Every length of pipe that leaves a mill has been pressure-tested or electrically examined, and the certificate says so. What that does not tell you is which method was used, what each method can find, and where the blind spots are. For buyers of stainless steel and nickel alloy pipe — particularly where the service is corrosive, the wall is thin, or the schedule is large — that difference is not academic: a hydrostatic test and an eddy current examination are not two ways of asking the same question. The limits below are quoted from the ASME specifications.
What each method can and cannot find
On the ultrasonic test: it can detect both longitudinally and circumferentially oriented imperfections, and the specification is explicit that different techniques should be used for differently oriented imperfections. The examination may not detect short, deep imperfections — a physics statement, not a quality shortcoming, since a tight, short, deep defect reflects the sound wave back before it can reveal itself.
On the eddy current test: it detects significant imperfections, especially of the short abrupt type. The specification adds a limitation routinely missed commercially — sensitivity decreases with wall thickness over 0.250 in. [6.4 mm], so on heavy-wall pipe eddy current is the least sensitive of the electrical methods.
On the hydrostatic test: it finds defects of a size permitting the test fluid to leak through the pipe wall, and may be either visually seen or detected by a loss of pressure. The specification says the important thing plainly — hydrostatic testing is not necessarily capable of detecting very tight, through-the-wall imperfections or imperfections that extend an appreciable distance into the wall without complete penetration.
Read those together and the trade-off is clear. Hydrostatic testing proves the pipe holds pressure; it does not prove the wall is free of tight laminar defects. Ultrasonic testing locates defects through the wall but needs the right technique for the orientation. Eddy current is fast and sensitive to the abrupt surface-type defect typical of handling damage, but falls off on thick walls.
| Method | Strong at finding | Limited by |
|---|---|---|
| Hydrostatic test | Defects that permit the test fluid to leak through the wall; visible or detected by loss of pressure | Very tight through-the-wall imperfections; defects extending an appreciable distance into the wall without complete penetration |
| Ultrasonic test | Both longitudinally and circumferentially oriented imperfections, with position | Short, deep imperfections; orientation requires the matching technique |
| Eddy current test | Significant imperfections, especially of the short abrupt type | Sensitivity decreases with wall thickness over 0.250 in. [6.4 mm] |
| Flux leakage examination | Presence and location of significant longitudinal or transverse imperfections | Sensitivity to imperfection type is affected by the calibration |
The hydrostatic test, in numbers
The test pressure is not a fixed figure; it is calculated from the material’s yield strength and the pipe geometry. The general requirements specify that each length of pipe is tested to a hydrostatic pressure producing a pipe wall stress of not less than 60 per cent of the specified minimum yield strength for ferritic alloy steel and stainless steel pipe, and 50 per cent for austenitic alloy and stainless steel pipe and for ferritic/austenitic stainless steel pipe. That 60 versus 50 split follows from how austenitic alloys behave: they work harden, do not show a sharp yield point in the way ferritic steel does, and carry lower allowable stresses relative to their measured strength.
The pressure follows from P = 2St/D, where P is the test pressure, S is the pipe wall stress, t is the specified wall thickness and D is the specified outside diameter. Two details affect the number. Wall thickness is the specified wall thickness — the nominal wall thickness according to the specified ANSI schedule number, or 1.143 times the specified minimum wall thickness. And the pressure is rounded to the nearest 50 psi [0.5 MPa] below 1000 psi [7 MPa], and to the nearest 100 psi [1 MPa] at 1000 psi and above.
There is also a practical ceiling. Regardless of the wall stress calculated from the equation, the minimum hydrostatic test pressure need not exceed 2500 psi [17.0 MPa] for outside diameters of 3.5 in. [88.9 mm] or less, or 2800 psi [19.0 MPa] for larger diameters, and a higher minimum may be agreed in the purchase order. For large-diameter line pipe at thin wall the calculated stress is often the governing factor rather than the pressure.
Two further conditions matter. The test pressure is held for a minimum of 5 seconds, and for welded pipe long enough to permit the entire welded seam to be inspected — so the weld is inspected under load, not after it. And the specification acknowledges a real limitation: the hydrostatic test may not be capable of testing the end portion of the pipe. The untestable length is determined by the manufacturer and, if the purchase order says so, reported to the purchaser — which is why an order specifying only a hydrostatic test receives a certificate with an uninspected end zone unless it asks.
How the product specifications use these methods
The product specifications do not all treat the choice the same way, and the difference tells you what the standard considers the default for that material. The table is a summary; the specification text governs.
| Product form and specification | Default method | Who chooses | Certification and marking |
|---|---|---|---|
| Stainless steel pipe (SA-312); duplex tubing (SA-789); low-temperature carbon and alloy steel pipe (SA-333) | Hydrostatic or nondestructive electric test | Manufacturer, unless the purchase order specifies | Marking letters on lengths without a completed hydrostatic test; visual imperfection criteria apply to the SA-333 electrical test |
| Carbon steel pipe (SA-106) | Hydrostatic test without leakage through the wall | Manufacturer, or the purchase order for the alternative | NH marking and “Not Hydrostatically Tested” on the certificate |
| Carbon steel forgings (SA-105) | Hydrostatic only when Supplementary Requirement S57 is specified | Purchaser, through the supplementary requirement | Mechanical test results to the table |
| Alloy and stainless forgings (SA-182) | Mechanical testing with hardness verification | Not applicable — not a pressure test | Hardness results against grade limits |
Three clauses are worth pulling out. For stainless steel pipe whose dimensions equal or exceed NPS 10, the purchaser, with the manufacturer’s agreement, may waive the hydrostatic test when in lieu of it the purchaser performs a system test, and each length furnished without the completed hydrostatic test must carry the marking “NH.”
For duplex tubing, the hydrostatic test follows the seamless pressure tube requirements, except that in the calculation of the test pressure, 64000 [441.2] is substituted for 32000 [220.6] — twice the coefficient that basis would give. For forgings — flanges and fittings — the hydrostatic test on carbon steel forgings is conducted only when Supplementary Requirement S57 is specified, while for stainless and alloy forgings the burden sits on mechanical testing, with hardness testing required to confirm the forgings fall within each grade’s limits in the tensile and hardness table.
What to write into the purchase order
The specifications make the method the manufacturer’s option unless you say otherwise, so an unspecified order gets whichever method that mill runs by default. Three items close it.
Name the method, and ask for the end-zone information. “Hydrostatic test to the requirements of the applicable product specification” and “nondestructive electric test” are not interchangeable, and one has a detection profile the other does not. If the service is chloride-laden or the failure mode you are guarding against is wall thinning, that choice should follow from the service and be written down. Where electrical testing is specified, name the practice too: the general requirements permit the ultrasonic, eddy current and flux leakage examinations, with E213, E309, E426 and E570 as the recognised practices referenced. And where you accept a length without a completed hydrostatic test, the specification requires the untestable end length to be reported if the purchase order says so — that report costs nothing and closes the one genuine gap in the hydrostatic route.
Ask for the calibration reference. The ultrasonic reference notches may be any one of the three common notch shapes, and the depth of the notch shall not exceed 1½ per cent of the specified wall thickness of the pipe or 0.004 in. [0.1 mm], whichever is greater. The eddy current calibration pipe shall contain a defined discontinuity to establish a minimum sensitivity level for rejection. These are the yardsticks against which your pipe was judged, and belong on the certificate rather than in the mill’s file.
All of this is specification reading, and it is what lets you write a purchase order which pins the verification method instead of accepting whichever arrives. Our Alloy 800 welded pipe page shows one grade where the manufacturing route changes the test options, and Alloy 625 seamless pipe and tube covers the austenitic case. If you want a quotation with the test method, marking and certification written in, send the grade, size, schedule and method to our WhatsApp and contact page and we will quote it back line by line.
