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Once annulus pressure pushes the mandrel up, the same ratchet locks the mandrel to keep the tool open. Because the rupture disc vents to an atmospheric chamber, it must withstand hydrostatic pressure plus PCTV operating pressure. The greater the hydrostatic pressure, the stronger the disc must be. There are 83 disc ratings available to cover the range of hydrostatic pressures from 900 to 24,000 psi. Table 15. 13 15,000 425 H2S/acid 23⁄8 Reg. 25 17,500 500 H2S/acid 31⁄2 PH6 42 Collet Piston mandrel Rupture disc Reversing ports Closed Open Figure 22.

Diameter ports. When direct circulation starts, the reversing port restrictors limit the flow, causing a pressure difference that moves the mandrel into the new position for spotting nitrogen or stimulation fluids. The MCCV is reclosed by bleeding off the tubing pressure or increasing the annulus pressure, which causes a 500-psi differential pressure. The MCCV is not affected by the operation of the annular pressure operated tools and is not limited in operation by available surface pump horsepower.

Table 12. 13 15,000 425 H2S/acid 23⁄8 Reg. or PH6 36 PCT reference chamber Drain valve Relief valve Reference port Seal mandrel Atmospheric chamber Rupture disc Bypass port Before Trapping Reference Pressure After Trapping Reference Pressure Figure 19. PORT Pressure Operated Reference Tool. Downhole Testing Services ■ Pressure Controlled Tools 37 Formation Protector Module The FPM (Fig. 20 and Table 13) is usually run with the PORT tool when there are open perforations. This module prevents the annulus overpressure applied to close the PORT tool from communicating with the rathole.