A pressure relief valve is a single-loop mechanical controller with no electronics in it. Inlet pressure acting on the disc area is the measured variable, spring preload is the setpoint, spring rate is the proportional gain, and disc lift is the final element. Swap the spring material and you have changed the gain and the setpoint calibration at the same time — the coil geometry that produced 150 psig in carbon steel will not produce 150 psig in 316 stainless, because the modulus of elasticity is different. That is why spring selection is a part-number exercise against a material-specific chart, never a like-for-like visual match.
What does the spring actually control, and what breaks when it is wrong?
Preload sets the pop point. Rate sets how far the adjusting screw has to travel per psi and how the valve behaves between simmer and full lift. Free height and solid height set whether the disc can reach rated lift before the coils bind. Corrosion pitting changes all three, and it changes them slowly, which is why a valve can pass one shop test and drift out of tolerance before the next turnaround.
Look at the signal chain before you touch the adjusting screw. Adjusting does not fix a spring that is out of range, and it never fixes a corroded one.
| Signal | Source of truth | Symptom when wrong |
|---|---|---|
| Spring preload (set pressure) | Valve nameplate + manufacturer spring chart for that material | Early simmer, or no lift at set; pop test misses target and will not trim in |
| Spring rate (wire diameter, active coils, material modulus) | Manufacturer part number, material-specific | Adjusting screw runs out of travel; set pressure moves far more or far less per turn than the chart predicts |
| Metal temperature at the spring | Relieving temperature, bonnet venting arrangement, body material | Relaxation in service; set pressure decays between inspections |
| Free height / solid height | Direct measurement against the parts list | Coil bind before rated lift, so measured capacity falls short of the stamped capacity |
| Surface condition of coils | Visual and pit-depth inspection during teardown | Set pressure drift, chatter, failure to reseat, or spring fracture on demand |
| Body/bonnet material | Nameplate and API 526 pressure-temperature limits | Valve temperature limit far below the spring's own limit |
Check 1: Does the nameplate spring range bracket the required set pressure?
Start with the nameplate and the parts list, not the catalog. Individual spring ranges are manufacturer-held data. API 526 tells you the maximum pressure a given size, orifice and flange rating combination is capable of; it does not tell you which of the vendor's springs covers 165 psig on a 1 x D x 2, 150 x 150 conventional valve. One manufacturer may carry roughly 35 distinct springs to cover 15 to 285 psig on that single valve in one material alone; another manufacturer splits the same band differently. There is no cross-vendor equivalence.
Approved repair houses and some end users hold controlled copies of those spring tables, issued and revision-controlled by the manufacturer. If you are inspecting rather than repairing, get the controlled table through your repair contractor or directly from the OEM and file it with the valve record.
Branch: if the required set pressure falls inside the stamped range, the existing spring stays. If it falls outside, you need a new spring part number — and a new set pressure means the relief scenario and the required orifice area have to be recalculated, because changing set pressure changes relieving capacity and back pressure margins. Do not treat a set-pressure change as a shop task.
Check 2: Which component actually limits the temperature?
The valve's temperature rating is the minimum rating of all its components, not the rating of the best one. Fit an Inconel spring into a carbon steel (WCB) body and the assembly is still limited by the body — around 800 °F — while the spring alloy on its own would go far higher.
The historical shift matters here. Manufacturers and API 526 moved from plain carbon steel springs to chrome alloy springs, which extend usable temperature to roughly 650 °F. A large installed population still has original carbon steel springs, and the routine replacement during overhaul is chrome steel. That substitution is normally a documented OEM upgrade with its own part number and its own set-pressure range — confirm the range, do not assume it is identical to the carbon steel spring it replaces.
Branch: if the relieving temperature at the spring exceeds the spring material's limit, the spring will relax and set pressure will fall. If the body limits first, the spring alloy choice is being driven by corrosion, and the next check applies.
Check 3: Why did the spring corrode in the first place?
Ask what wetted the coils. On a conventional valve, the bonnet is vented internally to the discharge side, so the spring, spindle and guide see relieving fluid and whatever back-pressure atmosphere sits in the header. On a balanced-bellows valve the bonnet is vented to atmosphere and the spring should stay dry — a corroded spring in a bellows valve is evidence the bellows has failed, and that failure also invalidates the valve's back-pressure compensation. Do not replace the spring and close the job.
Material selection for corrosion belongs to your materials engineer, not to the shop. Austenitic stainless resists general corrosion in many streams but is vulnerable to chloride stress corrosion cracking, which attacks a highly stressed helical spring preferentially. Nickel alloys are the usual answer for sour or chloride service. API 526 tables give the normal set of spring materials offered for each valve configuration and are a reasonable starting point for the conversation; the LESER Engineering Handbook, Chapter 9, covers spring and material reference codes if you want a vendor-neutral cross-reference to work from.
Check 4: Is a carbon-steel-to-stainless swap legitimate?
Only if the OEM lists a stainless spring for that valve, that orifice, and that set pressure — and you fit that part number. A stainless spring is not a plated carbon steel spring in different metal. Lower elastic modulus and lower allowable stress mean different wire diameter, different active coil count, and a narrower pressure band per spring, so the vendor's stainless table splits the same 15–285 psig span into a different set of ranges than the carbon steel table does.
If a stainless spring is already installed and the record does not show which part number it is, treat the valve as uncertified until it is bench-tested and the spring is identified. Measure free height and wire diameter, compare against the material-specific parts list, and if it does not match anything on the list, replace it.
Requalifying the valve after a spring change
- Confirm the required set pressure against the current relief scenario and, if the set pressure changed, re-run the sizing to verify the orifice designation and installed capacity still cover the governing case.
- Select the spring by OEM part number from the controlled chart for the correct material, and confirm the chart range brackets the set pressure with margin at both ends.
- Verify the assembly temperature limit as the minimum of body, bonnet, spring, guide, disc and seat ratings, cross-checked against the API 526 pressure-temperature limits for the body material and flange class.
- Inspect and replace the spring buttons and washers with the spring; a hardened button running against a softer replacement spring will gall and shift the effective preload.
- Bench-test to the required set pressure, then verify seat tightness at the specified percentage of set and confirm the valve reseats without chatter.
- Restamp and update the repair nameplate per the repair organization's National Board VR or ASME scope, and file the spring part number, material and test record with the valve history. A spring change that is not in the record is a spring change that will be argued about at the next audit.
Verification is the pop test plus the record, not the pop test alone. If two consecutive tests give you different set pressures on the same spring, the spring is relaxing or corroding and material selection is the open question, not calibration.
Stop and go to the manufacturer when the installed spring cannot be identified against a controlled chart, when the required set pressure falls outside every listed range for the valve, or when a material change is proposed that the OEM does not catalog for that model. Route sour-service and chloride-service material selection through your materials engineer and the OEM's application group together, and get the answer in writing before the valve goes back on the line.
Frequently Asked Questions
How do I find the spring range for a specific PSV spring?
Spring range tables are manufacturer-held and revision-controlled; get them from the OEM or through an approved repair house. API 526 only tells you the maximum pressure the valve size, orifice and flange class can handle, not which individual spring covers your set pressure.
How do I know if a carbon steel spring can be replaced with chrome steel?
Chrome alloy springs are the standard OEM replacement for legacy carbon steel springs and extend usable temperature to roughly 650 °F, but the part number and pressure range differ. Pull the chrome steel spring chart for that valve model and confirm it brackets your set pressure before ordering.
How do I determine the maximum temperature of a PSV after changing to an Inconel spring?
Take the minimum rating across every component. A WCB carbon steel body caps the assembly near 800 °F regardless of the spring alloy, so an Inconel spring in that body buys corrosion or sour-service resistance, not extra temperature.