Objectives

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!

Measurement

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.

ΔP

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.

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.

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.)

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.

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.

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.

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!

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.

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.

• Flow volume can be adjusted in three ways.
  1. Constrictions (Orifices).
  2. Pressure Differentials.
  3. 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.