Stay Safe with a Soft Start Valve

Soft-start dump valves allow downstream pressure to gradually increase on initial start-up before fully opening to deliver line pressure. The dump function allows the system to be exhausted quickly when required.

Do you design or use machines with air cylinders that have a 6” stroke or longer? If so, it is possible for someone to get injured or the equipment to be damaged without a soft start valve. Please use one located with the main air filter/regulator unit.

A soft start valve prevents air cylinders from slamming back to their home position after the main air supply is exhausted from the machine. Typically at the end of a workday air is exhausted from the machine, or it is exhausted when the maintenance team needs to make repairs or service.

Air cylinders mounted vertically will most likely have dropped down due to the loss of compressed air and/or horizontally mounted cylinders may have been moved to service the machine. As soon as the main air is turned back on, the air cylinders will accelerate and slam hard to their home position. The air cylinder has no back-pressure, so there is nothing to slow them down and the flow controls will not work. Longer air cylinders (6” or greater) have the distance to gain a lot of speed and are likely to cause injury or damage.

By including a soft start valve with the other main air preparation devices, you now have a pneumatic component that will slow the air pressure being introduced to the equipment. Therefore reducing the air cylinder speed to prevent hard slamming to a home position.

Soft Start Valve Circuit – Cylinder Position

The vertical air and load have fallen because the air pressure was evacuated from the machine. Luckily, a soft start valve, 3-way normally closed, is installed in front of the solenoid and cylinder. In this de-energized state, the main air supply feeds the normally closed port of the soft start valve, which means downstream there is no air pressure.

Soft Start Valve Circuit – Return to Home Position

When the main power is turned “ON” the soft start valve will allow the compressed air to bleed slowly into the machine and allow the air cylinder with load to return slowly to its home position.

Slow Start Valve Circuit – Full Pressure

The soft-start valve will fully open after the air pressure has reached approximately 70% of set air pressure and allow the compressed air to flow normally.

There are many different sizes and voltages of Soft Start Valves available, and I would be happy to help size one for you. In these diagrams, the selected soft start valve is a 3-way normally-closed valve. So when power is removed, all the compressed air downstream of the valve will be dumped through the exhaust port of the soft start valve. This is important in an E-Stop condition when the air cylinders need to stop fast to avoid injury or damage. If you do not want your vertical cylinders to fall in an E-Stop condition, check out my article published on 7/3/2019.

I hope this article was helpful to understand why you would want to use a soft start valve on your machine or equipment. If you have questions regarding this article or have any questions regarding any new pneumatic application – Please e-mail me at [email protected]

Congratulations to September’s Quiz Contest Winner: Robert Aikins

September’s quiz, which prompted the above article, was a contest asking readers how to keep air cylinders from slamming to the home position. Robert Aikins answered, “Use a slow start solenoid.” Robert won a $25 Amazon gift card. Congratulations!

October’s quiz contest asks readers why a rodless air cylinder is still moving after its sensor was tripped? Please join our conversations and provide your answers. Next month’s article will share the best-practice for pneumatic system design and the quiz contest’s winner. Good luck!

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