Superlok® USA · Valves
Check Valve Cracking Pressure, Explained
The spring behind the poppet decides whether your instrument sees flow, sees nothing, or hears a rattle you will be chasing for a week.
Cracking pressure is the one number on a check valve data sheet that people quote confidently and specify carelessly. According to a check valve tech tip published by Swagelok®, a valve and fitting manufacturer writing about its own products, cracking pressure is “the pressure in which you first get an indication of flow,” and the manufacturer is careful to add that this is not the same thing as the valve being fully open. That distinction is the root of most cracking pressure problems in instrument and sample lines: a valve can be cracked, passing a trickle and chattering against its seat, and still be a long way from doing the job you bought it for.
What cracking pressure actually measures
It is a differential, not an absolute. The figure describes the pressure at the inlet minus the pressure at the outlet at the moment flow becomes detectable. That matters because the outlet of a check valve in a real system is rarely at atmosphere. If you have a 5 psi cracking pressure valve discharging into a header sitting at 30 psi, the valve does not open at 5 psi upstream. It opens when the upstream side is roughly 35 psi. Engineers who read the number as an absolute inlet pressure end up with valves that never lift.
The word “detectable” is doing work too. Manufacturers verify first flow with different methods and sensitivities, so treat the published figure as the design intent of the spring rather than a guaranteed threshold. Superlok® lists fixed and adjustable check valve configurations in 316 stainless steel, with set points confirmed per order rather than assumed from a catalogue line.
How spring selection sets the number
In a poppet, piston or ball type check valve, the spring holds the sealing element against the seat, and flow must generate enough force across that element to compress it. A stiffer spring, or a smaller sealing area, means a higher cracking pressure. Because seat area is fixed by the body size, the spring is the variable the manufacturer changes to offer low and high set points in the same housing.
So cracking pressure is a component choice on a fixed spring valve, and a preload adjustment on an adjustable one. Adjustable is useful at commissioning and a liability if nobody records where it was left. Either way, put the set point on the line list next to the pressure rating, not just the valve model.
Three ways a cracking pressure specification goes wrong, and what each one sounds and looks like in the field.
Set too low
An oversized valve, or a spring too soft for the flow available, leaves the sealing element hovering instead of parked open. Valve Magazine, published by the Valve Manufacturers Association of America, puts it plainly: the valve or spring “is oversized for your application,” and any check valve that chatters “will likely wear prematurely.” Soft springs also reseal badly. Published distributor guidance puts the practical threshold near 3 to 5 psig: above it spring force alone tends to shut off tightly, below it you depend on system back pressure.
Set too high
The opposite error starves whatever sits downstream. A low differential sample line may never build enough pressure drop to lift a stiff spring, so the analyser sees a dead leg and a flat reading. Every psi spent on the spring is also a psi taken from the budget for the filter, the regulator and the tubing run, and on a long small bore line that budget is thin already.
Mounted without thinking
Orientation changes the arithmetic. Flowing vertically upward, the weight of the liquid column and the sealing element adds to what flow must overcome, so effective cracking pressure rises. Flowing downward, gravity helps opening and can hinder reseal. Springless designs depend on orientation to work at all. Decide the mounting attitude with the spring, not on the scaffold.
Reseal pressure deserves its own line on the spec
Cracking pressure tells you when the valve starts to pass flow. Reseal pressure tells you when it stops. The Swagelok® tech tip describes resealing as the point where “there’s no indication of flow,” and notes it typically needs back pressure to happen properly. Both come from the same spring, but they are not interchangeable: a low cracking pressure valve has little spring force left to push the element back onto its seat.
If the point of the valve is protecting an instrument against reverse flow, reseal is the number you care about. Ask for it, record it, and test in that direction. A valve that cracks at 1 psi and dribbles backwards at 0.5 psi has met its catalogue description and failed your application.
Where check valves sit in an instrument or sample line
In instrumentation the check valve is rarely the main event. It sits downstream of a sample take off to stop process fluid returning up a purge or calibration leg, ahead of a shared header so one stream cannot push into another, on the vent side of a relief valve arrangement, or upstream of an inline filter so collected debris cannot wash back. All low flow, small bore duties, which is exactly where an oversized valve chatters and an overstiff spring starves the loop. The same discipline that governs analyser sample system layout applies: size for the flow you actually have.
Cleanliness matters at these flows too, because a rough bore holds particulate that eventually reaches a seat, and a seat that will not seal makes cracking pressure academic. Superlok® fittings are finished to give the finest interior surface finish of any tube and pipe fitting on the market, and the same logic applies to valve internals and to the tube fittings either side. Specifying corrosion-resistant 316 stainless steel throughout removes one more reason for a seat to degrade.
| Symptom | Likely cause | What to change |
|---|---|---|
| Rapid clicking or rattle at the valve | Valve or spring oversized for the available flow, element hovering off the seat | Smaller valve body, or a lower cracking pressure spring so the element goes fully open |
| Instrument reads flat or lags | Cracking pressure too high for the differential the line can produce | Lower set point, shorter run, or recover pressure elsewhere in the budget |
| Valve passes slowly in reverse | Reseal pressure never met, soft spring with no back pressure behind it | Higher cracking pressure spring, or add back pressure downstream |
| Opens later than expected | Outlet pressure not accounted for, or vertical upward mounting | Re-specify against true differential, or change mounting attitude |
| Set point drifts over time | Adjustable preload disturbed, or seat and element wear from chatter | Record and lock the set point, then fix the underlying sizing error |
Triage guidance only. Cracking and reseal set points, materials and pressure ratings are confirmed per order.
Common questions
Is cracking pressure measured at the inlet or across the valve?
Across it. The figure is the differential between inlet and outlet at first detectable flow, so any back pressure at the outlet adds directly to the upstream pressure needed to open the valve.
Can I lower cracking pressure by cutting the spring?
No. Altering a spring changes rate and free length in ways nobody has characterised, voids the manufacturer’s rating, and usually ruins reseal. Specify a different spring or an adjustable design instead.
Why does my valve reseal on the bench but not in the plant?
The bench almost certainly had back pressure the plant does not. On low cracking pressure valves the spring alone may not give a tight shutoff, so removing back pressure removes the mechanism that was closing it.
Does mounting orientation really change the set point?
Yes, and more than people expect on light springs. Vertical upward flow adds the weight of the element and the liquid column to what flow must overcome. Vertical downward flow subtracts it and can delay reseal.
What should I record on the line list?
Valve model, cracking pressure, reseal pressure if it is critical, flow direction, mounting attitude, and body and seal materials. The installation guide covers the makeup side, and the catalog gives the configuration options.
Need a cracking pressure confirmed, not guessed?
Send the differential you actually have and the direction that matters, and we will quote against it.
Superlok® North America
Instrumentation valves and fittings listed in 316 stainless steel.
Sources
- Swagelok®, “Check Valve Basics: Cracking and Resealing Pressure,” Swagelok Tech Tips, a valve and fitting manufacturer’s own technical publication. Supports the definitions of cracking pressure and resealing pressure. safst.swagelok.com
- Industrial Specialties Mfg., “What is Cracking Pressure?” a fluid component manufacturer and distributor publication. Supports the differential definition, the practical 3 to 5 psig reseal threshold, and the effect of mounting orientation. industrialspec.com
- Valve Magazine, “What’s the Chatter About Check Valves?” published by the Valve Manufacturers Association of America. Supports oversizing as the cause of chatter and premature wear as its consequence. valvemagazine.com
Technical note: the sources cited above establish how cracking pressure and reseal pressure are defined, why an oversized valve or spring chatters, and how mounting orientation and outlet pressure shift the effective set point. They do not establish a set point for your line, and no figure in this article should be read as a specification for any particular Superlok® part. Cracking pressure, reseal pressure, seal and body materials, grade and pressure rating are confirmed per order.
Trademark disclaimer: Superlok® is a registered trademark of its owner. Swagelok®, Parker® and Ham-Let® are registered trademarks of their respective owners. No affiliation, endorsement, sponsorship or approval by those companies is claimed or implied, and nothing here states or suggests that Superlok® products are the same as, equivalent to, compatible with or a replacement for any other brand. Cross-references are made by form, fit, and function only.