text: The maximum pressure rating of a metering pump is actually the top of the discharge pressure range the pump is guaranteed to pump against at a reasonably controllable flow rate.  The pump itself is a pressurizing device often capable of exceeding its pressure rating, although not guaranteed to.   For this reason, if there is any stop valve downstream of the pump, a pressure relief valve should be placed in between to prevent overpressuring of the tubing or piping line in case the stop valve is inadvertently shut while the pump is running.  The relief valve setting should be below the maximum pressure rating that the piping, tubing, or any other components there could withstand.
  Liquids are only very slightly compressible.  This property of liquids lets metering pumps discharge liquids at high pressure.  Since a liquid can be only slightly compressed during a discharge stroke, it is forced out of the pump head.  Gases are much more compressible.  Metering pumps are not good at pumping gases.  Sometimes, a metering or similar pump has to be primed before operation, i. e. the pump head filled with the liquid to be pumped.  When gas bubbles enter a pump head, the compression motion compresses the gas but has a hard time forcing it out of the pump head.  The pump may stop pumping liquid with gas bubbles in the pump head even though mechanically the pump is going through the motions, repeatedly compressing and decompressing the bubbles.  To prevent this type of "vapor lock", chromatography solvents are often degassed before pumping.  
  If the pressure at the outlet is lower than the pressure at the inlet and remains that way in spite of the pumping, then this pressure difference opens both check valves simultaneously and the liquid flows through the pump head uncontrollably from inlet to outlet.  This can happen whether the pump is working or not.  This situation can be avoided by placing a correctly rated positive pressure differential check valve downstream of the pump.  Such a valve will only open if a minimum rated pressure differential across the valve is exceeded, something which most high-pressure metering pumps can easily exceed.