The outer seal of the gate valve generally refers to the seal at the bonnet and the valve stem. The seal at the bonnet refers to the seal between the bonnet and the valve body, which is divided into forced seal and self-tightening seal.

1. Forced sealing of gate valve

The forced sealing of the gate valve relies on tightening the bonnet bolt, applying a compressive force to the preloaded gasket, and compressing the uneven micro-gap of the sealing surface, thus creating an initial condition for preventing medium leakage (sealing surface). form a preload specific pressure). When the medium pressure rises and the valve is operated, the preload specific pressure on the sealing surface drops, and the gasket springs back. If the gasket has enough resilience so that the working sealing specific pressure on the sealing surface is always greater than the medium and operating specific pressure, the sealing surface will maintain a good sealing state. The necessary condition for forced sealing is that a certain residual pressing force can still be maintained on the sealing surface under the action of medium pressure.

The typical structures of gate valve forced sealing are flat gasket, wound gasket and toothed gasket, etc., which are usually used for bonnet sealing of medium and low pressure and small and medium diameter valves.

2. Self-tightening sealing of gate valve

As a high-pressure sealing technology, the self-tightening sealing of gate valves is often used in high-temperature, high-pressure large-diameter valves. The self-tightening sealing structure usually used on these valves is a wedge-shaped gasket combined self-tightening sealing structure.

The sealing principle is as follows: Before boosting the pressure, first tighten the pre-tightening bolts to make the bonnet rise, so that the initial sealing condition is formed between the bonnet and the wedge-shaped gasket and between the valve body and the wedge-shaped gasket, that is, the preload on the sealing surface. tighter pressure. When the medium pressure rises, the bonnet moves upward under the action of the medium pressure, and the sealing specific pressure between the bonnet and the wedge-shaped gasket and between the valve body and the wedge-shaped gasket increases gradually with the increase of the pressure. In the self-tightening seal, the specific pressure of the working seal on the sealing surface is composed of two parts: one is the specific pressure of the pre-tightening seal; the other is the specific pressure formed by the medium pressure. The characteristic of the self-tightening seal is that the medium pressure in the seal always tends to increase the specific pressure of the pre-tightening seal, which increases the sealing performance. The higher the medium pressure, the greater the specific pressure of the working seal and the better the sealing performance. In oil refining units, such as hydrocracking, hydrorefining, residue hydrodesulfurization and other units in the high pressure valve sealing is the use of this sealing structure. In order to make the seal have a good sealing effect at high temperature, austenitic stainless steel or hard alloy can be surfacing on the contact part (sealing surface) of the valve body, the bonnet and the wedge gasket to form a cladding. The thickness of the coating is about 2 mm. The function of the coating is to increase the hardness of the sealing surface and improve the corrosion resistance and erosion resistance of the sealing surface. It is easy to form a good contact surface and angle difference with the wedge-shaped gasket, thereby improving the sealing effect.