Pressure Relief Devices (PRDs) play a critical role in plant safety, but under the EPA’s HON regulations (Hazardous Organic National Emission Standards for Hazardous Air Pollutants), they are also now firmly in the spotlight as regulated emission points. Facilities operating under 40 CFR Part 63 must be able to detect, timestamp, record and report every PRD release event, while also notifying operators in real time and verifying the valve’s return to compliant operation.

On paper, these requirements are clear. In practice, meeting them across an entire facility remains one of the most difficult compliance challenges facing refineries, petrochemical plants and chemical manufacturers today.

Our new whitepaper explores why PRD monitoring has historically been so hard to achieve and how recent advances in non‑intrusive, high‑frequency acoustic monitoring are finally making plant‑wide, defensible compliance achievable.

This blog post outlines the key themes from that paper and explains why many facilities are now rethinking their approach to PRD monitoring.

HON has changed what ‘adequate monitoring’ really means

Under the HON rule, PRDs in gas, vapor or light liquid service are treated as regulated emission sources. Facilities must be able to:

• Detect every PRD release when it occurs
• Record the exact time and duration of each event
• Alert operators immediately
• Restore the valve to compliant operation
• Verify emissions have returned to less than 500 ppm above background (Method 21)

Missing an event or being unable to demonstrate precisely when it happened and how long it lasted, creates compliance exposure by undermining emissions reporting. In an enforcement environment where record accuracy matters as much as intent, this creates significant risk.

The challenge is scale. Large plants often operate hundreds (or thousands) of PRDs, with many installed in difficult-to-access locations. Yet the regulation makes no allowance for partial coverage; every relevant PRD must be monitored.

Why traditional PRD monitoring struggles to scale

Most PRDs today are not permanently monitored and the reasons are largely practical:

• Intrusive instrumentation (pressure taps, flow meters) requires process modifications, engineering approvals and sometimes even shutdowns
• Access constraints make installation and maintenance slow and costly
• Low-frequency acoustic or vibration systems suffer from high false‑positive rates in noisy industrial environments
• Manual or periodic inspections miss short, unpredictable release events entirely

Even handheld ultrasonic tools, while operating in the correct frequency range, depend on people being present at the right moment, which is incompatible with HON’s requirement for continuous event detection.

The result is a gap between regulatory expectations and what most legacy approaches can realistically deliver.

Non‑intrusive monitoring changes the design equation

The whitepaper makes a strong case that non‑intrusive monitoring is not just a convenience, but a necessity for plant‑wide compliance.

External acoustic sensors can be clamped to the outside of a PRD or adjacent pipework without breaking containment or modifying certified pressure equipment. This allows monitoring to be deployed while maintaining the integrity of safety‑critical systems.

From a practical standpoint, this approach enables:

• Installation in minutes rather than hours
• Minimal engineering approvals
• Deployment across hundreds or thousands of valves without production interruptions
• Monitoring in locations where intrusive instrumentation is impractical

This design philosophy underpins our acoustoSENSE platform, which is used as a reference example throughout the paper.

Why acoustic monitoring works and why frequency matters

When a PRD lifts, high‑pressure fluid passing through the valve creates turbulent flow, which generates microscopic elastic stress waves in the valve body and connected pipework.

These signals occur in the ultrasonic frequency range, often well above 100 kHz.

This is a crucial distinction. Most background plant noise (rotating equipment, structural vibration, rainfall, maintenance activity) exists at much lower frequencies. By operating in a high‑frequency band, acoustic monitoring can:

• Avoid environmental noise
• Improve signal‑to‑noise ratio
• Reduce false positives
• Detect very short release events

High‑frequency ultrasonic signals also attenuate rapidly, meaning sensors primarily ‘listen’ to the specific valve they are mounted on, further improving event discrimination.

The whitepaper explains how this alignment between the physics of PRD operation and sensor design allows acoustic monitoring systems to detect the precise moment a valve opens, how long it remains open and when it reseats; exactly the data HON requires.

Always‑on monitoring captures what sampling misses

PRD releases are rarely convenient. Some last seconds; others only tenths of a second. Monitoring systems that rely on periodic sampling or duty cycles introduce blind spots where real events can go undetected.

Always‑on monitoring ensures that every acoustic event has an opportunity to be captured, timestamped and recorded. Over time, this creates a behavioral history for each valve, revealing patterns such as simmering, chattering or repeated minor lifts.

This kind of dataset supports compliance, but it also enables risk‑based inspection (RBI) and better maintenance prioritization, turning a regulatory obligation into an operational advantage.

From sensors to systems: making PRD data usable

Detection alone is not enough. PRD monitoring must integrate seamlessly into existing plant systems to support compliance, operations and audits.

The whitepaper explores how modern systems use LoRaWAN connectivity, cloud analytics and standard industrial data protocols (such as MQTT and OPC UA) to embed PRD event data into:

• DCS and SCADA systems
• Asset performance management platforms
• Historian databases
• Environmental reporting tools

For sites without mature digital infrastructure, standalone dashboards can still deliver real‑time alerts, event logs and auditable records.

What engineers should look for in a HON‑ready solution

The paper concludes with a practical evaluation framework, highlighting key questions engineers should ask when selecting a PRD monitoring system, including:

• Does it operate above 100 kHz to avoid noise?
• Is monitoring truly continuous?
• How well does it discriminate real events from false positives?
• Can it scale across hundreds of devices?
• Does it integrate cleanly with existing OT and IT systems?

Download the full whitepaper to learn more about:

• What the updated HON regulations require in practice
• Why traditional monitoring approaches fall short
• How high-frequency acoustic sensing detects real PRD events
• How to eliminate false positives while capturing every release
• Best practices for deploying monitoring across hundreds of valves
• How to generate audit-ready, compliant event records

If your facility is working toward defensible, plant‑wide PRD compliance under the EPA HON regulations, this practical guide is an essential read.

Download the full whitepaper: Solving the PRD monitoring challenge under updated EPA HON regulations