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All about LP pressure, flow rate, and the demand regulator...

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5.4K views 11 replies 5 participants last post by  OrlyP  
#1 ·
Let's assume for the moment that we have a main LP regulator that puts out exactly 11 in. WC with an unlimited flow rate potential (ideal).... just to put pipe diameter and other constraints out of the equation.

On the other end of the line is an LP appliance, a generator, that has a demand regulator and with the load block set to sustain the genny up to 100% full load.

Given that the demand regulator responds to engine vacuum (or lack of), does it also work to regulate the flow rate that's fed into the combustion chamber depending on the engine load?

Another way to put it, so if the load block was tuned to sustain the engine at 100% load, there's a certain flow rate associated with it. What happens to the flow rate when the engine ramps down to say, 75% load or 50% load.... does the flow rate:

a) Go down, or
b) Remain constant (as per load block setting)?

The thing I'm getting at is LP fuel consumption. Will a generator consume less LP when it is partially loaded VS fully loaded?
 
Discussion starter · #4 ·
Thanks. Just interested in the basic concepts. I just realized that the main regulator is just maintaining 11 in. WC but the flow rate will change depending on demand.
 
Discussion starter · #8 ·
The LP nozzle is about a third of the way from the intake side of the carb throat. And yes, the demand regulator stops the gas from flowing whenever the engine shuts off.

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Help me understand the following... (my notes in BOLD)

The main regulator attached to the LP bottle has the following specs: (Pretty standard, right?)
  • Output pressure: 300mm. WC (about 11.8 in. WC or 2.94 kPa)
  • Capacity: 1.5kkg per hour (or 3.3 lbs per hr)
  • 3/8" barbed fitting
I'm using a 3/8" SS-reinforced LP hose from the main regulator to the demand regulator. It's 6ft long.

The demand regulator has the following specs:
  • Fit for 5KW 5000W GX390 188F (My engine is the 188F and the power head is 5kW peak, 4.5kW running)
  • LPG pressure range: 2.8 KPA ---3.8 KPA (Is this the allowable input pressure?)
  • Pressure regulating valve closing pressure: 3.50KPA (Not sure what this means)
  • NG pressure range: 2.0 KPA ---2.6 KPA (Ignore this, we don't have NG)
  • Pressure regulating valve output pressure: 0.03-1.56KPA (see note below)
  • 3/8" barbed fitting
Note: I believe this is adjustable by turning the screw in the middle of the orange (LP ↔ NG) knob. AFAIK, it regulates the gaseous fuel flow rate sent to the carb, much like an A/F set screw for gasoline. Would this be the equivalent of a load block?
 
Discussion starter · #12 ·
There are the two settings for the knob (CNG LPG). Each setting has an adjustment screw.

View attachment 10926
Yup, this one. The lower screw is for LP while the one on top is for NG. Turning the orange knob changes which screw the gas passes through. I had to figure this out myself because my kit didn't came with a manual. Youtube to the rescue!

Standard pressures though the use of barb fittings arent typical. NPT and flare fittings would be best.
Yeah, barbed fittings are the norm here.