Use of steel vs. gray iron gate valves in-plant for low pressure water services

[springwaterbr]

Dear all,

When used within the pressure-temperature rating given by an applicable standard, such as MSS SP-70, is there a code related reason for most gate valves to be specified in carbon steel instead of cheaper gray iron? I looked through ASME B31.3 and found only this paragraph related to the use of gray iron:

Gray iron shall not be used above ground within process unit limits in hydrocarbon or other flammable fluid service at temperatures above 149°C (300°F) nor at gage pressures above 1 035 kPa (150 psi). In other locations the pressure limit shall be 2 760 kPa (400 psi).

What are the reasons for steel valves to be preferred over gray iron ones, when the pressure-temperature rating and chemical compatibility are ensured? Is it mostly related to field situations and experiences? Faulty handling of a gray iron valve could lead to cracks, whereas the same wouldn't happen with a steel one, for example?

And is there anything specific to firewater service? I couldn't find anything on it, but, even though I've seen past specifications allow for gray iron and bronze valves for this application, the thought of it leaves me uneasy.

What are your experiences?

 

[bimr]

Cast iron has a long history for use in valves that have low-pressure and ambient-temperature applications.

Cast iron valves are used in a variety of applications such as water supply, sewage treatment, gas supply, district heating piping, irrigation projects, air conditioning systems, as well as food processing industry, chemical and petrochemical industry, pharmaceutical, power, and industrial plants.

Because of its wide range of working temperatures and pressures, cast steel is commonly used for pipelines in oil fields, refineries and petrochemical plants. Cast iron is not used because the stress developed from high temperatures, pressure, and fire exposure may cause instantaneous cracks and failure.

 

[TugboatEng]

In my experience, cast iron resists pitting corrosion better than steel in systems that may experience corrosion such as fire mains.

https://www.nature.com/articles/s41598-017-07245-1