Field Service Bulletin
Defueling 26610000 Behind the Cab Systems
ENP-521
Rev. A: September 8, 2016

1. Introduction
The 26610000 behind the cab systems have different plumbing and require some changes
to the defueling process. This document explains how to prepare and operate the
26610000 BTC and similar systems for defueling, however, it does not cover where to put
the fuel. To minimize methane discharge, the best practice is to consume as much fuel as
possible (drive it), and then perform system defueling.
Since it can take several hours to defuel a large and completely full system, it is important
to know when to defuel or when to depressurize the system.
Defueling is necessary when servicing components connected directly to the fuel cylinders
which cannot be isolated using valves or system components.
However, if components can be isolated from the cylinders, defueling is not necessary,
and depressurizing is needed.

Figure 1 A quick reference guide to decide if defueling is necessary.

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2. Affected Units
Agility Fuel Systems 26610000 BTC and similar fuel
systems with simplified defuel plumbing.

3. Corrective Action
None. This is a procedure / instruction only.

4. Parts, Tools and Materials
•
•
•

Wrench to open/close the bleed valve in the fuel
management module (FMM).
CNG-approved defuel hose with NGV-1 fitting
on one side and a compatible connector for the
defuel station.
Alternative: Vehicle to Vehicle Fuel transfer
hose, Agility Fuel Systems part number 20100021.

Figure 2 CNG vehicle to vehicle
transfer hose 20100021.

5. Things to Know
1. Pressure in the vehicle must be higher than the pressure in the defueling system.
2. ALWAYS open or close valves slowly to prevent damage.
3. Consider whether or not to defuel completely (vehicle requires towing) – or –
partially defuel so the vehicle can be driven to the service facility or motor pool.
4. Pressures and times may vary depending on system capacity, starting pressure and
desired ending (vehicle) pressure.
5. Best practice: Drive the vehicle to near-empty to reduce the amount and time for
defueling.
6. An Agility Fuel Systems Pony Tank can be helpful if the vehicle must be moved a
short distance after defueling. Refer to Section 13.
7. Most defuel stations are not capable of fueling a vehicle.

6. Staying Safe
1. Always wear personal protective equipment (PPE), including safety glasses,
visibility vests, proper footwear and fall arrestment harnesses as necessary.
2. Defueling must be done in an approved natural gas service facility or well ventilated
area.
3. Fire extinguishers should be charged, ready and accessible.
4. Know where the emergency stop switches are.
5. Chock wheels to prevent movement.
6. No smoking.

7. Defueling Options
The best and safest way to defuel a CNG system is to transfer fuel back to a dispensing
station, if so equipped. Check with your filling station for specific procedures.
There are three common defueling options:
(a) Vent to Atmosphere
First, verify whether or not this method is legal. There may be local air quality regulations
restricting the release of methane into the atmosphere. If atmospheric venting is
acceptable in your area, then a venting facility compliant to regulations must be used.

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(b) Compressor Transfer
In this defueling method, a compressor at the fueling station extracts the gas from
vehicle.
(c) Return to Supply
The third method returns fuel to a distribution system. When using this process, pressure
in the vehicle cylinder and the supply system pressure will equalize, so some pressure
may remain in the vehicle fuel system.

8. Defueling the 26610000 Behind the Cab System

UPPER
CABINET

DEFUEL PANEL
AND
LOW PRESSURE GAUGE

LOWER
CABINET
FMM AND
HIGH
PRESSURE
GAUGE
Figure 3 The defuel controls are located behind the system cabinet lower door. The fill
receptacles, high pressure gauge and quarter-turn valve are on the lower right corner.

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CLOSED
OPEN

BLEED
VALVE
Figure 4 Left: The manual shutoff valve on the FMM should be CLOSED. The high pressure
gauge reads cylinder pressure. Right: Next, OPEN the defuel valve. (Photo shows an empty
system, so gauge shows zero pressure.)

1. The manual valves on all fuel cylinders should be OPEN.
2. Refer to Figure 4. CLOSE the red handle 1/4-turn shutoff valve. The high pressure
gauge should read current cylinder pressure.
3. OPEN the defuel valve.
a. The low pressure gauge should read a nominal pressure of about 125 psi.
4. OPEN the bleed valve in Mani-Filter™. This relieves pressure in the manifold.
5. CLOSE the Mani-Filter bleed valve when pressure is relieved.
6. Connect an approved, electrically-conductive defuel hose to the defuel receptacle
(Figure 5).

Figure 5 Connect a CNG approved defuel hose to the defuel receptacle.

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OPEN

Figure 6 OPEN the 1/4-turn shutoff valve. Watch the high pressure gauge as the fuel moves
through the system.

7. See Figure 6. OPEN the red handle 1/4-turn shutoff valve on the FMM and
commence defueling.
a. The high pressure gauge should indicate pressure is decreasing.
b. Depending on how much fuel is being transferred, the valve may freeze. If
this occurs, stop the process and wait until the valve thaws. Water from a
garden hose may be used to thaw the defueling valve.
8. Once defueling is complete, CLOSE 1/4-turn shut off valve on the FMM.
9. Next, OPEN the bleed valve in the Mani-Filter to relieve manifold pressure. This will
allow safe removal of the defueling hose.
10. CLOSE the bleed valve.
11. Disconnect the defuel hose.
12. CLOSE the defuel valve and OPEN the 1/4-turn shut off valve on the FMM.

A passive fuel transfer process like this does not completely remove fuel. There will
always be some fuel left in the system. This fuel pressure must be vented into the
atmosphere. See Section 12.

9. If You Must Drive Away
Refer to the table in Figure 6. If you must drive the vehicle away from the defueling station,
watch the high pressure gauge carefully and stay within the “OK to Drive” zone shown in
green.

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For example, defueling a 60 DGE system to 500 psi will leave about 4.2 DGE in the
cylinder.
Storage Pressure (PSI)
1000
900
800
700
600
500
400
300
200

60
12.5
10.8
9.2
7.5
5.8
4.2
2.5
0.8
0.0

Useable Fuel for Various System Capacities at Given Presssure (DGE)
75
100
120
160
15.6
20.8
25.0
33.3
13.5
18.1
21.7
28.9
11.5
15.3
18.3
24.4
9.4
12.5
15.0
20.0
7.3
9.7
11.7
15.6
5.2
6.9
8.3
11.1
3.1
4.2
5.0
6.7
1.0
1.4
1.7
2.2
0.0
0.0
0.0
0.0

175
36.5
31.6
26.7
21.9
17.0
12.2
7.3
2.4
0.0

Figure 7 Useable fuel for various system DGE capacities.

11. In Case of Emergency

Figure 8 A typical emergency shut-off switch on the CNG compressor building.

If a fuel emergency happens – a major gas leak or fire - Hit the emergency cut-off switch
and call 9-1-1.

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12. Vent Post for Venting to Atmosphere

Figure 9 A vent post is needed to safely discharge CNG into the air.

When venting residual gas to the atmosphere, a vent post should be used. There should
be no ignition sources near the venting system and it should be located away from people
and vehicle traffic. Other requirements:
1.
2.
3.
4.

The pipe must extend at least 10 feet above ground.
The vent pipe must be connected to an electrical ground.
A high pressure electrically conductive flexible hose.
A hand valve to control the flow of gas.

13. Temporary Fuel Solutions
A temporary fuel source is needed when work is completed. Agility Fuel Systems Pony
Tanks are available to supply fuel for short runs from the defueling station to the service
bay or a staging/vehicle storage area or fuel station.

NOTE
Pony tanks are for temporary use when the vehicle is empty. They are NOT approved for
use on public roads.

Figure 10 Various Pony Tanks are available to temporarily supply fuel to a de-fueled vehicle.

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14. Transferring Fuel from One Vehicle to Another
If transferring fuel from one vehicle to another, the receiving vehicle must have a lower
pressure than the vehicle being defueled. Follow the defueling procedure but connect the
fuel transfer hose (part number 20100021) to the defuel receptacle and the other end to
the vehicle receiving fuel.
Remember to reinstall dust caps on both vehicles.
Transfer hose instructions are found in Field Service Bulletin ENP-425, “Vehicle to Vehicle
CNG Fuel Transfer with the Fuel Transfer Hose”

Figure 11 The fuel transfer hose, Agility Fuel Systems part number 20100021.

14. After Vehicle Repairs are Completed
Before the vehicle can be returned to operation, perform a quality control check of the
vehicle fuel system, and include the observations in the vehicle service records. This
should take from 10 to 20 minutes.
Perform a leak test and focus on the repair area. Fix any leaks.
Make sure all valves are set to OPEN or ON.
1) All cylinder valves - open
2) FMM front panel shut-off valve – on
Perform a General Visual Inspection
1) System and vehicle decals
2) Cylinder mounts
3) Cylinder valves
4) Emergency/Manual Shutoff valve
5) High pressure gauge
6) Low pressure filter
7) Low pressure gauge
8) Dashboard fuel gauge
9) Hoses and tubes
10) PRD components
11) Drive-away prevention caps

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6. Warranty Information
Does not apply.

7. References – Agility Fuel Systems Bulletins and Manuals
•
•
•
•
•
•

“Safely Working on CNG Fuel Systems,” ENP-391
“Service Facility Fuel Handling Equipment,” ENP-380
“Vehicle to Vehicle CNG Fuel Transfer with the Fuel Transfer Hose,” ENP-425
“Pony Tank Operation Manual,” ENP-005
“Pony Pallet Operation Manual,” ENP-372
“Type 2 Pony Tank Operation Manual,” ENP-249

The Agility Fuel Systems Video Library
“CNG Fueling and Defueling”
http://www.agilityfuelsystems.com/agility-fuel-systems-academy-video-library.html
If you have any questions, contact Agility Fuel Systems Product Support at 949-267-7745
or [email protected].
Agility Fuel Systems Product Support Group, 1815 Carnegie Ave., Santa Ana, CA 92705
Proprietary Statement
The information provided within this document is proprietary and confidential. All prior versions,
including updates and revisions forwarded separately, are proprietary. The information provided by
Agility Fuel Systems to its customers and clients is solely for the use of those customers and
clients. No portion of this manual may be reproduced or distributed without express written consent
of Agility Fuel Systems. Agility Fuel Systems reserves the right to utilize the intellectual property
contained within this publication as content for any other publication produced by Agility Fuel
Systems.
Trademark Notice
Agility® is a registered trademark of Agility Fuel Systems. Drop-N-Go™ and Mani-Filter™ are
trademarks of Agility Fuel Systems. Trademarks of other manufacturers are the property of their
respective companies.
Revision

Description

Author

Approved

--

Initial Release

W. Yoshida

A. Bhakta

7/13/16

A

Fig. 7: Added 175 DGE

W. Yoshida

A. Bhakta

9/8/2016

ENP-521: Defuel Procedure for 26610000 BTC Systems

Date

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