This article discusses some of the scenarios for an upstream control valve failure that may arise from the failure or inadvertent opening of a control or block valve segregating high and low pressure equipment and examples of mitigating the failure.
Typical situation is that the controlling case are high pressure production separator level control valve, rich Glycol level control valve, fuel gas system pressure let down station, and a gas lift supply header off higher design pressure supply source. For these systems, the safety relief valves should be sized based on:
Flow through the control valve should be calculated based on the maximum operating upstream pressure, a wide-open Control Valve (CV), and the relieving downstream pressure and as follows.
- Any normally open downstream outlets can be taken into account, so the relief load will be the difference between flow through the control valve and the minimum normal outflow from the lower pressure equipment.
- A check of the scenario with the control valve trim at the largest possible size should be made. However, the relief scenario should make sense. For instance, do not assume a larger trim size than required for the maximum hydrocarbon production rate that could be supported by the wells or upstream sources. Assuming a trim smaller than the largest possible requires the permission of the Owner's Engineer.
- For split range control (parallel control valves), assume both valves are wide open.
When the normal function of the control valve is to control the level or flow of a liquid, and the high-pressure side contains vapor, consideration must be given to all credible combinations of the fluid phase flowing through the control valve and the phase flowing through the Pressure Relief Valve (PRV). The fluid passing through the control valve will vary from liquid immediately following the failure, to two-phase when the liquid has drained from the upstream vessel (assuming liquid is present in the normal upstream feed), to 100% vapor when the feed is blocked in response to the overpressure scenario or if the feed contains negligible liquid fraction. The fluid entering the PRV may also be liquid, two-phase, or vapor, depending on the initial liquid inventory in the upstream vessel, the initial vapor volume in the relieving vessel, and the capacity available for outflow from the relieving vessel. At a minimum, the evaluation should consider and document the following potential combinations of control valve and PRV fluids:
- Vapor flow through both the CV and the PRV
- Two-phase (liquid/vapor) flow through both the CV and the PRV
- Vapor flow through the CV with liquid flow through the PRV. This combination may be deemed not credible if the free-board volume or outflow capacity available downstream are sufficient to prevent the downstream equipment from becoming liquid full
If a control valve is fitted with a bypass, the larger of the flow through either the wide-open control valve or the full-open bypass should be taken as the basis. It is typically not allowed the use of the control valve bypass for debottlenecking or for control of excursions. However, in some rare cases this may need to be considered in the design.
- It is assumed that the bypass will not be operated simultaneously with the control valve
- Scenarios involving assumed valve arrangements should be documented so the Ops guys can determine appropriate interim controls for overpressure during maintenance when bypass valves would be used