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I am designing a resin dosing & mixing room for a production plant. This will use pneumatic diaphragm pumps to pump the resin & additives, flow meters to measure the dose, and pneumatic ball valves to shut off flow when the correct does is reached.

Talking with pump suppliers, Google etc, everyone suggests that pressure relief valves should be incorporated, since the system can potentially shut off flow via the pneumatic ball valve while the pump is still pressurising the line. This seems like a logical safety measure to me as well. However, the experienced maintenance fitters at my work say that if the fittings and pipe in the process line are all rated higher than the pump output pressure, there is no need for PRV because the pump will just reach dead-head pressure and stall. This reasoning seems sound to me, and it would reduce a LOT of cost and complexity to remove the PRV return lines (about 14 in total), but it also feels uncomortable removing a safety measure which seems like common practice to include.

System specs for reference: Pneumatic diaphragm pump maximum output = 115psi (needs 120psi air supply to achieve this), Air supply = 100psi maximum, poly process line MOP (safety factors included) = 115psi, poly compression fittings = 180psi, The rest of the components (SS fittings, flow meters etc) are rated to at least 1000psi.

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    $\begingroup$ You really need to post a set of prints to answer this question. Wouldn't a simple high pressure cutout switch upstream of each ball valve handle the issue? (not for normal pump shutdown, just for misadventures) $\endgroup$
    – Phil Sweet
    Commented Jul 27, 2023 at 0:00
  • $\begingroup$ Any places where the ambient temperature will derate the pipe? $\endgroup$
    – Solar Mike
    Commented Jul 27, 2023 at 5:56
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    $\begingroup$ What about: Pump (including air supply) is replaced/repaired upgraded and can produce 1.5 to 3x current specs (or already can exceed the listed specs by that amount). Poly compression fittings that are below spec (due to wear, environment, improper part replacement, etc.). $\endgroup$ Commented Jul 27, 2023 at 20:54
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    $\begingroup$ I'd also be worried about legal here - if it's a common safety feature for all these systems, and you leave it out and something goes wrong? You've put the burden of proof on you to prove, conclusively, that your design was safe and did not need this feature. Doing the work to show this might cost more than just adding in the safety valves. $\endgroup$
    – lupe
    Commented Jul 28, 2023 at 11:12

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Not a plumbing engineer, but:

What about thermal expansion?
You reach a dead end, pump stops. Pumps often have a checkvalve that prevents backflow, so you have now created a pressurized container.
Imagine a small fire breaks out.
Would you rather have the contents of the pipe heat up and shoot off a dedicated pressure relief valve in controlled area, or have the pipes burst with potentially flammable materials?

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  • $\begingroup$ if a fire breaks out, its game over for the whole system. We have organic peroxides in the mixing room, so the whole environment is hazardous zoned with multiple redundancy. The pipes are polyethylene, so pressurised or not, they will melt and leak before thermal pressure can build up. $\endgroup$ Commented Jul 28, 2023 at 0:04
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    $\begingroup$ @KendricRendle-Short It doesnt have to be literal fire. Could be just temperature gradient between cold pipe in the morning and hot pipe at the noon. Something to consider. $\endgroup$
    – Thomas
    Commented Jul 28, 2023 at 6:32
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Certainly not all piping systems need relief valves. The specifics of the application matter. Relief valves typically protect vessels that can store significant energy and release it violently at rupture. Pipes are seldom such a risk.

Systems at risk of low/no flow conditions that could damage a pump (like feedwater pumps) typically have recirculation loops, possibly via an orifice, to prevent zero flow. Hydraulic systems with positive displacement pumps kind of use a relief valve to set system pressure and provide the recirc function. Your industry might have different standards, but at my paper plant I wouldn't have hesitated to put in piping systems without relief valves.

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  • $\begingroup$ AODD pumps are not at risk from zero-flow conditions, so I'm not worried about stress on the pumps. $\endgroup$ Commented Jul 28, 2023 at 0:07
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Another factor to consider--hammer. The valve closes while there's substantial flow, the pressure is going to spike above what the pump can produce.

And just because the pump has a max output pressure are you sure it can't produce more if something happened and it got fed more air than it should be getting? Sure the pump fails before the pipe?

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