damage to composite shell including cuts, gouges, scrapes, scuffs, chips, punctures,
and change in coloration or appearance. Surface cracks may appear circular or linear.

permanent
deformation

depression of composite surface resembling a dent

softness or
deflection

composite surface moves in response to pressure applied with hands

damage to cylinder
end bosses and /or
hardware
damage to dome
protection

signs of physical scrapes and scuffs to cylinder components including valves, plumbing,
fittings, pressure relief devices, and mount hardware

cracks in or loss to cylinder end dome caps

If signs of impact damage are observed, a tap test may be performed to help determine whether impact damage
is present.
The tap test is a method to identify delamination within the composite and may be performed on areas of possible
damage by tapping the surface with a quarter, or similarly size metal object, grasped between the fingers.
Differences in sound are evidence of damage.
NOTICE
False indications may be heard if the tap test is performed on the dome (end) areas. The tap test should
be limited to the main portion of the cylinder and not the dome ends.
WARNING
Decommission and dispose of the cylinder if a known impact has occurred, if signs of impact damage
are present, or if there is any uncertainty in assessing damage.

Weathering Damage
The outside layer of resin will naturally yellow over time. This is normal and does not affect cylinder performance.
Weathering damage can occur when the cylinder has prolonged exposure to sunlight and weather. Weathering
damage is evident by areas of resin degradation (chalking) and loose or broken fibers. FIGURE 7
NOTICE
Contact Hexagon Agility for evaluation of weathering damage.
a

b

c

FIGURE 7. a) cylinder not exposed to weathering, b) a weathered cylinder with yellowed resin and loose surface

fibers, c) a macro of the weathered cylinder showing the visible fibers.

Excessive Heat Damage
Hexagon Agility cylinders have a maximum service temperature as noted in the Statement of Service.
Heat damage may occur if the cylinder is involved in a fire or is near a heat source such as an exhaust system.
Evidence of heat damage includes discoloration, charring, soot, melted resin, loose fibers, melted labels, or
activation of a T-PRD. FIGURE 8
WARNING
Cylinders exhibiting evidence of heat damage must be decommissioned and disposed of.

FIGURE 8. Cylinder exposed to excessive heat. Left: surface charring, melted resin; right: melted label and resin.

Chemical Damage
Hexagon Agility cylinders are resistant to common chemicals encountered in the typical fuel service environment.
Chemical damage types are listed in TABLE 3.
TABLE 3. Chemical Damage Types

Chemical damage categories
corrosion

swelling

discoloration

softening

pitting

cracks

blistering

loose fiber

WARNING
Cylinders exhibiting evidence of chemical damage must be decommissioned and disposed of.
Leakage
Prior to leaving a Hexagon Agility production facility, cylinders are leak tested with a trace gas detector. This
testing method can detect extremely small amounts of gas and is more accurate than any possible leak check
done in the field or at the installer. No cylinder leaves the factory without a passing leak test.
Signs of leakage include the following:
1.

an audible sound—hissing—of a leak

2.

a strong smell of CNG

3.

icing, frost, or condensation visible on fuel system fittings and lines

4.

an inexplicable drop in fuel pressure over time

When investigating a potential leak in the field or at the installer, Hexagon Agility recommends performing a
bubble leak check. Using handheld electronic leak detectors often results in false-positive readings due to the
sensitivity of the equipment. A bubble leak check is performed by simply applying a certified leak detecting
solution (e.g., Swagelok Snoop®) to potential leak sources, waiting three minutes, and looking for bubbles.
WARNING
If a leak is found, remove cylinder / vehicle from service.
WARNING
If the leak source is in the plumbing or associated hardware, defuel cylinder before tightening fitting /
component connections or replace per OEM specifications, as necessary.
If the source of the leak has been isolated to the cylinder, the procedure below may be used to confirm a cylinder
leak.
False Indications of a Leak
Often, when a cylinder is filled for the first time, some amount of air will be present in the space between the liner
and composite. With time and pressure cycling, the air will work its way out. If a bubble leak check is performed
during this time, the escaping air can generate bubbles around the metallic end boss or on the surface of the
composite. This is not a leak. The number of bubbles and how long it takes to dissipate depends on the size of
the cylinder and variation in porosity of the composite. Normally, trapped air escapes within 24 hours. If bubbles
are observed during the first fill, no action is necessary unless there is a strong smell of natural gas.

A similar condition can occur if a cylinder is at low pressure or defueled. In this case, permeated gas evolving
from the liner can collect and mix with the air. When filled again, a handheld electronic leak detector may have
the sensitivity to pick up low concentrations of gas. While gas is present, the source is not a leak. Like the firstfill scenario, trapped gas normally escapes within 24 hours. If bubbles are observed during this inspection
scenario, no action is necessary unless there is a strong smell of natural gas.
Confirming a Cylinder Leak
If uncertain whether a cylinder has trapped gas or is leaking, first keep the cylinder at pressure for 24 hours, in
a well-ventilated area (preferably outdoors), then repeat the bubble leak check.
If bubbling appears the same, reduce cylinder pressure to 500 psi (35 bar) to help lower the interface pressure
between the liner and composite. This can sometimes help the trapped gas to escape more quickly. Hold at this
pressure for 24 hours and then repeat the bubble leak check.
If there are noticeably fewer bubbles, this indicates the source is trapped gas and will work its way out naturally.
The cylinder may return to service. If bubbling appears the same, contact Hexagon Agility.
WARNING
A cylinder with a confirmed gas leak must be decommissioned and disposed of.
Unraveling
Damage to cylinders can lead to unravelling when broken fiber progressively delaminates from the composite
surface. FIGURE 9
If there is evidence of lifted fiber without unraveling, it may be repaired.
WARNING
If fiber has begun to unravel, cylinder must be decommissioned and disposed of. Refer to CNG Cylinder
Decommissioning and Disposal Manual.

FIGURE 9. Fiber unraveling with a section of the composite lifted.

Composite Finish Imperfections
The method of manufacturing composite cylinders sometimes leaves surface characteristics which look like
defects, but which are simply cosmetic and do not affect the performance or integrity of the cylinder. TABLE 4
TABLE 4. Composite Finish Imperfection Types and Descriptions

Imperfection category Description

Image

hairline micro-cracking of the resin; common on composite
cylinders.

FIGURE
10

resin bubbles or
pock marks

formed as entrapped air works its way through the composite

FIGURE
11

resin and paint runs

formed when excess resin or paint drips down the composite
surface

FIGURE
12

raised fiber

occurs when fiber is lifted from the surface of the composite
creating a raised edge

FIGURE
13

tucked fiber

creates a bump where the cut fiber is tucked underneath the
final layer at the end of the winding process

FIGURE
14

appear around the dome; may be present on cylinders that
utilize wound-in dome protection

FIGURE
15

crazing

fiber gaps

NOTICE
If it is unclear whether a cylinder has a cosmetic defect or damage, contact Hexagon Agility.

FIGURE 10. Resin crazing on the cylinder composite (circled).

FIGURE 11. Solidified resin bubbles on the cylinder surface (circled).

FIGURE 12. Resin run on the cylinder surface (circled).

raised fiber

FIGURE 13. Raised fiber on the cylinder surface.

fiber tuck

FIGURE 14. Fiber tuck on the cylinder surface.

FIGURE 15. Dome end of cylinder displaying gaps in a fiber band (arrows) showing the band underneath.

End Mounted Cylinders
Visually inspect cylinder bosses, mount blocks, and hardware for damage and loosening.
Fixed End Inspection
Inspect the fixed end to ensure the cylinder is properly secured.
Signs of wear or a loose mount include the following:


block or cylinder movement



metallic dust around the boss/block interface.

WARNING
If damage is present, remove vehicle from service.
Disassemble loose mounts and inspect for damage (e.g., missing nuts, worn threads, broken bolts). Contact
Hexagon Agility for replacement components.
Sliding End Inspection
Inspect the sliding boss surface, block, and bushing for wear. Signs of wear include the following:


exposed aluminum FIGURE 16



variation in diameter along the length of the boss FIGURE 16



plastic shavings



contact between the boss and block.



a gap between the bushing and boss FIGURE 17

Wear severity and associated disposition for each sliding boss end component is shown in TABLE 5.
TABLE 5. Sliding Boss Wear Damage Severity and Disposition

Component
sliding
boss

block

bushing

Wear Depth
inches
millimeters

Disposition

≤ 0.050 in

≤ 1.27 mm

note in inspection record

0.051 ≤ 0.125 in

1.30 ≤ 3.18 mm

> 0.125 in

> 3.18 mm

decommission and dispose of cylinder

≤ 0.050 in

≤ 1.27 mm

note in inspection record

> 0.050 in

> 1.27 mm

replace

≤ 0.050 in

≤ 1.27 mm

note in inspection record

> 0.050 in

> 1.27 mm

replace

refer to “Resurfacing Sliding Boss Wear”
in CNG Cylinder Repair Procedure

WARNING
Inspect block bolts and nuts for signs of loosening. Tighten or replace hardware as necessary. Contact
Hexagon Agility for replacement components.

check for variation in diameter across boss

exposed aluminum
FIGURE 16. Indicators of sliding end cylinder boss wear.

bushing

sliding block
cylinder neck
mount

go no-go
gauge

Gap (arrow) exceeds allowed
tolerance of 1/16-in (1.6 mm) and
requires repair. Remove vehicle
from service and contact
®

Hexagon Agility CCTS.
plug
FIGURE 17. Gap indicative of bushing wear.

Strap Mounted Cylinders
Strap mount cylinders should be inspected for signs of movement relative to the strap and cracked or displaced
rubber.
WARNING
If signs of movement are present, or the rubber is cracked or displaced it must be inspected and possibly
replaced. Contact Hexagon Agility for replacement.
Inspect straps for signs of damage including corrosion, cracks, deformations, breakages, and broken bolts.


Do not torque strap bolts.



If straps are disassembled or loosened, replace rubber pads.

WARNING
If damage is present, remove vehicle from service and contact Hexagon Agility.

Labels
Check OEM cylinder labels for signs of damage including tears and weathering.
WARNING
If label is missing or illegible, remove vehicle from service and contact Hexagon Agility.
NOTICE
Contact Hexagon Agility for replacement.