Most piping specifications avoid cast iron for pressure applications. Steel is better because cast iron cracks with lower stress. A crack in a 125 psig steam could cause injury or kill people. The lost downtime is expensive. The installation labor is not much different.
What is the safety valve setting? There are lots of steam systems running at 125#, but the safety valves lift at 150#. Very often, these systems are filled with 125# cast iron valves & strainers - because they're cheaper than cast steel. However, they don't meet code. Even if the safety valves lift at 125, unless the raised faces are removed from the flanges on the piping, they STILL don't meet code.
So is it just a matter of pressure rating, or is there any other reason for choosing one over the other? In my case you're right, the PRV lifts at 130 psig. I was going to use a 250# or 300# valve anyway, which is why I asked which is better. Both are well above my relief valve setting.
You are correct, for 125# steam it is a good idea to use either 250# cast iron or 300# steel. The main difference is that cast iron is much more brittle than steel.
So, if you drop a cast iron valve it may crack but not a steel.
My customers prefer also steel in steam service because of the chance of water hammer. Large banging in the pipes could crack the flange of cast iron and not steel.
In refineries and chemical plants they often do not allow cast iron, but for another reason. If there is a fire, the cast iron tensile strength breaks down at a lower temperature than that of steel.
Ductile Iron is more malleable than cast iron, has many of the same properties of steel at a reduced cost. It is a great material!
A gate valve for steam is the last thing I look into. I used globe, piston, diaphragm and even ball valves successfully but never a gate. Do you guys have any exploded views or pictures of the gate valves you are speaking?
Are they for control or shut off? Not that it's anything wrong but my experience has been pretty bad. Particularly, the ones that were to bypass a pressure regulating stations.
I would agree that in general a gate valve is not the best for steam as it likely will erode on the bottom of the gate.
Gate valves do not throttle well like globes. However, gates do offer large Cv's for the size and if used for shut-off only and maintained full open for flowing they may be appropriate. However, do look at the design to assure the gate is out of the flow stream and talk to the manufacturer.
Further, regardless what you select the valve should be for steam service in addition to having the T & P capabilities.
The materials gray cast iron and later developed ductile cast iron have of course been mentioned on this thread (along with some references with regard to same). Gray cast iron is of course generally a quite venerable construction material that has been around for hundreds of years in piping, valves, and other applications.
However, while I don’t think the particular all ductile iron valves pictured should be used for steam applications you may nevertheless be interested in the general material-related features and special testing of some gate valves contained in the four minutes or so video "Ductile versus Gray Presentation" near the bottom of the page
under the heading “Product Presentation Files”. This video provides some more information/clues as to how/why gray cast iron valves might be more easily overloaded or broken in handling, installation, and or service events than the much stronger/tougher ductile material, and probably also why some manufacturers and users have opted instead for ductile iron constructions.
One more bit of information. Jordan Valve makes a sliding gate valve specifically for steam service. It is available in DI, Steel and other materials. You might look at it for your application.
(I don't work there but I did at one time.)
steam has its own requirements. There are valves specifically manufactured for steam service. Always use a steam valve in steam service. Also, I am a great believer in calling the manufacturer and getting their expertise. You would be surprised how helpful the sales engineer can be when trying to learn about different products.
