If air dumps safely release pneumatic pressure from an industrial machine when an E-Stop is pressed, why is it also standard practice to cut electrical control power from the valves with a safety relay? This would be useful if the air dump failed, but those are already incredibly safe so I feel like there must be a better reason to reasonably justify the added cost and complexity.
The air dump removes pneumatic potential energy from the system (assuming no closed-center valves or check valves). How does a safety relay reduce the potential for motion?
Maybe it has nothing to do with machine safety, but is rather for maintenance reasons? I have seen machines that can be put in "maintenance mode" that leaves air enabled but disables electrical control, and then trained maintenance personnel can enter the machine without fear of it automatically moving for any reason but can still manually move actuators using the override buttons present on most valves.
Cat 4 Dump Valve $\mathrm{MTTF_D}=95 \ \textrm{yr}$
Edit: Actually going through the calculation, I found that Category 3 / Category 4 dump valves are not nearly as safe as I thought. For example, the SMC VP544-X538 dual residual pressure release valve (air dump) is rated with $B_{10\text{D}} = 10\,000\,000 \ \mathrm{cycles}$, where $B_{10\text{D}}$ is the number of cycles until a dangerous failure occurs for 10% of the components. Assuming 100% uptime, a 30 second cycle time, and an operator entering a machine every cycle to load/unload it, we can calculate the dump completes about one million operations every year.
$$ \begin{align} \mathrm{n_{op}} &= \frac{365 \ \mathrm{day/yr}\times24 \ \mathrm{hr/day}\times3600 \ \mathrm{s/hr}}{30 \ \mathrm{s/cycle}}\\ &= 1\,051\,200 \ \left.\mathrm{cycle}\middle/\mathrm{yr}\right.\\ \end{align} $$
Then using the formulas helpfully laid out in the Festo Guideline: Functional Safety (and originally from ISO 13849-1), the mean time to dangerous failure $\mathrm{MTTF_D}$ is calculated at just 95 years.
$$ \begin{align} \text{MTTF}_\text{D} &= \frac{B_{10\text{D}}}{0.1\times\mathrm{n_{op}}}\\ &= \frac{10\,000\,000 \ \text{ cycles}}{0.1\times1\,051\,200 \ \left.\mathrm{cycle}\middle/\mathrm{yr}\right.}\\ &= 95.1 \ \mathrm{yr} \end{align} $$
Assuming a factory has at least 100 of these dump valves, then on average a dangerous failure would be expected every 10 years (assuming only 10% of the components have a dangerous failure).
Although I am slightly confused SMC has the same $B_{10\text{D}}$ rating for both their single-valve Cat 1 and dual-valve Cat 3 dumps. I'm not sure how to account for increased safety from dual valves versus a single valve, so perhaps I am missing something else with the $\mathrm{MTTF_D}$ calculation that would significantly reduce the risk for dual-valve dumps.