11.2: How Delta P Describes Hydraulic Resistance
- Page ID
- 117048
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)In fluid power, we use the term ΔP (Delta P) to describe the pressure drop across a hydraulic component or section of the system. It simply means "change in pressure", and it's a very useful tool for analyzing system performance.
ΔP (Delta P) = Pressure inlet − Pressure outlet
For example, suppose we place pressure gauges ahead and behind a needle valve:
- Pressure at Gauge A (upstream): 540 psi
- Pressure at Gauge B (just before the valve): 520 psi
ΔP = 540 − 520 = 20 psi
This 20 psi is the resistance caused by drag within the fluid lines.
Now let’s measure again across the needle valve:
- Gauge B (inlet to the valve): 520 psi
- Gauge C (outlet of the valve): 20 psi
ΔP = 520 − 20 = 500 psi
This 500 kPa is the resistance caused by the needle valve.
What Causes Delta P?
- Frictional Resistance – Any component (valve, fitting, hose) has internal surfaces that restrict flow.
- Load Resistance – Actuators like cylinders and motors also create ΔP based on the force needed to move the load.
Why Delta P Matters
Delta P gives you a localized view of pressure loss across each component. This is critical when:
- Designing a system – You need to ensure components can handle the expected flow and pressure drops without overheating or stalling.
- Sizing components – Manufacturers provide performance curves showing how ΔP varies with flow rate. You use these to select the right valves, hoses, and filters.
- Troubleshooting – A higher-than-normal ΔP can signal a clogged or failing component. If a filter’s inlet pressure stays steady but its outlet pressure drops, ΔP increases, indicating it might be clogged.
Imagine you have a hose connected downstream of the needle valve, and it adds 20 psi of resistance. That 20 psi shows up after the valve and has nothing to do with the valve’s resistance. The needle valve’s ΔP is still 500 psi —it just happens that the pressure after the valve is not zero because of further downstream components.
This separation is what makes ΔP such a useful metric. It allows you to measure each component’s impact on pressure, regardless of what's happening further down the line.