One of the more complex situations for control valve failure is associated with the level control on the liquid bottoms of an upstream vessel operating at a pressure significantly higher than the downstream system.  Opening of the level control valve or its bypass would result in flowing vapor from the High Pressure Vessel (HPS) into the downstream Low Pressure Vessel (LPS) upon loss of liquid level in the upstream vessel, commonly referred to as ballooning, gas blow-by, or gas breakthrough.

API Standard 521 6^th edition (January 2014) §4.4.8.3 has added new information on gas breakthrough which points out that this overpressure scenario may not be credible if the downstream LPS has sufficient volume to absorb the additional vapor from HPS without causing overpressure.  It also mentions the consideration of using a lower upstream pressure in quantifying the gas breakthrough required rate. 

Based on our experience, the following considerations for determining the credibility of a gas breakthrough scenario and for quantifying the required rate for control valve failure should be made:

• Just like ‘normal’ control valve failure cases, the capability of the downstream system to handle abnormal flows dictates the potential for credit for outflow from the LPS.
• If the maximum HPS operating pressure is independent of the LPS operation, as in most typical cases, then the initial HPS pressure to consider in the analysis is the operating pressure.  If the initial HPS pressure is higher than the maximum allowable accumulation pressure (MAAP) of the LPS, a downstream relief device protecting the LPS is typically designed for gas breakthrough.  In reality, overpressure due to gas breakthrough may not occur if downstream LPS has sufficient volume to absorb the additional vapor from HPS and can be confirmed by a settle-out pressure calculation.
• A check for overfilling of the LPS is usually needed, as liquid displacement by gas breakthrough resulting in rapid transient pressure increase requires additional consideration, and a settle-out pressure calculation may be of limited value.
• A settle-out pressure calculation is usually warranted for low ratios of HPS to LPS pressures and/or low ratios of HPS to LPS volumes, and is calculated based on the available vapor volumes and operating conditions in upstream HPS and downstream LPS assuming isolation of these systems.  During design, these parameters can be manipulated to make the systems inherently safer.  During rating, the settle-out pressure calculation can be useful as a ‘sharpening the pencil’ exercise for sizing and mitigation.
• If the settle-out pressure is lower than the MAAP of the LPS, then overpressure due to gas breakthrough is not credible; however, flashing of the HPS liquid prior to loss of upstream liquid level remains a potential overpressure source.  The buildup of gas in the LPS is based on that which flashes from the HPS liquid flowing through the fully opened level control valve (or bypass) from its operating pressure to the LPS relief pressure, minus any potential outflow credit.
• If the settle-out pressure is greater than the MAAP of the LPS, then overpressure due to gas breakthrough may be credible.  The buildup of gas in the LPS is based on that which can pass through the fully opened level control valve (or bypass), being driven by the HPS operating pressure to the LPS relief pressure, minus any potential outflow credit.
• As another ‘sharpening the pencil’ exercise, the HPS driving pressure may be better estimated by accounting for the depletion of the HPS vapor inventory used to raise the LPS pressure to its relief pressure.  It is important to note that API requires an allowance be made for the normal flow to HPS that maintains the upstream pressure which may counteract the credit taken for using the reduced HPS pressure to recalculate the gas breakthrough required rate; nonetheless, credit can also be taken for LPS vapor outlet.  This re-evaluation may be worthwhile for a LPS relief device that is marginally undersized.  Note that the reduced HPS pressure is not the same as and should be higher than the settle out pressure.

As stated in the opening paragraph, and illustrated in the bullets above, the gas breakthrough scenario is complex.  Having an understanding of how the pressures and volumes of the HPS and LPS compare can help in determining the credibility of the case and also in calculating the required relief rate.  The settle-out pressure calculation can be very useful when the pressure and/or volume ratios of the HPS to LPS are low to avoid sizing a relief device for the gas breakthrough case when it is not actually credible.  Always take care to ensure any assumptions made or credits taken to declare a gas breakthrough case not credible are as accurate as possible and are physically possible within the system.