Sizing Up A Valve: Which One Works Best For Your Application?

Written by Jeff Hopkins | 10/16/19 4:00 PM

"Some valves are designed to provide good control of flow or direction, or guard against over-pressure"

- From Choosing the Right Swagelok Valve for the Job: Part 2

You need to pay attention to more than just the size of the end connection

Even if a valve's end connections match up to the rest of your fluid system, you still might not have the right size valve for the job. When we talk about the size of a valve, we really mean the amount of flow that the valve can provide.

Basic flow calculations are simple, as illustrated by the common orifice flow meter (Fig. 1). We need to know only the size and shape of the orifice, the diameter of the pipe, and the fluid density. With that information, we can calculate the flow rate for any value of pressure drop across the orifice (the difference between inlet and outlet pressures).

For a valve, we also need to know all the valve passage dimensions and all the changes in size and direction of flow through the valve. Fortunately, you don't have to make complex calculations. The valve manufacturer has done that for you already, combining the effects of all the flow restrictions into a single number called a valve flow coefficient.

Valve manufacturers determine the valve flow coefficient by testing the valve with water at several flow rates, using a standard test method developed by the Instrument Society of America.

Liquid and gas flow

Because liquids are incompressible fluids, their flow rate depends only on the difference between the inlet and outlet pressures, called the pressure drop. The flow is the same whether the system pressure is low or high, so long as the difference between the inlet and outlet pressures is the same.

Gas flow calculations are slightly more complex. Gases are compressible fluids, so their density changes with pressure. In addition, there are two conditions that must be considered—low pressure drop flow and high pressure drop flow.

In low pressure drop flow—when the outlet pressure is greater than half of the inlet pressure—the outlet pressure restricts flow through the orifice: as outlet pressure decreases, flow increases, and so does the velocity of the gas leaving the orifice.

When the outlet pressure decreases to half of the inlet pressure, the gas leaves the orifice at the velocity of sound. The gas cannot exceed the velocity of sound, so this becomes the maximum flow rate. The maximum flow rate is also known as choked flow or critical flow.

No matter how much you decrease outlet pressure after that, you can't increase flow, even if the outlet pressure is reduced to zero.

Consequently, high pressure drop flow only depends on inlet pressure and not outlet pressure.

All the details

We have a special technical bulletin with all the equations and graphs you need for valve sizing. They include correction factors for the specific gravity of various fluids, and for the effects of temperature. It's available as a free download, along with a tutorial on valve selection and a valve-selection worksheet. Get all three PDFs by clicking here and filling out the form.

As always, the associates at Swagelok Northern California are glad to talk with you about what you are trying to accomplish, so we can help you make the right product selections.

Just ask

Swagelok Northern California has a great deal of exposure to all aspects of fluid system design and engineering. Whether you have a simple question or a complex challenge, we're glad to hear from you.