CNG Cylinder Hydro Testing: Rules, Cost and Validity in India

A CNG cylinder goes through repeated pressure changes every time you refuel. Gas enters under high pressure, and the cylinder expands slightly. As the gas is used, it contracts again. This cycle keeps repeating, and over the years, it can put stress on the metal. Rust can also develop if moisture gets inside. Since you cannot always spot this kind of damage from the outside, the cylinder needs to be checked at regular intervals.

This is where hydro testing comes in.

The cylinder is taken out of the car, filled with water and pressurised beyond its normal operating pressure. The test checks for leaks, damage and any permanent expansion that could make the cylinder unsafe to use.

In India, these tests fall under the Gas Cylinder Rules, 2016, and relevant Indian Standards, including IS 8451. PESO (Petroleum and Explosives Safety Organisation) oversees the safety requirements for CNG cylinders.

How Often Does a CNG Cylinder Need Hydro Testing?

A CNG cylinder must undergo hydro testing once every three years. This applies to both factory-fitted and aftermarket CNG kits. The date you need to check is the one stamped on the cylinder, not simply your car's purchase date. In some cases, the cylinder may have been manufactured a few months before the car was delivered. This means its first test could be due before the car turns three.

You should also have the cylinder inspected after an accident or fire if it may have been damaged. Do not wait for the regular testing date if you notice visible rust, dents or damage around the cylinder.

How Long Is a CNG Cylinder Valid?

A successful hydro test generally keeps the cylinder compliant for another three years, after which it must be tested again.

However, the cylinder itself has a separate service life. Many automotive CNG cylinders have a maximum approved life of up to 20 years from the date of manufacture, subject to the manufacturer's specifications and applicable rules. Passing a hydro test does not extend the cylinder beyond its approved service life.

So, there are two dates to keep in mind:

• Hydro test due date: When the next three-year safety test is required.
• Cylinder expiry date: The end of the cylinder's approved service life.
  

How Much Does CNG Cylinder Hydro Testing Cost?

The cost depends on your car's CNG setup, the number of cylinders and the testing centre. A single-cylinder setup generally costs less than a twin-cylinder setup, as the latter requires testing two separate cylinders.

These are indicative costs, and the final amount may vary by city and testing centre. If the cylinder valve is worn out or damaged, you may have to pay extra for its replacement.

For factory-fitted CNG cars such as the Tata Punch iCNG, Hyundai Exter Hy-CNG Duo and Maruti Suzuki S-CNG models, the cost can also depend on how the cylinders are arranged and how much work is needed to remove them.

How Is CNG Cylinder Hydro Testing Done?

The test must be carried out at an authorised cylinder testing centre. A local garage cannot simply inspect your cylinder and issue a valid hydro test certificate.

The process usually involves these steps:

1. Cylinder removal: The technician releases the remaining gas safely, disconnects the fuel lines and removes the cylinder from the car.
2. Visual inspection: The cylinder is checked for rust, dents, cracks and other visible damage. The inside may also be inspected for corrosion.
3. Hydro pressure test: The cylinder is filled with water and pressurised in a testing machine. This checks if the metal can handle pressure without excessive expansion or leakage.
4. Drying and valve check: After testing, the water is removed and the cylinder is dried properly. The valve is also checked and replaced if necessary.
5. New test markings: If the cylinder passes, the testing date is stamped on it and the updated compliance details are issued.
6. Refitting and leak check: The cylinder is installed back in the car, and the connections are checked for gas leaks.

The process generally takes a few hours, although the time can vary depending on the testing centre and the car's CNG setup.

Where Can You Get Your CNG Cylinder Tested?

You must visit a PESO-approved CNG cylinder testing centre. Not every CNG workshop or car service centre is authorised to carry out the actual hydro test.

You can contact your CNG kit supplier, car manufacturer or city gas distributor to find an authorised testing facility. Some dealerships may also help arrange the test by removing the cylinder and sending it to an approved centre. For more details you can check out this govt. Approved list of testing stations.

Before handing over your car, check that the facility can provide the updated test certificate and compliance plate after the cylinder passes.

What Happens If You Do Not Get Your CNG Cylinder Tested?

Missing the hydro test deadline is not something you should ignore. An expired test can create problems when you try to refill your car or get its documents checked. Here is what you need to know:

1. CNG Pumps May Refuse to Refill Your Car

This is the most immediate problem. CNG stations check the cylinder's compliance details before refuelling. If the test date has expired or the required plate is missing, you may be refused a refill until the cylinder is tested and updated.

2. You Could Face Problems During Vehicle Inspections

A valid cylinder test certificate may be checked during vehicle inspections and other compliance-related processes. An expired test can create difficulties when you need to establish that your CNG setup is safe and compliant.

3. Insurance Claims Could Become Complicated

If your CNG cylinder has an expired test certificate and the car is involved in an accident, the insurer may question whether the vehicle was being used in compliance with safety requirements. The effect on your claim will depend on the circumstances, policy terms and applicable rules. Do not assume that every claim will automatically be rejected.

4. You Risk Using an Unsafe Cylinder

The biggest concern is safety. A cylinder with internal corrosion or weakened metal may not be safe to keep using under high pressure. Regular testing helps identify problems before they become serious.

What If Your CNG Cylinder Fails the Hydro Test?

Not every cylinder passes. If the testing centre finds excessive corrosion, damage or expansion beyond the permitted limit, the cylinder can be declared unfit for further use. A failed cylinder cannot simply be repaired and put back into service. It must be condemned according to the applicable safety procedure and replaced with an approved cylinder.

A new CNG cylinder can cost around Rs. 12,000 to Rs. 18,000, depending on its capacity and type. You may also have to pay for fitting and update the vehicle's records with the new cylinder details. Remember that a hydro test cannot make an old or damaged cylinder usable beyond its approved service life.

How to Check Your CNG Cylinder's Next Test Date

You do not have to wait for a CNG pump attendant or mechanic to tell you that your test is due. Check the metal compliance plate fitted near the CNG filling point, fuel flap or another designated location in your car.

It generally carries details such as the cylinder serial number, capacity, previous test date and next test due date. You can also check the manufacturing details stamped on the cylinder itself. If the plate is missing, damaged or difficult to read, ask an authorised testing centre or your CNG kit supplier to help verify the details.

Keep Your CNG Cylinder's Test Date in Check

CNG can help reduce your running costs, but the cylinder needs regular safety checks. Get it hydro-tested every three years at an authorised facility, keep the updated certificate and compliance plate, and pay attention to the cylinder's overall service life.

It is a pretty small maintenance expense compared with the risks of using an untested high-pressure cylinder. More importantly, do not wait until a CNG pump refuses to refill your car before checking the test date.