Excess Inflow or Outflow
In the event of an imbalance in the volumetric flows in and out of a container, a change of pressure can result within the container. The flow rate entering and exiting the container can be determined using common engineering calculation techniques, either by evaluating the capacity of a single limiting source term or single piping component, multiple piping components in series, the limitations of a fluid driver, or the combined effect of the fluid driver and the intervening piping system. Several cases of overpressure scenarios are evaluated in this manner, including heat exchanger tube failure, inlet control valve failure, and inadvertent valve opening.
Required relief rate. The relief flow resulting from these imbalances is given by1
Where W is the mass flow rate, v is the specific volume, and the subscripts F and E refer to the feed and exit streams, respectively. For vacuum relief, W is negative and υ0 is the specific volume of the gas drawn in (usually ambient air, but may be another gas); otherwise, this equation applies to the conditions of the system at the relief pressure, and υ0 is the specific volume of the fluid exiting the container and entering the relief system. The fluid that is entering the container is not necessarily the fluid being relieved, although that is a commonly employed assumption. The hydrodynamics of the container should be determined in response to this inflow, especially if the inflow creates bubbles in the liquid (gas sparging or boiling).
Calculation models. To determine imbalance of volumetric flow, the flow rate entering the container should be characterized. In many cases, the assumption that no flow continues out of the container other than from the relief device provides a reasonable basis for estimating the venting requirements (especially for causes of overpressure related to a closed outlet or where the ability of the downstream system to adequately handle the additional flow from the container is unknown). Cases in which flow out of the container is assumed to continue should ensure that there is no means for blockage, the flow path is not affected by the scenario, and the downstream system can handle the flow of potentially different fluids during the scenario.
To determine the flow rate entering the container from a higher pressure reservoir, the flow limiting element needs to be identified. These flow limiting elements may be a specific piping element between the high pressure reservoir and the container to be protected, a source term that can be modeled as a piping element, the entire piping system, or the limitations of a fluid driver. These models include the following:
- Flow through an orifice
- Flow through a control valve
- Flow through piping
- Fluid driver limitation
When selecting the flow limiting element, one should consider the potential for future modifications to the flow limiting element and procedures to avoid such modifications. For example, an orifice plate can be easily changed to a larger size or a valve can be modified to have a larger port, while changing pipe sizes is usually more difficult. Care should be taken to identify the flow limiting element as part of the safety system so that review of future changes during the Management of Change process clearly evaluate the effect on the overpressure protection system. In addition, consideration should be given to the reliability of that flow limiting element. For example, an orifice plate that is not thick enough to withstand the expected pressure differentials during operation or during the relieving event may fatigue or bend, creating a larger opening and allowing greater flow than expected.
Overpressure scenarios. Common overpressure scenarios that can often be modeled using these techniques include the following:
- Structural failure
- Heat exchanger tube failure
- Inlet control valve failure
- Inadvertent valve opening
Blog series information. This blog is part of a series on the proposed updates to the CCPS Guidelines 2nd edition §3.3 Venting Requirements for Nonreacting Cases that were removed during final editing. See the general CCPS Guidelines for Pressure Relief and Effluent Handling 2nd Edition review for more information.
 AIChE Center for Chemical Process Safety. “CCPS Guidelines for Pressure Relief and Effluent Handling Systems”. 2nd Edition, 2017; New Jersey: John Wiley & Sons, Inc.