Industrial pressure switches have many functions. They are used to regulate water pressure in homes and buildings, maintain water levels in reservoirs and tanks, monitor and control pressure in HVAC systems and pumps, to name a few. However, at Ashcroft, a majority of our customers use pressure switches as a safety mechanism to set off an alarm or trigger a system shutdown when high- or low-pressure limits are reached.
Ashcroft is an industry leader in pressure measurement instrumentation. We created this article for new customers like you who want to know how pressure switches can function as an alarm or to shut down their systems.
Read on to learn about the different types of pressure switches commonly used in these applications and the additional approvals these switches may need to be used in this capacity. When you’re done reading, you will also find additional resources about switches that may interest you.
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4 ways pressure switches are used for safety.
Regulating pressure limits in high-pressure and low-pressure systems is essential for safety and operational integrity in industrial applications. Pressure switches effectively detect pressure changes, triggering responses from simple adjustments, operator alarms, or if needed, process shutdowns to prevent damage and hazards. Here's how:
1. High-pressure alarm.
To use a pressure switch as a high-pressure alarm, the instrument is configured to activate a microswitch when the pressure increases to a pre-determined alarm level. This switch can be directly connected to a visible alarm or light for operator notification or integrated into the control system. The control system can alert the operator or automatically adjust the process to restore normal pressure levels.
2. High-pressure alarm and shutdown activation.
When you need the pressure switch to serve as both an alarm and a shutdown device, you will need either two single set-point switches or a dual set-point switch. The first set point switch is directly connected to a visible alarm or light for operator notification or integrated into the control system, the same as above.
The second set point is calibrated to a higher pressure threshold. Once the system reaches this higher pressure threshold, it will automatically shut down, either directly through the pressure switch or through the control system. This added level of protection from the pressure switch stops the entire system to prevent potential damage or injuries.
Figure 1: Ashcroft® B-Series Pressure Switch with a single setpoint.
Figure 2: Ashcroft® L-Series Pressure Switch with dual setpoint.
3. A low-pressure alarm.
When it comes to using a pressure switch as an alarm when the pressure drops too low in a process, similar to how a high-pressure alarm is done, with a few additional considerations.
The pressure switch is set to activate the microswitch on decreasing pressure at the alarm level. This switch can be directly connected to a visible alarm or light for operator notification or integrated into the control system. The control system will then alarm the operator or make an automatic process adjustment to try to bring the process pressure back to normal.
However, when setting a low-pressure alarm, it is also important to determine whether the normal operating pressure is significantly high and the low-pressure alarm level is considerably low. In such scenarios, you may not be able to set the alarm pressure value within a range suitable for the high operating pressure.
A common example of this is an oxygen tank. The full tank pressure may be over 2,000 psi while the alarm value may be as low as 100 psi. In this case, you will need to use a piston-activated switch rather than a diaphragm actuator switch. For example, an Ashcroft® A-series Switch with a 1,000 psi range can be set as low as 100 psi and also has a maximum working pressure of 5,000 psi.
Figure 3: Ashcroft® A-Series Pressure Switch.
4. A low-pressure alarm and shutdown activation.
To effectively implement a low-pressure alarm and shutdown activation, you can use either two single-set point switches or a dual-set point switch. One set point is designated for the alarm function, as previously described, and the second set point is calibrated to a lower pressure threshold. When this specific pressure is reached, the system will automatically shut down, either directly through the pressure switch or through the control system.
5. High- and Low-Pressure Alarms.
For systems requiring both high- and low-pressure alarms, a dual set point switch can be configured to manage both functions. One set point microswitch is set increasing at the high alarm value and the other is set decreasing at the low alarm value.
Approvals for alarms and shutdowns that may be required
1. Safety Integrity Level (SIL) rating
Certain systems may require further certifications, such as a Safety Integrity Level (SIL) rating. Many shutdown systems incorporate this rating to ensure reliability and safety. Ashcroft’s A-Series and B-Series pressure switches are capable of achieving SIL 3 certification. To obtain a SIL 3 rating for your safety shutdown system, you would use multiple switches configured in a voting arrangement.
Figure 4: Ashcroft® A-Series SIL certificate.
2. Steam limit control for boilers (UL-385 – Boiler Limit Control)
Another safety application for pressure switches involves regulating steam pressure in boilers. For example, the Ashcroft® B-Series option for steam limit control is engineered for use on boilers equipped with electrically operated burners. The limit control switch is adjustable and can stop burner operation when the boiler's pressure surpasses the recommended safe operating levels.
Figure 5: U.L. Listed Steam Limit Control Switch
3. Gas and Oil Limit Control Switch (UL-385 – Boiler Limit Control)
The Ashcroft® B-Series Switch option for medium-pressure gas and oil limit control is designed for use with air, LP gas, natural gas, number #1 & #2 fuel oil and No. #6 oil preheated to 240 °F. This limit control is an adjustable pressure-operated switch with a secondary chamber to prevent fuel from entering the switch enclosure in the unlikely event that the diaphragm develops a leak. The control shuts down a fuel pump in high or low-pressure conditions.
Figure 6: U.L. Listed Limit Control Switch
Go beyond the basics.
Now that you have a basic understanding of the different ways pressure switches can be used for alarms and shutdown activations in multiple industrial applications, you may be ready to dig in a little deeper. Here are a few related articles that can get you started.
- What is a Pressure Switch
- Choosing a Pressure Switch: 9 Factors to Consider
- Pressure Switch Fundamentals (webinar)
- Class and Division Codes on Explosion-Proof Pressure Switches
- What is SIL Certification for Pressure Switches?
Or, to speak with someone directly, feel free to contact one of our product experts with any questions you have. In the meantime, download our e-book to learn more.