Wednesday, December 5, 2007
Shutdown valve (SDV) is commonly used in oil and gas facilities for safe and isolation to avoid escalation of hazardous material from one system to another system. It is commonly controlled by Emergency Shutdown System (ESDS) which is highly reliable system. In earlier post, i have listed 12 main features required by a Shutdown valve. Normally the Shutdown valve is maintained in open position using Instrument Air. In the event of Instrument air failure, the Shutdown valve will shut closed.
Bypass Pressurization Line
During start-up, the shutdown valve in closed position. The upstream of shutdown valve will be pressurized at high pressure (i.e 100 barg) whilst the shutdown valve may still kept at 2-3 barg. High differential pressure across the shutdown will results high torque against the actuator. It may not be cost effective and justifiable to provide an large actuator just to open the Shutdown valve. A more reasonable approach will be providing a bypass line with a small shutdown valve and a throttling valve across the main shutdown valve for initial pressurization purpose. See below image.
Shutdown Bypass Valve during Emergency
There may be an emergency situation e.g. plant fire, serious leakage, etc take place during pressurization period and demand downstream system to be isolated immediately and reliably for safety purpose, thus a the bypass valve shall be connected to the Emergency Shutdown System (ESDS) to enable proper shutdown.
Distance between Bypass Shutdown Valve and Throttling Valve
High pressure drop across the throttling valve may potential leads to very low temperature downstream of the throttling valve due to Joule-Thompson (JT) effect. The fluid temperature downstream of throttling may drops below subzero (Less than zero degree Celsius). Sensible heat in the piping will transfer to cold fluid and slowly approaching this subzero temperature. The "coldness" will travel back to upstream of throttling valve and may reaches the Shutdown valve. It potentially cause the upstream Shutdown valve body temperature drops below subzero as well. Moisture from atmosphere will freeze at the Shutdown valve body and potentially cause the stem stuck at open position. This will poses hazard to the system as the shutdown may not able to close when it is demanded. It is recommended to provide to locate throttling valve 600mm downstream of the Shutdown valve. Similar arrangement in the blowdown line.
Heat Tracing Downstream of Shutdown valve
JT effect may leads to sweating / ice form at external pipe. Application of anti-condensation coating and heat tracing on the bypass pipe will help to avoid sweating / icing.
Depends on fluid composition, low temperature may leads to hydrate formation or ice formed and potentially partially/totally block the small pressurization line. Heat tracing may help (may not avoid) keep temperature above hydrate formation temperature.
Bypass Line Size
Bypass line size is very much subject to fluid composition, downstream inventory, differential pressure, etc. The line may be sized such that the flowrate is maximum but without hydrate fromation or ice formed at reasonable pressurization time. In some event, noise level may be an issue and needs additional assessment.
As downstream pressure slowly increases, the differential pressure will drops and lead to low flow passing the throttling valve. Operator may needs to slowly increase the opening to increase the throttling valve. Some plant imposing minimum manning and increase automation, those the opening of throttling valve may be automated with a pressure control with a bypass control valve. This type of arrangement shall be properly engineered as there are many issues such as hydrate, ice-form, noise associate with this arrangement.
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