AutoQuiz: How Is Fluid Velocity Affected When the Pressure Drop Across a Valve Increases?

AutoQuiz is edited by Joel Don, ISA’s community manager.

Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for information about the CCST program. This question comes from the Level I study guide, Domain 3, Troubleshooting. Level I represents a professional who has a five-year total of education, training, and/or experience.

If the pressure drop (delta P) across a valve increases, the velocity of fluid through the valve will normally:

a) decrease
b) increase
c) remain the same
d) change direction
e) none of the above

   

Click Here to Reveal the Answer

The pressure drop refers to the change or drop in pressure from the upstream side of the valve to the downstream side. If there were no difference in pressure, the stuff would just lie there in the pipe not moving. Therefore, to move something through a pipe there has to be pressure, and the content of the pipe moves from the high-pressure end to the low-pressure end. Intuitively, one can work through this and see as the pressure difference gets larger and larger, more material moves through the pipe and the valve.

Mathematically, this argument manifests in this equation for volumetric flow across a valve.

 

Q is volumetric flow, and its units are meters cubed over time (m3/sec). Av is the cross sectional area of the valve (m2), P is pressure in Pascals ((kg•m)/(sec2))/(m2), and ρ is density of the fluid (kg/m3).

Since more is moving through the pipe and across the valve, it must be moving faster and therefore the velocity is increasing.

The correct answer is B.

 

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