Pressure Relief Valve vs Pressure Reducing Valve

Last updated: 7 October 2026

A relief valve limits maximum system pressure; a reducing valve holds a lower pressure in one branch. That is the whole difference between a pressure relief valve and a pressure reducing valve, and everything else follows from it: where each one sits, which way it moves and where its oil goes. If a clamp circuit on your press has to run at a lower pressure than the main cylinder, the valve doing that job is a reducing valve. The valve that stops the pump from over-pressurizing the whole machine is a relief valve. This guide shows how to tell them apart on a schematic, on a nameplate and on the machine.

Relief valve vs reducing valve: the differences at a glance

A relief valve and a reducing valve differ on every row of this table, and any one row is enough to identify the valve in front of you. The Rexroth rows come from the manufacturer's data sheets RE 25802 (types DB and DBW) and RE 26892 (type DR).

How a hydraulic pressure relief valve works

A hydraulic pressure relief valve stays closed until the pressure at its inlet reaches the set value, then opens a path to tank. It protects everything connected to the pressure line: pump, hoses, cylinders and the people standing near them. Bosch Rexroth describes its pilot operated DB valve this way in data sheet RE 25802: pressure in channel P acts on the main spool, and once the set pressure is passed, the connection from channel P to channel T opens and oil flows from P to T while the set operating pressure is maintained.

The DBW version of the same valve adds a small directional valve on top. Switching its solenoid unloads the main spool, so the pump can circulate oil to tank at low pressure when the machine has nothing to do. Oil that crosses a relief valve gives up its pressure as heat, which is why a machine that sits on its relief valve for long periods runs hot.

How a hydraulic pressure reducing valve works

A hydraulic pressure reducing valve stays open until the pressure at its outlet reaches the set value, then closes down to hold that pressure. It is fitted in line, ahead of the part of the machine that must run at a lower pressure than the rest. Bosch Rexroth's data sheet RE 26892 for the pilot operated DR valve says the valve is open in its initial position, with oil flowing from channel B through the main spool to channel A without obstruction. The pressure in channel A, the outlet, is what acts on the pilot stage.

Two details on the DR catch people out. Pilot oil from the spring chamber always returns to tank through a separate port, Y, so the valve has three connections, not two. And an optional check valve lets oil flow freely back from A to B, which matters when the actuator in that branch has to retract through the valve.

Relief and reducing valve symbols on a hydraulic schematic

On a schematic, a relief valve is drawn with its arrow offset from the flow line and a reducing valve with its arrow in line. Both symbols are a single square with a spring on one side. Three details separate them.

• The arrow. Offset from the line means the path is closed at rest (relief). In line means it is open at rest (reducing).
• The dashed pilot line. Taken from the inlet side on a relief valve, from the outlet side on a reducing valve.
• The drain. A reducing valve shows a separate dashed line to tank. A relief valve sends its main flow to tank instead.

The drawings above are simplified for comparison. Work from the schematic that came with your machine.

Which valve do you have? Four checks on the machine

The fastest way to identify a pressure valve is the type code on its nameplate. If the nameplate is gone or painted over, the piping and the schematic give the same answer.

1. Read the nameplate. On a Bosch Rexroth valve, DB or DBW at the start of the type code is a relief valve and DR is a reducing valve. Write down the R9 material number as well.
2. Count the connections. A reducing valve has a supply, an outlet and a separate small drain to tank. A relief valve has a pressure connection and a tank connection.
3. Find it on the schematic. A relief valve branches off the pump line and returns to tank. A reducing valve sits in line, ahead of the actuator that runs at the lower pressure.
4. Look at what is affected. Low pressure across the whole machine points toward the supply side: the pump or the main relief valve. Low pressure in one branch only points toward that branch, including its reducing valve.

Bosch Rexroth relief and reducing valves by type code

Bosch Rexroth names its pressure valves by function: DB types relieve and DR types reduce. The table lists the main families, the data sheet for each and where to find the valves in our store. For the full code, field by field, see our guide to Bosch Rexroth pressure valve model codes.

Two valves with the same first letters can still differ in size, pressure stage, pilot oil routing and seal material. Do not treat one as a replacement for another on the strength of the type letters alone. Confirm fit with our engineering team first.

What you need, and where to go

Livingston & Haven splits this work across two sites: part numbers and ordering are on the store, and engineering, repair and training are here. Livingston & Haven was founded in 1947 and is based in Charlotte, North Carolina. We are an authorized Bosch Rexroth distributor.

Common questions

What is the main difference between a pressure relief valve and a pressure reducing valve?

A pressure relief valve limits the maximum pressure in a circuit by opening a path to tank. A pressure reducing valve holds a lower pressure in one branch by throttling the oil that enters it. The relief valve reacts to its inlet pressure and the reducing valve reacts to its outlet pressure.

Is a pressure relief valve normally open or normally closed?

A pressure relief valve is normally closed. It opens only when the pressure at its inlet reaches the set value. A pressure reducing valve is the opposite: it is normally open and closes down as its outlet pressure reaches the set value.

Can a relief valve be used as a pressure reducing valve?

No. A relief valve reacts to the pressure at its inlet and sends oil to tank, so it cannot hold a set pressure downstream of itself. To run one branch at a lower pressure than the rest of the system, fit a reducing valve in line with that branch.

Does a pressure reducing valve need a drain line?

On the Bosch Rexroth DR it does. Data sheet RE 26892 states that pilot oil from the spring chamber always returns to tank externally, through port Y. Check the data sheet for any other make or model before assuming the same.

Is a pressure regulator the same as a pressure reducing valve?

They do the same job. Pressure reducing valve is the usual name in hydraulics. Pressure regulator is the more common name in pneumatic and process systems.

What do DB and DR mean on a Bosch Rexroth valve?

DB is Bosch Rexroth's type code for a pilot operated pressure relief valve, covered by data sheet RE 25802. DR is the type code for a pilot operated pressure reducing valve, covered by data sheet RE 26892. DBW is a DB with solenoid unloading.