MDE-4334C Atlas Start-up and Service Manual · April 2008
Page 7-19
Measuring Pressure and Vacuum
Hydraulic/Mechanical Components
3
Compare the observed flow rate to the chart and to the actual inlet pressure for the dispenser
model being checked. If the flow rate is low relative to the pressure, it is likely that a dispenser
or hanging hardware problem exists. If the readings are similar to the specification, it is likely
that the dispenser is working properly and it is a
system
problem.
Pressure at shear valve or filter cap with no flow
Maximum pressure capability of the STP
Pressure at shear valve with full flow
Actual capability of STP and in-ground piping to supply
pressure at maximum flow rate
Pressure at shear valve or filter cap minus pressure at hose
at full flow
Pressure drop across dispenser meter and valve at
maximum flow rate
Pressure at shear valve or filter cap minus pressure at hose
at no flow
Maximum pressure available to force fuel through
meter and valve
Pressure at hose at full flow
Pressure drop across nozzle and hose at full flow
Use this table to evaluate the probable cause of dispenser low flow rate.
Low Flow Rate Symptom
Probable Causes
Observed pressure drop across the meter and valve is
more than a few psi higher than indicated in the charts
Flow restriction or defective pump/dispenser
component, assuming that the filter is new
Observed and expected pressure drop across the pump/
dispenser components is close to specification
Hanging hardware or dispenser pressure supply (STP)
problem upstream or issue with pumping unit/
installation for pumps
Observed pressure at the hose and nozzle during full flow
is too high
Undersized or defective hanging hardware
Pressure at the dispenser inlet or pumping unit is too low at
full flow
Undersized or defective STP/piping to dispenser or
pumping unit hardware or installation problem
Reaching the Desired Flow Rate
Suggested methods for achieving the desired flow rate are:
• Correct and replace faulty components
• Use a larger STP or manifolded STP
• Use larger inlet piping to the pump or dispenser
• Use less restrictive hoses and nozzles
• Reduce the maximum lift of product from the tank
• For a pump, reduce the maximum lift of product from the tank and minimize feed line
lengths