Introduction
A main steam stop valve (crown valve) connects the boiler steam system to the steam main/header. When the valve is open, it connects the boiler to the steam main. When the valve is closed, it isolates the boiler from the steam main. Main steam stop valves are usually installed on top of the boiler shell or within close proximity to it; they should be either fully open or fully closed at all times (not throttled/regulated). Each interconnected boiler (boilers within a battery/range) should have its own main steam stop valve i.e. a single valve should not be responsible for isolating multiple boilers from the steam main.
Boiler Crown Valve
Materials
Main steam stop valves should be made of cast steel rather than cast iron because cast iron valves have been known to fail as a result of water hammer; in some countries, the use of cast iron main steam stop valves is forbidden.
Design
Main steam stop valves are usually of the angle-flow globe valve design.
Globe Valve Body Designs
It is important personnel can visually see the position of a main steam stop valve from a distance, and without needing to actuate the valve. To make this possible, stop valves are fitted with a rising stem. As the valve is opened, the stem of the valve rises i.e. the stem moves upwards out of the valve bonnet. The GIF below shows this occurring, although the valve shown is a gate valve, not a globe valve.
Gate Valve Rising Stem Animation
Note
In the Americas, gate valves with a rising stem are favoured as main steam stop valves; this type of valve is referred to as an outside stem and yoke valve (os&y valve).
Although globe valves are often used for regulating/throttling flow, they are not used for this purpose when used as main steam stop valves. Gate valves are better suited for fully open and fully closed service, but mounting this type of valve directly onto the boiler shell is difficult. To circumvent this problem, a pipe bend is installed between the boiler shell and main steam stop valve; this method is favoured in the Americas where gate type main steam stop valves are common.
Enjoying this article? Then be sure to check out our Introduction to Valves Video Course! The course has a quiz, handbook, and you will receive a certificate when you finish the course. Enjoy!
Maintenance
As with most boiler fittings, main steam stop valves must be periodically maintained. They should be removed during statutory inspection periods to check the valve-to-boiler-nozzle studs, sealing pressure between the valve disc and seat, valve operating mechanism, and to overhaul or replace as necessary.
Additional Resources
https://www.spiraxsarco.com/learn-about-steam/the-boiler-house/boiler-fittings-and-mountings