Testing of Hydrogen Valves

Body-bonnet connections must be assembled correctly and gaskets selected and sized properly to ensure that small-molecule-sized gases are contained during helium testing and in hydrogen service.

With the excitement and optimism over clean and green hydrogen fuel cells, multiple industries are working to make hydrogen a key in future energy planning. Almost all the processes to accomplish this goal involve piping and valves operating at high pressure and in most cases high temperatures.

The challenge is controlling the flow of these tiniest of molecules without perceptible leakage. Substantial leakage of hydrogen can be a dangerous situation, because hydrogen in conjunction with oxygen and ignition source burns very aggressively, witnessed by heavier-than-air, hydrogen-filled dirigible disasters, such as the Hindenburg at Lakehurst, NJ in 1937.

Leakage in any valve is characterized in one of two ways: leakage across the closure members (seats, disc, gate, etc.) or fugitive leakage through the pressure-retaining shell, around gaskets, or through the packing, if so equipped. Leakage from either method is even harder to mitigate when dealing with hydrogen’s tiny molecular size.

Fugitve-emsissions leakage through the packing or between the stem and packing can be cause for concern. Sealing helium or hydrogen is tougher because of the tiny molecule size, so stem straightness and smoothness is critical during helium testing and in hydrogen service.

Any hydrogen-service valve user expects the valves they purchase to perform as required. The expectation is that leakage will be zero. Another challenge with valves designed for a gas like hydrogen is proving they don’t leak before they leave the factory.

HELIUM TESTING

In the industrial valve sector, the most stringent leak-testing techniques involve safe-to-use helium as a test medium. Helium tests can be monitored via visual cues such as bubbles or with leak detectors such as mass spectrometers. Helium testing is a mature test method and has traditionally been used to verify the performance and leakage rates in valves carrying more viscous fluids or needing tighter scrutiny. The thought being that if it will hold helium (with its very small molecule size), then it will surely hold the fluid that will be passing through the valve while in service. However, in the case of hydrogen, there is no smaller-molecule-fluid to use as a tougher-to-seal test fluid.

The primary test gas utilized for testing valves in hydrogen service is helium. Even at low pressures, helium gas can find the smallest of leak paths.

Another issue with utilizing micro-molecule gases for testing is their permeability through materials such as seals and gaskets. A gasket or packing that may hold pressure easily with a liquid or a gas such as nitrogen may allow hydrogen to pass easily through its cross-section.

Decades of valve experience in the petroleum sector with valves in high temperature and high-pressure hydrogen applications have proven that valves in that service often need tougher than normal pressure tests and sometimes even additional volumetric inspections such as radiography to ensure their soundness.

METAL-SEATED BALL VALVES

A popular valve design for high-temperature, high-pressure hydrogen service, such as encountered in hydrogen separation facilities or fuel cell charging stations, is the metal-seated ball valve. The standard design test for this type of valve designed and manufactured in accordance with American Petroleum Institute (API) 608, Metal Ball Valves—Flanged, Threaded and Welding Ends, requires testing per API 598, Valve Inspection and Testing. This calls for a required seat test at 90 psi using inert gas. Optionally, a water test at 110% of the rated working pressure may be called for as well. Neither of these prescribed and optional closure tests are much good at verifying the sealing integrity in hydrogen service. As an alternate, the test fluid may be changed to an inert gas for these tests.

By changing the test fluid to helium, the test comes closer to emulating the containment of hydrogen. However, the consensus from many in the industry is that the prescribed testing times at the API 598 test pressures may not be stringent enough. The only options at this point, if requiring zero leakage, are to increase the pressure holding time and/or utilize a much more precise leak detection method, such as a helium mass spectrometer.

The Manufacturers Standardization Society (MSS) created a standard practice designed to provide a more sensitive test method for valves in critical services such as hydrogen. Standard Practice (SP) 158, Supplemental High-Pressure Gas Test Procedures for Valves, provides procedures, including holding times, much greater than those prescribed in API 598.

To adequately test valves for hydrogen service, the use of helium gas as a test fluid and extended pressure holding times may be required.

The second issue involves testing of the entire pressure envelope of the valve, including the packing and gasket. In the case of the API 608 designed metal-seated ball valve, this test is normally performed at 150% of the rated working pressure with water, although inert gas may be specified. For hydrogen, helium is again the best choice. An extended duration high-pressure gas test procedure for the verifying the integrity of the valve pressure envelope is also detailed in MSS SP-158.

Fugitive emissions testing per se is not required of valves in hydrogen service. Although hydrogen is not a greenhouse gas, it has the energy potential and volatility to blow up green-houses, out-houses or high rises.

Because of the permeability of the helium into non-metallics, care must be taken when taking readings of stem seals or non-metallic gaskets with a helium mass spectrometer, as helium gas can enter these materials and affect the instrument’s readings for a substantial period of time after the initial pressure charge.

HYDROGEN PIPELINE VALVES

One of the initial issues within the growing hydrogen energy sector is the transportation of hydrogen gas and pipelines are the most logical mode of transport. There are pipeline companies currently transporting pure hydrogen in newer dedicated hydrogen pipelines as well as pipeliners blending hydrogen with natural gas in existing pipelines. The added flammability and danger of this activity has caused some users to invoke the tougher options present in the current API 6D, Pipeline Valves, design and testing document. These additional tests include the use of inert gas, combined with the already longer API 6D holding times.

Currently no hydrogen-specific valve design or testing standards exist in North America. With the increasing interest in the hydrogen and carbon capture and sequestration (CC&S) industry, various groups outside the valve industry realm are hinting at creating valve and valve-testing standards for these green and growing energy sectors. It behooves the valve standards development organizations to take the lead and create these specifications based upon their extensive valve knowledge and experience, rather than let a group of bureaucrats or scientists dream them up.

The future of hydrogen energy use and the design and construction of hydrogen-specific piping systems is bright. The opportunity for valve manufacturers is also excellent. Since we are still near the embryonic stage of the venture, it is important that the end users, manufacturers and regulating agencies work together and get the details right. While testing procedures are not the center of the discussion, agreement on testing protocols could go a long way in ensuring the budding industry’s long and especially safe future.

About the Author

Greg Johnson is president of United Valve. He is a contributing editor to VALVE Magazine and a current Valve Repair Council board member. He also serves as chairman of the VALVE Magazine Advisory Board, is a founding member of the VMA Education and Training Committee and is pas president of the Manufacturers Standardization Society. Reach him at greg1950@unitedvalve.com.