If you operate a vacuum truck, septic truck, or liquid waste unit, you’ve probably heard the terms vacuum relief valve and pressure relief valve.
They sound similar. They’re often installed as part of the same vacuum system. But they do two very different jobs. The simplest way to remember it is:
A vacuum relief valve protects against too much vacuum. A pressure relief valve protects against too much positive pressure.
Both are important safeguards, and understanding how they work can help protect your vacuum pump, tank, plumbing, and other components. Be sure to checkout our selection of valves at The Pumper Store.
What Is a Vacuum Relief Valve?
A vacuum relief valve is designed to open when the vacuum level reaches its set point.
Rather than allowing the system to continue pulling deeper vacuum, the valve allows atmospheric air into the system. On certain rotary vane vacuum pump applications, this incoming air can also help provide the airflow needed to keep the pump cool. A vane pump is cooled by air traveling through it and that a properly functioning vacuum relief valve helps introduce cooling air at higher vacuum levels.
Think of it as the system saying:
“That’s enough vacuum. Let some air in.”
What Does a Vacuum Relief Valve Help Protect?
Depending on the vacuum system and pump design, a properly selected and functioning vacuum relief valve can help:
- Limit excessive vacuum
- Protect vacuum-system components
- Provide additional airflow through certain vacuum pumps
- Reduce the risk of overheating in applicable rotary vane pump systems
- Keep the system operating within its intended vacuum range
Vacuum relief valves are commonly available specifically for vacuum tanks, trailers, storage tanks, and other vacuum-system applications.
What Is a Pressure Relief Valve?
A pressure relief valve works on the opposite side of the equation.
Instead of reacting to excessive vacuum, it reacts when positive pressure gets too high.
When pressure reaches the valve's set point, the relief valve opens to vent excess pressure. KeeVac Industries describes the pressure relief valve as a safeguard that activates when pressure exceeds safe operating limits, helping protect the tank, pump, and plumbing system from overpressure.
In simple terms:
“There’s too much pressure. Let some of it out.”
Pressure can develop in vacuum truck systems during certain operating conditions, which is why an appropriate relief system is critical.
Vacuum Relief vs. Pressure Relief: The Easy Way to Remember
| Vacuum Relief Valve | Pressure Relief Valve |
|---|---|
| Reacts to excessive vacuum | Reacts to excessive positive pressure |
| Allows atmospheric air into the system | Allows excess pressure out of the system |
| Helps limit maximum vacuum | Helps limit maximum positive pressure |
| Can help maintain cooling airflow on certain vane pumps | Helps protect equipment against overpressure |
So, if you only remember one thing:
Vacuum relief lets air IN. Pressure relief lets air OUT.
That simple difference explains why one valve isn't necessarily a substitute for the other.
Why Does a Vacuum Truck Need Both?
A vacuum truck isn't always operating under just one condition.
During normal vacuum operation, the system creates negative pressure to pull material into the tank. Under other operating conditions, positive pressure may be present.
That means the equipment needs protection on both sides of atmospheric pressure.
The vacuum relief valve deals with excessive vacuum.
The pressure relief valve deals with excessive positive pressure.
Together, appropriately specified relief devices provide safeguards against operating outside the system's intended range.
Relief Valves Need Maintenance Too
Because these valves are there to protect expensive equipment, they shouldn't be forgotten until something goes wrong.
As part of routine vacuum truck maintenance:
- Inspect relief valves regularly
- Watch for corrosion, contamination, or damage
- Confirm valves aren't obstructed
- Monitor vacuum and pressure gauges during operation
- Investigate unexpected changes in operating vacuum or pressure
- Replace worn or damaged components
- Follow the equipment manufacturer's inspection and adjustment requirements
Be sure to checkout our guide for How to Test a Vacuum Relief Valve. Regularly checking the operation of vacuum and pressure relief valves is recommended as part of vacuum truck maintenance.
The Bottom Line
Vacuum relief valves and pressure relief valves might look similar, but their jobs are fundamentally different.
Vacuum relief valve: protects the system from excessive vacuum by allowing air in.
Pressure relief valve: protects the system from excessive positive pressure by allowing air or gas out.
On a hard-working vacuum truck, both can be critical pieces of the overall system.
They're small components compared with a tank or vacuum pump, but they're components you definitely don't want to overlook.
Need a Replacement Vacuum or Pressure Relief Valve?
Whether you're replacing a worn valve, troubleshooting your vacuum system, or keeping spare parts on the shelf, The Pumper Store can help you find the vacuum truck parts you need.
