I'm an EE trying to interpret a pneumatic schematic and have a few questions regarding the symbols being used.

I've come across two different regulator symbols (or so I believe them to be regulator symbols). I'm curious if someone could break down the difference between each.

Here are my interpretations and questions for each symbol:

  1. I believe this symbol represents a pressure regulator. The spring indicates that it is a pressure regulator valve. However, I'm somewhat confused as to what the dashed line is meant to represent. Is it just meant to represent a feedback line for regulation purposes?

  2. This symbol is similar to symbol 1, however, it differs in how the arrow is oriented. What is the significance of the arrow direction?

enter image description here

  • $\begingroup$ The dashed line represent measurement signals, they are hypothetical and they don't represent anything physical at all in the circuit. $\endgroup$
    – user14407
    Aug 14, 2018 at 19:23
  • $\begingroup$ @Sam If I’m remembering these pictures correctly, the dashed line is a real part of the system. It represents a pilot line and does serve a purpose in schematics. It’s a physical pathway for the fluid to go somewhere else, generally an extremely low flow that’s only used as a reference pressure. Sometimes the fluid can’t actually flow through it, but the pressure of the fluid can be measured through the pilot. $\endgroup$
    – JMac
    Aug 14, 2018 at 20:36
  • $\begingroup$ @JMac, you are right, i meant no flow passes through the dashed lines, they can represent an electrical signal of hydraulic, mechanic or pneumatic signals. Since i can't edit my comment anymore, here is my apology for any confusion. $\endgroup$
    – user14407
    Aug 14, 2018 at 20:39

1 Answer 1


Both of symbols represent 'relief valves', to my knowledge.

The shape of arrow in the first schematics represent the pilot operated relief valve and the second one is direct operated relief valve.

The first schematic, the pressure valve is in its normal state, no flow is passing, once the pressure exceeds the pre-defined value, it will change its state to the second schematic. The second schematic also has a pressure indicator, the circle with an arrow inside.

The dashed line are signal lines, no flow passe through the dashed line.

Direct operated relief valve: enter image description here

Pilot operated relief vale:

enter image description here

  • $\begingroup$ Hmm... so is there a difference between a relief valve and a pressure regulator? It seems like a relief valve can perform step-down regulation. $\endgroup$
    – Izzo
    Aug 14, 2018 at 21:03
  • $\begingroup$ Both can regulate pressure, the relief valve acts as a safety valve however the pressure regulator acts as a locking system, i.e car elevators. With pressure regulator you can indeed accomplish the desire pressure, but safety valves just protect the circuit elements agains over-pressure. Btw, i use always NBN and DIN, symbols can also be different in different norm systems. $\endgroup$
    – user14407
    Aug 14, 2018 at 21:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.