Flanges or Unions ?

delcrossv

What determines when to use a flange and when to use a union? On installs I've seen it's seems hit or miss. ??? Seems Unions would be longer lasting -not having a gasket.

Hap_Hazzard

It seems to be a matter of personal preference, or how someone was trained. When you find something that works, you stick with it, and they both work, so I guess it depends on which one they find first. ¯\_(ツ)_/¯

JUGHNE

One thing to consider is how big of pipe wrenches do you have?
A 3" union needs at least a 3' wrench.
A 4" union would need a 4' ....maybe 2 of them.
Then compare costs of materials also.

EBEBRATT-Ed

JMHO

Flanges are better but a little harder to work with. Your more likely to find flanges on high pressure steam so that tells you something.

For regular low pressure steam or hot water or chilled water. I would use unions 2" and under, flanges 5" and larger. 2 1/2", 3" & 4" you can use either one

STEAM DOCTOR

Flanges are good for making repairs in  strait runs of pipe where there is no give in the pipe

STEAM DOCTOR

4" is biggest that I have worked on. Always use unions, besides for above mentioned scenarios

ScottSecor

When using steel threaded pipe, we use unions on 2" inch pipe and less. For threaded pipe between between 2.5" and 4" we typically use flanges. For five and six inch threaded pipe, we always use flanges.
When we weld pipe (almost always above 2') we always use flanges.

With copper we use unions for 2" and below. For 2.5" through 6" we use flanges.

ChrisJ

Are flanges allowed on gas piping?

ScottSecor

Yes, flanges are allowed on gas piping. As a matter of fact most commercial gas meters here in NJ are flanged. For example we see these regularly

SteamingatMohawk

Which handles pipe misalignment better, flanges or unions?

JUGHNE

The gas valves on all the ModCons I see have a "flange" for the pipe connection.
2-3 Allen screws with O ring.

Some HW gas valves have a flange adaptor for outlet increaser to the burner manifold.

ScottSecor

I would suggest flanges are better at being able to align things up. We've attempted to fix leaking unions many times, rarely with good results. We specify and install these relatively often, they only come flanged in the larger sizes.

dopey27177

Flanges are great when you install equipment such as boilers, heat exchangers, turbines and generators.
Should some of this equipment need maintenance (disassembly) spool pieces installed allow for disassembly or short term relocation to effect special maintenance which is nearly impossible to do when unions are installed.

Jake

JUGHNE

I reused some 4" flanges for steam project.
The most work was cleaning the old gaskets off after about 70 years.
Serious brush grinder.

New gaskets were coated with "Anti-seize", both sides, as recommended by people here.

Hoping to not have to find out if that helps for removing.

EBEBRATT-Ed

@JUGHNE

My brother hooked me on "Never Seize". he worked in a hospital boiler room for 18 years. 100 psi steam He said fitting he put together at the begining of his time came right apart 15 years later. Never seize is good pipe dope too regardless of what anyone else thinks. I put it on gaskets and bolts (especially tankless heater bolts that always break years later)

Also steel to aluminum threaded joints (gas valves) it prevents galling and ripping out the aluminum threads.

As far as unions go you used to be able to buy "Dart" unions which were heavy duty and the manufacturer said they would not leak even if the pipe s were misalligned,

They had a rounded face, I don't think they make them any more.

The other think I have done in some more critical spots is to buy 300# unions heavier and better quality


One thing I have found when using old flanges over, wire brushing is great except if the old gasket contains asbestos.

I had 22 flanges to clean once, they were all welded in so the cost of replacement was high.

I took an oxy actyelene torch and cooked the hell out of the gasket and it woud flake off,,,,,,,at least most of it. kept the wire brushing to a minimum

EBEBRATT-Ed

Dart Unions

They are still made. I think Anvil bought the rights to these as I recall someone in Rhode Island used to make these but I could be wrong, usually am.

Check out the attached if you want a union that won't leak. They are larger you need bigger wrenches. Page 73 at the bottom of the page

EBEBRATT-Ed

@ChrisJ

Flanges are allowed on gas. In fact although you can buy CI or steel threaded flanges (welding flanges are always steel) CI flanges are allowed on gas in certain sizes although in general CI fittings are not allowed on gas. One of the few exceptions.

delcrossv

So, what are people using for flange gaskets? Kalrez? TFE/P? Paper?

EBEBRATT-Ed

@delcrossv

Regular "Garlock" fiber gaskets are ok for gas, water or low pressure steam. They come in 1/16" & 1/8" thickness.

Don't use them on high pressure steam you need "Flexatallic" gaskets for that.

And don't use "red rubber" on anything is heated they will dry out and crack

ScottSecor

https://www.supplyhouse.com/Matco-Norca-GNAX4-4-Full-Face-Non-Asbestos-Ring-Gasket
We used to call them Durabla or Garlock

ChrisJ

EBEBRATT-Ed said:

@delcrossv

Regular "Garlock" fiber gaskets are ok for gas, water or low pressure steam. They come in 1/16" & 1/8" thickness.

Don't use them on high pressure steam you need "Flexatallic" gaskets for that.

And don't use "red rubber" on anything is heated they will dry out and crack

The red rubber ones I see are silicone?
I can't see that drying out and cracking at steam temperatures?

What about PTFE gaskets?

Hap_Hazzard

PTFE creeps under pressure.

ChrisJ

Hap_Hazzard said:

PTFE creeps under pressure.

No i mean they literally make PTFE gaskets for them.

Hap_Hazzard

ChrisJ said:

No i mean they literally make PTFE gaskets for them.

It can't be pure PTFE then. PTFE isn't crosslinked, so if you compressed it between two flanges it would squeeze right out of the joint, and it doesn't rebound, so after a couple of expansion cycles, the joint would leak.

ChrisJ

Hap_Hazzard said:

No i mean they literally make PTFE gaskets for them.

It can't be pure PTFE then. PTFE isn't crosslinked, so if you compressed it between two flanges it would squeeze right out of the joint, and it doesn't rebound, so after a couple of expansion cycles, the joint would leak.

No idea.

BobC

PFTE does creep under pressure but can be pretty resilient. I worked on oil filled radar power supplies for decades, these had a large C core high voltage (20KV) transformer that was in the oil bath. That transformer was bolted through the bottom of the case with 4ea #8 SS flathead screws. The countersunk holes were a little oversized and we used teflon shoulder bushings made for insulating TO3 transistors. They were tightened till they started to "flow" out from under the flathead screw.

Those cans were subject to some vibration and thermal cycling but after decades of use they did not leak. They were built to military specs and used all over the world, we built the last 5 units in 1997 and shipped them over to the Suez canal. We tried nylon shoulder bushings but they usually cracked and leaked. They were advertised as teflon but I don't know for certain they were just teflon, I think we got the bushings through Cramer Electronics at the time. This is a heaver duty version of what we used -

https://www.mcmaster.com/ptfe-shoulder-washers/

Bob

Hap_Hazzard

I think the rationale for using PTFE in that application is that it can be compressed almost infinitely without tearing. It will squish down to 1 molecule thickness and then resist further compression. That's why they tell you not to use PTFE tape on LWCO probes. Unless you get lucky and there's a burr that cuts through and makes contact, that single molecule layer of PTFE insulates the probe from ground. The only part I'm having trouble with is that PTFE is really not very resilient. When you stretch plumber's tape, it doesn't pull back like a rubber band.

Pumpguy

Where common red rubber or fabric inserted rubber gaskets were used to connect flange mounted vacuum and condensate pumps to receiver tanks, I have been using Ethylene Propylene Rubber (EPR) gaskets with great success. EPR elastomer is good for 300* F. and is resistant to Amine based water treatment chemicals.

At 400* F. VITON is even better, but VITON is not recommended for use where AMINE based water treatment chemicals are used.