1 June 2005

Wireless relief

Valve monitoring system can offer operational benefits.

By Clifford Lewis

The purpose of relief valves are, simply enough, to relieve pressure and provide safe operation. They typically function by opening at a given set pressure, venting, and then resealing after establishing a safe pressure.

Very frequently, relief valves see use in gas service where the gas vents to the atmosphere or to a safety flare. These valves are frequently installed in remote locations where monitoring of the valves is difficult.

Wireless technology allows for continuous monitoring of these valves without significant capital expense. The non-invasive installation of an acoustic sensor coupled with wireless transmission of data on the relief valve operation provides an easy and inexpensive monitoring solution.

In many monitoring situations, it is best to estimate the amount of gas that has discharged from the relief valve. A theoretical estimate of the discharged gas can be determined by knowing the flow characteristics of the relief valve, the pressure at any given time, and the time duration of the relief event.

This test can conclude how effectively a wireless acoustic monitor could not only determine that a relief valve had discharged, but also estimate the amount of discharge. You can calculate the estimated discharge from the standard relief valve flow sizing formulas and the duration of the event.

In the test that follows, the event duration came from the installed instrumentation. The actual discharge measurement then underwent comparison to the estimated value. The comparison to estimated and actual discharge was very consistent. From this comparison, we can conclude that wireless monitors may reasonably estimate the amount of gas discharged during a pressure relief event.

Test is on

We designed a laboratory test program to validate the effectiveness of the wireless acoustic monitor for pressure relief valve monitoring. There were five objectives of the test:

1) To confirm the acoustic monitor can validate a properly seated, non-leaking pressure relief valve.

2) To confirm the acoustic monitor can reliably detect a valve leak.

3) To confirm the acoustic monitor can detect the opening of a pressure relief valve.

4) To confirm the acoustic monitor can repeatedly and accurately measure the duration of pressure excursions of relief valves.

5) To estimate the discharge of the relief valve using the standard relief valve sizing formulas and compare this estimated discharge with the actual discharge.

The data from the tests confirmed the effectiveness of the acoustic monitor for validating all of these pressure relief valve conditions.

All five of the test objectives showed:

1) The acoustic monitor effectively verified a properly seated, non-leaking valve with the absence of either leakage or overpressure.

2) The test program validated the suitability of the acoustic monitors to reliably detect a relief valve leak.

3) The test program validated the suitability of the acoustic monitors to reliably detect a relief valve overpressure event.

4) The acoustic monitor was able to determine the time duration of an over-pressure event. Using this time duration along with the available process data, you can calculate the mass of fluid that passed through a relief valve within a consistent relationship to the actual flow.

5) The actual flow consistently measured at 10% greater than the calculated flow. This is consistent with the ASME Pressure Vessel Code that requires pressure relief valves actually pass 10% more gas than their capacity size certification sizing.

Using the acoustic data to measure duration of overpressure event along with the actual system pressure at the relief valve, the test consistently produced actual flows that were 10% greater than the theoretically calculated flow, consistent with the ASME Pressure Code requirements for pressure relief valve sizing.

Acoustic monitor mounting on test valve
Acoustic monitor mounting on test valve

A series of tests occurred at an ASME authorized flow laboratory, and an ASME authorized observer supervised the testing. The flow lab sees regular use for capacity certification testing of pressure relief valves and gained accreditation from inspectors from the National Board of Boiler and Pressure Vessel Inspectors (NBBI) to meet the valve certification testing requirements of the ASME Boiler and Pressure Vessel Code, Sections I and VIII. The test laboratory consists of calibrated instruments for measuring pressure, temperature, and flow.

Wireless pressure instrument mounting on test vessel
Wireless pressure instrument mounting on test vessel

This equipment undergoes calibration before and after the test. In addition to the laboratory measuring instrumentation, the lab has the capability to generate and store a large amount of air at high pressure. The lab's compressor rates at 3,000 PSIG, and it can fill nine large high-pressure cylinders with air at 2,250 PSIG. A 2-inch metered line from the high-pressure cylinders feeds a 47 cubic foot pressure vessel drum. The pressure relief valve mounts to this pressure vessel drum. A throttling valve controls the pressure in the pressure vessel drum, and a sharp edge orifice meter monitors the air flow.

The valve was set to open at 120 PSIG. In addition to the laboratory instrumentation, several instruments used during the program included:

  • An acoustic monitor mounted on a 3-inch long straight bracket attached to a valve bonnet stud.
  • An acoustic monitor (SN 100521) mount ed on a 6-inch diameter split clamp bracket attached to the valve body.
  • A pressure monitor (SN 100322) mount ed on the pressure drum immediately below the valve inlet.
  • An acoustic monitor (SN 100427) mount ed on the valve cap using a 3-inch straight bracket for leak test runs only.

The instrument attachment occurs via installed brackets with no penetration or other invasive installation. The acoustic monitor is easy to install in the field under normal operating conditions. In addition to ready installation, acoustic monitors are FM certified for use in hazardous areas including Class I, Div 1, Group A. The objective was to simulate, as closely as possible, the non-invasive mounting that would actually occur in field installations.

Under fire

The first test verified the acoustic monitor can correctly determine when there is no leakage or overpressure flow. For the first test, the vessel drum pressurized to 115 PSIG or 95% of the set pressure, and the pressure maintained for 10 minutes. This "no activity" condition repeated between each overpressure and leakage event. The acoustic monitors transmitted zero or nominal trace ultrasound value of up to two, which is less than 1% of full-scale ultrasound value, validating that no fluid was escaping from the valve.

During the initial 10-minute test, we placed a small tangential force on the valve stem to induce minor leakage. This minor leakage came through as slightly higher ultrasound levels but still in single digits.

The second test was a trial run overpressure event to verify that all equipment was working properly. The pressure relief valve started to "warn" at a pressure of 119 PSIG when it detected an ultrasound value over 20. The valve popped open at a pressure of 121 PSIG, and the ultrasound value registered over 140. Pressure in the drum increased to a maximum level of 148 PSIG and then decayed by slowly reducing air flow to the vessel. The relief valve closed one minute after opening at a pressure of 106 PSIG, and the ultrasound immediately returned to its trace value of 0-2.

The valve characteristic mapped as a very short warn, opening pressure of 121 PSIG and reseat at 106 PSIG for a "blowdown" of 15 PSIG, or 12%.

All instrumentation recorded data. The laboratory pressure gauge was within ±0.1 PSIG of the wireless pressure field unit readings. Ultrasonic values ranged from 143 to 206 on sensor one and 130 to 190 on sensor two, clearly recording the duration of the event with qualitative certainty through each of the mounting brackets selected.

Summary of flow data

One more time

The next overpressure run, Test Run 2, was the first metered event. The valve popped open at a pressure of 121 PSIG. The ultrasound readings were consistent with Test Run 1. Pressure to the drum increased to a maximum of 132 PSIG (10% above opening point), which is the pressure for full rated capacity flow. The actual flow rate metered at this pressure was 892 SCFM. The rated capacity for this valve at this pressure is 811 SCFM. After sustaining steady state flow, the pressure decayed until the valve closed at a pressure of 106 PSIG. The duration of the overpressure event was 1 minute, 52 seconds. The theoretical calculated mass flow for the event was 87.25 pounds of air. The metered mass flow for the event was 96.42 pounds of air, or 110.5% of the theoretical flow. Remember the ASME code requires a de-rating of 10% for the valve, bringing the actual flow and the calculated flow to a very close correlation.

Two additional overpressure events verified the repeatability of results. Test run 3 was the second metered event. We increased the initial pressure, and a valve warning came on at a pressure of 114 PSIG. The valve popped open at a pressure of 121 PSIG. Over pressure added to a pressure of 139 PSIG and sustained for about one minute before decaying. The valve closed at a pressure of 106 PSIG. The duration of the event was 1 minute, 40 seconds. During this run, the actual flow rate at maximum overpressure was 957 SCFM. The actual metered mass flow was 91.84 pounds of air. The calculated mass flow for the event was 83.27 pounds of air or 110.29%. Again, this is an extremely close correlation given the 10% derating factor mandated by the ASME code.

Test run 4 was the third metered event. Slight warn occurred at a pressure of 114 PSIG. The valve popped open at a pressure of 121 PSIG. Pressure increased to 132 PSIG and held for about one minute until decay. The valve closed at a pressure of 106 PSIG. The duration of the event was 1 minute, 28 seconds. During this run, the actual flow rate metered at 133 PSIG was 925 SCFM. The total actual amount of air was 74.03 pounds. The calculated mass flow for the event was 67.29 pounds of air. Again, the actual mass flow was 110.0% of the calculated mass flow, consistent with the capacity derating factor.

The uncertainty analysis of the laboratory equipment predicted a maximum error of 1.6% of metered flow rate. The consistency of calculated versus metered flow was within this variance for each test.

The next series of tests were to validate that the acoustic monitor would detect a leakage condition. A gag restricted the valve stem to inhibit lift of the valve so it could not generate a measurable flow. The system pressure then increased to a point to induce leakage. The acoustic monitors effectively detected the ultrasonic activity associated with the leakage. The leakage event characteristic was different from the overpressure event in the shape of the ultrasound versus time graph. While the overpressure ultrasound curve consistently showed an ultrasonic activity peak just before closing, the leakage event was a steady level of ultrasound until pressure removed from the valve. In practice, leakage will continue until you repair a damaged valve, while an overpressure event will be of limited time duration. These characteristic patterns are more clearly evident in the expanded views of the traces.

By reviewing the ultrasonic date, you can identify the overpressure and leakage events, confirming the effectiveness of the acoustic monitors. CP