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CNG Fueling Station Maintenance: Draining Storage Vessels is Critical Yet Often Overlooked

By Leo Thomason, Executive Director, NGVi    

Performing routine maintenance on compressed natural gas (CNG) stations is essential, and helps protect the investment in both fueling stations and vehicles.  One of the most critical preventative maintenance procedures required at a CNG station is draining the high-pressure storage vessels.  In this article we will explore why this procedure is critical, how to properly drain the vessels and the serious consequences of NOT conducting the procedure.

CNG Starts With a Compressor

CNG is produced by compressing natural gas in a reciprocating compressor.  The natural gas is typically processed through a dryer before it enters the compressor to remove any unnecessary water.  In a fast-fill CNG station, the compressed gas is then stored in high-pressure storage vessels for later dispensing. 

ASME Pressure Vessels

The most common type of high-pressure storage used at CNG stations today is American Society of Mechanical Engineers (ASME) pressure vessels.  These are pressure vessels that have been designed according to the ASME Boiler and Pressure Vessel Code (BPVC) to have a long, useful service life, and one that ensures the protection of human life and property.  ASME works as an Accreditation Body and entitles independent third parties such as verification, testing and certification agencies to inspect and ensure compliance to the BPVC.  ASME pressure vessels must be periodically drained of any liquids that may accumulate from natural gas in the form of water, or from the compressor lubrication system in the form of oil. 

Managing Oil and Water at the CNG Station

Compressor oil and water are inevitable in a CNG station, but why?  They occur because any water or oil in a vapor phase that is entrained in the CNG as it enters the high-pressure storage system will become a liquid as they cool.  Although CNG stations use multiple coalescing filters to help remove any liquid or aerosols from the gas supply, coalescing filters cannot catch substances in a vapor phase. 

Outward symptoms of oil issues would include oil or water present when draining coalescing filters, or vehicles that fuel at the CNG station that are hard to start, hesitate under acceleration or stall.  Oil likely will not be visible when draining coalescing filters at a relatively new station, but it will happen eventually as the compressor ages and the packings, gaskets and rings wear, and lubricating oil will migrate into the compressor cylinder heads and become entrained in the natural gas that is being compressed.  Ultimately, it is the effects of pressure and the high temperature—along with the physics of gas being compressed in the presence of oil—that will cause some oil to become entrained in the CNG. 

Routine preventative maintenance procedures are the best solution to mitigating the oil problem.  In addition to ensuring that the coalescing filters at the CNG station are properly sized and maintained, each ASME pressure vessel should be periodically drained.  This requires that a block-and-bleed drain system be installed on each vessel, and it is important to specify the drains during the design phase of the CNG station.  These drains should include a ball valve connected to a length of stainless steel tubing that extends down the side of the ASME pressure vessel stack and connected to a needle valve at the end of each vessel’s drain tube. 

Steps for Draining ASME Vessels

To drain the ASME vessels, with the needle valve closed, open the ball valve and allow any liquid to drain from the vessel into the stainless steel tube system.  After allowing time for any liquid to drain, close the ball valve and open the needle valve, being sure to install a container below the needle valve to catch any liquid that may drain out.  Repeat this procedure until there is no additional liquid draining from the vessel. Then repeat this procedure with each ASME vessel at the fueling station.

No special tools are necessary to drain the ASME vessels if the block-and-bleed valve system is installed.  The only item required is a container to collect the liquid. 

When draining the ASME vessels, the technician should expect that once the needle valve is opened, there will be a small amount of CNG that will escape with the liquid being drained.  While it is not something to be concerned about, there will be a small amount of pressure present when the needle valve is first opened. 

It is also important that when liquid is drained from the ASME pressure vessels, the coalescing filters in the supply piping from each compressor should be drained.

The Importance of Record Keeping

In the maintenance records for the CNG station, the amount of liquid drained from the ASME pressure vessel system should be added to any other liquids drained from filters or the compressor blow-down tank.  Over time, the amount of oil placed in the compressor (crankcase and cylinders) and the amount of oil drained from all filters and ASME pressure vessels should equal each other.  Any difference between these two numbers is the amount being transferred to the vehicles being fueled at the CNG fueling station and this will cause vehicle performance problems.

What Happens if You Don’t Drain the ASME Pressure Vessels?

If the ASME pressure vessels are not drained as part of a routine maintenance program at the CNG station, any liquid (oil or water) in the storage system will eventually work its way into the dispenser and ultimately into the fuel system of the vehicles being fueled at that station. This can create serious operational and warranty problems for fleet operators.  In addition to engine and performance problems like difficulty starting, hesitation and engine stalling, unchecked carryover oil can accumulate in the onboard CNG fuel storage cylinders.   NGVi recently was contacted by one of our clients who was removing and purging the CNG storage cylinders from each vehicle in the fleet due to accumulation of oil—an expensive task that easily could have been prevented by proper maintenance procedures, including draining of the ASME vessels. 

Preventative maintenance for the CNG fueling station—specifically draining coalescing filters and ASME pressure vessels—is the number one solution to ensuring that vehicles fueling at that station perform well and do not experience the problems associated with carryover oil.  

If you’d like to learn more about draining the high-pressure storage vessels and other preventive maintenance procedures, sign up for NGVi’s CNG Fueling Station Operation and Maintenance Training.