More flange guards fail on fit than on material. A shield that is one size too large sags away from the flange face and lets a jet escape sideways; one that is too small will not close at all and ends up in a store room. Neither problem is expensive to avoid, but both are easy to create when a purchase order says nothing more specific than “6 inch flange covers”.
This guide covers exactly what information you need to order the right flange guard size the first time, how to measure a joint you do not have drawings for, and the special cases that catch people out.
What actually determines flange guard size
A flange guard is sized to the flange, not to the pipe. Two 6 inch lines can carry flanges of quite different outside diameters if they are built to different pressure classes or different standards. Three pieces of information define the shield:
- Flange standard — ANSI/ASME B16.5, DIN/EN 1092, JIS B2220, or a proprietary standard.
- Nominal size — NPS in inches for ANSI, DN in millimetres for DIN and JIS.
- Pressure class or rating — Class 150, 300, 600 and upward for ANSI; PN10, PN16, PN25, PN40 for DIN; 5K, 10K, 16K, 20K for JIS.
Give a supplier those three items and the shield can be specified without any measuring at all. It is only when drawings are missing, or the joint is non-standard, that you need to reach for a tape.
Typical ANSI Class 150 flange dimensions
The table below shows the dimensions most commonly encountered on ASME B16.5 Class 150 raised-face flanges. Use it as a sanity check on a measurement, not as a substitute for your own spec sheet.
| Nominal size | Flange OD | Bolt holes |
|---|---|---|
| 1 in / DN25 | 4.25 in | 4 |
| 2 in / DN50 | 6.00 in | 4 |
| 3 in / DN80 | 7.50 in | 4 |
| 4 in / DN100 | 9.00 in | 8 |
| 6 in / DN150 | 11.00 in | 8 |
| 8 in / DN200 | 13.50 in | 8 |
| 10 in / DN250 | 16.00 in | 12 |
| 12 in / DN300 | 19.00 in | 12 |
Notice that bolt count changes with size and class. This matters because bolt count is often the fastest way to confirm which class you are looking at when the paperwork is long gone.
How to measure a flange you have no drawing for
Work through these four measurements with a tape or callipers, and record them on the requisition:
- Flange outside diameter. Measure edge to edge across the widest part of the flange face. This is the single most important number.
- Overall joint thickness. Measure across both mating flanges together, including the gasket. This sets the depth of the shield.
- Number of bolts. Count them. Combined with OD, this usually pins down the standard and class.
- Bolt circle diameter. Measure centre to centre between two bolts directly opposite each other. On an odd bolt count, measure from one bolt centre to the midpoint of the opposite gap and double it.
Measure the flange itself, not any insulation or cladding around it. If insulation will stay in place, say so on the order, because the shield needs to be sized over the finished outside diameter.
Do not forget the projection around the joint
A flange guard has to be fitted and removed by a person wearing gloves, often at height or in a congested rack. Before ordering, check the clearance around the joint: how close is the adjacent pipe, structural steel or vessel wall, and can a technician reach right around the circumference?
Tight clearances point toward a two-piece clamshell design with a side closure rather than a wrap-around style that needs full access. Note the constraint at order time rather than discovering it on the scaffold.
Special cases that need extra information
Orifice flanges
Orifice flange assemblies carry tappings, and the shield must clear the pressure connections and impulse lines. Supply a photograph along with the dimensions.
Spectacle blinds and spacer plates
These increase the overall joint thickness significantly. Measure the assembled stack, not a single flange pair.
Valve bonnets and valve body joints
Valve joints are rarely a simple parallel flange pair. They usually need a purpose-made valve protector rather than a standard flange shield, sized from the valve model and body dimensions.
Non-standard and legacy flanges
Older plants and skid-mounted equipment often carry flanges that match no current standard. Adjustable shields with a banded or threaded strip closure handle a range of diameters and are the practical answer when a site has a long tail of oddities.
An ordering checklist
Copy this into your requisition template and the back-and-forth with suppliers largely disappears:
- Flange standard, nominal size and pressure class for each line
- Shield material, driven by fluid and peak temperature
- Quantity per service group
- Closure type: quick-release for frequently broken joints, permanent for set-and-forget
- Leak indicator patch required, yes or no
- Any clearance restriction or insulation to be shielded over
- Measured OD, joint thickness, bolt count and bolt circle for non-standard joints
Common sizing mistakes
Ordering by pipe size alone. A 4 inch Class 150 flange and a 4 inch Class 300 flange are different diameters. Always include the class.
Assuming metric and imperial lines are interchangeable. DN100 and 4 inch are nominally equivalent but the flange dimensions in EN 1092 and ASME B16.5 are not identical. Order to the standard the flange was actually made to.
Forgetting the gasket and blind stack. Joint thickness is measured assembled, with everything that sits between the two flange faces.
Sizing to the pipe under insulation. If cladding stays on, the shield must go over it.
What changes above Class 150
Higher pressure classes mean thicker, wider flanges with more and larger bolts. A Class 300 flange of the same nominal bore has a noticeably larger outside diameter and greater overall thickness than its Class 150 equivalent, and Class 600 more again. Three practical consequences follow.
The shield is physically bigger and heavier. Budget for two-person handling on larger high-class joints, and check that the access platform gives room to manoeuvre it.
Release energy is higher. Higher pressure service argues for a metal shield or a heavier gauge polymer, and for tighter attention to the closure design.
Stocking gets more complex. A plant with mixed classes cannot standardise on one shield per nominal size. Group your line list by size and class together, and you will usually find the real number of distinct shields is far smaller than the number of joints.
Standardising across a plant
Most sites discover that a few hundred flanged joints reduce to a dozen or so distinct shield specifications once they are grouped by standard, size, class and material. Building that grouping once has three benefits: purchase orders get simpler, spares holding drops, and technicians stop having to work out which shield belongs on which joint.
Keep the grouping in the same register you use for shield coverage and inspection, so specification, coverage and condition all live in one place.
Frequently asked questions
Can one flange guard fit more than one size?
Adjustable designs with a strip or band closure cover a range of diameters and are useful for legacy plant. For a standardised line list, fixed-size shields give a better seal and a cleaner fit.
Should I order spares?
Yes. Shields are consumables on frequently broken joints, and a small float in the store room means a joint is never left unshielded because the original was damaged during removal.
What if my flange sits at an awkward angle?
Orientation affects where the shield should drain. Tell the supplier whether the joint is horizontal, vertical or inclined so the discharge point is positioned safely.
Get the fit right the first time
Sizing a flange guard is a five minute exercise per service group, and it decides whether the shield does its job for years or becomes a store room ornament. Capture the standard, size, class, material and closure on every line, and treat measurement as the fallback rather than the default.
If you have a line list or a folder of site photographs, send them over and we will return a sized, priced schedule. You can also browse the flange guard range to see the available materials and closure styles.