Any advantage to having FF flanges on a CS pump?

[timbones]

I am specifying some pumps for the third train of a plant using the second train pump datsheets as a guide.

For some reason, the engineer for the second stage pumps specified flat faced flanges for a pump out of carbon steel. Piping is also carbon steel. The pump is a little 5 hp Moyno progressive cavity pumping a oil/water mixture. 4" suction and discharge.

Personally, I would have gone with raised faced. Is there any advantage to the flat faced design?

 

[pennpiper]

The general "Rule" is:
Raised face for Raised face
and
Flat face for Flat face

Now, having said this there will always be the exception. You have not given any infor mation that points to a possible exception so stick with the "Rule"

 

[timbones]

If I choose FF, the mating will be FF to FF. If I choose RF, the mating will be RF to RF. This is not the question.

Is there any advantage to using FF with a carbon steel pump and carbon steel FF mating flange?

 

[JJPellin]

I am not aware of any advantage with FF. The only thing that comes to mind would relate to how they are tightenend. If you knew in advance that your mechanics were going to drastically over-tighten the flanges using an impact gun, you might have a slightly lower chance of cracking a flange by using full face flanges with full face gaskets. But this would be a silly reason to go FF.

We have some blowers that we purchase with FF flanges. But they have rubber boot expansion joints bolted directly to each flange. The particular expansion joints used are desgined to bolt up to full faces, not raised faces.

Johnny Pellin

 

[dcasto]

Flat face flanges on pumps are typically cast iron, to try and get a raised face flange would be a huge expense.

 

[bingopin]

If the pump material is steel, as it is in this case, I would go with the face configuration that costs the least. If both flanges are steel, there's no difference in RF vs FF and RF vs RF. Different story if the FF is ductile or cast iron.

 

[legs]

dcasto is right on with his expaination. FF are typ on cast pumps.

 

[garfio]

ANSI B16.5 requires that flanges connected to cast iron flanges be FF.

Using two FF steel flanges has no advantage. In fact, it could reduce the pressure in the face of the gasket (depending in the type of gasket used), thus being more susceptible to leaks.

 

[timbones]

Bingopin, I disagree that RF vs FF is acceptable if both flanges are steel. Although the ductility is not a problem, unequal loading on the gasket is. As you bolt the flanges together, the deflection on the outside of the flange from the bolting will induce a moment from the FF flange to the gasket which will not be balanced from the mating flange because it is RF. The result is the joint is more likely to leak.

 

[rconner]

You have mentioned that the "Piping is also carbon steel". If, however, there happened to be an appurtenance of a different material (e.g. lets say a gray cast iron valve) between the discharge steel piping and the pump discharge, it may then be best to have both flat faced flanges and a full-faced gasket in the connection between the steel pump discharge and the appurtenance of different material (to minimize at least the vulnerability of breaking the less ductile flange by cupping in assembly). If that is not the case, I'm not sure like other responders I see any sort of advantage to the flat-faced flange.
You have not stated any of your pressure conditions, but in small piping like 4" at least if the pressures are not very high I'm not sure I see any real performance disadvantage of flat-faced flanges, either.

 

[sterl]

The Cast Iron and ductility/bolt load comments are sound, but the gasket load and clamp force is the real topic. "Soft" gaskets (low durometer, highly compressible)suit low pressure duties with relatively low stress cycles, that is small changes in temperature and small levels of implied pipe movements. Raised face flanges apply much higher gasket loads for a given bolt size/torque/tension and suit gaskets with higher bulk compression coefficients, for high pressure, high initial stress (bolt tension) duty.

This is covered in the ANSI B16.5 Appendices and repeated in a variety of detail levels by the gasket and in certain cases, the bolt manufacturers. There is also considerable train on the topic in the B & PV section of Eng Tips.

 

[pumpking]

The raised face flange will, as previously highlighted, give more force on the gasket material and is easier to seal than a full face flange. Flat face flanges are used on pump flanges which require full machining. Many modern cast Iron pumps in fact have raised face connections cast in, which simply requiring a simple facing machining process, as opposed to trying to machine a raised face which can prove to be difficult, and grain structure makes them weaker at the steps when absorbing the bolt torque.

Old British standards used to specify flat face flanges, but these have long been superseded by the common PN or ANSI specifications, due to the raised face simply being more reliable and better at sealing.

Not sure if it helps. but hope so !!

 

[rconner]

I basically agree that a flat-faced flange essentially wastes bolting load from the inside of the bolt holes outward; however, not to belabor the point but if e.g. a ring type gasket were specified, or say a special sealing design (e.g. of say 1/8" thickness or e.g. with o-rings or bulbs etc.) happened to be used between two flat-faced steel or ductile iron etc. flanges, would there then really be a "performance" advantage of a raised face over a flat-faced flange? It may be hard to make absolute or judgmental statements in such a complicated field (that cover all conditions)!

 

[rconner]

Oops, I meant to say in the first couple lines, "a flat-faced flange when used with a flat full-faced gasket essentially wastes bolting load from the inside of the bolt holes outward..."