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Fuel Pressure Regulator Sizing
Fuel pressure regulators are designed to automatically
adjust flow to meet downstream demand at a required
pressure. The typical regulator installed as the primary
regulator for a generator is of the direct acting, internally
registered design. "Direct acting" means the pressure
sensing element acts directly to open the fuel valve and
control flow to load while maintaining desired pressure.
The pressure sensing element is typically a diaphragm
which is opposed by a combination of spring pressure
and atmospheric pressure. The valve is the restricting
element and consists of some type of variable restriction
(cone, poppet, disc) which closes against a fixed seat.
Internal registration means the pressure used for sensing
comes from within the valve body, usually through a pas-
sage from the secondary side (outlet) to the sensing dia-
phragm.
The primary regulator must be sized to provide required
flow at rated pressure to generator at its full load capac-
ity. Generator fuel consumption values and required
operating pressures are listed on unit specification sheet.
The manufacturer recommends primary pressure regula-
tor be sized for at least 110% of generator's required fuel
consumption at 100% load, and pressure regulator pro-
vide no more than a 1–2 in water column (0.25–0.50 kPa)
pressure drop at each operating condition; static, crank-
ing, running at no load, and running at full load.
Various regulator manufacturers provide sizing tables,
flow capacity, pressure drop tables, and distributors who
will help size a regulator correctly to a system.
Recommended Fuel Pressure Regulators
Use only fuel pressure regulators marked as "direct act-
ing."
Primary Fuel Pressure Regulator
The following are the manufacturer's recommendations
for specifying, sizing, and installing the primary fuel pres-
sure regulator.
1. Verify regulator:
•
is sized to have a fuel flow delivery rating equal to
fuel consumption requirements of generator.
NOTE: The recommended selection for orifice diameters
is to use smallest orifice that will still provide a CFH fuel
flow rate at least 1.1 times greater than the required full
load CFH rating of generator.
•
is approved for a mechanized engine application.
•
has a spring rating within range of fuel pressure
listed in the generator specification sheet.
•
has an accuracy rating of 1% or less and/or have a
maximum allowable pressure droop rate of 1–2
water column (0.25–0.50 kPa).
NOTE: "Droop" is reduction of outlet pressure experi-
enced by pressure-reducing regulators as flow rate
Installation Guidelines For Spark-Ignited Stationary Generators
increases. It is stated as a percent, in inches of water col-
umn, or in kPa, and indicates the difference between out-
let pressure at low flow rates and outlet pressure at the
published maximum flow rate. Droop is also called offset
or proportional band. For correct generator operation, a
maximum of 1–2 in water column (0.25–0.50 kPa) droop
is required at each operating condition; static, cranking,
running at no load, and running at full load.
2. Verify generator has a dedicated fuel supply which
is not shared with any other appliances (furnace,
water heaters, ranges, etc.).
3. Verify inlet fuel pressure measured at pressure
regulator body inlet connection when pressure reg-
ulator appears unable to pass published flow rate.
Supply piping up to regulator can cause significant
flowing pressure losses.
4. Verify pressure regulator is flowing at least 5% of
normal operating flow when adjusting pressure set
point.
5. Expect approximately a 1 °F (0.5 °C) drop in fuel
temperature for every 15 psid (differential) across
regulator due to natural refrigeration effect.
NOTE: Freezing is often a problem when ambient tem-
perature is between 30–45 °F (-1–7 °C), particularly with
LP gas systems.
6. Point vents down to help avoid accumulation of
water condensation or other materials in spring
case.
7. Keep vents open. Do not use long, small diameter
vent lines. Follow the rule-of-thumb: use the next
nominal pipe size for every 10 ft (3 m) of vent line,
and use 3 ft (0.9 m) of vent line for every elbow in
the line.
8. The connection point on generator is the end of
manufacturer supplied flexible fuel line. The flexible
fuel line is the same size as the connection point
through base frame rail (see installation drawings).
It is permissible to install one elbow (90º) and a
short nipple between flexible fuel line and fuel con-
nection point to allow flexible fuel line to parallel
generator for installation purposes.
Gaseous Fuel Systems
21
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