- First, which check valve are you actually buying?
- The three pressures
- The spring table is the specification
- The DK-Lok range and what each series is for
- Seals set the temperature, and the seal is an order code
- Building the order code
- Sour service and Malaysian compliance, stated accurately
- What to confirm before the requisition leaves your desk
Three pressures, not one, specify a check valve. Cracking pressure opens it, reseal pressure is where it shuts bubble-tight again, and back pressure is what it can hold. On DK-Lok’s V33 range, those three come from a spring table, and the spring designator is part of the order code. Below about 5 psig cracking pressure, the valve may not reseal bubble-tight.
First, which check valve are you actually buying?
The phrase “check valve” covers two product families that share a principle and nothing else commercially.
Large-bore process check valves sit in a pipeline between flanges. These include swing, dual-plate, and tilting-disc valves covered by API 594, 9th edition (February 2022), which is predominantly a large process-piping standard starting around DN 50 / NPS 2. If you are buying a 6-inch valve for a pump discharge, that is the standard your requisition cites.
Small-bore instrument check valves sit in an impulse line, a sample line, a gauge hook-up or an analyser panel. They are poppet or lift designs in 1/8 to 3/4 inch, and you order them by manufacturer part number. API 594 does not cover them, and it explicitly excludes lift-type and nozzle check valves.
This article focuses on the second family because specification errors are easiest to make and hardest to spot there. If you are specifying a line valve, most of what follows will mislead you.
The three pressures
Most guides mention cracking pressure and stop. The DK-Lok catalogue defines three, and a requisition that names only the first is incomplete.
Cracking pressure is the differential between inlet and outlet at which the poppet lifts. It is a range, not a number: a spring with a nominal 10 psig designator actually cracks somewhere between 7 and 15 psig.
Reseal pressure is the pressure, in the same flow direction but lower than cracking, at which the valve returns to a bubble-tight seal. Higher-cracking springs reseal by spring force. A 25 psig spring, for instance, reseals at a minimum of 17 psig, so the line has to be able to sit below that figure for the valve to shut tight.
Back pressure is what the valve holds in reverse. And here is the constraint that catches people: DK-Lok states that valves with a cracking pressure of 5 psig (0.34 bar) and lower may not be able to return to a bubble-tight seal. If you specify a 1/3 psig spring because the line is low-head, you are choosing a valve that may weep on reversal. That is a legitimate design trade, provided you make it deliberately.
One more operational note from the same catalogue: a valve that has not been actuated for some time may require a higher cracking pressure than its set value to open the first time. On a standby or seasonal line, that matters, because the first opening after a long idle period is the one you are least likely to be watching.
The spring table is the specification
For the V33 series, this is the table your requisition is really choosing from.
| Spring designator (psig) | Cracking range (psig) | Reseal/back pressure |
|---|---|---|
| 1/3 | 0 to 3 | Up to 6 psig back pressure |
| 1 | 0 to 4 | Up to 6 psig back pressure |
| 3 | 2 to 7 | Up to 4 psig back pressure |
| 10 | 7 to 15 | Minimum 3 psig reseal |
| 25 | 20 to 30 | Minimum 17 psig reseal |
| 50 | 40 to 60 | Minimum 35 psig reseal |
| 75 | 60 to 90 | Minimum 53 psig reseal |
| 100 | 80 to 120 | Minimum 70 psig reseal |
Ratings at 70 °F (21 °C), per the DK-Lok V336 catalogue. Other series have their own spring tables; do not carry these figures across.
The first four rows are where the reseal caveat bites. The last four give you a positive reseal but cost you that differential every time the line flows.
The DK-Lok range and what each series is for
| Series | Type | Max working pressure @ 21 °C | Notes |
|---|---|---|---|
| V33 (A–D) | Poppet, fixed cracking | 3,000 psig (206 bar) | SS316 and brass; the general instrument workhorse |
| V33 (E, F) | Poppet, fixed cracking | 2,000 psig (137 bar) SS316; 1,500 (103) brass | Larger orifice sizes |
| VP33 | One-piece, fixed cracking | 3,000 psig (206 bar) | One-piece body, blow-out-proof O-ring seal |
| VA33 | One-piece, adjustable | 3,000 psig (206 bar) | Adjustable cracking pressure |
| VDA33 | In-line, adjustable | 3,000 psig (206 bar) | Adjustable, in-line pattern |
| VH36 (A, B) | High pressure, fixed | 6,000 psig (413 bar) | SS316 |
| VH36 (C) | High pressure, fixed | 5,000 psig (344 bar) | SS316 |
| VCH36 | Fixed cracking | — | CNG / NGV applications |
| VL36 | Lift check | 6,000 psig (413 bar) | Metal-to-metal seat, to 900 °F (482 °C) |
On the VH36 and VCH36, the catalogue warns that the end connection can cap the pressure rating below the body rating. The dimensions table carries the figure for the connection you are ordering.
The VL36 carries an orientation requirement, and it belongs on the drawing as well as the PO. It is gravity-assisted, so it must be mounted horizontally with the bonnet nut upward so the poppet operates vertically. It is primarily a liquid-service valve, and its reverse-flow Cv is under 0.1% of forward Cv.
Seals set the temperature, and the seal is an order code
| Seal | Designator | Rating |
|---|---|---|
| FKM | VT | −10 to 375 °F (−23 to 190 °C) — standard on SS316 |
| NBR | BN | −10 to 250 °F (−23 to 121 °C) — standard on brass |
| EPDM | EP | −50 to 300 °F (−45 to 148 °C) |
| FFKM | KZ | −10 to 600 °F (−23 to 315 °C) |
On the VH36 with FKM, the range extends to 400 °F (204 °C). Match the seal to the medium and the temperature: the designator appears in the order code, so getting it wrong means a re-order.
Building the order code
DK-Lok check valves are ordered as series, then end connection, then options. A V33A with 1/4-inch DK-Lok tube-fitting ends is V33A-D-4T-; with 1/4-inch male NPT it is V33A-M-4N-; with a 1/4-inch male NPT inlet and a 1/4-inch DK-Lok outlet it is V33A-MD-4N4T-. Metric tube ends follow the same pattern, with D-6M- for 6 mm.
That structure is the point. A requisition that says “1/4-inch stainless check valve” leaves the orifice, the Cv, the spring, the seal and both end connections for someone else to guess at, and the guess usually happens at the supplier’s desk rather than yours.
A complete instrument check-valve line reads: series and size (V33A), inlet connection, outlet connection, spring designator for the cracking pressure you need, seal designator, body material, and the tag number. Then the two things the catalogue cannot tell you: the medium, and the normal and minimum flow.
Sour service and Malaysian compliance, stated accurately
Where the line carries Hâ‚‚S, the material qualification comes from the project’s material class. NACE MR0175 / ISO 15156 addresses oil and gas production; ISO 17945 / NACE MR0103 addresses sour petroleum refining and related processing environments. Neither is a blanket standard for “downstream”, and which one applies is a project decision that geography does not settle.
On the plant side, the Occupational Safety and Health (Plant Requiring Certificate of Fitness) Regulations 2024 require a Certificate of Fitness for steam boilers, pressure vessels and lifting machinery, subject to the exclusions in the Regulations and their Schedules. The certificate attaches to the plant. What documentation a component must carry comes from the design code, the design verification requirements and the owner’s inspection and test plan.
DK-Lok factory-tests every check valve for cracking and reseal performance. Whether you receive a per-valve certificate, and what material documentation accompanies it, is settled at the order stage, so put it on the PO.
What to confirm before the requisition leaves your desk
The medium, and the normal and minimum flow. The differential the line can afford to lose. Whether the line reverses fast or sits idle for long periods. Whether a weep on reversal is tolerable, because below 5 psig cracking it may be. The end connection at both ends. The seal designator against the medium and the temperature. And on a VH36, whether the end connection limits the rating below the body rating.
Ask the supplier to confirm the cracking and reseal figures for the exact spring and series before you order.
Source: DK-Lok Check Valves catalogue, V33 / VP33 / VA33 / VDA33 / VH36 / VL36 / VCH36 Series, No. V336-9, Rev. 01-01 (Aug 2023). Figures quoted at 70 °F (21 °C). API 594, 9th ed. (Feb 2022) for the large-bore comparison.
