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ASME B16.48-2015 — Line Blanks
⛔ ASME B16.48-2015 covers pressure classes, dimensions, design, materials, marking, and testing of line blanks used to positively isolate piping at flanged joints. Line blanks — also known as figure-8 blinds or paddle-blank/paddle-spacer pairs — are among the most important tools in petrochemical, gas, and power piping for switching process configurations and isolating sections for maintenance. By turning the device over or swapping the blank for a spacer between the same pair of flanges, the engineer achieves either a full block (blank) or full flow (spacer) state. The Standard works hand-in-hand with ASME B16.5 (flanges) and B16.47 (large-diameter flanges) and is the standard component basis for the positive-isolation practice required by piping codes such as ASME B31.3.
⚠️ A blank is an “inherently safe” isolation device: unlike a valve it cannot leak internally or be operated by mistake, and it can substitute for valves where double isolation of hazardous media is required. But it is only effective when properly captured and gasketed between flanges — removal, turn-around, and re-clamping must follow the work-permit procedure.
1. Types and Pressure Classes 📌
Clause 2 (General) and Clause 4 (Design) define the two basic forms (Figure 1):
- Paddle blank: a solid circular plate with a long handle (paddle), used to fully block the line; the handle aids recognition, handling, and hanging.
- Paddle spacer: a ring with bore equal to the line inside diameter, inserted to keep the passage open while maintaining the same installed thickness as the blank (bolt lengths unchanged).
- Figure-8 blank: a blank end and a spacer end rigidly joined as an “8” — rotating the device switches between blocked and open states without disassembly.
Pressure classes cover Class 150, 300, 600, 900, 1500, and 2500, one-for-one with the flange class system. Dimension tables are provided per class and facing type (raised face, ring joint, flat face, etc.): Tables 1–4 give Class 150/300/600/900 blanks for raised-face flanges; later tables cover higher classes and other facings.
💡 Exact bolt-alignment compatibility is the key to interchangeability: the blank’s outside diameter, number of bolt holes, and bolt-circle diameter follow the flange drilling template of the matching class, so the same flange pair accepts either a blank or a spacer with the same bolts.
2. Pressure-Temperature Ratings and Design Thickness 🔥
Clause 3 coordinates the blank’s pressure-temperature rating with the flange class system. Blanks are not simply assigned the flange’s class number; their minimum thickness is derived from plate-bending strength as a function of design pressure, temperature, material allowable stress, and gasket-span distance. The standard’s thickness tables cover common duty for every class and size; special conditions require a plate-theory stress check.
| Rating Class | Typical Applications | Typical Form | Matching Facing |
|---|---|---|---|
| Class 150 | Low-pressure water, fire protection, general process | Figure-8 / paddle | Raised face (RF), flat face (FF) |
| Class 300 | Medium-pressure chemical piping | Paddle blank + spacer | Raised face (RF) |
| Class 600 | High-pressure steam, oil and gas lines | Figure-8 | Raised face (RF) |
| Class 900 / 1500 | High-pressure hydrotreating, ultra-high process | Paddle blank (thicker) | Ring joint (RTJ), etc. |
| Class 2500 | Ultra-high-pressure service | Paddle blank (thickest) | RTJ, etc. |
🚨 Blank thickness is not “the thicker the safer”: an oversized plate absorbs bolt preload, starves the gasket, and bows the flanges — leaking. An undersized plate bulges and fails. Use the tabulated thickness for the class and size; never improvise.
3. Materials, Marking, and Identification 📏
Clause 6 requires the same steel families as the flange system (carbon, alloy, stainless), meeting the corresponding specification’s mechanical properties and compatible with the line media to avoid galvanic corrosion. Clauses 7 and 8 give the most practically important requirements of the Standard:
- Each blank carries the manufacturer’s name or trademark, class, size, and material identification;
- Paddle blanks and spacers must be clearly distinguishable by shape and marking so that a spacer is never mistaken for a blank — or the line is left open when it should be blocked;
- Figure-8 blanks carry explicit “OPEN” and “BLANK” legends on each end for instant field verification of the current state.
Dimensional tolerances per Clause 5 cover outside diameter, thickness, bolt-hole diameter, and pitch; testing per Clause 9 includes hydrostatic verification where required.
4. Engineering Practice Essentials 🛠️
- 🔍 Selection: set class from design pressure/temperature, choose the size table by facing (RF/RTJ/FF), and verify thickness, outside diameter, and bolt layout against the flange drilling template;
- 🔧 Installation: clean the faces, center the blank, use service-matched gaskets on both sides, and tighten bolts evenly in a criss-cross pattern;
- 📋 Change-over: before rotating a figure-8, confirm the line is depressurized, purged, and the blind-installation work permit is issued; inspect facing surfaces afterward;
- 🏷️ Records: number every blind location and maintain a blind list (blind log) with media, class, and current state (blocked/open), so maintenance crews never leave a blank installed — which would cause trapped-pressure incidents.
5. Frequently Asked Questions ❓
❓ Q: How do you tell the “BLANK” end from the “OPEN” end of a figure-8?
A: Besides the shape itself (solid plate vs. open ring), the Standard requires “BLANK” and “OPEN” legends on the respective ends; additionally, cross-check the blind-list number with two-person verification at the job site.
A: Besides the shape itself (solid plate vs. open ring), the Standard requires “BLANK” and “OPEN” legends on the respective ends; additionally, cross-check the blind-list number with two-person verification at the job site.
❓ Q: Can a line blank replace a blind flange?
A: They perform different functions. A blind flange is itself a flanged component bolted to a mating flange; a line blank is captured between two existing flanges (e.g., at a valve). B16.5 blind flanges and B16.48 blanks serve different design positions and are not interchangeable.
A: They perform different functions. A blind flange is itself a flanged component bolted to a mating flange; a line blank is captured between two existing flanges (e.g., at a valve). B16.5 blind flanges and B16.48 blanks serve different design positions and are not interchangeable.
❓ Q: Why must blank thickness never be altered in the field?
A: Thickness governs plate strength and the bolting force balance. Slimming risks bulging failure; thickening makes the plate too rigid, starving gasket stress. Both destroy the seal — use the standard table.
A: Thickness governs plate strength and the bolting force balance. Slimming risks bulging failure; thickening makes the plate too rigid, starving gasket stress. Both destroy the seal — use the standard table.
🔍 Q: How are RTJ blanks used?
A: RTJ blanks are machined with a ring groove on each face to accept metallic ring gaskets; never force a raised-face blank into an RTJ flange — the joint will leak.
A: RTJ blanks are machined with a ring groove on each face to accept metallic ring gaskets; never force a raised-face blank into an RTJ flange — the joint will leak.
