Tubing retrievable subsurface safety valve · flapper type · conceptual model

TRSV Cutaway

A fail-safe valve held open only by hydraulic control-line pressure. Take that pressure away and the power spring lifts the flow tube, the flapper swings shut, and well pressure from below presses it onto its seat. Drag to rotate, scroll or pinch to zoom.

Closed Flow tube stroke 0%
Shut in

Closed. 4,700 psi of well pressure below the flapper holds it on its seat. Press Open valve to run the opening procedure.

Why it fails closed

Nothing holds the valve open except control-line pressure acting on a small rod piston. The piston pushes the flow tube down, compressing the power spring and swinging the flapper out of the bore. Bleed the line, cut it, or lose the surface panel, and stored spring energy returns the flow tube. The flapper's torsion spring then snaps it shut.

Once shut, the valve seals harder the more pressure the well has: well pressure acts upward on the whole flapper area and pushes it onto its seat.

What the control line has to beat

At the piston, pressure is the surface pressure plus the hydrostatic head of the control line. It must overcome the spring and the tubing pressure acting on the other side of the piston.

psurface = pvo + pt + pmo − Dset·ρcl pvo valve opening pressure (spring) · pt tubing pressure at valve · pmo opening margin incl. friction · Dset·ρcl control-line hydrostatic

Example: 1,800 + 4,700 + 500 − (2,000 × 0.433 × 0.87) = 6,247 psi. The surface panel needs at least 6,250 psi. Holding the valve open takes less than opening it, so the applied pressure can be reduced as the reservoir depletes.

Why you equalise first

With the tubing above bled off, the full well pressure acts on the flapper. The flapper's area is about 45 times the piston's, so the piston cannot win.

4,700 psi × 18 in² ≈ 84,600 lbf closing
7,000 psi × 0.40 in² ≈ 2,800 lbf opening

So the tubing is pressured from surface (adjacent wells, pumps or methanol) until the pressure above matches the pressure below. Self-equalising valves do it with a small poppet that the flow tube pushes off seat first. Try Open without equalising to see the flow tube stall.

Fail-safe setting depth

If the valve is set too deep, the hydrostatic column in the control line (or heavier annulus fluid, if the line parts) can hold the piston down on its own. The spring then can't close the valve, even with zero surface pressure.

Dmax = (pvc − pmc) ÷ ρf pvc valve closing pressure · pmc closing safety margin · ρf the heavier of control-line or annulus fluid gradient. Tubing pressure is ignored: worst case is a well open to atmosphere.

Example: (1,500 − 200) ÷ (0.433 × 1.2) = 2,502 ft. Set the valve no deeper than about 2,500 ft. A stronger spring, a nitrogen charge or a balanced dual-line piston allows deeper setting.

Opening sequence

  1. Shut in the well at surface and note flowing rate and pressure.
  2. Equalise: pressure the tubing from surface until the pressure above the flapper matches the pressure below.
  3. Pressure the control line to shut-in tubing pressure + opening pressure, plus a margin (typically 500 psi).
  4. The flow tube strokes fully down, opens the flapper and shields it and the seat from the flow.
  5. Open the well. Compare rate and pressure with the earlier values; a big difference suggests a partly open valve, which can cut the flapper and seat.

Closing sequence

  1. Close the wing valve so the well is not flowing through the closing flapper.
  2. Bleed the control line to zero. The spring and tubing pressure push the piston back, and control fluid returns up the line.
  3. The flow tube clears the flapper, which snaps onto its seat.
  4. Bleed the tubing above the valve. Pressure stays low if the flapper is holding, which is the inflow test.