Types of PRV: A Complete Guide to Pressure Relief and Reducing Valves

2025-09-08

Updated on 2025-09-15
William Leo
Pressure Valve

If you're searching for "types of PRV," you might be confused about what PRV actually means. That's because PRV can stand for two completely different things in the valve world: Pressure Relief Valves and Pressure Reducing Valves. Don't worry – this guide will clear up all the confusion and help you understand both types.

What Does PRV Mean? Clearing Up the Confusion

The biggest challenge when talking about PRV types is that the same three letters can mean two totally different valves:

Pressure Relief Valve (PRV)

A safety device that opens to prevent dangerous overpressure

Pressure Reducing Valve (PRV)

A control device that lowers and maintains steady downstream pressure

Think of it this way: one PRV protects you FROM too much pressure (relief valve), while the other PRV gives you the RIGHT amount of pressure (reducing valve). Let's explore both types.

Part 1: Pressure Relief Valve Types - Your Safety Guards

Pressure relief valves are like emergency exits for your pressure systems. They stay closed during normal operation but spring into action when pressure gets dangerously high.

1. Spring-Loaded Relief Valves (Direct Acting)

How they work: These are the most common type. A spring holds the valve closed against system pressure. When pressure gets too high, it overcomes the spring force and the valve pops open.

Key Features:

Simple design with few moving parts
Fast response time
Lower cost
Works well for clean fluids

Limitations:

Sensitive to backpressure changes
May "chatter" (rapidly open and close) near set pressure
Accuracy around ±10%

Best for: General industrial applications where backpressure is constant and high precision isn't critical.

2. Pilot-Operated Relief Valves (PORV)

How they work: These use a small pilot valve to control a larger main valve. The pilot senses pressure and controls whether the main valve opens or closes. It's like having a smart assistant that decides when to open the big door. [Compare with safety relief valve types]

Key Features:

Excellent sealing (can operate at 98% of set pressure without leaking)
Not affected by backpressure changes
High accuracy (±1% to ±5%)
Can handle large flow rates

Limitations:

More complex and expensive
Slower response than spring-loaded valves
Sensitive to dirty fluids that can clog small passages

Best for: High-pressure, large-capacity applications where tight sealing and accuracy are crucial.

3. Balanced Bellows Relief Valves

How they work: These spring-loaded valves have a special bellows (accordion-like metal tube) that cancels out the effects of changing backpressure.

		
			Key Features:
		
				Eliminates backpressure effects on set pressure
			
				Protects internal parts from corrosive fluids
			
				Maintains accuracy despite outlet pressure changes
			
			Limitations:
		
				More expensive than standard spring-loaded valves
			
				Bellows can be damaged by particles or corrosion

Best for: Systems where multiple relief valves discharge to a common header, or when dealing with corrosive fluids.

4. Specialty Relief Valve Types

Thermal Relief Valves:

Handle small volume increases from liquid thermal expansion
Common in aircraft hydraulic systems and isolated liquid lines

Nozzle-Type Safety Valves:

Use nozzle design instead of flat seat
Better for fluids with particles or debris
Higher cost but longer service life

Temperature-Activated Valves:

Respond to temperature rise or combined temperature/pressure
Used in water heaters and steam systems

Part 2: Pressure Reducing Valve Types - Your Pressure Controllers

Pressure reducing valves are like governors that take high, variable upstream pressure and deliver steady, lower downstream pressure.

1. Direct-Acting Reducing Valves

How they work: A spring pushes against a diaphragm or piston that senses downstream pressure. When downstream pressure drops (more flow needed), the spring opens the valve more. When pressure rises (less flow needed), the valve closes.

Key Features:

Simple, reliable design
Fast response to pressure changes
Lower cost
No minimum pressure drop required

Limitations:

"Droop" - downstream pressure drops as flow increases
Limited accuracy (±5% to ±10%)
Best for smaller flow rates

Best for: Home water systems, small pneumatic tools, applications where some pressure variation is acceptable.

2. Pilot-Operated Reducing Valves

How they work: A small pilot valve senses downstream pressure and controls the main valve. The pilot valve adjusts the pressure above the main valve's diaphragm, which then moves the main valve to maintain steady downstream pressure.

Key Features:

Excellent accuracy (±1% to ±3%)
Maintains steady pressure despite flow changes
Can handle large flow rates and high pressure drops
Very stable control

Limitations:

More complex and expensive
Slower response than direct-acting valves
Requires minimum pressure drop to operate

Best for: Industrial steam systems, municipal water distribution, large pneumatic systems where precise pressure control is essential.

Quick Comparison: Relief vs. Reducing Valves

Feature	Pressure Relief Valves	Pressure Reducing Valves
Purpose	Emergency safety protection	Continuous pressure control
Normal state	Closed	Modulating (partly open)
Senses pressure from	Upstream (inlet)	Downstream (outlet)
When they work	Only during overpressure	Continuously during operation
Flow direction	OUT of system	THROUGH system

How to Choose the Right Type

For Relief Valves:

Simple applications: Spring-loaded valves work fine
Variable backpressure: Use balanced bellows or pilot-operated
High accuracy needed: Choose pilot-operated
Dirty fluids: Stick with spring-loaded
Large capacity: Consider pilot-operated

For Reducing Valves:

Small, steady loads: Direct-acting is sufficient
Variable loads: Pilot-operated gives better control
High accuracy required: Definitely pilot-operated
Cost-sensitive: Direct-acting is more economical

See all pressure valve types for complete options
Sizing and Standards

Cv Method

For both valve types, you'll often see "Cv" ratings. This tells you how much water (in gallons per minute) flows through the valve with a 1 psi pressure drop. Higher Cv = more flow capacity.

Key Standards

		
				ASME Section VIII: Requires pressure vessels to have relief protection
			
				API 521: Helps identify what could cause overpressure
			
				API 520: Provides formulas to calculate required valve size
			
				API 526: Standardizes relief valve sizes and specifications

Real-World Applications

Relief Valves Are Found On:

Pressure vessels and tanks
Boilers and steam systems
Pump discharge lines
Compressed air receivers
Chemical reactors

Reducing Valves Are Found In:

Home water systems (reducing city pressure)
Building steam heating systems
Pneumatic tool air supplies
Industrial gas distribution
HVAC systems

Maintenance and Troubleshooting

Common Problems:

Relief Valves:

Chattering (rapid open/close) - usually oversized valve
Leaking - damaged seat or wrong pressure setting
Won't open - wrong size, damaged spring, or blocked

Reducing Valves:

Pressure drift - worn internal parts or wrong pilot setting
Hunting (pressure oscillation) - improper sizing or installation
Won't reduce pressure - blocked pilot line or damaged diaphragm

Best Practices:

		
				Install relief valves close to protected equipment
			
				Keep inlet piping short and straight
			
				For reducing valves, take pressure sensing downstream of any turbulence
			
				Regular testing and maintenance are essential

The Future: Smart Pressure Control

Modern PRVs (both types) are getting smarter with:

Smart Features:

Built-in pressure sensors and wireless monitoring
Predictive maintenance alerts
Data logging for compliance and optimization
Remote adjustment capabilities

Conclusion

Understanding the different types of PRV – whether you mean pressure relief or pressure reducing valves – is crucial for safe and efficient system operation. Here's the key takeaway:

Relief valves = Safety devices that protect against overpressure
Reducing valves = Control devices that maintain steady downstream pressure

Choose spring-loaded for simplicity and cost, pilot-operated for precision and capacity, and balanced designs when backpressure varies. Always follow proper sizing calculations and industry standards to ensure your pressure control system works safely and effectively.

Remember: the right valve type depends on your specific application, accuracy requirements, and budget. When in doubt, consult with a qualified engineer or valve manufacturer to ensure you get the protection and performance you need.

Relief Valve Working Principle: How These Safety Devices Protect Your Systems

What is a Main Relief Valve?