You should know the procedures for starting and stopping a Roots vacuum pump

The Roots vacuum pump is one of the most widely used vacuum technologies across modern industry. From chemical processing and pharmaceutical manufacturing to metallurgy, semiconductor production, and aerospace simulation, the Roots vacuum pump delivers high pumping speeds and reliable performance in the medium‑to‑high vacuum range. However, despite its robust design, the Roots vacuum pump has one critical operational limitation: it cannot be used alone.

Unlike pumps that can discharge directly to atmosphere, a Roots vacuum pump must always be combined with a backing pump (also called a fore‑vacuum pump or pre‑stage pump) to form a complete vacuum unit. The backing pump lowers the system pressure to a level at which the Roots vacuum pump can operate safely. This is not merely a recommendation—it is a fundamental requirement to prevent excessive pressure differentials that can lead to motor overload, overheating, rotor seizure, and catastrophic equipment failure.

Equally important as the correct startup sequence is the shutdown procedure. Many operators focus on getting the Roots vacuum pump running but give insufficient attention to how they stop it. An incorrect shutdown order—particularly stopping the Roots vacuum pump after the backing pump—can cause oil backflow, system contamination, and serious damage to both pumps.

This article provides a comprehensive guide to the correct startup and shutdown procedures for Roots vacuum pumps. Whether you are an operator, maintenance engineer, or plant manager, understanding and strictly following these sequences is essential for protecting your equipment, ensuring consistent performance, and extending service life.

Understanding the Fundamental Principle – Why a Backing Pump Is Essential

Before examining the startup and shutdown procedures in detail, it is important to understand the underlying principle that makes these sequences so critical.

A Roots vacuum pump operates with two intermeshing lobe‑shaped rotors that rotate in opposite directions. The rotors maintain extremely tight clearances—typically measured in tenths of a millimeter—between themselves and the pump casing. This non‑contact design allows for high‑speed operation without mechanical wear, but it also creates a fundamental limitation: the Roots vacuum pump has a limited compression ratio and cannot discharge directly against atmospheric pressure.

If a Roots vacuum pump is started with its inlet at atmospheric pressure, the pressure difference between the suction and discharge sides becomes too large. The motor draws excessive current, overheating occurs, and the rotors expand due to thermal expansion, closing the critical clearances. The result can be rotor seizure, motor burnout, or both.

The backing pump serves two essential functions:

1. Pre‑evacuation: The backing pump first evacuates the system from atmospheric pressure down to the Roots vacuum pump's allowable inlet pressure—typically below 1,330 Pa (10 Torr).

2. Pressure maintenance: During normal operation, the backing pump continues to handle the gas load at the discharge side of the Roots vacuum pump, maintaining the pressure differential within safe limits.

The critical point: All conventional Roots vacuum pumps—with the exception of multistage, wet, and air‑cooled designs—require this combination with a backing pump. The startup and shutdown procedures are designed to ensure that this combination operates safely and reliably.

The Correct Startup Procedure for Roots Vacuum Pumps

The startup procedure for Roots vacuum pumps follows a strict, sequential order that must never be reversed or shortcut. This sequence ensures that the system pressure is reduced to a safe level before the Roots vacuum pump is engaged.

Step 1 – Start the Backing Pump

The very first action in starting a Roots vacuum pump unit is to start the backing pump. The backing pump may be an oil‑sealed mechanical pump, a water‑ring pump, a dry screw pump, or another suitable type, depending on the application.

What to do:

• Turn on the backing pump and allow it to run for a few seconds to stabilize.

• Verify that the backing pump is operating normally without unusual noise or vibration.

Why this matters: The backing pump must begin the pre‑evacuation process before any valves to the Roots vacuum pump are opened. Starting the Roots vacuum pump before the backing pump would expose it to atmospheric pressure, causing immediate overload.

Step 2 – Open the Pre‑Evacuation Valves

Once the backing pump is running, the next step is to open the valve on the backing pump's pre‑evacuation line and the valve on the Roots vacuum pump inlet pipe.

What to do:

• Open the pre‑evacuation valve (also called the bypass valve or roughing valve) on the backing pump's bypass line.

• Open the inlet valve on the Roots vacuum pump to connect it to the system.

• If the Roots vacuum pump has an exhaust valve, ensure it is also open.

Why this matters: Opening these valves connects the system volume to the backing pump, allowing the rough vacuum to be established. The backing pump begins reducing the system pressure toward the Roots vacuum pump's allowable inlet pressure.

Step 3 – Monitor the System Pressure

With the valves open, the backing pump continues to evacuate the system. The operator must monitor the system pressure until it reaches the Roots vacuum pump's allowable inlet pressure.

What to do:

• Use a vacuum gauge installed at the Roots vacuum pump inlet to monitor the pressure.

• The allowable inlet pressure varies by manufacturer and model—consult the pump's technical specifications.

• For most conventional Roots vacuum pumps, the allowable inlet pressure is typically below 1,330 Pa (10 Torr).

Why this matters: Starting the Roots vacuum pump before the system pressure has dropped to the allowable level will expose it to excessive pressure differential, leading to overheating, motor overload, and potential rotor seizure.

Step 4 – Close the Pre‑Evacuation Valve

Once the system pressure reaches the allowable inlet pressure for the Roots vacuum pump, the next step is to close the valve on the pre‑evacuation line.

What to do:

• Close the pre‑evacuation (bypass) valve on the backing pump's bypass line.

• The Roots vacuum pump inlet valve should remain open.

Why this matters: Closing the pre‑evacuation valve redirects the full pumping capacity of the backing pump through the Roots vacuum pump itself, preparing for the Roots vacuum pump to start.

Step 5 – Start the Roots Vacuum Pump

With the pressure condition satisfied and the pre‑evacuation valve closed, the final step is to start the Roots vacuum pump.

What to do:

• Start the Roots vacuum pump motor.

• Observe the pump for smooth operation, normal sound, and stable pressure readings.

Automatic control option: Many modern Roots vacuum pump systems are equipped with automatic control devices. Pressure sensors monitor the system pressure and start the Roots vacuum pump automatically once the allowable inlet pressure is reached. This automation reduces the risk of human error but does not eliminate the need for operators to understand the underlying sequence.

Special Consideration for Large Vacuum Systems

For large vacuum systems, a more cautious approach is required. Although many Roots vacuum pumps are equipped with bypass (overflow) valves that provide motor overload protection, this protection is primarily for the motor, not the pump itself. A Roots vacuum pump with a bypass valve can still suffer from overheating and seizure if it operates for an extended period at high inlet pressure. For large systems, always wait until the backing pump has reduced the system pressure to a safe level before starting the Roots vacuum pump.

If the system has no pre‑evacuation line: If there is no separate pre‑evacuation bypass line or valve, the Roots vacuum pump can be started as soon as the backing pump alone has pulled the pressure down to the allowable inlet level.

The Correct Shutdown Procedure for Roots Vacuum Pumps

The shutdown procedure for Roots vacuum pumps is equally critical—and perhaps even more frequently mishandled. An incorrect shutdown sequence can cause oil backflow, system contamination, and serious equipment damage.

Step 1 – Close the Air Ballast Valve (If Applicable)

If the backing pump of the Roots vacuum pump is an oil‑sealed mechanical pump and it has a gas ballast valve that is open, the first shutdown step is to close the air ballast valve on the backing pump.

What to do:

• Close the gas ballast valve on the front‑stage oil‑sealed mechanical pump.

• This step applies only if the gas ballast was open during operation.

Why this matters: Closing the gas ballast valve prevents air from being drawn into the system during the shutdown process, which could affect the vacuum integrity.

Step 2 – Close the Roots Vacuum Pump Inlet Valve

The next step is to close the inlet valve of the Roots vacuum pump.

What to do:

• Close the inlet valve on the Roots vacuum pump to isolate it from the vacuum system.

• This valve is typically located on the suction pipe between the vacuum chamber and the Roots vacuum pump.

Why this matters: Closing the inlet valve isolates the Roots vacuum pump from the process, preventing any backflow of gas or contamination from the system into the pump.

Step 3 – Stop the Roots Vacuum Pump First (Critical!)

This is the single most important step in the entire shutdown procedure: always stop the Roots vacuum pump first, then stop the backing pump.

What to do:

• Turn off the Roots vacuum pump motor.

• Confirm that the Roots vacuum pump has come to a complete stop before proceeding.

Why this matters – The critical error to avoid: Stopping the backing pump before the Roots vacuum pump will create a pressure differential that can cause oil from the backing pump to be sucked backward into the Roots vacuum pump chamber. This oil contamination can damage the rotors, seals, and internal surfaces of the Roots vacuum pump. As industry guidance emphasizes: "First switch off the Roots pump, then the backing pump". It is "strictly forbidden to mistake the shutdown procedure".

Step 4 – Stop the Backing Pump and Vent to Atmosphere

With the Roots vacuum pump stopped, the next step is to stop the backing pump and immediately vent the backing pump inlet to atmosphere.

What to do:

• Turn off the backing pump motor.

• Immediately open the vent valve on the backing pump inlet to admit air.

• This venting breaks the vacuum and equalizes the pressure.

Why this matters: Venting the backing pump inlet immediately after stopping prevents oil from being drawn up into the vacuum system due to the remaining pressure differential. It also makes it easier to restart the system later.

Step 5 – Close the Cooling Water Inlet Valve

For Roots vacuum pumps equipped with water‑cooling systems, the final step is to close the cooling water inlet valve.

What to do:

• Close the valve on the cooling water supply line.

• This step applies only to water‑cooled Roots vacuum pumps.

Step 6 – Drain Cooling Water for Extended Shutdown or Winter Storage

If the Roots vacuum pump will be out of service for an extended period, or if there is a risk of freezing temperatures, it is essential to drain any remaining water from the cooling water jacket.

What to do:

• Open the drain plugs or valves on the cooling water jacket.

• Allow all water to drain completely.

• Leave the drain valves open to prevent any trapped water from freezing and cracking the cooling jacket.

Why this matters: Water left in the cooling jacket during freezing temperatures will expand as it freezes, potentially cracking the casting and causing expensive damage.

Summary – The Startup and Shutdown Sequences at a Glance

Startup Sequence (5 Steps)

H3: Shutdown Sequence (6 Steps)

Common Mistakes to Avoid

Even experienced operators can make errors when starting or stopping Roots vacuum pumps. Avoid these common pitfalls:

The Role of Automatic Control Systems

Many modern Roots vacuum pump systems are equipped with automatic control that can manage the startup and shutdown sequences without manual intervention. Pressure sensors monitor the system pressure and start the Roots vacuum pump automatically once the allowable inlet pressure is reached. Automatic systems can also manage the shutdown sequence, ensuring that the pumps are stopped in the correct order.

However, operators must still understand the underlying sequences. Automatic systems can fail, and operators must be able to recognize when the sequence is proceeding correctly and intervene if it is not. Training on the correct startup and shutdown procedures is essential for all personnel who operate Roots vacuum pumps.

Conclusion – Discipline Is the Key to Long Service Life

The startup and shutdown procedures for Roots vacuum pumps are not arbitrary bureaucratic requirements—they are essential safeguards designed to protect the equipment from damage and ensure reliable operation.

The startup sequence is built on one fundamental principle: never start the Roots vacuum pump until the backing pump has reduced the system pressure to a safe level. This prevents excessive pressure differential, overheating, and rotor seizure.

The shutdown sequence is built on one equally critical principle: always stop the Roots vacuum pump before stopping the backing pump. This prevents oil backflow, contamination, and pump damage.

By following the procedures outlined in this article—starting the backing pump first, opening the pre‑evacuation valves, monitoring the pressure, closing the pre‑evacuation valve, and then starting the Roots vacuum pump—operators can ensure safe and reliable startup. By closing the gas ballast valve, closing the inlet valve, stopping the Roots vacuum pump first, then stopping the backing pump and venting, closing the cooling water, and draining for winter storage, operators can ensure proper shutdown.

For any facility that relies on Roots vacuum pumps for critical processes, these procedures are not optional—they are essential. The small investment of time in following the correct sequences is far less than the cost of premature equipment failure and unplanned production downtime. Train your operators, post the procedures prominently, and never allow shortcuts. Your Roots vacuum pumps—and your production schedule—will thank you.