We have put together a number of useful service tips that you can use to keep your NGV station online and in safe operating condition. If you have any additional questions pertaining to NGV stations, please email us.
- That the only load on the gas meter is the compressor you are clocking
- What the gas pressure is at the meter while you are clocking
- What the smallest rotary dial on the meter measures in one revolution
Next, record how long it takes for a given amount of gas to go through the meter and what pressure the meter was at when you clocked it.
Now the fun part; use the following formulas:
Correction Factor = 14.7 + P (pressure at meter in psi) / 14.7
Uncorrected Flow = Cubic feet clocked X 60 seconds / clocking time (seconds)
Corrected Flow (scfm) = Flow X Correction Factor
Example: You have determined that you need to know what the gas flow at your NGV station is. Pressure at the meter while running is 18 psig. It takes 25 seconds for the 10 cubic foot dial on the meter to rotate once. What is the corrected flow?
Correction Factor = 14.7 + 18 psig / 14.7 = 2.224
Uncorrected Flow = 10 scf X 60 / 25 sec = 24 scfm
Corrected Flow = 24 scfm X 2.224 = 53 scfm
Many of you are already documenting the performance of your NGV refueling stations. This provides you with a daily history, a means of comparing past performance with the present with an eye on future performance.
We encourage our customers to fill out log sheets on a daily basis. In one instance we had an individual that had never seen an NGV station before but was responsible for filling out the log sheet. We marked each point alphabetically for them and keyed this into our log sheet. Within weeks he could log the station in less than 10 minutes and was able to identify problem areas!
If you are not already completing daily checks of your NGV refueling station and recording your findings on a log-sheet, you may wish to start today! This information will prove most helpful in the efforts to keep your station up and running.
A daily log-sheet is simply a form that an operator completes on a daily basis by examining the station instrumentation and recording the findings. Some of these check points include:
When compared against past performance the operator can determine if there might be a problem developing. This ability to compare and predict is the foundation of Planned Preventative Maintenance.
There are a number benefits to this type of maintenance approach. First and foremost the station is kept on-line satisfying the customers that depend upon the station. Second it allows for more economical repairs. Compressor parts can be ordered and shipped via regular means and repairs maybe be scheduled for regular working hours. Both save money.
Another benefit is the increased safety of the station. A station that is maintained on a daily basis is less likely to experience a catastrophic failure than one that is neglected.
One aspect of maintenance that is often overlooked is station cleanliness. Clean equipment not only looks nice, but it also tends to show leaks much more quickly than dirty equipment. Clean equipment tends to send the message to both customers and supervisors that the personnel responsible for maintaining a site are doing exactly that
We have included a sample log-sheet that has been developed by Ingersoll Rand for their CNG compression packages.
Here is a log sheet that we suggest using >>
- Identify the sequence of operation.
- Determine the actual sequence of operation.
- Make corrections to operations required.
- Select onsite personnel to be responsible for daily maintenance. Then provide the training and time to perform the job properly.
- Set up and use a log sheet to track equipment performance.
- Keep wiring diagrams, flow schematics and OEM manuals onsite. They are much more useful there for troubleshooting purposes than sitting on a bookshelf miles away from the station.
- Check OEM maintenance schedules and draw up a plan to carry out required maintenance procedures. Document these procedures when they are completed.
- Test all equipment safety controls on a regular basis to verify their proper operation. Document these safety checks.
- If you do not have personnel within your organization qualified to troubleshoot the station equipment, contact qualified, reputable service companies and check the cost of having them run a regular maintenance check on your station. Specify what equipment checks and maintenance procedures you want carried out. Also, request that they state any additional checks they believe should be made. A standard simplex compressor with priority sequencing with a gas dryer and a dispenser can easily require an entire day to PM.
- Have your gas tested for water content (dewpoint testing) so possible freeze-up problems can be identified. Take dewpoint readings at the suction side as well as the discharge. Plan to stock a minimum of spare parts, such as gasket kits, O-rings, valve rebuild kits, control relays, etc. These should be kept onsite so minor repairs can be done with a minimum of downtime.
All natural gas contains some amount of water. This is most evident when you first light your stove top burner and see a small whiff of condensation on the stove. This is the water in the gas and at a burner tip, can not cause any problems. But compress this same gas and use it for fueling your natural gas powered vehicle and it is a different story.
Gas companies typically deliver gas with a 7#MMSCF which means 7 pounds of water per million cubic feet of gas. This is sufficiently low enough to avoid condensation, hydrate formation and freeze-ups at normal pipeline operating pressures. But 7#/MMSCF water content will cause problems for your refueling station and vehicle.
Many times we asked just how dry does the natural gas have to be. Gas companies typically deliver gas with a 7#MMSCF which means 7 pounds of water per million cubic feet of gas. This is sufficiently low enough to avoid condensation, hydrate formation and freeze-ups at normal pipeline operating pressures.
But, what does 7#/MMSCF mean for NGV applications? It means a +52 degree F. Dew point @ 3,600 psig. This can translate into potential condensation, hydrate formation and freeze ups during gas expansion due to the Joule – Thomson effect.
To eliminate any potential problems for your station or your customers, the gas should be dried to a pressure dew point (PDP) that is well below the minimum ambient temperature that will occur at the highest storage pressure.
The society of Automotive Engineers has issued a fuel specification
(SAE J1616) that specifies the gas be dried to a local dew point temperature of -10 degrees F. below the 99.0% winter design dry-bulb temperature as found in the 1989 ASHRAE handbook at the maximum operating tank pressure.
Here in Pittsburgh, PA the winter design dry bulb temperature is 1 degree F., therefore, we must dry our gas to a PDP of -11 degree F in order to meet fuel spec J1616. In San Diego, CA, the winter design dry bulb temperature is 35 degrees F., we then need to dry to a PDP of 25 degree F to meet J1616.
Contact us for the recommended PDP in your area.
Why Convert? The synthetic lubricant lasts longer and reduces the number of oil changes required. It also protects your equipment better. But beware – synthetic and petroleum lubricants are not compatible.
How to Convert: To convert from petroleum to synthetic, it is necessary to purge all residue of the petroleum lubricant from the crankcase (Mobile DTE r Extra Heavy residue cannot exceed 1% of the crankcase capacity or 1.7 oz in the IR blocks.) To accomplish this, the oil must be either wiped or flushed from the crankcase. Make sure you drain the lubricant from the crankcase while the oil is still hot.
We also suggest that you keep a supply of synthetic oil on hand. This is available from stock in five-gallon pails. Call or e-mail our parts department and ask for part number 32320426.
- Motor Horsepower
- Quantity of lubricant consumed. (see tip #2)
- Number of operating hours.
For example: we are running an Ingersoll Rand Model 05H25NGSX, a simplex 25 horsepower compressor package. Through our log sheets we know that we have added 5 ounces of lubricant for every 20 hours of operation. The formula looks like this: 25bhp X 20 hours / 5 ozs = 100 bhp hours per ounce.
Some customers may require oil consumption reporting in parts per million (ppm). The conversion of oil consumption from bhp hrs/oz to ppm can be done if the capacity and horsepower of the compressor is known. The equation for the conversion of bhp hrs/oz to ppm for air is different than that of natural gas due to the differences in density between the two gasses.
For natural gas it is PPM = 202.61 X specific Power at test conditions (bhp per 100 scfm) divided by Oil Consumption (bhp X hrs divided by ozs) So, for the same IR package listed above we would calculate as follows:
PPM = 202.61 X (25 bhp / 28 scfm) / 25 bhp X 20 hrs / 5 oz = 180.90 ppm
The equation for converting ounces of oil to PPM is as follows: PPM = 20261 X (oz. oil/hrs) / scfm of gas at test conditions.
This priority valve is really a back-pressure regulator, designed to maintain a minimum discharge pressure from the compressor package. The purpose of this is to assure that the final stage of the compressor is loaded.
It is important to keep this final stage loaded in order to prevent oil carryover.
Ingersoll Rand recommends 2500 psig as the minimum pressure to be maintained on the CNG packages and, of course, higher pressures are quite acceptable.
Compressors tested without a back-pressure regulator show as much as 8 times the amount of oil carry over as those that include the valve. In addition, loading the unit also allows the compressor to operate under a more consistent load, reducing problems that maybe associated with large swings in the rod loading.
Priority valves should never be adjusted at the compressor outlet with the compressor running. The Joule – Thompson effect from taking a large pressure drop across the valve causes a severe cooling effect that can cause the valve stem to momentarily “grab” its seating O-rings. Should this happen, the O-rings will move with the stem and tear. A damaged O-ring is easy to find. Gas vents will vent up the valve stem and exit out the top of the valve.
Remember; adjust the regulator with the compressor off!
- Understand and use good lockout and tag out procedures.
- Never work on compressor engine internals or rotating externals without locking out all electrical power and closing and securing with lock and tags all gas into and out of the skid. Know how to vent the gas off skid. Secure and lock out air start lines and vent same.
- Don’t vent down high pressure lines by loosening tubing connections. It is not only slow, IT IS DANGEROUS! Install vent purge plugs if they do not already exist. Most high pressure fitting supply houses stock them.
- Don’t use carbon steel compression fittings on stainless steel.
- Don’t intermix different manufacturer’s fitting parts on the same fitting.
- After having the compressor system open to air for repair, purge the air out of it before restarting.
- Leak test with soap or approved gas leak solution or electronic leak detectors.
- Avoid loose clothing. Don’t work with watches, rings or loose neck chains on.
- Always open high pressure vent and drain valves slowly and be sure that the discharge line is securely anchored and venting to a safe area. Avoid sudden high venting rates. Large discharges of high pressure gas can create a static charge, particularly if directed across a moveable surface such as a grass field or weed patch.
- Keep a fire extinguisher on site and know how to use it.
- Be aware that compressor piping becomes quite hot when running and remains so for a while after running.
- Repressurize high pressure lines slowly and in stages, checking for leaks at each stage. Generally this means stopping at 1000, 2000, 3000 and 4000 psig.
- NEVER TRY TO TIGHTEN A LEAKY FITTING WITH PRESSURE ON IT!!! High pressure compression fittings are designed to use pressure to seat them after they are torqued.
- Know where all the ESD shutdowns on a station are before performing work.
- Never jump out a safety control because it is a nuisance.
Vibration Analysis – All rotating and reciprocating compressors have a normal vibration pattern that can be analyzed. But sampling the vibration of a piece of equipment we can compare and trend the results. Vibration analysis can detect many problems long before equipment fails.
Thermographic Analysis – infrared images of mechanical or electrical equipment will often identify hot or cold spots that are indictive of problems. We have a hand held device that we use not only on compressors but also on boilers that we service.
Lube Oil Analysis – We all know that mechanical equipment wears with each revolution. By analyzing a sample of the lube oil we can often determine and correct abnormal wear before it becomes a catastrophic failure. Emission Spectrometric Analysis, Partical Count Analysis and Ferrographic Analysis can determine the type and source of the wear.
- Reduce lift valves with inconel springs
- Hydrostatically tested castings
- Forged steel 1st – 2nd – 3rd stage connecting rods
- Unique connecting rod bushings
- Explosion proof LOLS
- Locking crankpin bushing design
- Special gaskets
- Special oil seals
- Unique pressure gauge and pressure relief valve configuration
- Oversized stainless steel interstage and control tubing
- Unique interstage traps suitable for electrical solenoid controlled dumping
- 24 hour test on natural gas
The foundation bolts must be of sufficient length to project at least 1/2″ through the nuts to allow for proper leveling. We also suggest that our customers use MJ #1295, a vibration isolation material between the compressor skid and the foundation. This material is available in 30″ X 30′ rolls.
Email our parts department for this material. We normally have rolls in stock.
Also, make sure that the unit is positioned so the belt wheel is facing a fresh air source and allow for at least 36 inches between the compressor and any walls or obstruction. This not only helps air circulation but also makes it easier to access the unit for maintenance.
The compressor package needs to be properly leveled and securely bolted to the foundation to avoid any vibration. Go ahead and use shims to level if needed.