Total Control Systems 700-20 Engineering Manual page 16

LPG SYSTEM DESIGN
LPG will have a high vapor pressure and tends to vaporize easily. The successful meter
operation will depend on the care exercised to prevent this from happening.
In order to prevent vaporization of LPG, certain elements must be adhered to in the design,
fabricating and operation.
1) The piping from the storage tank to the pump inlet should be at least one pipe size
larger than the pipe size of the pump inlet. Any valves, strainers or other fittings in
the inlet piping system should be similarly sized. During operation the pressure at the
pump inlet must be equal to or greater than the vapor pressure of the product.
2) All shut-off valves should be full-port ball type or similar free-flow type.
3) An external pump bypass (pressure relief) line from the pump outlet to the storage
tank should be provided. This should include a pressure relief valve set to permit
circulation through this line at a pressure that is about 25 PSI lower than the pump
bypass relief valve setting. This is to prevent recirculation within the pump, which
may cause heating and formation of vapor.
4) The piping system from the pump to the meter should be the same pipe size as the
pump outlet. Shut-off valves should be the ball type or an equally free-flow type. A
minimum number of elbows and fittings should be employed.
5) Safety valves or pressure relief valves should be included in the installation so that
any section of the installation that might be isolated when valves are closed will be
protected.
6) There must be provision for determining product temperature as it pass through the
meter. The thermometer used should have no less than one graduation per degree F,
preferably five.
7) Calibration of LPG flow meters should follow National Institute of Standards and
Technology (NIST) Handbook 44 regulations pertaining to LPG.
LPG is flammable when mixed with air and the same safety precautions should be observed that
would be used in handling a liquid such as gasoline. Dangerous concentrations of LPG may not
be readily detected; therefore, adequate ventilation is a primary requirement.
LPG has a lighter coefficient of expansion than other petroleum liquids. A change in liquid
temperature of 1 degree F will result in a change in volume of 0.17%. Due to this characteristic,
it is common to use Automatic Temperature Compensators (ATC) on LPG meters used in the
resale of LPG. The use of an ATC will automatically correct the delivered volume to 60 degrees
F.
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