Conventional vs Balanced Relief Valve

Quick Answer

A conventional relief valve is affected by backpressure at its outlet, which can change the effective set pressure and reduce capacity. A balanced bellows relief valve uses a bellows element to isolate the disc from backpressure, maintaining a stable set pressure regardless of downstream conditions. Use balanced bellows when the valve discharges into a common header with variable backpressure.

How Backpressure Affects Relief Valves

Backpressure is the pressure at the outlet of the relief valve. It comes from two sources:

• Superimposed backpressure: pressure already present in the discharge header before the valve opens (from other valves discharging into the same header).
• Built-up backpressure: pressure generated by flow through the discharge piping after the valve opens.

In a conventional relief valve, backpressure acts on the back of the disc, opposing the spring force. This effectively increases the set pressure: the valve opens later than intended. If backpressure is high enough, the valve may not open at all, defeating its safety purpose.

Design Comparison

Parameter	Conventional Relief Valve	Balanced Bellows Relief Valve
Backpressure sensitivity	High (set pressure shifts with backpressure)	Low (bellows compensates)
Bellows element	None	Metal bellows surrounds the disc stem
Set pressure stability	Stable only if backpressure is constant and known	Stable up to ~50% of set pressure backpressure
Backpressure limit	Max 10% of set pressure (variable)	Up to 40-50% of set pressure
Bonnet type	Conventional (open or closed)	Closed with bellows vent
Bonnet vent	Not required	Required (vents bellows leakage)
Cost	Lower	20-40% higher
Maintenance	Simpler (no bellows to inspect)	Bellows integrity must be monitored
Failure mode	Shifts set pressure if backpressure changes	If bellows ruptures, reverts to conventional behavior
Standard	API 526, ASME Sec. VIII	API 526, ASME Sec. VIII
Typical application	Dedicated discharge (no shared header)	Shared flare/vent header, variable backpressure

When to Use Each Type

Conventional: use when the relief valve has a dedicated discharge pipe to atmosphere or to an individual drain, and backpressure is negligible or constant. Common on small vessels, utility systems, and individual equipment with independent discharge piping.

Balanced bellows: use when the valve discharges into a shared header (flare, vent, or blowdown system) where other relief valves may be discharging simultaneously. The bellows isolates the disc from backpressure fluctuations, ensuring the valve opens at the correct set pressure every time.

Bellows Failure and Monitoring

The bellows is a thin-walled metal element subject to fatigue and corrosion. If it ruptures, the valve reverts to conventional behavior: backpressure now acts on the disc. API 526 requires a bonnet vent on balanced bellows valves. If fluid discharges from this vent, the bellows has failed and must be replaced immediately.

Common Mistake

Connecting the bonnet vent of a balanced bellows valve to a pressurized header. The vent must be open to atmosphere or a safe, non-pressurized drain. Pressurizing the vent defeats the purpose of the bellows and creates a safety hazard.

For pilot-operated relief valves as an alternative to bellows designs, see the full safety and pressure relief valve guide.

Read the full guide to valve types