Evolution of torque technology in Assembly and Testing of Gas Cylinder Valves

On 16th April 2021, an overseas customer reported leakage through the Pressure Relief Device(PRD) threads in sixteen pin index medical cylinder valve threads. Even before investigating the complaint, our engineering team ruled out the most probable cause of leakage in such incidents – inaccurate tightening of the PRD plug at the factory. The confidence that the PRD has been installed accurately at the specified torque of 7 Nm (+/-3%) during assembly stems from the use of corded DC Nut runner tools for torque tightening of every component into the valve body.

Like most things in life and business, we were introduced to electric torque tools in 2005 by accident. The initiation happened while discussing the purchase of additional pneumatic torque tools for our growing assembly needs with the sales engineer of Chicago Pneumatic. The sales engineer showcased the impressive and colourful catalogue of DC tools not to make a sale but to make my jaws drop at the technology and price point. He informed me that aircraft manufacturers use these tools to tighten engine parts. My experience was similar to a jeweller showcasing his latest collection for his customer who cannot afford it but would be impressed nevertheless.

The importance of imparting torques in-cylinder valves with high precision has mainly been understated. Understandably cylinder valves cost only a few dollars per valve (90% of cylinder valves are sold for less than ten USD), and there is always pressure to reduce cost. Components that are accurately tightened look precisely the same as inaccurate ones. It is only during leakage or failure that the accuracy of torques come into question.

In the '80s and early '90s, our assemblers were experienced enough to assemble the valves by hand tools. Invariably they got the tightening correct, and even if they got it wrong, our customers were more forgiving. In the mid-'90s, we introduced click type manual tools to reduce operator error during assembly. These devices had a +/- 6% error when new and +/-10% after repeated use and needed to be replaced as they regularly failed periodic calibration.

We soon introduced pneumatic torque tools with a similar level of accuracy to the click type torque tools to improve productivity. Pneumatic tools are set up to impart a specific torque by adjustment using another torqued part as a reference. Their accuracy depends on the pressure of the compressed air pressure required to drive the tools.

Always eager to punch above our weight, we purchased our first DC nut runner tools from Chicago Pneumatic in 2006 for assembly in 2006. Within a few months, we bought additional tools to torque the operating mechanism during pressure testing of the valve assembly. The investment cost us thousands of dollars, but once we raised the benchmark for accuracy in torquing, we have not looked back. The investment in DC nut runner torque tools have made our assembly process error-proof, energy-efficient and flexible. It allows our engineers to adjust tool speed to match various applications and control key parameters such as torque and angle. In addition, they seamlessly support the need for data collection, connectivity and 'Industry 4.0'.

Transducer-controlled corded DC nut runner continues to occupy the top spot in the torque technology chain with +/- 3% accuracy with impact tools with no accuracy lying at the bottom. Other technologies like clutch tools, oil pulse tools, click-type tools and pneumatic tools occupy middle positions and offer +/- 7-15% accuracy.

I spoke to Mr Ashish Kaul, owner of Diya Technologies and a leading distributor of Ingersoll Rand, to know about the latest trends in torque technology before writing this blog. He informed me that the use of precision torque technology continues to be driven by automobile OEM's who use DC Nut runner technology for critical assemblies and force their vendors to do the same. However, even Tier 1 vendors are reluctant to invest in DC Nut runner technology if they can get away.

We both agreed that any design with close tolerances requires accurate torques. More so if the assembly is critical for public safety as with gas cylinder valves. An under tightened valve assembly can loosen over in service and result in leakage through the valve, while an overtightened part would result in high stresses and increase chances of stress cracking.

The investigation concluded that the leakage was due to the mechanical inference of the customer valving tool with the PRD during the valve installation.

Reach me at rb@teknovalves.com