All you need to stay in the KNO
When discussing a piping system, the term “pressure” is often used to describe a key fluid property that plays an important role in the operation of equipment like pumps, control valves, tanks, and vessels.
But like many terms used in engineering, there are nuances in meaning that must be taken into account to avoid miscommunication, confusion, and costly mistakes.
Quite often, key qualifiers that distinguish between “stagnation pressure”, “static pressure,” and “dynamic pressure” are not used.
But sometimes the distinction is important, just as the difference between “mass flow rate” and “volumetric flow rate” must be made to be concise when discussing “flow rate“. Here we go over the definition, units, and application of common fluid dynamic equations:
Stagnation pressure or Total Pressure is the force per unit area that is felt when a flowing fluid is brought to rest and is usually measured with a pitot tube type instrument, shown in Figure 2:
The Stagnation Pressure is the sum of the Static Pressure and the Dynamic Pressure, also shown in the equation below:
Static Pressure is felt when the fluid is at rest or when the measurement is taken when traveling along with the fluid flow.
It is the force exerted on a fluid particle from all directions and is typically measured with gauges and transmitters attached to the side of a pipe or tank wall.
Because this is what most pressure gauges measure, static pressure is usually what is implied when just the term “pressure” is used in discussions.
The difference between Stagnation and Static Pressure is the Dynamic Pressure, which represents the kinetic energy of the flowing fluid. Dynamic pressure is a function of the fluid velocity and its density and can be calculated from*:
Depending on the application, the difference between total and static pressure may be negligible, but for others, neglecting the difference may result in costly mistakes.
For many liquid applications, the pipelines are sized to ensure low fluid velocities to reduce the head loss and pressure drop for a given flow rate, resulting in a small value of dynamic pressure. Also, because of the accuracy and scale of the instrument used to measure the pressure, the distinction between total and static pressure may be neglected.
Also, because of the accuracy and scale of the instrument used to measure the pressure, the distinction between total and static pressure may be neglected.
In Figure 3, the pipe size is changed to result in different fluid velocities for 700 gpm of water flow, resulting in different amounts of dynamic and static pressure for an inlet total pressure of 100 psig.
Here we understand the different cases by velocity:
For gas applications the distinction between total and static pressure again will again depend on the amount of dynamic pressure. But because the density of a gas is much lower than that of a liquid, a much higher velocity is needed before the difference between total and static pressure needs to be made. This is seen here in Figure 4:
Notice the various pipe sizes, fluid velocities, and static pressures for an inlet total pressure of 100 psig and a mass flow rate of 7500 lb/h of 350F steam with a density of 0.248 lb/ft3
When evaluating the operating parameters of a piping system, the distinction between stagnation and static fluid properties may or may not be important.
For most liquid applications, fluid velocities are intentionally kept low to minimize the amount of head loss and power consumption of the system.
This results in a small amount of dynamic fluid energy, making the difference between total and static pressures indiscernible on most industrial pressure gauges.
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