It is sometimes possible for the duty of a heat exchanger to exceed its specified value. For example, a reboiler’s duty may be specified under the assumption of the presence of some deposits on the heat exchange surfaces; when the exchanger is freshly cleaned, these deposits will be absent, and the duty can be higher than specified. In such situations, the abnormally high heating can produce greater than design vapor flow rates, which may require pressure relief. The required relief flow rate is simply calculated as the vaporization rate in excess of the normal (design) rate.
In other situations, a loss of heating can lead to a cause of overpressure. For example, in series distillation systems, the bottom product of one column provides the feed to the next column. If the reboiler for the upstream column loses heating, the bottom product of that tower will be unusually rich in volatile material. When this altered feed enters the normally-operating downstream column, it will yield an above-average vapor flow rate, which the downstream column may not be able to handle.
Abnormal vapor input may result, for example, from an abnormally high feed rate to a distillation tower. In this case, the required relief flow rate is taken as equal to the excess vapor flow rate.