Thursday, June 25, 2026

A control valve on the inlet to the container that is connected to a high pressure reservoir can fail open to introduce excess material. The calculation methodologies described in Control valve flow are typically employed to determine the amount of flow entering the container. In some cases, designers may take credit for continued outflow from the system, usually on the basis of the minimum operating flow through the control valve; however, care should be taken that the downstream system can handle this flow and that there is no instrumentation that will tend to close the exit in response to the overpressure conditions.

Vapor breakthrough. A special case of inlet control valve failure that results in vapor breakthrough warrants further discussion. Consider the case of a control valve on the liquid line exiting a vapor/liquid separator that has a vapor headspace above the liquid level. In the event of a control valve failing open, the initial period of the event results in the liquid passing through the control valve at a higher than normal flow rate. At the point when the upstream liquid level is depleted, the vapor that was in the headspace then has the opportunity to pass through the wide-open control valve and into the container. This situation is often referred to as vapor breakthrough or gas blow-by. This is especially problematic considering the size of the control valve and the compressible nature of the fluid passing through the control valve. In some cases, consideration should be given to providing a means to limit the amount of gas that can flow to the downstream system (e.g. installing a restriction orifice that can choke the compressible fluid flow).

Three possible liquid levels in the downstream equipment can occur as the result of the upstream liquid inventory entering the container depending on the relative volumes of liquid and the location of the nozzle at which the excess fluid is entering: the final downstream level is below the inlet nozzle, above the inlet nozzle yet below the top of the vessel, or above the top of the vessel (essentially resulting in overfilling of the downstream system).

Final level below inlet nozzle: In this case, the gas entering the equipment has an opportunity to disengage from the liquid in the vessel, and the flow rate of gas entering the equipment is typically used as the basis for relief.

Final level above inlet nozzle yet below vessel top: In this case, the gas entering the equipment mixes with the liquid and may not have an opportunity to disengage from the liquid before reaching the pressure relief device. The potential vapor-liquid disengagement may be evaluated as discussed in detail in the GPREH 2nd Edition §3.2.8-9.¹ In most cases, assuming that the vapor and liquid are well mixed throughout the vessel is sufficient and homogeneous vessel venting is commonly used.

Final level above vessel top: In this case, the gas that passes through the control valve displaces the liquid in the downstream system; therefore, the initial relief should be based on this liquid displacement. After a sufficient amount of liquid has been displaced, the event may proceed as described for the cases above; however, the sizing for the liquid displacement is usually much larger. The breakthrough of the gas presents additional hazards for the piping. With the gas from the valve displacing the liquid, the pipe may see a pressure close to the pressure in the upstream vessel. If there is a pipe specification change after the control valve, the pressure may exceed the piping design pressure. In addition, the pressure of the gas downstream of the valve may cause rapid acceleration of the liquid. Depending on the piping geometry, operating pressures and fluid properties, significant transient forces may be generated.

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.

 

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[1] AIChE Center for Chemical Process Safety. “CCPS Guidelines for Pressure Relief and Effluent Handling Systems”. 2nd Edition, 2017; New Jersey: John Wiley & Sons, Inc.

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