Chat with us, powered by LiveChat

Bleed Valves, Purge Valves and Proportional Relief Valves: What’s the Difference?

Today we discuss the difference in the three main types of pressure release valves commonly found in modern fluid networks.

Pressure-control valves are essential parts of the instrumentation and control world. They’re also commonly used in hydraulic systems assisting other products to, as the name states, control pressure.

Bleed valves, purge valves, and relief valves are all pressure-control valves, but they each have a special purpose. Let’s find out what the difference is for these products and how to choose the one you need.

What is a Bleed Valve?

A bleed valve is a valve that typically utilizes a threaded bleed screw to open or close. Generally, these valves vent pressure or release media from within a system. This is often necessary before starting maintenance work on a line that contains pressurized liquids or gasses. At other times, system operators use bleed valves to drain off media for analysis or while calibrating other control equipment.

This Image shows a cut away of a bleed valve. At the bottom of the image is the threaded inlet. To the side is the vent tube and at the top is the screw head that turns to open the valve

Bleed valves are compatible with many other Superlok products, such as Superlok Instrumentation Manifolds and Gauge valves. When used together, these valves safely vent line pressure from a system to the atmosphere or into a containment vessel.

Superlok bleed valves are capable of working with up to 10,000 PSI and beyond. Additionally, they feature a back-stop screw opposite the vent tube. This accessory prevents accidental removal of the stem.

What is a Purge Valve?

The purge valve is similar to a bleed valve. It features a vent hole in the bottom of the captive cap of the purge valve. This hole allows for venting of excess liquid or gas into the atmosphere from the build-up of pressure within systems.

Because of the built-up pressure flowing through the purge valve, accidental disassembly is an issue that can occur. Superlok permanently clamps their purge valve caps onto the valve body to prevent this.

This Image shows a cut away of a purge valve. At the top is the cap that turns to open the internal seal. Under the cap, a vent allows media to escape downward toward the threaded input.

The purge valve uses a poppet and spring system under the cap to open and close the flow. To achieve leak-tight closure on first make-up, installers simply tightening the cap an additional quarter turn with a wrench from finger tight.

Superlok offers purge valves in an assortment of end configurations. From NPT and SAE threads to tube adapters, purge valves are available in any connection you may have. They allow for pressures up to 4,000 PSI at 100°F and are viable at higher temperatures with less pressure.

What is a Proportional Relief Valve?

In fluid power systems, there’s a maximum pressure range that must be controlled. Relief valves are the safeguards which limit maximum pressure in a system by diverting excess fluid when pressures get too high.

They control pressure using adjustable internal springs that can be set for a full range of pressures. Using eight interchangeable springs, Superlok relief valve settings are infinitely variable between 10 PSI and 6,000 PSI. Additionally, these valves feature a lock-wire that, when used, can prevent vibrations or other environmental factors from changing the setting.  

This Image shows a cut away of a relief valve. At the top of the image is the cap that contains the spring.

With the help of the preset pressure from the spring range, relief valves remain closed until their set pressure is matched by the upstream pressure. This action forces the lower stem open allowing flow through the valve that increases in proportion to the upstream pressure. This is why they are called proportional relief valves.

Once open, the valve redirects fluids or gases are through the valve outlet. As a result, system pressure decreases until it is below the set cracking pressure, or reseal pressure. The relief valve then automatically closes and the process is complete.

Though relief valves vent media similarly to the others there is one major difference compared to bleed and purge valves. The output they provide is not a simple vent tube or cap, it’s a full size end connection. If desired, Installers can even use this output to direct flow back into a system.

How to Choose Which Pressure-Control Valve?

Choosing the pressure-control valve for your application depends on several important factors. What working pressure and temperature are you managing? Do you need to use a precisely set pressure or not? What vent or redirection style do you prefer for the released media? All of these are important questions when considering system venting solutions.

Bleed Valves & Purge Valves

Bleed Valve

Release Style:

Bleed valves are manually operated and used specifically for bleeding liquids or gases out of an instrument or system. These valves also give you slight control of the outflow direction via the vent tube.

Scope of Operation:

Instrumentation bleed valves are capable of working at higher temperatures and with higher pressures. They function efficiently at a wider range of pressures with a working pressure of 10,000 PSI at -65°F to around 6,085 PSI at 850°F.

Purge Valve

Release Style:

Purge valves are also used to control the bleeding of liquids or gases. Additionally, they have the capability to empty and drain liquids or gases out of an instrument or system. However, purge valves do not direct the outflow in any specific direction.

Scope of Operation:

Instrumentation purge valves perform well up to 4,000 PSI at the lowest temperature and have a max temperature rating of 600°F with a working pressure of 2,500 PSI.

Proportional Relief Valves

Proportional relief valves differ from bleed and purge valves because they are not manually operated. After setting your desired pressure range, relief valves continuously regulate automatically.

Release Style:

  • Relief valves are used to direct flow out of the system or into a containment area like the bleed valves and purge valves. However, unlike bleed valves and purge valves, they have the additional ability to direct flow back into a system.

Scope of Operation:

  • In relief valves, the spring choice determines the range of pressure allowed through the valve by setting the max pressure. The Superlok relief valves have a max working pressure of 6,000 PSI and cracking spring selections range from 10 PSI to 6,000 PSI. Their max temperature rating is 1,200°F.

Superlok has a Quality Selection of Pressure-Control Valves to Choose From

Mako Products is the source for your instrumentation and control product needs and the home of the Superlok products in the US. If you’ve concluded which pressure-control valve you’re interested in purchasing, contact us and receive your product in the next few days.

We ship same-day and would love to offer you one of our Superlok bleed valves, purge valves, or proportional relief valves today. 


Superlok Relief Valves

Superlok Bleed Valves and Purge Valves

Collins Dictionary

Understanding Pressure Control Valves

Free Patents Online

The Author

Sarah Myers
Customer Care Manager
Sarah is our four star customer care team general. She's seen it all when it comes to customer needs and questions, so we let her write articles about them from time to time. We've never seen here away from her desk. We think she might sleep there...

Suggest a Topic

Tell us what you would like to see covered and we might write an article about it in the future. Seriously, just try us.
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

Our Blog

Check out our blog for the latest updates and how-to's from Mako Products!

View All