Roots Blower Surge Control

Roots blower surge control is a critical concern in industrial applications – but here's the key fact: roots blowers do not surge. Surge is a phenomenon that affects dynamic compressors like centrifugal blowers. Roots blowers are positive displacement machines – they have no surge limit and operate stably at any flow or pressure within their design range.

However, the systems that roots blowers feed can experience pressure fluctuations, pulsation, and instability. And roots blowers can experience over-pressure, over-speed, and other conditions that mimic surge. Understanding these differences is essential for proper system design.

Based on commissioning experience across hundreds of installations, surge-related problems in roots blower systems trace to system design issues, not the blower itself. This guide covers what surge is, why roots blowers don't surge, and how to prevent system instability.

Table of Contents

• What Is Surge in Industrial Systems?

• Do Roots Blowers Surge?

• Why Roots Blowers Don't Surge – The Engineering Explanation

• What Causes System Instability with Roots Blowers?

• Surge Control Strategies for Roots Blower Systems

• Comparison – Roots Blower vs Centrifugal Blower Surge

• Common Problems and Troubleshooting

• Frequently Asked Questions

• Final Thoughts

What Is Surge in Industrial Systems?

Surge definition:
Surge is a flow instability that occurs in dynamic compressors (centrifugal, axial) when the flow drops below a critical minimum. The compressor cannot maintain pressure, flow reverses, pressure collapses, then the cycle repeats – creating violent pressure and flow oscillations.

Surge characteristics:

• Loud, rhythmic noise (chugging, hammering)

• Pressure fluctuations (10–50% of operating pressure)

• Flow reversals (air moves backward through the compressor)

• High vibration (can damage bearings, seals, and piping)

• Rapid temperature rise

Surge causes:

• Flow drops below minimum (system restriction)

• Pressure setpoint too high for flow

• Inlet conditions change (temperature, density)

• Speed too low for required pressure

Surge is destructive. It can destroy a centrifugal blower in minutes. It is the primary reason centrifugal blowers require surge control systems.

Do Roots Blowers Surge?

No – roots blowers do not surge.

Roots blowers are positive displacement machines. They trap a fixed volume of air and push it from inlet to discharge. They maintain flow regardless of discharge pressure (within design limits). There is no minimum flow requirement – the blower operates stably at any flow.

Key facts:

• Roots blowers have no surge limit

• Roots blowers operate stably from full flow to zero flow

• There is no minimum flow requirement

• Flow reversal does not occur (check valve prevents backflow)

• Pressure pulsation is inherent (from backflow) but not surge

The engineering reason:
Surge requires a dynamic pressure rise characteristic (pressure vs flow curve with a negative slope). Roots blowers have a positive displacement characteristic – flow is independent of pressure. There is no instability point.

Why Roots Blowers Don't Surge – The Engineering Explanation

Centrifugal blower pressure characteristic:

• Flow decreases as pressure rises

• The pressure vs flow curve has a negative slope

• At low flow, the curve becomes unstable

• Surge occurs at the left side of the curve

Roots blower pressure characteristic:

• Flow is independent of pressure (constant volume)

• The pressure vs flow curve is nearly vertical

• No negative slope – no instability

• No surge limit

The comparison:

What Causes System Instability with Roots Blowers?

While roots blowers don't surge, system instability can occur:

1. Over-pressure.
If discharge pressure exceeds the relief valve setting, the relief valve cycles open and closed. This creates pressure fluctuations that can be mistaken for surge.

Causes:

• Blocked discharge piping

• Closed valves

• Clogged filters or diffusers

• Relief valve set too high or stuck

Solutions:

• Set relief valve properly

• Clean filters and diffusers

• Add pressure margin to system design

2. Pulsation.
Roots blowers have inherent pulsation from the discharge backflow. In extreme cases, pulsation can cause piping vibration and instrument damage.

Causes:

• Inadequate silencer sizing

• Damaged silencer baffles

• Long unsupported piping

• High pressure operation

Solutions:

• Install properly sized silencers

• Support piping adequately

• Use flexible connectors

• Consider helical rotors (lower pulsation)

3. Check valve chattering.
Rapid opening and closing of discharge check valve creates noise and pressure fluctuations.

Causes:

• Incorrect check valve selection (swing type)

• Low flow conditions

• Damaged valve internals

Solutions:

• Use silent check valves

• Size valve for minimum flow

• Regular inspection

4. Relief valve cycling.
Relief valve opening and closing creates pressure fluctuation.

Causes:

• System pressure near relief set point

• Relief valve undersized

• Stickiness in valve mechanism

Solutions:

• Set relief valve 2 psig above operating pressure

• Size relief valve for full blower flow

• Test relief valve regularly

5. VFD instability.
At very low speeds, the blower may not maintain stable pressure.

Causes:

• Speed below 30% of rated

• Insufficient pressure margin

• Control loop instability

Solutions:

• Maintain speed above 30% of rated

• Add receiver tank for damping

• Tune control loop

Surge Control Strategies for Roots Blower Systems

Since roots blowers don't surge, "surge control" for roots blowers means preventing system instability:

1. Proper relief valve sizing and setting.

• Set relief valve 2 psig above maximum operating pressure

• Size relief valve for full blower flow

• Test relief valve monthly

• Vent relief valve to safe location

2. Proper silencer selection.

• Inlet and discharge silencers required

• Size for pressure and flow

• Use reactive type for pulsation damping

• Inspect silencer annually

3. Check valve selection.

• Use silent check valve (not swing type)

• Stainless steel internals for corrosion resistance

• Inspect annually

4. Piping design.

• Flexible connectors within 18 inches of blower

• Adequate pipe support

• Avoid sharp bends

• Use schedule 40 or heavier pipe

5. VFD control.

• Maintain speed above 30% of rated

• Use PID control with pressure feedback

• Add receiver tank for damping

6. Pressure control.

• Install pressure gauge at blower discharge

• Monitor pressure trend

• Investigate pressure increases

7. Filter/diffuser maintenance.

• Change filters before pressure rises 2 psig

• Clean diffusers on schedule

• Monitor pressure drop

Comparison – Roots Blower vs Centrifugal Blower Surge

Common Problems and Troubleshooting

Frequently Asked Questions

1. Can a roots blower surge?
No – roots blowers are positive displacement machines. They do not have a surge limit. They operate stably at any flow from full flow to zero flow. Surge is a phenomenon of dynamic compressors like centrifugal blowers.

2. Why don't roots blowers surge?
Roots blowers have a constant volume characteristic – flow is independent of pressure. The pressure vs flow curve is nearly vertical. There is no negative slope – no instability point. Centrifugal blowers have a negative slope on their pressure vs flow curve – that's where surge occurs.

3. What causes instability in roots blower systems?
Over-pressure (relief valve cycling), pulsation (inadequate silencers), check valve chattering, VFD instability at low speed, and filter/diffuser fouling. These are system issues, not surge.

4. Do roots blowers need surge control?
No – roots blowers do not require surge control. However, they require proper relief valves, silencers, check valves, and system design to prevent other types of instability.

5. What is the difference between roots blower pulsation and surge?
Pulsation is inherent in roots blowers – pressure fluctuations at the discharge frequency (4–6 pulses per revolution). Surge is a violent flow instability in centrifugal blowers with pressure fluctuations of 10–50% of operating pressure. Pulsation is normal and manageable. Surge is destructive.

6. Can VFD cause surge-like behavior in roots blowers?
At very low speeds (below 30% of rated), roots blowers may not maintain stable pressure. This can cause pressure fluctuations that resemble surge. Maintain speed above 30% of rated to avoid this. Add receiver tank for damping.

7. How do I prevent pressure fluctuations in roots blower systems?
Use properly sized relief valve (2 psig above operating pressure), discharge silencer, silent check valve, and adequate piping support. Monitor pressure trend and clean filters/diffusers before pressure rises 2 psig.

8. Why does my relief valve cycle frequently?
The system pressure is too close to the relief valve set point. Increase set point to 2 psig above operating pressure. Or the relief valve is undersized – size it for full blower flow. Or the valve is sticky – test and clean it.

9. Why does my check valve make noise?
Swing check valves chatter at low flow. Install a silent check valve (spring-loaded) that closes smoothly. Check valve noise is not surge – it's a system issue.

10. Why does my piping vibrate?
Pulsation from the blower is not adequately dampened. Install or replace discharge silencer. Support piping adequately. Use flexible connectors within 18 inches of blower.

11. Can roots blowers operate at zero flow?
Yes – but the relief valve should open to prevent over-pressure. Roots blowers can operate with blocked discharge if the relief valve is properly sized and set. This is not surge – it's over-pressure protection.

12. What is the surge margin for a centrifugal blower?
Centrifugal blowers typically require 10–20% surge margin – they must operate at least 10–20% above the minimum flow. Roots blowers have 0% surge margin requirement – they operate at any flow.

13. Do helical roots blowers surge?
No – helical roots blowers are still positive displacement machines. They have lower pulsation but still no surge limit.

14. How do I know if my system is surging?
If you have a centrifugal blower: loud rhythmic noise, pressure fluctuations, high vibration. If you have a roots blower: you don't have surge. You may have over-pressure, pulsation, or check valve chatter – but not surge.

15. What safety devices are required for roots blower systems?
Relief valve (set 2 psig above operating pressure), pressure gauge at discharge, temperature gauge at discharge, and check valve on discharge. For high-pressure systems: temperature shutdown at 250°F.

Final Thoughts

After decades of commissioning roots blower systems, here is my practical advice:

Roots blowers do not surge. This is the most important fact to understand. Roots blowers are positive displacement machines – they have no surge limit. They operate stably at any flow. If a roots blower system is unstable, the problem is elsewhere – relief valve cycling, pulsation, check valve chatter, or VFD instability.

Don't look for surge – look for the real problem. If your roots blower system has pressure fluctuations, check the relief valve, silencer, check valve, and filters. These are the common causes of instability. Roots blowers don't surge – so don't waste time trying to fix surge that doesn't exist.

Proper system design prevents instability. Relief valve set 2 psig above operating pressure. Silencers on inlet and discharge. Silent check valve. Adequate piping support. These prevent the problems that can be mistaken for surge.

The bottom line. Roots blower surge control is about understanding that roots blowers don't surge. The engineering explanation is clear: positive displacement machines have no surge limit. The system instability that does occur has other causes. Zhanggu and other manufacturers design blowers that operate stably – but the system must be designed correctly to prevent relief valve cycling, pulsation, and check valve issues. Focus on system design, not surge control.