What is pressure drop?

What is pressure drop?

Pressure drop is the amount of line pressure that is permanently lost as gas passes through an instrument in the gas line. This pressure loss is due to the frictional resistance of the components the gas touches. Every instrument and fitting in a line induces some pressure drop; even the pipe walls generate some friction, which causes a small amount of pressure drop.

How to calculate pressure drop

Setup with a mass flow meter, an absolute pressure controller, and a differential pressure gauge to explain what is pressure drop

Pressure drop is determined by calculating the difference between the pressure of the gas when it enters the instrument and the pressure of the gas when it leaves the instrument. The easiest way to make this measurement is to plumb the inlet and outlet of the device to a differential pressure gauge, like one of our P-PSID Series gauges. When making these measurements, it is important to consider the following relationships:

  • Under laminar flow conditions, pressure drop is proportional to volumetric flow rate. At double the flow rate, there is double the pressure drop.
  • Under turbulent flow conditions, pressure drop increases as the square of the volumetric flow rate. At double the flow rate, there is four times the pressure drop.
  • Pressure drop decreases as common mode pressure increases.
  • Pressure drop increases as gas viscosity increases. Since increasing the temperature of the gas increases its viscosity, pressure drop also increases as gas temperature increases.

To standardize our pressure drop specifications, Alicat’s instrument data sheets always denotes the maximum pressure drop at the instrument’s full-scale flow when venting to atmosphere under standard conditions. Said another way, our pressure drop specification identifies the minimum amount of inlet pressure to run the instrument at full-scale flows while venting to atmosphere.

Why pressure drop matters

For any gas process to work, the available system pressure must be greater than the total pressure drop of the components in the system at the expected operating flow rates and temperatures. If too little pressure is provided at the inlet of the entire system, there will not be enough gas pressure to pass through all the components of the process at full-scale flows. Likewise, if multiple gases are being used in the same process, pressure drop will be the highest for the most viscous gas.

This principle is a primary driver for Alicat’s many valve customizations. The wider the valve orifice, the lower the pressure drop of the valve when it is wide open. However, greatest control precision is achieved when we use the largest portion of the valve’s operating range. So our goal when building a flow or pressure controller is to select the smallest valve that will allow full-scale flows of all gases being used in the system.

When customers have little available inlet pressure, or perhaps a very large amount of back pressure, sometimes our standard line of mass flow instruments exhibit too much pressure drop to reach full-scale flows. In these cases, we recommend our Whisper-Series mass flow meters and controllers. By making the flow body design more open and by changing the spacing and configuration of our laminar flow elements, we are able to achieve much lower pressure drops, usually by a factor of ten. The Whisper-Series also employs a much more sensitive pressure sensor package to take full advantage of the lower pressure readings.

A pressure drop demonstration

The following video shows how to measure pressure drop under various flow conditions. It also displays the difference that using a Whisper-Series flow meter can make under low differential pressure conditions.