Flow Control Fundamentals


Flow controls like the humble needle valve look simple, but there's a bit more going on than you might expect. We'll learn some vocabulary and get into the principles behind a classic flow control valve's operation.

We're going to (virtually) crack a needle valve open and see what's happening inside.

We'll even run our own experiment on a virtual system!

Flow Basics


Before we get to controlling flow, let's take a minute to review how we measure it.

A typical flow meter, like the one pictured here, tells us how much volume is moving though the meter in a given amount of time. This gives us units like L/min, or gpm. Note that it is not telling us how fast the fluid is moving; that would be flow velocity.

Volume is like a basic traffic car count. It can tell us how many cars went by the sensor, but it doesn't tell us anything about how fast those cars are going.


The Δ symbol (pronounced "delta") indicates a measure of difference. For this reason, it's also called the pressure differential. In hydraulics, we often refer to ΔP ("delta P"), which is shorthand for saying, "the net difference in pressure between 2 points." In the drawing shown here, the ΔP between point A and point B is 50 psi.

What Does Flow Do?

Why are we concerned with flow? Why do we care enough to measure it and modify it with flow control devices?

While the ability to work against pressure provides the power to move hydraulic actuators, it is the volume of flow to the actuators that determines the actuator speed. Very simply, more flow means more speed. If you want a cylinder to extend faster, or a motor to spin more quickly, you need to send more flow. And if you want to limit the speed of an actuator, you'll need to reduce the flow.

All About Flow

There are three ways to modify flow.

  1. Restrict it with an orifice.
  2. Push or pull it with a pressure differential.
  3. Change the viscosity of the fluid. (The higher the fluid viscosity, the greater its resistance to flow.)
You won't be at all surprised to hear that needle valves make use of an orifice to control flow — after all, they belong to a class of flow control called "adjustable orifices" — but the push/pull from the pressure differential might be less obvious.
As for viscosity changes, the effect is determined at the point of manufacture by the orifice's leading edge profile. It's not something you can adjust.

Sharp-Edge & Bell-Mouth Orifices

Most hydraulic orifices can be generalized as sharp-edged or bell-mouthed. This is a description of the upstream orifice edge.

Bell-Mouth Orifices

Upstream Edge
Upstream Edge
Flow In
Flow Out

Bell-mouth orifices have a rounded taper to guide flow. The taper helps to keep the flow laminar, which means that relatively little energy is lost to turbulent flow.

Sharp-Edge Orifices

Flow In
Flow Out

Sharp-edge orifices do not promote laminar flow, but flow through a sharp-edge orifice is far less affected by changes in fluid viscosity than flow through bell-mouth orifices. Sharp-edge orifices have a single point of contact with the fluid as it flows through the opening.

A single point of contact around the orifice.

The bell-mouth has a lot of surface area contacted by the fluid.

This reduced contact is the key to the sharp-edged orifice's reduced response to fluid viscosity change. Compared to the bell-mouth taper, there is almost no surface area in the orifice. This means that fluid viscosity changes are relatively unimportant, as there is barely any surface area for fluid to drag against.

Trivia Night Achievement Unlocked

The narrowest point of the fluid jet is actually just behind a sharp-edge orifice, and it has a fancy name - the vena contracta. If you ever get to casually throw out this term in conversation, you are a legend. Write us.


As with most system design decisions, these two categories of orifices offer trade-offs. If keeping flow as consistent as possible in a variable temperature environment is important, you'll probably chose a sharp-edge orifice, and sacrifice energy efficiency. But if you're operating in a temperature-stable environment, or if it's not too important to keep flow consistent, a bell-mouth orifice might be the better choice for improved efficiency.

If you'd like to learn more about viscosity, check out Fluid Basics: Part 1! For our purposes, it's enough to know that viscosity is tied to temperature. This is why it's important to use the correct fluids and operate hydraulic systems within specified temperature ranges if you want your flow control valves to perform as expected.

Schematic Symbols

A fixed diameter bell-mouth orifice.

Angle brackets, instead of rounded parenthesis, indicate a sharp-edge orifice.

If diameter is adjustable, as in the case of our classic needle valve, an arrow is added.

What's in a Needle Valve?

A flow control is a limiting device. It won't magically produce more flow — that's the pump's job — but it is extremely useful in reducing flow.

Unlike many pressure-controlling devices, in terms of flow, the area of effect of a flow control is downstream.

Let's open up this simple needle valve and see what's happening inside.

There isn't too much going on here. There's a bore for flow, and an adjustable needle to open/close the orifice.

If we put it into a simple circuit, it allows us to control the motor speed by limiting flow. Try it out!

What happens when you close the flow control?

This system uses a fixed displacement pump. Where is the extra flow going when the flow control is closed?

It's easy to see how the orifice is adjusted by changing the needle position, but where does Δ P and the pressure differential come in?
Let's change the rules of our circuit just a bit to demonstrate.

ΔP Demo

This time, you're going to leave the flow control alone. (It has been arbitrarily set at about 50% closed.) Instead, you'll adjust the pressure relief valve to induce different pressures, and cause a greater ΔP across the flow control. The relationship between pressure ahead of the needle valve, and flow allowed through the needle valve will be graphed.

Make a Prediction

Before you start, take a minute and imagine what you're about to do. What will happen to the flow at F2 (flow through the motor) as you increase the ΔP across the needle valve? What do you think your graph will look like?

Graph 1
Graph 2
Graph 3
Graph 4

Try It

What's Happening?

The flow control orifice diameter didn't change, but the volume of flow through the orifice did. That is entirely the work of the pressure differential. When the pressure differential across the orifice increases, it pushes/pulls flow through faster, and the flow rate increases, even though the restriction remains the same.

This orifice diameter did not change at all.
This isn't an issue in many applications. When pressure remains relatively constant (for example, when load pressure at P2 doesn't change), it's easy to adjust the flow control to produce the desired actuator speed, and the pressure differential won't cause problems. The simple needle valve is all you need!
Ok, great. So the needle valve is all we need for several of our systems. But sometimes system pressure or load pressure is going to fluctuate! What do we do then?
Shh! They haven't seen Pressure Compensated Flow Controls yet!


What does a flow volume measurement tell us?

How is flow rate adjusted?

Why do we need to control flow in a hydraulic system?


    1. Flow volume can be adjusted in three ways.
    2. Constrictions (Orifices).
    3. Pressure Differentials.
    4. Viscosity Changes.
  • A change in temperature will change fluid viscosity.
  • Adjustable orifices, like needle valves, limit flow.
  • Bell-mouth orifices are generally the most energy-efficient orifice choice.
  • Sharp-edge orifices remain relatively consistent through fluid viscosity changes.
  • Flow through an orifice corresponds to the size of the orifice, but also to the degree of pressure differential across the orifice.
  • Simple flow control valves, like the ones we've discussed here, are vulnerable to pressure fluctuations.

We hope you enjoyed Flow Control Fundamentals

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