logic. Even more, the cost to implement and maintain an intelligent control system over time can be orders of magnitude greater than a simple control scheme achieved with a pressure switch. Standards compliance within intelligent control systems can also add additional cost in some applications.

When pressure switches can be applied in an application, it is important to follow some basic guidelines in choosing the right switch for the application. Up front, it is important to consider all the basic application requirements to narrow the selection of a device. Pressure and temperature requirements of the application can quickly guide users into devices that will perform well. Materials of construction and media compatibility should also be a first consideration when choosing a pressure switch device. From this point, the details of how the switch will operate in the application should be considered.

In general, switch selection can be broken down into three different categories.

The first category is gross, binary pressure sensing. These are applications where it is valuable to know if pressure is present or absent across a wide pressure domain. In this case, creep action switches are a very good choice. These devices are cost effective and quite capable to handle simple on/off applications. An example application might be using a creep action pressure switch to sense the state of a compressed air system and indicate the state with a lighted beacon. Caution should be applied when the set and reset state of the application is very narrow, however. Creep action pressure switches can fail in applications where the electrical contacts hover near the switch make/break point. In this case, the electrical switch can repeatedly arc and erode the contacts.

Snap action pressure switches are a better choice when the sensing pressure domain is in a narrow band or when the pressure moves slowly up to the switch point. In applications like these, a snap action switch will positively change state with a more deliberate switching action. Additionally, snap action switches have a more definite reset window by their nature. Therefore, in applications where the pressure creeps up to the set point, the electrical switch does not cycle between make and break repeatedly. In this case the snap action switch positively switches and a perceptible reduction in pressure is required for the switch to reset. This is the delay of the switch.

A good application for snap action pressure switches might include a pressure limit cut off with a manual reset. This type of application can be routinely found on mobile hydraulic cranes. These cranes typically have a maximum load carrying capability that can be sensed by the pressure in the hydraulic system. When the hydraulic pressure extends beyond the safe working limit of the crane, a pressure switch is used to cut off the hydraulic motor.

When the application requires a little more control than a basic mechanical pressure switch can provide, electronic pressure switches should be considered. These devices can be considered the step between a full automated control system and a simple on/off pressure sensing device. Many electronic pressure switches are available with extended functionality beyond simple pressure sensing and switching. Often, these devices also offer explicit reset pressure setting as well as delay times when the set points have been reached. With these extended functions, more sophisticated applications can be addressed while still maintaining a cost effective and simple solution.

One example application that could be a good fit for an electronic pressure switch includes tank filling applications. In some holding tank applications, it is simple and convenient to control the level in the tank with a simple electronic pressure switch connected directly to the filling pump. The pressure switch can then be set for separate high and low-level set points. The low-level set point can initiate a filling cycle that is turned off at the high set point. Delay timers can also be applied to ensure that the level in the tank is positively beyond the intended switch set point (taking into account product agitation in the tank).

With a basic technical knowledge regarding pressure switch types and capabilities, these instruments can be an excellent choice for control applications when the sophistication of an intelligent control system is not necessary.

Source: Noshok

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