How to size soft-start and dump valves correctly?
Soft-start and dump valves deliver controlled filling and fast, safe exhaust of pneumatic systems. This guide explains sizing to EN ISO 4414: port size and flow, switching pressure, exhaust time and correct integration into the safety chain.
View soft-start valvesWhat do soft-start and dump valves do?
A soft-start dump valve combines two safety functions in one unit. On start-up it fills the system slowly through a throttled bypass until the switching pressure is reached. The full cross-section then opens for the working flow.
On shut-down or in an emergency the valve isolates the compressed air and simultaneously opens a large exhaust port to atmosphere. Cylinders and lines become depressurised in a fraction of a second, so no uncontrolled motion is driven by stored energy.
How to combine filter, regulator and soft-start into a compliant unit.
Read the guideHow do you size port and switching pressure?
Flow, port size and the switching point are decisive. The port size (G1/4 to G1) follows the system flow so the pressure drop across the valve stays low. The switching pressure is usually around 50 % of the working pressure.
- Sum the flow of the largest consumers first, then choose the port.
- Set the fill time so cylinders do not extend abruptly.
- Check the switching pressure: full section should open at ~50 %.
- Exhaust port large enough to release residual energy fast.
How is the valve integrated safely?
Under EN ISO 4414 depressurised switching is part of energy isolation. The dump valve is therefore mounted as close as possible to the system inlet, directly after the air preparation unit. Electrically it is wired into the safety chain so an emergency stop reliably exhausts the system.
For higher demands, redundant monitored valves with feedback are used. The achievable safety level is assessed to EN ISO 13849‑1 (Performance Level) and matched with the rest of the control system.
- Mount close to the system inlet, after filter and regulator.
- Wire the electrical control into the emergency-stop chain.
- For high PL choose a monitored, redundant design.
- Check and document the exhaust time regularly.
Frequently asked questions
Why should the system be filled slowly?
If a cylinder sits at end position it can extend uncontrolled during sudden pressure build-up. The soft-start valve fills throttled up to the switching pressure and so prevents dangerous motion per EN ISO 4414.
At what pressure does the valve switch to full flow?
The switching pressure is typically around 50 % of the working pressure. Up to that point the throttled bypass fills; then the full cross-section opens for operation.
How fast must the system exhaust?
Usually in under one second. For that the exhaust cross-section must be at least as large as the supply and the silencer must pass air freely.
Is a simple valve enough or is redundancy needed?
For simple applications a single-channel valve is sufficient. At a high Performance Level to EN ISO 13849‑1 monitored, redundant valves with feedback are required.
Looking for the right soft-start dump valve?
We supply soft-start and dump valves in all port sizes - sized for gentle filling and fast depressurised switching to EN ISO 4414.
Standards compliant
Sized to EN ISO 4414 for safe pneumatics.
Fast to zero
Large exhaust port depressurises in under one second.
Two functions
Soft-start and fast exhaust in one unit.
Expert advice
Our specialists support you with sizing.