The ball of a trunnion mounted bal valve is fixed and the valve seat is floating. The valve seat can be divided into single piston effect (SPE) or self-relieving action,

and double piston effect, (DPE.) A single piston valve seat can only be sealed in one direction. The dual piston valve seat can achieve sealing in both directions.

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If we use the → │ symbol for the SPE piston and the → │← symbol for the DPE, the four types of valves listed above can be identified using Figures 1-4

Fig 1 ?DBB (SPE-SPE)

Fig.2? DIB (DPE+DPE)

Fig.3 DIB-1 (SPE+DPE)

Fig4. DIB-2 (DPE+SPE)

In Figure 1, when the fluid flows from left to right, the upstream valve seat (SPE) plays a sealing role, and under the effect of fluid pressure,

the upstream valve seat adheres to the ball to achieve sealing. At this time, the downstream valve seat does not play a sealing role.

When a large amount of high-pressure gas is generated in the valve chamber and the pressure generated is greater than the spring force of the downstream valve seat,

the downstream valve seat will be popped open to achieve pressure relief. On the contrary, the downstream valve seat serves as a sealing function,

while the upstream valve seat serves as an overpressure relief function. This is what we called double block and Bleed valve.

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In Figure 2, when the fluid flows from left to right, the upstream valve seat (DEP) will play a sealing role,

while the downstream valve seat can also play a sealing role. In actual production applications, the downstream valve seat actually plays a dual safety role.

When the upstream valve seat leaks, the downstream valve seat can still remain sealed. Similarly, when the fluid flows from left to right,

the downstream valve seat plays a major sealing role, while the upstream valve seat plays a dual safety role. The disadvantage is that when high-pressure gas

is generated in the valve chamber, neither upstream nor downstream valve seats can achieve pressure relief, which may require the use of a safety relief valve

connected to the outside of the valve, so that the rising pressure in the cavity can be released to the outside, but at the same time, it adds a leakage point.

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In Figure 3, when the fluid flows from left to right, the upstream valve seat can play a sealing role, and the downstream two-way valve seat can also

play a dual sealing role. This way, even if the upstream valve seat is damaged, the downstream valve seat can still remain sealed. When the pressure inside

the cavity suddenly rises, the pressure can be released through the upstream valve seat, which can be said to have a similar sealing effect as the two two-way valve seats DIB-1,

However, it can achieve spontaneous pressure relief at the upstream valve seat end, combining the advantages of both DBB and DIB-1 valves.

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In Figure 4, it is almost the same as that of Figure 3. The only difference is that when the pressure in the valve chamber rises, the downstream valve seat end realizes

spontaneous pressure relief. Generally speaking, from the perspective of technology, it is more reasonable and safer to release the abnormal pressure in the middle

chamber to the upstream. Therefore, the former design will be used, while the latter design is basically of no practical value, which is very rare in practical applications.

It should be emphasized that generally, the upstream valve seat plays a major sealing role and is used frequently, resulting in a high probability of damage.

If the downstream valve seat can also play a sealing role at this time, it is a continuation of the life of the valve. This is also the reason why DIB-1 and DIB-2 (SPE+DEP)

valves have a long service life compared to other valves.

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