Maximum Working Pressure in Steel/ Brass Fittings

[Roy S Ramirez]

TL;DR Summary

Is it safe to operate a fitting slightly above its "maximum operating pressure"

Hello everybody!

I bought some fittings for a feed system I'm building, and I need to handle pressures between 150 and 200 psi. I noticed some of them have max working pressures of about 1000 or 3000 psi, but others have relatively lower ones (150 psi, see https://www.amazon.com/dp/B003JQJST6/?tag=pfamazon01-20)

So I have the following questions: is the max working/ operating pressure the same as the "burst pressure"? Is there a safety factor taken into account when deciding these pressure ratings?, and thus, would it be safe for me to use a 150 MOP fitting at about 185 ish psi ?

Thanks!

Roy

 

[Asymptotic]

Summary: Is it safe to operate a fitting slightly above its "maximum operating pressure"

is the max working/ operating pressure the same as the "burst pressure"?

No. Burst pressure is the pressure at which the fitting is expected to fail.

Definitions per Wheatland Tube
https://www.wheatland.com/wheatland-standard/review-test-burst-working-pressures/

would it be safe for me to use a 150 MOP fitting at about 185 ish psi

By definition, no, and is almost certainly a code violation. It makes more sense to use schedule 80 and not have to worry than to go with schedule 40 and have one foot already on a banana peel.

 

[jrmichler]

You need to be careful with pipe and fitting pressure ratings. For example, a 150 lb class malleable iron fitting is rated for 300 PSI at temperatures up to 150 deg F, and 300 PSI at 350 deg F: https://www.engineeringtoolbox.com/iron-threaded-fittings-d_855.html. Note that "malleable iron" is not the same as "cast iron".

Fitting pressure ratings come under ANSI B16 standards, and there are a lot of them: https://www.engineeringtoolbox.com/ansi-b16-pipes-fittings-standard-d_215.html. If you want a copy of the ANSI standards, start here: https://webstore.ansi.org/standards/asme/asmeb16pipeflangesfittings. In your case, your university library should have a copy or be able to get you a copy.

For steel pipe, start by searching maximum working pressure schedule 40 steel pipe. That will give you allowable working pressure for the pipe, however that may not apply to threaded ends. Keep looking.

 

[Roy S Ramirez]

Thanks! I'll try to get fittings that are rated for the pressures I want to deal with. But just out of curiosity, supposing the fittings are well tightened, what are the risks of flowing a fluid at 180ish psi when the max pressure is 150 psi? Could a stainless steel fitting just blow up?

 

[anorlunda]

Thanks! I'll try to get fittings that are rated for the pressures I want to deal with. But just out of curiosity, supposing the fittings are well tightened, what are the risks of flowing a fluid at 180ish psi when the max pressure is 150 psi? Could a stainless steel fitting just blow up?

I think you have the wrong idea about safety factors. One purpose of safety factor is to allow for uncertainty and variability in the devices. Therefore, there can be no single numeric answer to that question.

 

[JBA]

You need to be careful with pipe and fitting pressure ratings. For example, a 150 lb class malleable iron fitting is rated for 300 PSI at temperatures up to 150 deg F, and 300 PSI at 350 deg F:

There is a misquote in the above (see ref document), it should read: "... and 150 PSI at 350 deg F:"

 

[Asymptotic]

Thanks! I'll try to get fittings that are rated for the pressures I want to deal with. But just out of curiosity, supposing the fittings are well tightened, what are the risks of flowing a fluid at 180ish psi when the max pressure is 150 psi? Could a stainless steel fitting just blow up?

It depends on layout, installation and service conditions. For example, components in a line that cycles on once per day, and operates at 180 PSI, +/- 5 PSI with a consistent fluid temperature of 30°C are much less stressed than those in a circuit that cycles twice per minute with significant surge pressures and wide temperature variations.

A poorly supported pipe network is more prone to failure because the fittings encounter more force, and force variations than a properly supported system, and so on.

 

[jrmichler]

@JBA: Good catch. This was one of those times when my typing was faster than my proofreading. Glad to see that somebody was paying attention.

And do not use numbers from Ebay or Amazon when looking at pipe and fittings. Size 1/2" NPT Schedule 40 pipe is 0.62" ID and 0.84" OD.

 

[Roy S Ramirez]

Thank you very much to all of you. My system is meant to run for only 3 seconds perhaps twice or three times a day. I'll do my best to follow your advice!