High- and low-pressure sensors are used to monitor the flowline pressure of the well downstream of the choke. The high-pressure sensor is used to protect both the final flowline segment and the downstream process equipment. The low-pressure sensor is used to detect a leak or flowline rupture. Requirements for pressure sensors are covered in API RP 14C Section A1 for offshore platforms and by established Company practices.
The settings for these sensors must be carefully determined, reviewed, and documented. Because of the pressures normally encountered in flowline service and the proximity to the workover operations, the most commonly used pressure sensors are called “stick pilots.” See Figure 1700-2.
These pressure sensors come in a wide selection of ranges, which can be easily adjusted. The sensors have 1/2 NPT connections and are typically color-coded to identify the spring range. See Figure 1700-3.
The pressure sensors are usually connected in tandem. The holding circuit supply pressure is connected to the inlet of the high-pressure pilot. The low connection of the high-pressure pilot is connected to the inlet connection of the low-pressure pilot. The upper connection of the low-pressure pilot continues through the system holding circuit. A typical pneumatic hookup is shown in Figure 1700-4.
Should the flowline pressure downstream of the choke exceed the preset limit, the high-pressure sensor internal piston will shift upwards, blocking its supply port and venting the holding circuit pressure, thus triggering a wellhead shutdown. Should the flowline pressure decrease below the low limit, the low-pressure sensor internal piston will shift down, blocking its supply port and venting the holding circuit pressure, thus triggering a wellhead shutdown.
Pressure sensors are normally mounted on a manifold on the flowline and on the pneumatic signal tubing sent to the wellhead control panel. Some of the reasons for remote mounting are as follows.
• To avoid high wellhead pressures and process fluids in the control panel
• To reliably measure viscous liquid hydrocarbon pressures by having the sensors mounted close to the process
• To minimize the risk of chloride or sulfide stress cracking when corrosive fluids are present
• To avoid plugging problems in the tubing when paraffins are present
• To avoid plugging problems in the tubing when the formation of hydrates is possible
• To avoid freezing in long process leads in cold environments
• To avoid mechanical damage during workover operations
Dual pressure pilot sensors, such as the Fisher Model 4660, have been directly mounted in the control panel in some locations.