Cast Iron Pressure Relief Valves

[UtilityLouie]

Hello everyone,

Thanks for all who have answer my questions on relief valves in the past. I have one more.

I have received bids on relief valves for a shell and tube heat exchanger. I requested that the valves be sized for full steam flow into the exchanger. The valves will relieve 60 psig steam.

I received quotes of CS body valves and cast iron valves because I didn't specify which I wanted and there is a HUGE cost difference.

In the condensate side of things, I try not to use cast iron. On 10# systems or lower, I can see using them. On higher pressure systems (between 10# and 100#), I usually use ductile iron as a minimum. I am not sure about the application of cast iron on the steam side of things. From my understanding, cast iron is only problematic under shock loading like water hammer that is frequent on the condensate side of things because water is less compressible than steam.

The "project manager" has seen the price difference and if I want the CS reliefs I need to justify. This guy is a micro-manager.

Please help by giving me opinions. I think that use cast iron on this steam application, but need more confidence in this decision.

Thanks

 

[Keef]

UtilityLouie,

I know that cast iron valves are not allowed on steam boilers built to BS standards, but cast iron is not excluded from the rest of the system. I also gather that ASME I allows cast iron safety valves on boilers.

Having said this, as there is generally very little cost difference between cast iron and SG iron valves, the tendency these days is to go for SG iron (unless you are in Africa, India or parts of the Middle East). Certainly, you do not have to go for a steel valve for the pressures you are working on.

Keef

 

[deanc]

See ASME SecVIII Div1 UG-136b3 and ASME SecI PG-73.2
Make sure the valves have the required marking per the Construction Code and the "NB" stamp.

 

[TBP]

Class 250 cast iron is pretty hard to break. The scrap yard guys don't like 250 valves for that reason. Class 125 is another matter. I've seen water hammer in low pressure steam and condensate system break 125 fittings, and there have been instances of 125 flanges being broken away from the valve body under waterhammer conditions. (And this was in an application where the 125 stuff was, in fact properly rated, and met code.) This is aside from the fact that a huge number of industrial steam systems are filled with 125 valves and strainers, when the steam systems operate at 125 PSIG, but the safety valves lift at 150. Class 125 cast iron components probably get mis-applied more than all other classes combined.