Credit for Outflow
Many of the relieving requirements for credible overpressure scenarios we calculate implicitly assume that the only mass/energy allowed to leave the system is through the relief device; however, there are some instances of reasonable attempts to take ‘credit’ for other outflows, including various cases specifically outlined in API Standard 521.1 In fact, API Standard 521 §220.127.116.11.4.1 explicitly states (with caveats discussed below): “Crediting for flow paths that remain open during an overpressure event is generally an acceptable practice.”1
While we refer to this as a ‘credit’, it is placed in quotes here as there is often an inappropriate connotation associated with the term credit, as if what we are doing is being liberal with our calculations to get a ‘better’ answer. This is not the case, as the fundamental basis for the generation of pressure during various causes of overpressure is the volumetric accumulation of mass and/or energy in the system. And to determine the accumulation within a system, one must evaluate the mass and energy entering and leaving the system.
On the other hand, it is very important to note that accounting for outflow from paths other than through the relief device does require an evaluation of the ability of those alternative paths to provide outflow. This evaluation includes the following:
- There can be no means of blockage, particularly farther downstream
- The alternative flow path cannot be affected by the scenario being evaluated
- The downstream system needs to be able to handle the flow
- The effect of outflow involving abnormal fluid phases needs to be analyzed
Because of some difficulties encountered, it is easier to assume other flow paths do not exist, hence the common implicit assumption stated initially above; however, there are instances where the engineering effort is worth the analysis.
No means of blockage. The potential for blocking normally opened flow paths is actually the basis for why the external pool fire scenario is commonly evaluated without outflow, as indicated in API Standard 521 §18.104.22.168.4.1 (the caveat alluded to above): “…it should be recognized that operators and/or emergency responders may attempt to isolate certain lines and vessels during a fire condition in order to limit the fire spread and to safely shutdown the unit. There can also be actuated valves that fail in the closed condition when exposed to a fire. It can be difficult to establish with a degree of certainty whether a particular line will indeed remain open under all fire conditions.”1
We often check a similar rationale when evaluating outflow during tube rupture scenarios involving low pressure utilities. If it is common practice at a facility to close off a cooling service to find the source of the leak evidenced at the cooling tower, then we cannot be reasonably assured of an open path in the event of a tube rupture without some additional safeguards.
We will also not take credit for flow through automated valves that may be closed during normal operation, such as pressure spill valves, consistent with the philosophy of no credit for favorable response of instrumentation and considering all ranges of normal operation. API Standard 521 §22.214.171.124 specifically indicates “…no credit should be taken for any favorable instrument response. Minimum normal valve position is the expected position of the valve prior to the upset incident, that is, the position of the valve when at minimum design flow rates (unit turndown conditions). Therefore, unless the condition of flow through the control valve changes, credit can be taken for the normal minimum flow of these valves, corrected to relieving conditions, provided that the downstream system is capable of handling any increased flow.”1
Not affected by scenario. The excerpt above highlights the potential for alternative flow paths to be affected by the overpressure scenario, specifically for instances where a downstream automated valve may reasonably respond in such a way as to limit the outflow. For example, a shutdown valve that actuates on high pressure downstream of a letdown valve into a unit may be expected to close on elevated pressures in the event of a failure open of that letdown valve.
This type of situation is not limited to automated valves downstream. As an example, compressor systems often have emergency shutdowns on the detection of liquid in the inlet (or high liquid level upstream). A case involving a dump of liquid from a high pressure separator with subsequent gas breakthrough may not have an open outlet flow path for the vapor from the low pressure separator through a vapor recovery compressor if the compressor shuts down on high liquid level.
Downstream capacity. The ability of the downstream system to handle the flow is an important consideration, particularly for situations involving abnormal inputs, such as during tube rupture scenarios. For example, we once encountered a case where credit for flow to the steam header was taken from a tube rupture, yet the capacity of the steam header to handle the flow was not evaluated. We found that the low pressure steam header had limited volume, a normally closed pressure spill valve going to atmosphere, and no pressure relief device to relieve any excess fluid. In this case, the steam header did not have the capability to handle the excess flow of hydrocarbons from the tube rupture.
For cases where the flow path goes to a closed system, the capability of that downstream system to handle the flow often relies on its overpressure protection. We encounter this often in cases involving hot oil circulation systems with tube rupture scenarios – the capacity of the intervening piping and the capability of the relief devices on the hot oil surge drum have to be evaluated before we can accept the hot oil system as a viable outflow path.
The evaluation of the capability of downstream systems is not limited to tube rupture scenarios. When evaluating scenarios involving the loss of heat in series fractionation, the ability of the downstream tower’s condensing capability can be challenged by the introduction of more volatile components. In some cases, the cooling medium does not have an approach temperature low enough to condense the lighter components, regardless of the normal heat transfer capabilities (highlighting the need to check actual heat exchanger performance in these cases, as opposed to simply applying a heat transfer duty).
Abnormal fluid phase. The effects of a potentially different phase through a normally open path also need to be evaluated for the hydraulic and mechanical capabilities of that alternate flow path. The potential for a gas relief into a low-pressure liquid-filled system during tube rupture is a common example for this consideration. After the impulse associated with the tube rupture is evaluated (see our Fireside Chat regarding the Energy Institute guidelines for this evaluation), one may look into whether the gas can be relieved through the normally-opened liquid-filled path, such as is common with cooling water headers. In addition to frictional effects, the pressures developed as the liquid is displaced must take into account the effect of the acceleration of the liquid by the gas. Because of the additional (usually significant) pressures caused by the acceleration of the liquid, it is common to impose a ‘no acceleration’ constraint on the credit for outflow in these cases.
In addition to the hydraulic effects, one should also determine whether the piping itself can handle the different fluid phase. This may either be the weight associated with liquid-filled piping that is normally vapor-filled, or the impact of slug flow when gas is pushing through a normally liquid-filled piping.
Conclusion. Accounting for the potential for outflow from a pressurized system other than through a pressure relief device in the analysis for a potential cause of overpressure is acceptable practice with plenty of precedence; however, the availability, reliability, and capability of any alternative flow paths needs to be evaluated before doing so.
Example excerpts for outflow credit from API Standard 521, 6th Edition1
§126.96.36.199.3 (loss of cooling, partial condensing systems) “The required relieving rate is the difference between the incoming and outgoing vapor rate.”
§188.8.131.52 (inlet control valve failure) “The required relieving rate is the difference between the maximum expected inlet flow and the normal outlet flow, adjusted for relieving conditions and considering unit turndown, assuming that the other valves in the system are still in operating position at normal flow.”
§184.108.40.206 (abnormal process heat input) “The required relieving rate is the maximum rate of vapor generation at relieving conditions (including any noncondensables produced from overheating) less the rate of normal condensation or vapor outflow.”
§220.127.116.11 (inadvertent valve opening) “the vessel should have a PRD large enough to pass a rate equal to the flow through the open valve; credit may be taken for the flow capacity of vessel outlets that can reasonably be expected to remain open.”
§18.104.22.168.2, 22.214.171.124.4 (tube rupture) “Capacity credit can be taken for the low-pressure side piping … To determine the influence of piping, either in eliminating the need for a relieving device or in reducing relieving requirements, the configuration of the discharge piping and the contents (liquid or vapor) of the low-pressure side should be considered. If the low-pressure side is in the vapor phase, full credit can be taken for the vapor-handling capacity of the outlet and inlet lines, provided that the inlet lines do not contain check valves or other equipment that could prevent backflow. If the low-pressure side is liquid-full, the effective relieving capacity for which the piping system may be credited shall be based on the volumetric flow rate of the low-pressure side liquid that existed prior to the tube rupture. However, if a detailed analysis is performed, a capacity credit may be taken for acceleration of the low-pressure side liquid.
If the piping system to the low-pressure side of heat transfer equipment contains valves, their effect on the capacity of the system when overpressure occurs should be taken into account. Valves provided only for isolation may be assumed to be fully opened. In calculating relieving-capacity credit for the piping system, one should consider the valves used for control purposes to be in a position equivalent to the minimum normal flow requirements of the specific process. However, this assumption cannot be made if the valve could automatically close because of the emergency situation.”
 American Petroleum Institute. “API Standard 521: Pressure-relieving and Depressuring Systems”. 6th Edition, 2014 Jan.