In the chemical and petrochemical industry, measuring and monitoring pressure is critical to ensuring your operation runs safely and efficiently. Inaccurate pressure measurements can have serious consequences, including employee injury, equipment loss, environmental issues and costly downtime. That’s why it is essential to use instruments engineered to withstand the severe demands of chemical processing operations.
Ashcroft has been making accurate and reliable pressure and temperature measurement instruments for more than 170 years. During that time, we have seen how pressure gauges perform in the most demanding conditions and have become an industry authority across the globe.
In this article, you will learn about the unique challenges pressure gauges encounter in chemical and petrochemical operations and gain a better understanding of how to ensure your gauge is safe to use in these demanding applications. When you are done reading, you will have access to additional articles and resources to deepen your knowledge even further.
How to ensure your pressure gauge is safe to use in hazardous material applications.
The U.S. Environmental Protection Agency (EPA) defines a toxic chemical as any substance that may be harmful to the environment or hazardous to your health if inhaled, ingested or absorbed through the skin. In chemical and petrochemical industries this includes corrosive, reactive, oxidizing and toxic/carcinogenic agents that convert raw materials such as air, water, metals, minerals, oil, and natural gas—into usable forms for manufacturers.
Due to the harsh nature of these processes, it is important to consider how your pressure gauge will stand up to these five primary factors:
1. Corrosion and wetted parts material compatibility.
Corrosion is a leading cause of pressure gauge failure in chemical and petrochemical applications. Incompatible process media can result in media leakage, instrument failure and serious safety hazards for operators and the environment.
To prevent corrosion-related damage to your gauge, you must ensure the wetted components of your gauge are compatible with the process media. For example, on a process gauge, the sensing element, such as the Bourdon Tube system, the socket, and the tip all come into contact with the process and must be compatible.
Material selection considers the process media’s composition, concentration, and temperature range. Figure 1 is an example of the wetted components of the Ashcroft® 1279 Duragauge® pressure gauge, which is designed for maximum safety and reliability. However, before you choose this or any other pressure gauge for your process, double-check the requirements of the application.
Figure 1. Ashcroft® 1279 Duragauge® wetted materials.
If a suitable wetted material is not available, using a diaphragm seal/isolator is recommended to protect your instrument from corrosion damage. The diaphragm seal you select must also be compatible with the process media. On a diaphragm seal, it is the lower housing (if applicable) and the diaphragm that are wetted. For additional guidance on material compatibility, refer to the Ashcroft® Material Selection & Corrosion Guide.
You can also download the Complete Guide to Pressure Instrument Assembly to learn more about each of the mounting options for pressure gauge assemblies.
2. Temperature limits.
Pressure gauges have specific process, ambient, and storage temperature ranges for optimal use. Using them outside these ranges can result in significant damage. For instance,
- Dry gauges are typically suitable for ambient temperatures below 200°F (93 °C), while process temperatures for dry gauges are usually between -20°F (-29 °C) to 250 °F (121 °C).
- Liquid-filled gauges are generally suitable for ambient temperatures up to 150 °F (66 °C).
The chart below shows the ambient, process and storage temperature limits of pressure gauges, considering the type of gauge.
Figure 2. Ashcroft® 1279 Duragauge® temperature limits.
If process temperatures go beyond these limits, adjustments can be made to help dissipate the process temperature and maintain instrument integrity and safety. For instance, you can:
- Use a temperature dissipator. With a direct-mount gauge, you can use an Ashcroft® MicroTube™ siphon for process temperatures to 800 °F (427 °C), a finned siphon for process temperatures to 700 °F (371 °C) or a steam siphon for live steam applications.
- Mount the gauge remotely. Using a capillary line, the instrument can be installed away from the process to protect it from high or low temperatures.
- Use a diaphragm seal. Using a diaphragm seal in conjunction with a temperature dissipator is also an option.
Read, How Does Temperature Affect Pressure Gauge Performance to learn more.
3. Ingress Protection (IP) rating for instrument housing.
When selecting instrumentation for outdoor use exposed to weather conditions, it's crucial to consider the Ingress Protection (IP) rating of the gauge housing to ensure it is adequately sealed. The pressure gauge's 2-digit IP rating considers the ability of dust or liquids to enter the gauge housing in these settings. For these applications, you will want your gauge to have a higher IP rating of IP66 or IP65 to provide better protection against moisture ingress.
Gauges with low IP ratings like IP54, may be prone to water infiltration from rain or washdowns. Selecting the right case for the application is crucial to prevent damage and ensure the gauge's longevity in outdoor settings.
4. Overpressure and pressure spikes
Overpressure or pressure spikes (surges beyond the full-scale range of a gauge) in a process can result in accuracy degradation, gauge failure or rupture of the bourdon tube. Proof pressure, typically 130-150% of the range, should not be surpassed to maintain gauge integrity. Before you select a pressure gauge for these applications, be sure to check the manufacturer's datasheet to find the maximum operating pressure of the instrument.
A few ways to prevent overpressure from damaging your gauge, include:
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Using a pressure limiting valve (PLV). A pressure-limiting valve is designed to limit pressure up to a specific set value and only open once the pressure drops down below a certain value. The valve will prevent overpressure damage to the gauge and help ensure equipment safety and functionality.
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Incorporating an internal stop. An internal overload stop option, like in the Ashcroft 1279 option "OS" can typically increase proof pressure by an additional 20%.
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Using a solid-front gauge. The Ashcroft 1279 offers a solid front design with a pressure relief back to redirect pressure to the back of the gauge, safeguarding against front-facing bursts.
In processing equipment, fluctuations in process pressure can cause pulsation and vibration to occur in the gauge, leading to issues like pointer flutter and component damage, affecting gauge readability and integrity. Vibrations can be categorized as high-frequency/low-amplitude or low-frequency/high-amplitude, each impacting the gauge differently:
- High-frequency vibrations cause pointer flutter and localized gear teeth damage
- Low-frequency vibrations lead to widespread gear teeth damage and increased stress on the Bourdon tube
A general guideline is to address pulsations or vibrations if they cause pointer movement exceeding 5% of the full-scale range. To mitigate their impact on the instrument's performance and longevity, a diaphragm seal can also help protect your gauge from pulsation and vibration. Other options include:
- Dampening movement in the gauge. Liquid-filled gauges with silicone, glycerin or halocarbon will help dampen movement. If you prefer a dry gauge, you can use the Ashcroft Duragauge with PLUS!™ Performance which is designed to dampen pulsation and vibration and improve gauge readability without the liquid fill.
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Restricting the flow of the process material. Another way to reduce pulsation to the gauge is with a pulsation dampener, steel needle valve or a diaphragm seal to dampen the flow of process material. These accessories dissipate pulsation before it reaches the instrument.
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Installing the instrument assembly away from the source. A capillary line (available up to 100 feet) can be added to a mounted gauge, putting more distance between the gauge and the process, thus reducing the effects of pulsation and vibration.
Ready to learn more?
Now that you have a better understanding of the safety factors to consider when choosing the right pressure gauge for chemical and petrochemical applications, you can use this information to make a more informed purchasing decision. If you want to dig a little deeper, here are a few more articles you may find interesting.
- Six Reasons Pressure Gauges Fail Instrument Audits
- How Does Temperature Affect Pressure Gauge Performance?
- Fill-Fluid Options for Pressure Gauges
You can also talk to one of our industry experts and get your questions answered.
In the meantime, download our solutions guide to learn more about the common applications, and factors to consider when selecting pressure and temperature instruments for chemical and petrochemical processes. 