Back Pressure Effect

In the previous article, two kinds of back pressure have been discussed. We have already understood that back pressure has several impacts to PSV performance. In this article, we will learn the impact of back pressure on the relief valve opening, operation and flow capacity.

My friend, If you don’t have a good understanding on the definition of terminology such as; overpressure, accumulation, set pressure, re seat pressure etc, I guest you will be confused with my explanation. In this article, I won’t to explain each of them, so please refer to API 520 part I by your self. It’s very important to understand the meaning of its definition.

PRESSURE RELIEF VALVE OPENING

Superimposed back pressure has impact to opening of conventional relief valve type. This back pressure will give additional spring force onto valve disk in closed position. Therefore, the actual spring setting can be reduced by an amount equal to the amount of superimposed back pressure.

Hopefully, this picture will help us.

For balance and pilot type, the compensation of the constant superimposed back pressure is not required

PRESSURE RELIEF VALVE OPERATION

Excessive of built up back pressure has impact the conventional valve operates in unstable condition. Its may be chatter or flutter. Chatter is rapid motion of closing and opening valve where the disc contacts with the relief valve seat during cycling, whereas flutter is not contact with the seat. Chatter cause damage to the valve.

PRESSURE RELIEF VALVE CAPACITY

Built up back pressure has impact reducing the valve capacity. High back pressure reduces the lifting of disc result in reduction of flow capacity. For conventional type, built up back pressure shall not exceed 10% of set pressure at 10% allowable overpressure. For application that allowable overpressure is higher than 10%, say 16% of multiple valve application, then the built up back pressure up to 16% of set pressure is allowed for conventional type.

See the picture below of capacity correction factor due to back pressure for conventional type where the spring setting compensation for superimposed back pressure is required.

The figure below shows that the capacity of balance below type will be reduced significantly due to back pressure. Look at this picture, hopefully you get the answer why we choose to use balance below type for back pressure up to 30% at 10% allowable overpressure.

Finally, I can share this topic for you. Hopefully we get better understanding on the importance of the back pressure. The back pressure is one of consideration for optimization flare system. For new design system, which one do you prefer, use larger size of tail pipe to reduce back pressure or use balance below type to overcome it?

Friends, Thank you for reading. Please correct me if I am wrong.