Problems with globe valves in liquid ammonia pipelines
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Some pipeline design manuals state that only ammonia stop valves can be used, and gate valves are not permitted. For example, a carbon steel-specific stop valve for liquid ammonia can be used, but a carbon steel gate valve cannot be used, nor can valves made of other materials. What are the specific reasons? Is it because the sealing surface of gate valves is prone to scratches? I can think of no other reasons. Also, can carbon steel soft-sealed ball valves be used for liquid ammonia pipelines?
The main reason why gate valves are not recommended for liquid ammonia pipelines is indeed related to their sealing performance. As a highly corrosive and volatile chemical, liquid ammonia demands high levels of sealing performance and corrosion resistance from pipeline valves. Gate valves are typically designed to achieve sealing through tight contact between the valve disc and the valve seat, but this design can lead to friction and wear between them during frequent opening and closing, thereby affecting the sealing performance. The design of a globe valve is different; it closes the fluid by having the valve disc perpendicular to the flow direction. The contact pressure on its sealing surfaces is high and relatively uniform, which helps to achieve a better sealing effect. Furthermore, when opening and closing, the gate valve has a small contact area between its valve disc and seat, resulting in less wear compared to gate valves; this makes it more suitable for handling media such as liquid ammonia that require high sealing performance. As for ball valves with soft seals made of carbon steel, they are typically used in applications that require good sealing performance and fast opening and closing. For liquid ammonia pipelines, carbon steel ball valves can theoretically be used if equipped with appropriate soft sealing materials, but the specific choice still requires consideration of the corrosivity of the medium, operating conditions, and the pressure rating of the valve. It is advisable to consult a professional valve manufacturer or refer to more detailed design specifications before making a choice. .
After the ball valve is closed, liquid will remain; as the liquid ammonia vaporizes, the pressure increases, posing a risk of leakage.
The same principle applies to gate valves as well; when a gate valve is closed, liquid remains in the upper valve chamber. As the liquid ammonia vaporizes, the pressure in this upper chamber increases, which raises the risk of leakage through the packing. Additionally, an excessive pressure difference can prevent the gate valve from opening.
For the insulation design of liquid ammonia pipelines, even at normal temperature the vaporization pressure does not reach 1.6 Mpa; ordinary steel valves are sufficient to meet the strength requirements. It shouldn’t be that reason