Pressure Vessel Steel Plate Thickness Tolerance: Nominal Thickness vs Actual Thickness
A buyer orders a 20 mm pressure vessel steel plate. Upon arrival, the actual measurement shows 19.x mm.
Is the plate under-thickness? Why does the nominal thickness differ from the actual thickness?
This is not a defect. It's a question of thickness tolerance — a standard allowance that every steel plate manufacturer works with.
1. What Is Nominal Thickness?
Nominal thickness is the specified thickness listed on the purchase order and the standard. It is the target thickness that the steel plate is designed to meet.
- ✔ 20 mm, 25 mm, 30 mm — these are nominal thickness values
- ✔ The nominal thickness is what appears on the order and the Mill Test Certificate (MTC)
- ✔ It is the reference thickness for design and fabrication
📌 Example: A buyer orders a 20 mm Q345R pressure vessel steel plate. The 20 mm refers to the nominal thickness.
2. What Is Actual Thickness?
Actual thickness is the thickness measured on the finished steel plate. It is a measured value, not a specified value.
- ✔ Measured using tools like calipers, micrometers, or ultrasonic thickness gauges
- ✔ May vary slightly across different positions on the same plate
- ✔ Should be within the tolerance range specified by the applicable standard
💡 Key distinction: Nominal thickness is the specified size. Actual thickness is the measured value. They are not expected to be exactly the same.
3. What Is Steel Plate Thickness Tolerance?
Thickness tolerance is the allowable deviation between the nominal thickness and the actual thickness. It is defined by the applicable steel standard and varies by:
- ✔ Steel grade
- ✔ Nominal thickness range
- ✔ Applicable standard
- ✔ Rolling process and delivery condition
📌 Important: Thickness tolerance is not a defect. It is a standard allowance that every steel plate manufacturer works with. The specific tolerance values must be checked against the applicable standard and thickness range.
4. Nominal Thickness vs Actual Thickness vs Tolerance
Simple logic:
Nominal Thickness + Permitted Tolerance → Acceptable Actual Thickness Range
5. Why Can a 20 mm Pressure Vessel Steel Plate Be Less Than 20 mm?
Several factors contribute to the difference between nominal and actual thickness:
6.1 Rolling Process
Steel plates are produced by rolling slabs to the required thickness. It is not possible to maintain exactly the same thickness across every point of the plate. Minor variations are normal.
6.2 Thickness Control
Steel mills use online thickness measurement and rolling control systems to maintain thickness within the allowable tolerance range. However, some deviation is expected.
6.3 Standard Tolerance
Every applicable standard specifies a thickness tolerance. As long as the actual thickness is within this tolerance, the plate is considered acceptable.
6.4 Measurement Location
Thickness measurements taken at different positions on the same plate may show slightly different values. This is normal and expected.
⚠️ What we've seen: A buyer once rejected a 20 mm plate because the edge measurement was 19.7 mm. The standard allowed down to 19.5 mm for that thickness range. The plate was acceptable — but the buyer didn't know the tolerance. Checking the standard before measuring would have saved time and frustration.
6. How Is Pressure Vessel Steel Plate Thickness Checked?
Buyers and inspectors typically use the following tools to measure steel plate thickness:
- ✔ Caliper — for edge and near-edge measurements
- ✔ Micrometer — for more precise thickness measurements
- ✔ Ultrasonic thickness gauge — for non-destructive thickness measurement
📌 Note: The measurement tool, measurement position, and measurement method can all affect the final reading. Standardized measurement practices are important for consistent results.
7. Which Standards Define Pressure Vessel Steel Plate Thickness Tolerance?
For pressure vessel steel plates, thickness tolerance is typically defined by:
- ✔ GB/T 713 and GB/T 713.2 — Chinese standards for boiler and pressure vessel steel plates
- ✔ ASME SA-516 — American standard for pressure vessel plates
- ✔ EN 10028 — European standard for pressure vessel steels
📌 Important: The applicable thickness tolerance should always be checked against the specific product standard and thickness range. Tolerance values vary by standard, grade, and thickness.
8. Does Thickness Tolerance Affect Pressure Vessel Design?
Yes — thickness tolerance is not just a manufacturing detail. It has design implications.
- Design thickness: The thickness used in pressure vessel design calculations
- Nominal thickness: The thickness specified on the purchase order
- Corrosion allowance: Additional thickness for expected corrosion
- Manufacturing allowance: Additional thickness for fabrication tolerances
💡 Key point: The purchase thickness should not be viewed simply as a "20 mm" plate. It must be evaluated together with the design requirements, applicable standard, and tolerance allowance.
9. What Should Buyers Specify When Ordering Pressure Vessel Steel Plates?
Before placing an order, confirm these details:
- ✓ Steel grade — Q245R, Q345R, Q370R, SA516 Gr.70, etc.
- ✓ Applicable standard — GB/T 713, ASME SA-516, EN 10028
- ✓ Nominal thickness — the specified thickness
- ✓ Thickness tolerance — standard tolerance or tighter requirement
- ✓ Plate dimensions — width and length
- ✓ Delivery condition — AR, N, NR, TMCP
- ✓ Ultrasonic testing — if required
- ✓ Impact test — temperature and energy
- ✓ MTC / EN 10204 — 3.1 or 3.2
💡 Pro tip: If tighter thickness control is required — such as for critical fabrication — it should be confirmed before production. The supplier needs to know if you need tighter-than-standard tolerance.
10. Frequently Asked Questions
❓ Is 20 mm the actual thickness of a 20 mm steel plate?
No — 20 mm is the nominal thickness. The actual thickness may be slightly different, within the tolerance range specified by the applicable standard.
❓ Why does my 20 mm steel plate measure 19.x mm?
This is usually within the thickness tolerance allowed by the standard. The actual thickness can be slightly less or slightly more than the nominal thickness, provided it falls within the tolerance range.
❓ What is the difference between nominal and actual thickness?
Nominal thickness is the specified thickness on the order and standard. Actual thickness is the measured thickness on the finished plate. They are not expected to be exactly the same.
❓ What is steel plate thickness tolerance?
Thickness tolerance is the allowable deviation between the nominal thickness and the actual thickness. It is defined by the applicable steel standard and varies by grade, thickness range, and standard.
❓ Is thickness tolerance included in the steel standard?
Yes — steel standards such as GB/T 713, ASME SA-516, and EN 10028 specify thickness tolerance requirements. The tolerance depends on the grade, thickness range, and standard.
❓ Can I request tighter thickness tolerance?
Yes — tighter thickness tolerance can often be requested, but it should be confirmed with the supplier before production. It may affect cost and availability.
❓ How should I check the thickness of a pressure vessel steel plate?
Use appropriate measuring tools such as calipers, micrometers, or ultrasonic thickness gauges. Follow the measurement procedure specified in the applicable standard for consistent results.
❓ Does thickness tolerance affect pressure vessel fabrication?
Yes — thickness tolerance affects welding, fitting, and the final assembly of pressure vessels. The design thickness should account for tolerance allowances, corrosion, and fabrication requirements.
11. Final Thoughts: What Buyers Should Remember
- ✔ Nominal thickness is the specified thickness — the number on the order.
- ✔ Actual thickness is the measured thickness — the number on the inspection report.
- ✔ The applicable standard determines whether the actual thickness is acceptable.
💡 Our view: When purchasing Q345R, Q245R, Q370R, 16MnDR, or other pressure vessel steel plates from China, clearly specifying the grade, standard, nominal thickness, and tolerance requirements can help avoid problems during inspection and fabrication. And if you're not sure whether a measured thickness is acceptable — check the standard.
📚 Further Reading & Related Content
- 🔗 How to Read a Steel Plate Mill Test Certificate (MTC)
- 🔗 Q345R vs SA516 Grade 70 – Comparison Guide
- 🔗 Ultrasonic Testing (UT) for Steel Plates – Buyer Guide
- 🔗 Hot Rolled Steel Plate vs Normalized Steel Plate – Buyer's Guide
- 🔗 Pressure Vessel Steel Plate Standards Explained: ASME vs EN vs GB
- 📄 GB/T 713.2-2023, ASME SA-516, EN 10028, GB/T 709
Need Pressure Vessel Steel Plates with Clear Specifications?
CEHE STEEL supplies pressure vessel steel plates including Q245R, Q345R, Q370R, and SA516 Grade 70. Send us your grade, nominal thickness, and tolerance requirements — we'll provide a quotation with full MTC documentation.