Gray Cast iron VS Ductile iron

[PAN]

For diaphragm valve lined with PTFE, I understand that the fluid will not contact body material. I would like to use gray cast iron instead of ductile iron. Are there any disadvantages for this applicatoin.

 

[BillBirch]

It depends on your application. Grey cast iron is more brittle than ductile iron so piping should impose minimal forces on the valves. If the fluid contained is flammable you should not used cast iron.

 

[MJCronin]

PAM,

The only set of rules that should concern you is the design code that governs your installation.

Both ASME B31.1 and B31.3 put limitations on use of cast iron in toxic, flammable and steam service. Limitations imposed typically are in the form of pressure/temperature maxima or inclusion only in non-shock service.

If a material is deemed suitable for service (by the piping code) and the component is design rated for that combination of temperature and pressure, then you should be able to use it. Valve manufacturers have excellent experience with succesful and non-successful installations and are usually happy to share it with a customer..

MJC

 

[Hush]

Gray cast iron, regard;ess of what you coat your wetted surfaces with, won't meet NACE MR0175 if that is a consideration.

 

The Iron body is not wetted as it is fully insulated by the teflon lining. So there is no discussion on the chemical properties.
Why should you consider cast iron. It will not save you money. The standard for any valve manufacturer is ductile.

My advise, stick to the standard material configuration.

ood luck !
Ed

 

[stanier]

A thing you like to consider is if the lining is glass reinforced there may be a danger of cracking the flange in tighteneing the bolts. The teflon will form its own gasket and as such will be raised above the face of the flange. When the bolts are tightened tha CI is subject to a bending moment and shear at the "gasket". CI is weak in tension and examples exist of CI flanges cracking when in a raised face scenario.

For critical service I would always use DI over CI. I beleive that Saunders diaphragm valves are only available in CI but other manufacturers such as Kim or Gemu could well have DI. Of course you would expect to pay more for a DI bodied valve. But this will be a lot less than the repair of a cracked flange.

 

[PAN]

Thanks for all comments. In our plant's specification, there are both ductile iron and gray cast iron body. I will study more to update the specification.