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Valve Packing Sealing for Emissions Service

Methane (CH4) is the second-largest greenhouse gas emitted in the United States. In 2019, CH4 accounted for approximately 10% of all U.S. greenhouse gas emissions from human activities. Methane’s lifetime in the atmosphere is much shorter than carbon dioxide (CO2), however, it is significantly more efficient at trapping radiation. Pound for pound, the comparative impact of CH4 on climate change is 25 times greater than CO2 over a 100-year period.
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One area that is responsible for the increase in CH4 is leaking equipment in the refinery sector. The largest component that accounts for this leakage are valves that leak CH4 (and sub derivatives called volatile organic compounds) from the valve packing gland. Over the last 30 years, governmental agencies have worked with industry to address these leaks by increasing the implementation of new technology. This focus on new technology has pressured packing manufacturers to ramp up research and development of new materials. State and federal agencies have also tightened standards for maximum leak rate measurements of VOC valves from 10,000 PPM twenty years ago to 100 PPM in most places in the U.S. today. As a comparison, that is like changing the CAFE limits for MPG of cars from 1990 levels of 27.5 to 2,750 MPG today. That is going from New York to Los Angeles on one gallon of gas!

One of the main challenges that needed to be overcome was the lack of a unified testing standard. Twenty-five years ago it was similar to the Wild West with many refineries and packing manufacturers using their own testing standards with varying media (nitrogen, helium or methane at different concentrations), temperature ranges, and thermal cycles. This made it very hard to compare the effectiveness between low emissions valve packings. A few standards organizations created tests for valve emissions to start to tackle this issue. The American Petrochemical Institute (API) has specifically focused on a detailed testing procedure for packing performance in methane. This standard, API 622, has become a benchmark for packing performance here in the United States for a variety of reasons, including the EPA statement that packing performance testing needs to be conducted in methane instead of other gases. This standard became the bullseye for research & development working towards the goal of a <100PPM valve packing.

The API 622 requires five thermal cycles from ambient temperature to 260 C (500 F), 1,510 mechanical cycles, and methane as the test gas. Thermal cycles are done once per day, and are divided into 300 mechanical cycles (150 at ambient and 150 at 260 C), with a final 10 mechanical cycles at ambient temperature where the final leakage measurement is taken. This test has an extreme amount of mechanical and thermal cycles and is designed to challenge packing manufacturers to improve development. Without setting such a high bar it is hard to believe the industry could have achieved the technical changes we have seen.
One important point is that API 622 is NOT a valve test but a packing test, so it would be a mistake to assume that all valves will have the same performance with the same packing. This is because all valves have their own design considerations that affect overall sealing, primarily tolerances and unique valve design. With this in mind, the API turned their attention toward creating a testing protocol that focused on emissions capability by each valve design by manufacturer, API 624. One important factor in API standards is that they dovetail into each other by complementing and building on work previously done. In the case of API 624, one of the requirements is that the packing used has already been tested to the API 622 standard. This is one reason API 624 has fewer thermal cycles and stem actuations than API 622 (The 624 test procedure requires 310 mechanical cycles and three thermal cycles to 260 C (500 F)).

Some of the lessons learned with API 624 have focused on the machining tolerances of components. Methane molecules are small and leak rapidly through small gaps compared to steam and other media. With this knowledge, you can’t use standard gland to valve dimensioning for steam service in emissions service. The main areas of concern are stem and box size tolerance, the packing gland tolerance to the stuffing box and stem, and the stem to the ID of the bottom of the stuffing box. All of these dimensions are critical to successful emissions sealing.

Another sometimes overlooked area of concern that has shown to be extremely important in sealing methane is the bolting on the valve. One of the most important issues that came out of testing is getting an accurate gland load on the packing, and the condition of the bolts plays a big part in that. Lubrication is critical and the use of new bolts compared to used bolts can drastically affect how the valve seals. This is because, in most cases, the applied gland load is estimated based on torque—a measurement not of tension but of force over the threads and nut. When using unlubricated used bolts compared to new bolts the same torque will result in a much lower tension, and therefore an under-loaded gland. One way to mitigate this is to use a load indicator, such as disc spring height, that measures specific tension on the connection.

The combination of the work the refinery industry, regulatory agencies, packing manufacturers, and the valve industry have completed has served to reduce methane emissions. The development of new packing materials, new testing standards, and valve design improvements has allowed the refinery industry to lower its environmental footprint by lowering the emissions of greenhouse gases.


Post time: Dec-31-2021