(9c) Can I Use My Cooling Water Header As a Relief Device?

According to current industry practice, a heat exchanger tube rupture scenario is considered when the design pressure or maximum possible system pressure of the high-pressure side exceeds the corrected hydrotest pressure of the low-pressure side. Typically, an adequately-sized Pressure Relief Device, PRD, is installed to relieve the fluid from the high pressure side. In some older facilities, overpressure protection credit was given to the low-pressure side’s hydraulic circuit (e.g., a cooling water header) as long as the tube rupture relief load did not accelerate the normal flow in the low-pressure side’s hydraulic circuit. In other words, if the volumetric relief load into a cooling water header was less than the volumetric cooling water rate, the cooling water header was considered adequate and utilized as the sole means of overpressure protection.

In cases where external fire was the only source of overpressure for the cooling water side of an exchanger, isolation valves upstream and downstream of the exchanger were considered acceptable as long as the exchanger was drained during isolation. Normal cooling water flow is often considered adequate overpressure protection without rigorous analysis or consideration of potential effects on other interconnected exchangers. Many operating companies use the above conditions to mitigate external fire scenario on the cooling water side. Review of previous literature on the subject provides minimal design criteria, simplified assumptions, and does not shed much light on the topic. This paper provides a contemporary design and calculation methodology that allows relief system engineers to take accurate overpressure protection credit for the low-pressure side hydraulic circuits.