Valves Commonly Used in Natural Gas Pipelines: Flat Gate Valve

In general, ball valves are the most widely used in natural gas pipelines, and flat valves are often used in pipelines for crude oil and refined oil. When the nominal pressure of the medium in the natural gas pipeline is CL900~2500Lb, the ball valve cannot be used because the switching torque of the ball valve is too large. At this time, a high-pressure hard-sealed flat gate valve should be used.

1. Structural characteristics of the flat gate valve

Flat gate valves can be divided into flat gate valves with diversion holes and flat gate valves without diversion holes according to the gate structure. The main pipeline for conveying oil and natural gas adopts balanced plate valves with diversion holes. For pipelines that do not require the sweeper, the flat gate valve without a diversion hole can be used.

Flat gate valves without diversion holes are lighter in weight than flat gate valves with diversion holes, and the price is about 15% lower. But the sealing surface of the valve seat of flat gate valves without diversion holes is not always protected by the gate, so it can possibly be washed by medium or worn by debris.

Moreover, there is debris attached to the gate, and the debris may be transferred to the bottom of the valve cavity when the gate is opened and closed. Therefore, the quality reliability of the flat gate valve without the diversion hole is lower than that of the flat gate valve with the diversion hole.

When the natural gas is delivered at low temperature, the valve body and bonnet of the flat gate valve should be made of ASTM A217 LCC or ASTM A350 LF2, the material of the valve stem should be ASTM A182 F304, the material of the valve plate should be A350 LF2 or A182 F304, and the surface should be treated by glow ion nitriding.

The valve seat is generally a metal valve seat, inlaid with PTFE. The material of metal valve seat is A350 LF2, the surface is treated with glow ion nitriding or is hard alloy with F304 surface overlaying, the material of soft sealing valve seat is PTFE, and the back of the valve seat is Inconel X750.

When the conveying medium is high-sulfur natural gas, the valve body and bonnet of the flat gate valve should be made of high-quality sulfur-resistant steel specially smelted against high-sulfur. The main material of the valve must be pure dead steel with low carbon, ultra-low sulfur and low phosphorus fine grains, made by electric arc furnace. Use Si-Ca alloy as deoxidation material, and use Al or Ti modification for deoxidation.

The inclusions must be controlled during smelting and casting, so that the possibility of non-metallic inclusions appearing in the metallographic structure of the WKM gate valve is minimal, and they cannot have obvious columnar segregation.

The heat treatment method is annealing or normalizing + high temperature tempering to obtain tempered sorbite with uniform sulfur resistance. HB hardness is not more than 200.

2. The installation requirements of flat gate valve

During the installation process of the flat gate valve, in addition to ensuring the same accuracy as that of other valves, it should also be noted that the flat gate valve cannot be installed horizontally.

When the flat gate valve is placed horizontally (meaning that the large plane of the valve plate is parallel to the ground), due to the action of gravity, the valve stem will inevitably become a cantilever beam and flexibly deform, causing the valve plate of the valve to tilt and sink. Flat gate valves may produce contacts that cannot be mated.

Almost all the weight and the deformed displacement of the gate plate of the flat gate valve are all pressed on the lower semicircle of the valve seat below, so that the O-ring and the back spring of the valve seat cannot move. And because the valve seat and seat hole has a high degree of concentricity, so the it is impossible for the lower valve seat to tilt and match the valve plate by itself.

In this way, there is a situation where the lower valve seat and the flat gate valve are compacted on the inside and there is a gap on the outside. Under the combined action of gravity and the back spring, the valve seat on the flat gate valve contacts the valve plate that produces flexible deformation on the outside. And in the valve plate, because the valve seat and the valve seat hole have high concentricity, it is impossible for the upper valve seat to tilt and match the valve plate by itself.

In this way, the outer half circle is tightly attached between the upper valve seat and the parallel valve plate, and there is a gap inside. Therefore, flat gate valves installed horizontally tend to produce "internal leakage".