Flowline Pressure Rating, Ten Foot Rule

Flowlines transport hydrocarbons from the wellhead to the first downstream process equipment or vessel. Flowlines are divided into flowline segments. A flow-line segment is a portion of the flowline that has an assigned operating pressure from the other portions of the flowline. Thus, most wells have an initial and a final segment with an inline pressure reducing device or choke separating the segments.

API RP 14C Section A1 describes the requirements for safety devices and their location for offshore platforms. These rules follow the Company requirements and should apply on land as well as offshore. The following discussion is an abbreviation of RP 14C. It assumes that the flowline has only one choke and therefore is divided into two segments. Refer to Figure 1700-15.

Recommended Safety Devices for Wellhead Flowlines

Two important considerations determine the requirements for safety devices on wellheads and flowlines.
• Is the first choke device in the initial flowline segment less than 10 feet from the wellhead?
• Is the maximum allowable working pressure of the final flowline segment (after the choke) greater than or less than the shut-in tubing pressure (SITP) of the well?

When the distance to the choke is less than 10 feet, then pressure sensors in the initial segment of the flowline upstream of the choke are not required. When the distance is greater than 10 feet, then API RP 14C only requires a low-pressure sensor to detect leaks and ruptures. High- and low-pressure sensors are always required in the final flowline segment downstream of the choke. When the maximum allowable working pressure (MAWP) of the final segment of the flow line after the choke is greater than the shut-in tubing pressure (SITP), then both high- and low-pressure sensors are required to detect a blocked line or flow control failure as well as a leak or rupture.

However, when the MAWP of the final flowline segment is less than the SITP, then a pressure relief valve as well as both high- and low-pressure sensors are required. This requirement follows the concept of an independent backup device discussed in Section 1300, “Process Alarm and Shutdown Systems.”

In the previous case, where the MAWP of the final segment is less than the SITP, API RP 14C allows the substitution of a second shutdown valve and an independent high-pressure sensor in place of a pressure relief valve.

If the flowline has no choke then the MAWP for the entire flowline must be greater than the SITP. Both high- and low-pressure sensors are required in this case where there is only one segment.

Flowlines may have more than two segments. API RP 14C should be consulted to determine the required safety equipment.

For all flowlines, the following safety devices should be included:
• Check valve in the final flowline segment to prevent any backflow
• ESD shutdown
• Fire loop shutdown
• Downstream process equipment shutdown

In some locations fusible plugs have been installed in the pressure line to the SSV actuator and in other locations they have been installed inside every control panel.

Leave a Reply

Your email address will not be published. Required fields are marked *