When do I need balanced bellows?

When variable backpressure would upset a conventional valve — follow the relief philosophy.

What must be on the RFQ?

Set pressure, relieving medium, orifice/API letter, inlet/outlet size, and backpressure.

Backpressure on the discharge side changes which PSV design you should RFQ.

Topic	Conventional	Balanced bellows
Backpressure	Limited built-up/variable backpressure tolerance	Better for variable backpressure
Bellows	None	Isolates spring from discharge
Cost	Lower	Higher
RFQ cue	Conventional SRV	Balanced bellows SRV
Docs	Set P + orifice	Set P + orifice + backpressure

Attach the relief load calculation summary or orifice letter when available.