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Replace Press Fit Component

10/10/2011 10:49 AM

I have been asked to replace a small pressure relief valve with a seal/plug. The valve is composed of a brass case and some internal components, essentially a hollow cylinder with a spring loaded seal inside. The valve is press fit into a cast iron piece.

I have gone through the calculations and found dimensions for a plug that yield a similar assembly press force but I am wondering if this is best way to find a component with equivalent assembly properties. My concern is that the valve is hollow whereas the plug will be solid which changes internal stresses, among other things.

Would it be better to compare the ratio of assembly force to diameter, internal stresses, or something else?

Perhaps I'm overthinking this but, as I have limited experience with interference fits, I wanted to request input. I searched for similar situations on CR4 but didn't find anything.

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#1

Re: Replace Press Fit Component

10/10/2011 3:59 PM

Please tell me you arn't simply removing a pressure relief valve from the system. You are replacing this valve with something else that perform the relieving function... right? Just in another location? The relief valve is there to prevent catastrophic failure of the system, and is likely a needed component.

You could get a good welder to weld a NPT bung onto that cast iron... but he needs to be a darn good welder.

Now for the important question:

What kind of pressures are you dealing with?

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#2
In reply to #1

Re: Replace Press Fit Component

10/10/2011 10:05 PM

Thanks for your reply.

I'm not sure of the reasoning behind removing the valve, perhaps for testing or perhaps the valve is being moved, but I'm not sure. All I got was an email with two sentences asking me to come up with a concept for sealing the port.

As far as pressure, its a pressure relief valve designed to open at 50psi.

I originally thought a plug made of the same material (brass) and similar press fit force would be acceptable but then I realized the press fit calculation for the valve assumed a hollow diameter whereas a plug would only have outside diameter. This causes the dimensions of the plug to be different from the valve in order to get the same press force. I was wondering if this will affect the components as far as stress of perhaps something else I'm not thinking of.

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#3
In reply to #2

Re: Replace Press Fit Component

10/10/2011 10:28 PM

I sure would ask a lot more questions before you commit to doing this. Too many open issues to proceed with confidence something bad's not going to happen.

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#4

Re: Replace Press Fit Component

10/10/2011 11:49 PM

Is it against the regulation if we fix a gate valve between the cylinder/tank and the PRV?.

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#5
In reply to #4

Re: Replace Press Fit Component

10/11/2011 12:22 AM

Yes, it is against the regulations in my jurisdiction, but local regulations could vary. One way around this is a three-port valve that allows one of two relief valves to be isolated while leaving the other connected to the vessel.

To the OP, there are some cases in which a rupture disk or fusible plug can substitute for a relief valve, but it depends on specific circumstances and local Codes.

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#6

Re: Replace Press Fit Component

10/11/2011 8:32 AM

Without getting into the reasons why, or the regulations. Depending on the pressures you will be dealing with, freeze plugs that are pressed into cast iron engine blocks have worked well for decades.

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#7
In reply to #6

Re: Replace Press Fit Component

10/11/2011 10:41 AM

Ah, that's a detail I forgot to mention. I looked at using something like a freeze plug or a a cup plug. But after looking at the details, the mating surface of the interference is only 4mm deep which makes for a very shallow cup.

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#8
In reply to #7

Re: Replace Press Fit Component

10/11/2011 11:37 AM

Well, since the pressure relief valve is already in place, do you have to replace it?

My thought is to simply cut it off, remove the guts, thread the remaining brass cylinder and screw a plug into that. Or does it have to be flush with the cast iron?

If you could pull this off, you could probably use a common size brass bolt/screw as the plug.

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#9
In reply to #8

Re: Replace Press Fit Component

10/12/2011 7:25 AM

I'll look into this. I'm not sure if this is a one-off for something like test or if this will be a change on the production line but I will check on it.

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#10

Re: Replace Press Fit Component

10/12/2011 7:26 AM

Thank you to everyone for your comments and input.

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