Types of Pressure Relief Valves - A Complete Overview

Home Types of Pressure Relief Valves - A Complete Overview

Pressure relief valves – they might sound as technical as a page from an engineering manual, but hey, they’re actually pretty simple… and incredibly important. Hang tight while we take a dive into the world of pressure relief. Ready? Awesome.

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What are Pressure Relief Valves and How Do They Work?

A pressure relief valve, commonly referred to as a PRV, is a safety device. It’s kinda like a pressure watchdog, always on the lookout for pressure going haywire. It’s a crucial part of many systems where pressure can build up due to an operational mishap, like in boilers, pressure vessels, and piping systems.

These clever little devices are designed to open at a predetermined set pressure to protect equipment from being subjected to pressures that exceed their design limits. Once the set pressure is reached… whoosh! The valve opens, allowing the process fluid to flow out from the system, thereby averting a possible explosion. Just like your trusty sidekick, right?

When the pressure drops back to a safe level, the valve closes, preventing the further flow of fluid. This is due to the spring force that keeps the main valve in place under normal conditions. The closing and opening pressure of a PRV are usually the same, but sometimes there can be a slight difference. The closing pressure might be a smidge lower due to something called “hysteresis”… but that’s a topic for another day.

Now, here’s the cool part: the primary purpose of a pressure relief valve is self-actuated. That’s right, no outside force needed. It’s all about the pressure and the set-up of the valve. Clever, huh?

Different Types of Pressure Relief Valves

Before we go any further, let’s clear something up. You might hear the terms “pressure relief valve” and “safety valve” used interchangeably. Now, while they’re quite similar, there’s a key difference between them.

A pressure relief valve, or PRV, is designed to open gradually, allowing fluid to escape gradually as the system pressure rises. On the other hand, a safety valve (or safety relief valve, to be more precise) pops open in an instant when the pressure exceeds the set limit. Think of it like a soda can bursting open… just a lot more controlled and a lot less messy.

So, when we talk about the types of pressure relief valves, we’re including both PRVs and safety relief valves in the mix. But hey, enough chit-chat. Let’s get to know the different types of pressure relief valves!

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Spring-Loaded Pressure Relief Valves

First up on our grand tour of pressure relief valves is the spring-loaded type. Now, don’t let the name fool you – these valves aren’t loaded with actual springs. Well, not in the way you’re thinking.

The term “spring-loaded” here refers to the design of the valve. It uses a spring force to hold the valve closed under normal conditions. Once the system pressure exceeds the force exerted by the spring (the set pressure), the valve opens up, allowing excess process fluid to escape. Then, once the pressure’s back to a safe level, the valve closes back up again. Neat, huh?

These valves are the most common types of pressure relief valves, and they’re used in a wide range of applications, from boilers and pressure vessels to piping systems and more. They’re reliable, straightforward, and – best of all – they don’t require an external power source. That’s right, they’re self-actuated.

Pilot-Operated Pressure Relief Valves

Next up are the pilot-operated pressure relief valves. Now, these are a bit more complex than their spring-loaded counterparts, but they’re also more versatile. Let me explain.

In a pilot-operated valve, the main valve is controlled by a secondary valve (the “pilot”). The pilot valve senses the system pressure and opens or closes accordingly. This, in turn, controls the operation of the main valve. Cool, huh?

But why bother with a pilot valve, you ask? Well, the design of the pilot-operated valve allows for more accurate control over the set pressure. This means the valve can be used in systems with very high pressures or where a small amount of leakage could be disastrous. Think nuclear power plants, and you’ll get the idea.

Temperature-Activated Pressure Relief Valves

Just as their name suggests, these valves are all about temperature. They’re designed to open when the temperature of the process fluid exceeds a certain limit. This makes them perfect for systems where heat build-up can be a problem.

Temperature-activated pressure relief valves are usually self-actuated and don’t require an external power source. They’re most often used in heating systems and hot water tanks – anywhere a sudden surge in temperature could cause pressure to go haywire.

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Balanced Bellows Pressure Relief Valves

Imagine a spring-loaded pressure relief valve. Now, imagine that valve with an added component: a bellows. That’s a balanced bellows pressure relief valve in a nutshell.

The bellows is a flexible, accordion-like component that moves with the disc of the valve. This design helps to balance out the effect of back pressure on the valve’s performance, making these valves ideal for systems where back pressure can vary.

Nozzle Type Pressure Relief Valves

Last but not least, we have nozzle type pressure relief valves. Now, these are a bit different. Instead of a disc and seat, these valves use a nozzle and orifice to control the flow of process fluid.

Nozzle type valves are often used in systems where the fluid is dirty or contains particulates. The design of the valve prevents it from becoming clogged, ensuring reliable performance even under tough conditions.

Advantages and Disadvantages of Different Valve Types

Pressure relief valves come in a variety of designs, each with their own unique advantages and disadvantages. The optimal valve type depends on the specifics of the application and system conditions. Key factors to consider when selecting a pressure relief valve include required set pressure, fluid properties, allowable backpressure, cost, maintenance requirements, and operating conditions. The table below provides a comparison of common pressure relief valve types, summarizing the key pros and cons of each design:

Valve TypeAdvantagesDisadvantages
Spring-Loaded Relief Valves* Simple and reliable design 
* Suitable for wide range of applications 
* Self-actuated – no external power required
* Sensitive to back pressure 
* May not handle very high pressures
Pilot-Operated Relief Valves* Allow precise control of set pressure 
* Can handle very high pressures 
* Less prone to chattering
* More complex design 
* Higher maintenance requirements 
* Increased cost
Temperature-Activated Relief Valves* Activated by rise in temperature 
* Do not require external power 
* Suited for systems prone to heat build-up
* May not respond quickly to sudden pressure spikes
Balanced Bellows Relief Valves* Can accommodate fluctuating back pressure 
* Bellows balances effects of back pressure
* Potential for bellows wear or damage 
* Higher cost than basic spring-loaded valves
Nozzle Type Relief Valves* Resistant to clogging from dirty fluids 
* Reliable in applications with particulates
* More expensive than other designs 
* Increased complexity and maintenance
Piston-Type Relief Valves* Handle very high pressures 
* Extremely accurate set pressures
* Complex design 
* Prone to wear and erosion 
* Require regular maintenance
Diaphragm-Type Relief Valves* Excellent for liquid service 
* Balanced design 
* Rapid opening
* Potential for diaphragm damage 
* Generally higher cost

Safety Relief Valves for Gas, Air and Liquids

Safety relief valves are like the Swiss Army knives of the valve world. They’re designed to handle all sorts of fluids, from gases and air to liquids. But they’re not just versatile – they’re also very precise, designed to pop open in an instant when the pressure exceeds the set limit.

Gas and air are compressible, meaning their volume can change under pressure. On the other hand, liquids are incompressible – their volume stays the same no matter how much pressure you apply. So, dealing with these different types of fluids requires a bit of engineering finesse.

Safety relief valves for gases and air are usually set to open at a pressure slightly above the maximum system pressure. This is to prevent the valve from chattering (rapid opening and closing) due to minor pressure fluctuations.

On the flip side, safety relief valves for liquids need a slightly different approach. They’re typically set to open at a pressure just below the maximum allowable system pressure. This is because a sudden pressure surge in a liquid can cause water hammer – a shock wave that can damage equipment.

Regulating station with pressure relief valves. Regulating station with pressure relief valves. Pressure "Relief Valves" stock pictures, royalty-free photos & images

Selecting the Right Pressure Setting

Now that we’ve met the different types of pressure relief valves, let’s talk about picking the right pressure setting. It’s a bit like picking the right temperature for your oven – too high, and your cake could end up a charred mess; too low, and it might not cook at all.

The “set pressure” of a pressure relief valve is the pressure at which the valve starts to open. It’s usually set to a value slightly above the maximum allowable working pressure (MAWP) of the equipment it’s protecting.

Remember, the idea is to prevent the pressure from exceeding the equipment’s design limit, not to maintain a certain pressure. So, the set pressure should be high enough to avoid unnecessary openings but low enough to protect the equipment from overpressure.

Typical Pressure Relief Valve Materials & Key Components

Pressure relief valves are made of some tough stuff. They have to be, given the job they do. But what exactly are they made of?

Most pressure relief valves are made of materials like stainless steel, bronze, or cast iron. These materials are chosen for their strength, durability, and resistance to corrosion. After all, you don’t want your valve rusting or breaking down when you need it most.

As for the key components of a pressure relief valve, you’ve got the main valve (or disc), the seat, the spring, the bonnet, and – in some valves – the bellows. Each component has its own job in the operation of the valve. For example, the main valve controls the flow of process fluid, while the spring provides the force needed to keep the valve closed under normal conditions.

Installation Considerations for Pressure Relief Valves

Now, let’s talk about installing these little heroes. You can’t just stick a pressure relief valve anywhere and expect it to do its job. Nope, there are some key things to consider.

Firstly, the valve needs to be installed at the highest point in the system or on the pressurized vessel. This is because pressure relief valves work based on the static pressure of the fluid, which is highest at the lowest point in the system.

Secondly, the inlet side of the valve needs to be connected to the system, and the outlet side needs to be open to the atmosphere or connected to a discharge pipe. You also need to make sure that the piping doesn’t restrict the flow of fluid from the valve.

Lastly, pressure relief valves need to be installed vertically with the bonnet on top. This is to allow the process fluid to drain from the valve and prevent buildup.

Regulating station with pressure relief valves. Regulating station with pressure relief valves. Pressure "Relief Valves" stock pictures, royalty-free photos & images

Inspection and Maintenance Requirements

Like any piece of equipment, pressure relief valves need a bit of TLC to keep them in tip-top shape. Regular inspections and maintenance can help prevent failure and extend the life of the valve.

Inspections usually involve checking the valve for signs of damage or wear, as well as checking the set pressure and the relieving pressure. Maintenance, on the other hand, can include cleaning the valve, replacing worn components, and recalibrating the set pressure.

Don’t forget, safety relief valves are there for your protection. So, it’s well worth taking the time to look after them.

Pressure Relief Valve Standards and Certifications

When it comes to pressure relief valves, you can’t just wing it. Nope, there are strict standards and certifications in place to ensure that these devices do their job effectively and safely.

One of the main bodies that oversees these standards is the American Society of Mechanical Engineers (ASME). They have a specific section in their Boiler and Pressure Vessel Code (BPVC) dedicated to pressure relief devices. The section, known as ASME Section VIII, outlines the requirements for the design, materials, and testing of pressure relief valves.

Other notable standards include API 520 and API 526 from the American Petroleum Institute, and ISO 4126 from the International Organization for Standardization.

Applications of Pressure Relief Valves by Industry

Piston CompressorsPiston safety valves installed on cylinder heads prevent damage from over-pressurization. The piston rises against a calibrated spring when process pressure exceeds the set pressure, allowing excess gas to escape.
Diaphragm Safety ValvesDiaphragm valves provide reliable, precise pop action. The diaphragm pops open when inlet pressure overcomes spring force. Outlet port diameter is sized for required flow rate at full lift.
Conventional Safety Relief ValvesContain inlet nozzle, outlet port, spring housing, and main valve. Pilot-operated versions add external pilot to control main valve opening for greater set pressure accuracy.
Oil & Gas, Chemical, Power Generation, ManufacturingProtect equipment from catastrophic failures due to excessive internal pressure. Must meet rigorous industry standards, especially at high temperatures.
ASME BPVC Section VIIIProvides requirements for safety valve design, materials, and testing for boilers, pressure vessels, and piping in the United States. Valves stamped with set pressure based on MAWP.
Liquid ServiceSet pressure just under waterline rating to avoid chatter.
Forced-Flow Steam GeneratorsSet pressure exceeds fixed steam dome pressure.
Dome Pressure ChambersPilot-operated valves allow tighter set pressures for large bore diameters.
Bellows Safety Relief ValvesHandle varying backpressures well. Protect from excess pressure and excessive internal vacuum.
Inspection, Testing & MaintenanceCritical across industries to prevent failure and ensure proper functioning when required. Protect assets and personnel.


So there you have it – a complete overview of the types of pressure relief valves. These little devices might seem complicated, but really, they’re all about safety. They’re like the silent superheroes of the industrial world, always on guard and ready to step in if pressure gets out of hand.

So next time you’re near a boiler or a pipeline, take a moment to appreciate the humble pressure relief valve. It might not look like much, but it’s doing an incredibly important job.

And with that, our journey into the world of pressure relief valves comes to an end. But remember, this is just the tip of the iceberg. There’s a whole world of valves out there, each with their own unique designs and functions. So keep exploring, and you never know what you might discover!