Carbon Steel RF to Carbon Steel FF Flange Connection a Poor Practice? 1

[DGrayPPD]

Although mechanically I know that this is okay, it is my understanding that connecting a raised face flange to flat face flange is a poor piping practice and should be avoided regardless of what material the flanges are.

Am I correct?

 

[pennpiper]

Give some thought to which Gasket you will use. I don't think anybody makes a half Raised Face (one side) and half Flat Face (other side) Gasket.

You also did not give us any detail on the:
- Commodity
- Operating Pressure
- Operating Temperature

Sometimes its possible to do all the right things and still get bad results

 

[DGrayPPD]

It was just a general question. My current application is a fire water system with FF tie-point flanges and I am being asked to bolt with RF flanges because they are both CS and I am trying to tell them that although it is okay, it is not a good practice for examples like you have given with the gasket issue.

My other thought is that, why would someone deliberately bolt two different flange facings together when they don't need to?

Op. Temp = 80 deg. F
Op. Press = 20 PSIG

 

[rconner]

fyi

http://www.eng-tips.com/viewthread.cfm?qid=290249

 

[DGrayPPD]

I read the forum but it pretty much did nothing to answer my question. I basically got the notion that there could be confusion with the gasket selection, making sure the flange surface finish was correct, and a couple other things.

As I stated, I understand that it CAN be done, I'm just curious if it is a bad practice and why anyone would deliberately select a raised face flange over a flat face flange if they are tying into a flat face flange. Seems much simpler to avoid any potential confusion with gasket selection, surface finishes, and relaying the message 10 years from now when maintenance has to disconnect the flanges and sees two different flange facings.

All can be avoided by just mating FF to FF regardless of material. It's just my opinion but I still appreciate any opinions telling me why otherwise.

Thanks again for the reference thread.

 

[Gator]

How about this: there's no FF flange in the CAD catalog database. I'm only partially joking.

 

[moltenmetal]

FF to RF is a definite problem when one of the flanges (the FF one) is CAST IRON.

When both are ductile materials, it's really not a problem unless the people doing your fitting are incompetent.

 

[rconner]

<<<why anyone would deliberately select a raised face flange over a flat face flange if they are tying into a flat face flange>>>

Availability? Price? Necessity, or at least perceived necessity (e.g. they can get the item quickly, or off the shelf with raised face, but have to wait for flat)?? A raised face flange after all requires less facing/machining than a full/flat-faced flange.

While flat-face to raised face has been used for decades for such reasons, it can be argued a bad practice for sure when too strong bolting may result in breakage of a very strong flange material bolted to a weaker or less tough material. It is harder to argue against a practice that works.

 

[LittleInch]

If your two flanges are carbon steel and your gasket is good enough then there is no issue - the previous post noted has all the arguments.

The poor practice is simply that the uneducated look at it and either condemn it as not being allowed or think that if it's ok in that location then it's ok in other situations where the material is different ( but looks the same) and where you can overstress the flange (classically class 125 cast or ductile Iron FF flanges).

It's difficult to show in the field that both materials are C Steel and it's Ok so will / could cause confusion and alarm in 10 years time when everyone has gone away and then a check or breaking of the flange shows up this apparent anomaly.

So is it poor practice? Weellll no, but I would try and avoid it if possible simply for the long term consequences. It also looks better.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DGrayPPD]

Thanks guys, that helps.

 

[KevinNZ]

The problem others have indicated is the bending in the flange caused by the bolts bending the flange at the edge of the gasket/raised face. The RF flange would be design for this, the FF flange may be only designed for a full face gasket where is no edge to bend the flange about. If the FF gasket is a different material or thinner than the RF flange be warned.

 

[Gator]

moltenmetal had it with cast iron FF flanges, which are often found on pumps. Crack an integral pump flange and you have real problems.

 

[curtis2004]

DGrayPPD,

You said it is CS FF flange. As all people pointed out, it is usually cast iron pump or other equipment which comes with FF flange. You have to buy RF flange and cut off RF in order to get matching FF flange. So it cost more money.

Do you know why other side of tie in is FF?

This simple doesn't make sense to me...

Curtis

 

[DGrayPPD]

Curtis,

It is a vendor supplied fire water pump house with single stage horizontal split case pump and all vendor supplied suction, discharge, relief, and drain piping inside the pump house is galvanized carbon steel. All vendor tie-point flanges are also FF galvanized carbon steel flanges.

 

[MickMc]

In water systems I occasionally see CS piping, sometimes lined depending on service, with Ductile Iron Valves therefore all flanges specified FF and that is what is specified in purchase order.

 

[LittleInch]

I'm still struggling to understand why someone else is being so obstinate that they won't let you specify a FF CS flange to mate with the FF CS flange being supplied by the package vendor??

Can't you just ask the package vendor to supply a loose FF flange to fit?

I'm not surprised the pump vendor just uses FF flanges regardless of material because then he doesn't have this issue.

I would suggest you join him.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DGrayPPD]

I have no problem specifying FF flanges. I have included them in my design. And ultimately, I am responsible for the final decision. My rule will still continue to be FF to FF and RF to RF whenever possible.

What it ultimately boils down to is lack of knowledge on the everyday issues involved with certain design practices. Some people only focus on if it CAN be done instead of focusing on if it SHOULD be done and what implications come along with designing something a certain way. I prefer to take the route that makes more sense and avoids more confusion.

And luckily, I get very valuable tips and advice through eng-tips that continue to aid me in areas that I may need second opinions.

 

[LittleInch]

Good to hear it, hope the discussion was useful.

LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DGrayPPD]

LI,
You would be surprised how often I have to talk coworkers out of poor designs or practices.

 

[Gator]

It would seem that a lot of common knowledge and common sense has been driven out, possibly due to increased reliance on software and commoditization of design. There also seems to be a lot less mentoring going on, with "leads" often being (and preferring to be) administrative rather than technical, and it's difficult to mentor someone when they are 10,000 miles away in a different time zone. More experienced people are being funemployed in favor of fast-fingered software users with 10 years' experience in 5 year-old programs (but only from one specific vendor, and only experienced with version 5.355 R2.7x).

Do we have a mass-production mentality applied to process plant design even though almost all are custom designs?

I'll now go back to my cave, mumble to myself and wait for lawn trespassers.