All control valves, regardless of their fail-safe position, have the potential to go wide open.  Failure open of an inlet control valve is typically considered an applicable overpressure scenario for a pressure vessel when the pressure upstream of the control valve is higher than the downstream vessel’s maximum allowable working pressure, noting that credit for outflow can affect this determination.  One of the more complex situations for control valve failure is associated with the level control valve on the liquid bottoms of an upstream vessel, commonly referred to as gas blow-by or gas breakthrough.  We previously wrote about this topic in the blog Use of Settle Out Pressure in Gas Breakthrough Calculations, where various aspects of gas breakthrough credibility, and required relief rate calculation, were discussed.  A specific question that arose in response to the blog is a common one:

Do we need to consider bypass valve open, while calculating relief rate, in addition to control valve full open?

The potential for the bypass valve in a control valve station to be open when the control valve itself fails open should be considered, unless administrative controls, both mechanical locking elements (car-seal or chain-lock) and procedures, are in place to eliminate the root cause of opening the bypass valve simultaneously with the control valve in operation.  If the corrected hydrotest pressure of the downstream vessel can be exceeded due to an opening of the control valve station, then administrative controls alone are usually not considered sufficiently reliable, commensurate with the risks, to eliminate the root cause of inadvertently opening the bypass valve.

Even if administrative controls are used to eliminate inadvertent opening, the manual bypass valve may be opened during maintenance, non-routine operations such as start-up and shutdown, or other special conditions when the operators are using the bypass valve.  Potential causes of overpressure that may occur as a result of opening the bypass valve during these modes of operation need to be considered.

API Standard 521 6th Edition §4.4.8.3¹ provides general guidance on when to consider operation of the manual bypass valve simultaneously with the failure of automatic controls for overpressure scenarios, although we should note that operating companies generally have their own design guidelines for this case.

When evaluating the potential for overpressure, the pressure upstream of the control valve station is needed.  API Standard 521 §4.4.8.3 indicates the ‘normal’ operating pressure upstream should be considered unless a higher upstream pressure can occur coincident with the scenario.  The pressure actually selected varies from one company to another; some use a mechanical limitation such as a PSV set pressure, some use a safe operating limit such as a high pressure trip or alarm set point, while others use a process control set point.  The selection of the pressure should be defined in corporate design guidelines, and may be situation-dependent such as when a settle-out calculation would be appropriate.  In addition to normal operations, consideration should also be given to different operating pressures that may occur during non-routine operations mentioned above.

It is also useful to note that the flow through a control valve station may be limited by pressure drop through the inlet and outlet piping, as well as flow-limiting elements (such as a restriction orifice) that may be in the line. If these pressure losses are accounted for in the determination of the required relief rate, then one should consider identifying the piping and/or flow-limiting elements as pressure relief critical controls.