Gate valves with rising vs non-rising stem design

Classification of gate valves:

Gate valves have a variety of different structural forms, the main difference is the type of structure of the sealing element used is different. According to the structure of the sealing element, the gate valve is often divided into several different types.

For example, according to the type of structure classification, there are parallel gate valves and wedge gate valves; the structure of the stem can also be divided into open stem (lifting rod) gate valves and concealed stem (rotary rod) gate valves.

1）Classification by stem

①The stem is the operating part of the gate valve, and its role is to transfer the opening and closing force to the opening and closing parts. According to the classification of stem structure type, there are two kinds of open stem gate valves and concealed stem gate valves.

②By rotating the stem nut, the stem drives the gate plate to rise and fall simultaneously to open and close the valve, so it is easy to identify the opening and closing status of the valve and avoid misoperation. Since the stem nut is outside the body cavity, it is conducive to lubrication, and the opening and closing status is obvious and intuitive, so it is widely used. However, in harsh environments, the exposed threads of the valve stem are susceptible to damage and corrosion, even affecting operation. Its disadvantage is that the height of the valve after opening is large, usually on the basis of the original height of the valve to add a stroke, thus requiring a large operating space.

③Concealed stem gate valve stem nut is placed inside the valve body, often fixed on the gate, through the rotation of the stem to make the stem nut drive the gate to do lifting movement to complete opening and closing. Because the trapezoidal thread for transmission is located inside the valve body, although not affected by the external environment, but vulnerable to media corrosion and can not be lubricated, the degree of opening can not be directly observed, need to set up another indication device. But its stem does not do lifting movement, and the required operating space is small, so it is suitable for the location of limited pipeline dense occasions, such as ships, underground pipelines, etc.

2) Stem structure

Gate valve stem structure mainly has three different forms: lower thread lift stem (ISRS), non-lift stem (dark stem NRS), and upper thread with bracket (open stem OS&Y).

The lower threaded lift stem is the most commonly used stem structure for copper alloy-type valves today. Since the stem threads are exposed to the medium, there is a possibility of corrosion, which can lead to the failure of the stem and the gate. Therefore, this structure is generally not suitable for critical industrial use valves.

The non-lifting stem is a special type used when the vertical space above the handwheel is limited because the stem does not need to be raised when the valve is opened. Today, most valves made of cast iron or copper alloy use this type of stem construction. Steel non-lifting stem valves are used when operating space is limited (e.g., underground operation).

The threaded upper stem with bracket is mostly used in industry. Especially in corrosive conditions, because the stem threads are basically not in contact with the medium, they are also away from potential corrosion hazards. Unlike the other two types of stem construction, the handwheel is connected to the stem nut at the top of the valve bracket rather than directly to the stem itself, so the handwheel does not rise with the valve when it is opened.