Effect of orifice plate deformation on flow measurement

(1) Causes of orifice plate deformation

Orifice plate deformation does not occur often. Standard orifice plate deformation is often manifested as concave surface A (flow surface) and protruding surface B. In severe cases, it looks like an iron pot with the bottom removed.

Orifice plates that are deformed are often larger in diameter and have high fluid temperatures, such as superheated steam, and relatively thin orifice plates in throttling devices with annular chamber pressure measurement. Since the orifice plate and annular chamber temperatures are low and close to room temperature during processing and manufacturing, and the temperature is high during actual use, their geometric dimensions have changed significantly. Since the material of the annular chamber is generally carbon steel, the linear expansion coefficient is generally 11×10-6℃-1, while the material of the orifice plate is stainless steel, the linear expansion coefficient is generally 16×10-6℃-1, so the difference in expansion coefficient is 5×10-6℃-1. When the throttling device is put into use, it comes into contact with high-temperature fluid, and the orifice plate and annular chamber expand accordingly. The increase in the outer diameter of the orifice plate is larger than the increase in the inner diameter of the corresponding part of the annular chamber. Therefore, sufficient expansion gap must be left through calculation during processing and manufacturing.

When the deformed orifice plate is disassembled for inspection, it is often found that the above gap is not small. When the orifice plate and the annular chamber are calculated according to the highest working temperature, there is still a certain gap. In this case, why does the deformation still occur?

One thing is certain, that is, the deformation of the orifice plate is due to the disappearance of the gap between the outer diameter of the orifice plate and the annular chamber after thermal expansion. When the orifice plate continues to expand, it cannot expand in the direction of the outer diameter, so under the action of the differential pressure on both sides of the orifice plate, the outlet side is deformed.

When investigating the site where the orifice plate is deformed, related clues are often found.

① It is related to the leakage at the junction of the positive and negative annular chambers. The leakage cannot be eliminated by tightening the bolts, so the throttling device is removed and the sealing gasket is replaced, and then the orifice plate is found to be deformed.
② It is related to the weather. The throttling device originally had no leakage. Due to the extremely cold weather and the strong northwest wind, the junction of the positive and negative annular chambers leaked.
③ It is related to the poor insulation and heat preservation at the throttling device. The throttling device with good insulation did not leak.

The above related clues can be linked to make a deductive analysis of the cause of the orifice plate deformation. When designing and calculating the throttling device, the gap between the outer diameter of the orifice plate and the annular chamber is large enough, assuming that the temperature of the orifice plate and the annular chamber is the same, but in fact it is impossible. The orifice plate is surrounded by the annular chamber and is provided with heat by the high-temperature fluid, so the temperature is high and it is fully expanded. The inner circle of the annular chamber is in contact with the fluid, but the outer circle is in contact with the atmosphere. In addition, factors such as "poor insulation", "extremely cold weather" and "strong northwest wind" cause its temperature to drop very low, resulting in insufficient reserved gaps and causing deformation of the orifice plate.

Orifice plate deformation is related to leakage at the positive and negative annular chambers. If the leakage is not caused by damage to the sealing gasket, it is because the gap between the positive and negative annular chambers increases after the orifice plate is deformed, and a gap appears between the sealing gasket and the annular chamber.

(2) Solution to orifice plate deformation

After finding the root cause of orifice plate deformation, there is a simple and effective way to prevent orifice plate deformation, that is, when designing and calculating, consider the actual situation that the temperature of the annular chamber may be much lower than that of the orifice plate, and calculate the gap reasonably. When calculating, the temperature at the outer circle of the annular chamber should not be higher than 100℃.

Orifice plates that have been deformed and failed the inspection can only be scrapped.

(3) Estimation of flow measurement error caused by orifice deformation

There is no standard or experimental data on the effect of orifice deformation on flow measurement indication, but the direction of the effect is obvious.

The degree of orifice deformation varies greatly. When severely deformed, its shape is somewhat similar to that of a nozzle. From the standard of throttling devices, it is known that the discharge coefficient of a standard orifice is about 0.6, while the discharge coefficient of a nozzle is about 0.99. Based on this, the actual discharge coefficient of the orifice increases accordingly after deformation, and the flow indication is correspondingly lower. In the case of severe deformation, if it is assumed that its discharge coefficient is close to that of the nozzle, the flow indication will be reduced to about 60% of the indicated value. Therefore, it is reasonable to say that the flow indication after orifice deformation may be 0 to 40%R lower than the indicated value. (end)