Natural gas powered engine will not start and run

[RIMJ]

The engine is a one year old Wisconsin V465D natural gas fuelled unit [the derated hp for natural gas at 1000 feet above sea level at 1800 rpm is about 35 hp]. This engine ran without difficulty for about a year. Recently, however it has exhibited the following problem.

The driven load, a centrifugal pump has a brake hp of about 25 hp [when we can get the engine up to speed].

The engine will not start without the shutoff valve on the gas supply line being pinched almost closed. This did not use to be the case. Once the unit is idling, it can be brought up to speed by opening the shutoff valve very slowly. A tee has been installed right at the inlet to the carburettor, and a pressure gauge read while the unit is running displays rapid pulsations.

We suspect an issue with the regulator, but can an expert in the field be more specific in the diagnosis?

Thanks in advance for any ideas.

(The engine is fed by a fuel gas line with a regulator [brand/model not known, but it is a self operated type]. Orifice size is reported to be 3/16 inches. The piping to the engine consists of a gas meter and fuel lockoff valve, then 10 feet or less of 1 inch pipe which passes through the wall of the building, a shutoff valve inside the enclosure, then about 5 feet of 1/2 inch line and flex hose reducing to 1/4 inch at the engine. The engine is designed for an inlet gas pressure of 4 - 6 inches water column and using 1050 BTU gas requires about 25 x 12 = 300 cubic feet of fuel per hour.)

 

[abeltio]

It is very important to know that the motor was working ok before.
What kind of control does the motor have? e.g. if you have an external speed governor (Woodward for example) seems like you may be have a wrong start-up fuel setting.
If you have to bring the unit up to speed by opening the shutoff valve very slowly... may also indicate that the governor gain is whacked.
Summarizing: your best bet would be to bring in a tech rep for the speed governor, since your problems seem to be concentrated on the starting and acceleration characteristics of the motor.

Also to consider:
As the load is a centrifugal pump, confirm that the system always started with the discharge valve of the pump almost fully closed this will require the minimum power from the motor when starting (i.e. have the system conditions changed?).
A typical application for this type of arrangement is the back-up pump (diesel powered) for a fire-fighting system.
Hope this helps.
a.

 

[TBP]

The rapid pressure pulsations almost sounds like the PRV on the gas line is "machine-gunning", or rapidly hunting. Valves that are seriously oversized do this. Was any work done on the PRV station? Has the pressure on the inlet side of the PRV increased? (A large enough pressure increase on the inlet of a control valve can have the effect of making of making the valve "too large" for the load.