CPVC flange bolted to SS flange

Hi Nithya,

I would NOT use stainless steel for H3PO4 service! See: http://www.coleparmer.com/techinfo/ChemComp.asp and check the compatibility of 304 or 316 SS with phosphoric acid.

If they're worried about durability, Hastelloy C valves will work, but they're expensive. I would contact a valve supplier and see what they would recommend.

Best Regards,

Mike

Thanks for the guidance. But apparently, the client has insisted on an SS grade that is sutiable to this service according to him... ASTM A351CDM4Cu...I had sugegsted CPVC valves only but he is doubltful about th etsregth of these valves coz the design pressure is high 10Kg/cm2(g).

But if at all I do have to bolt CPVC flange with SS flange, do u forse a problem with the joint?

DEAR SIR,

I AM NONT AT ALL A PIPING EXPERT, IWORK IN FORGING COMPANY MANUFACTURING FLANGES OF ALL STANDARDS AND IN SS,CS,ALLY STEEL ETC.

I THINK , YOU MAY USE SMOOTH FINSH FLANGE ( WITH OUT SERRATION) OR SPECIAL SERRATIONS SUITABLE TO YOUR MEETING FLANGE. OFCOURS TAKE THE PROPER ADVISE FROM PIPINIG DESIGNER.

THANK YOU.

V.D.DEODHAR.

Please turn off Caps Lock. Shhhhhhhhhhhhhhhhhh!

As you can see by the spelling errors in his post that person gets very emotional over this subject and was probably pulling at his hair while he was yelling it at the screen with his voice activated interface.

I believe that plastic flanges are only used with full face gaskets - gaskets that extend to the OD of the flange and have holes for the bolts to pass through. With a raised face gasket, the bolt tightening would produce way too much moment and distort the plastic flange. The mating SS flange should be flat faced to match the plastic flange. The flange finish should be whatever is compatible with the gasket material. Soft composition or elastomer gaskets tend to need rough surfaces (250 RMS) for more bite on the gasket.

Hello. I think you can do this with no problem, I have run CPVC piping with metal (usually carpenter 20 for acids) valves and even put the straight runs of pipe inside of larger galvanized or stainless pipe to support and protect it and have had good success with it. If you are going to do this there are certain things you must be careful to address

(1) If the valve has a raised face then you will need full to raised face spacer or just simply order the valve with a full face. You will use a gasket that is suitable for the liquid in between the flanges so if each has a different profile finish it will be of no concern. I would personally use a 1/8" thick gasket.You can seal a raised face to a full face but there is a chance you may crack or distort the plastic flange when you do this.

(2) be sure you support the weight of the valve and do not rely on the pvc pipe to carry the load. Although it may seem ok at first the load can weaken the pvc over time and lead to a sudden failure and a safety issue.

(3) I would use a flange cover to prevent the acid from spraying out if it started to leak. They make these that change color when the acid touches the cover material so you will know when you have a leak.

(4) The good part about putting the pvc pipe into a larger galvanized pipe is that the metal pipe can be supported very easily with U-blots and metal hangers and it will support the pvc pipe as well as protect it from breaking in case someone accidently hits it with something. I would highly recommend doing this.

pipewelder

Thanks for the informative contributions for which I couldnt resist voting "good answer" for 3 of the posts here.

So i understand that, I can go ahead with SS valve/flanges and bolt them to CPVC flanges, both having a flat face. However, PTFE being not too soft, can a rough/serrated finish 125-250 AARH work? or a smooth finish should be more appropriate? The CPVC flange manufacturer that I am dealing with, doesnt seem to forsee a difficulty with serrated finish but a second opinion from experts here would help.

has anyone here used a PTFE gasket? If so, with what kind of flange facing?

Soft is a relative term. PTFE is somewhere in the middle of the gasket range, which runs from rubber to annealed nickel-chrome-iron alloys. There is a great synopsis of gasket materials and design criteria put out by Flexitallic, who make a full range of products. On page 47 of this link, there is a table of gasket style versus flange face style. I believe that the general tendency is that for harder gaskets, the face should be smoother, and that for softer gaskets, the face should be rougher. O-rings and other self energizing seals are an exception to this rule, though.

With respect to the CPVC flat faced flange under discussion; if PTFE is an appropriate material for the phosphoric acid service, then expanded PTFE would be a better choice of gasket material. It is chemically the same as PTFE, but is significantly softer and doesn't have the creep or stress relaxation problems of sheet PTFE.

The other way to go on PTFE is filled or fiber reinforced PTFE. This eliminates the the creep and stress relaxation problems, but increases the hardness. These are better choices for high pressure situations. I'm assuming that, since there are plastic flanges and piping, the pressures are low.

Thanks Lawrence!

This was really helpful in clearing many issues.

yes, the pressures are low-10kg/cm2g... and service as mentioned is phosphoric acid so plastic piping is suitable.

Just one more thing,... can you tell me how is "expanded" PTFE different from normal PTFE gasket? Are they compositional different due to which Expanded PTFE is softer?

If your client is comfortable with the grade of the stainless steel in the construction of the valve, I don't see a problem with the joint. The gasket material being somewhat pliable will effect a good seal between the two flanges whether it is a full-face gasket or simply a "bolt-circle" gasket. The concentric rings in the face of the flange will be pressed into the gasket to form a series of seals, each concentric to the next. This type of seal will be very forgiving if there is some small amount of mis-alignment in the two mating flanges. However, with the material you are transporting, I would be careful not to over-tighten the bolts, but make them sufficiently tight to bring the faces together with enough force that they don't leak. Perhaps you can get some help from the flange manufacturer about the torque requirements. I can understand your concern when transporting such a corrosive material.

Not funny. What's wrong with using <...CPVC...> valves instead, i.e. use valves made of the same material as the <...piping...>?