Do you know the startup procedure for a Roots vacuum pump?

Since its introduction to China, the Roots vacuum pump has rapidly gained widespread adoption across numerous industries. Its outstanding performance, operational stability, and long service life have earned high praise from users in petrochemical processing, pharmaceutical manufacturing, metallurgy, electronics, and countless other fields. However, despite its robust design and proven reliability, the Roots vacuum pump remains a precision piece of equipment that demands correct operational procedures—particularly during startup.

Yet many operators, whether due to haste, insufficient training, or simple oversight, frequently make mistakes during the startup sequence of a Roots vacuum pump. These errors can lead to motor overload, overheating, rotor seizure, and even catastrophic failure. The consequences extend beyond equipment damage: unplanned downtime, costly repairs, and production losses that could have been entirely avoided.

This article provides a comprehensive guide to the correct startup procedure for a Roots vacuum pump, explains why each step is critical, highlights common mistakes, and offers practical advice for ensuring safe and reliable operation. Whether you are a plant engineer, maintenance technician, or operator, understanding the correct startup sequence for your Roots vacuum pump is essential knowledge.

Why the Startup Procedure for a Roots Vacuum Pump Is So Critical

Understanding the Roots Vacuum Pump's Design Limitation

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: a Roots vacuum pump cannot discharge directly to atmosphere [9†L6-L7].

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 [9†L7-L8][10†L12-L13].

This is why, with the exception of multistage, wet, and air-cooled Roots vacuum pumps, conventional Roots vacuum pumps must always be used in combination with a backing pump (also called a fore-vacuum pump or pre-stage pump) [9†L6-L7]. The backing pump reduces the system pressure to a level at which the Roots vacuum pump can operate safely.

The Purpose of the Backing Pump

The backing pump serves two essential functions in a Roots vacuum pump system:

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) [10†L15].

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.

Without a properly functioning backing pump and a correct startup sequence, a Roots vacuum pump cannot operate safely or reliably [9†L7-L8].

The Correct Startup Procedure for a Roots Vacuum Pump

Pre-Startup Preparations

Before initiating the startup sequence for a Roots vacuum pump, operators must complete several essential preparatory steps:

1. Check the lubricating oil level:
The gearbox and bearing housings must contain the correct amount of lubricating oil. For most Roots vacuum pumps, the oil level should reach approximately 2/3 to 3/4 of the diameter of the oil sight window [7†L4-L5][10†L9]. The gearbox side should have the oil level covering 3–5 gear teeth, while the motor side should be slightly above the center of the sight window [10†L9]. Use only No. 1 vacuum pump oil as specified by the manufacturer [7†L5][10†L9]. If the oil appears milky, turbid, or dark, replace it immediately before proceeding [8†L16].

2. Verify the cooling water system:
For water-cooled Roots vacuum pumps, open the cooling water inlet valve and confirm that the cooling water flows smoothly [7†L7-L8][8†L17-L18]. Ensure that both inlet and outlet valves are functioning correctly.

3. Inspect the electrical system:
Confirm that the motor rotation direction matches the arrow indicated on the Roots vacuum pump housing [10†L5]. The electrical system must include interlock protection devices so that if the backing pump stops, the Roots vacuum pump also stops simultaneously [10†L8].

4. Check the system piping:
Ensure that all piping connections are clean, free of debris, and properly sealed [10†L6]. If the gas being pumped contains dust or particles, install a dust filter or debris screen at the suction port [7†L12-L13]. If the gas is corrosive, appropriate neutralization measures must be taken [7†L14].

5. Close all isolation valves except the inlet valve:
Before starting the Roots vacuum pump, close all isolation valves in the system except the inlet valve to prevent any backflow of air or gas into the pump during startup [0†L21-L23].

Step-by-Step Startup Sequence

Once the pre-startup preparations are complete, follow this exact sequence to start your Roots vacuum pump safely [7†L18-L24][9†L8-L15][10†L9-L11]:

Step 1: Start the backing pump (pre-stage pump).
The backing pump—whether a water-ring pump, rotary vane pump, or other type—must be started first. This begins the pre-evacuation process [7†L18][9†L9][10†L10].

Step 2: Open the pre-evacuation valves.
Open the valve on the backing pump's pre-evacuation bypass line and the valve on the Roots vacuum pump inlet pipe [7†L18-L20][9†L9-L10][10†L10]. This connects the system volume to the backing pump, allowing the rough vacuum to be established.

Step 3: Monitor the system pressure.
Allow the backing pump to reduce the system pressure to the Roots vacuum pump's allowable inlet pressure. This value varies by manufacturer and model but is typically below 1,330 Pa [10†L15]. The permissible cut-in pressure depends on the ratio between the Roots vacuum pump and the backing pump [6†L8-L9].

Step 4: Close the pre-evacuation valve (if applicable).
Once the system pressure reaches the allowable inlet pressure for the Roots vacuum pump, close the valve on the pre-evacuation bypass line [7†L21-L22][9†L11-L14][10†L10].

Step 5: Start the Roots vacuum pump.
With the pressure condition satisfied, start the Roots vacuum pump [7†L22][9†L14][10†L10-L11].

If your system has no pre-evacuation bypass line or valve, simply wait until the backing pump alone has pulled the pressure down to the allowable level, then start the Roots vacuum pump [7†L22-L24][9†L14-L15][10†L11].

Automatic Control Option

Many modern Roots vacuum pump systems are equipped with automatic control devices. When configured for automatic operation, pressure sensors monitor the system pressure and start the Roots vacuum pump automatically once the allowable inlet pressure is reached. This process can achieve automation rates of up to 92% in startup procedures [8†L33-L34][7†L24-L25][10†L11].

While automation reduces the risk of human error, operators must still understand the underlying sequence and be prepared to intervene manually if the automatic system fails.

Special Considerations for Large vs. Small Vacuum Systems

The startup procedure for a Roots vacuum pump may vary slightly depending on the size of the vacuum system.

Small Vacuum Systems

For small sealing systems, the Roots vacuum pump can be started immediately after the backing pump begins operation [7†L26-L27][10†L11-L12]. The system volume is small enough that the pressure quickly drops to the allowable level, and the risk of overheating is minimal.

Large Vacuum Systems

For larger vacuum systems, a more cautious approach is required. Although many Roots vacuum pumps are equipped with bypass (overflow) valves that provide overload protection, this protection is primarily for the motor, not the pump itself [7†L28-L30][10†L12-L13].

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 [10†L12-L13]. The bypass valve protects against differential pressure spikes, but it does not prevent the gradual heat buildup that occurs when the Roots vacuum pump runs at high pressure for too long.

For large vacuum systems, always wait until the backing pump has reduced the system pressure to a safe level before starting the Roots vacuum pump [7†L29-L30][10†L13].

Common Startup Mistakes to Avoid

Even experienced operators can make errors when starting a Roots vacuum pump. Avoid these common mistakes:

Operation Monitoring During Running

Once the Roots vacuum pump is running, continuous monitoring is essential to detect problems early and prevent failures.

Temperature Monitoring

Under normal operating conditions:

• The maximum temperature rise of the Roots vacuum pump should not exceed 40°C above ambient [7†L39-L40][8†L38-L39][10†L16].

• The maximum absolute operating temperature should not exceed 80°C [7†L40-L41][8†L39][10†L16].

• If a water-ring pump is used as the backing pump and the pumping speed ratio is relatively high, the temperature rise may be higher, but the maximum temperature should still not exceed 100°C [10†L16-L17].

Motor Load Monitoring

Regularly check the motor load using a power meter, ammeter, or voltmeter. An increase in motor current without a corresponding change in process conditions may indicate increased rotor-to-rotor or rotor-to-housing contact, contamination inside the pump, or backing pump inefficiency [7†L38].

Noise and Vibration Monitoring

During operation, the Roots vacuum pump should produce a uniform, smooth sound without irregular noise or abnormal vibration [7†L41-L42][10†L17]. Any grinding, rattling, or knocking sounds are red flags that require immediate investigation.

Emergency Response

If any of the following conditions are observed during operation, stop the Roots vacuum pump immediately and investigate the cause [7†L42-L44][10†L18]:

• Motor overload

• Excessive temperature rise

• Abnormal noise

• Excessive vibration

The Shutdown Procedure – A Critical Complement to Startup

Correct shutdown is just as important as correct startup. Follow this sequence when stopping a Roots vacuum pump [7†L45-L51][9†L17-L24][10†L13-L14]:

Step 1: Close the inlet valve of the Roots vacuum pump [10†L13].

Step 2: Stop the Roots vacuum pump first, then stop the backing pump [7†L48][9†L20-L21][10†L14]. The order must never be reversed. This is a critical safety requirement.

Step 3: Immediately after stopping the backing pump, vent the backing pump inlet to atmosphere [7†L49-L50][9†L22-L23][10†L14].

Step 4: Close the cooling water inlet valve [7†L51][9†L24].

Step 5: If the pump will be out of service for an extended period or if freezing temperatures are expected, drain any residual water from the cooling water jacket to prevent frost damage [9†L25-L26].

Shandong Zhangqiu Blower Co., Ltd. – Supporting Correct Roots Vacuum Pump Operation

Shandong Zhangqiu Blower Co., Ltd. , founded in 1968, has over 50 years of experience in the design, production, and manufacturing of Roots vacuum pumps and related equipment. The company understands that even the highest-quality Roots vacuum pump will underperform or fail prematurely if not operated correctly.

Shandong Zhangqiu Blower Co., Ltd. offers comprehensive technical support to help customers operate their Roots vacuum pumps safely and reliably, including:

• Detailed operation manuals covering startup, shutdown, and maintenance procedures

• On-site commissioning and training for new installations

• Technical consultation on operational best practices

• Original equipment manufacturer (OEM) spare parts for all models

The company's commitment to customer success extends beyond delivering high-quality Roots vacuum pumps—it includes ensuring that customers have the knowledge and support to operate them correctly.

Conclusion – Correct Startup Is the First Step to Long Service Life

The Roots vacuum pump is a remarkably reliable and efficient piece of equipment, but its performance depends entirely on correct operation—starting with the startup procedure. The sequence is clear and straightforward:

1. Start the backing pump first.

2. Open the pre-evacuation valves to establish rough vacuum.

3. Wait for the system pressure to reach the allowable inlet pressure for the Roots vacuum pump.

4. Close the pre-evacuation valve (if applicable).

5. Start the Roots vacuum pump.

This procedure is not optional. It is a fundamental requirement for safe and reliable operation of any Roots vacuum pump.

By following this sequence, monitoring operating conditions, and adhering to proper shutdown procedures, operators can ensure that their Roots vacuum pump delivers years of trouble-free service. Conversely, cutting corners or reversing the order invites motor overload, overheating, rotor seizure, and costly repairs.

For any facility that relies on Roots vacuum pumps for critical processes, the startup procedure deserves the same attention as any other safety-critical operation. Train your operators, post the procedure prominently, and never allow shortcuts. Your Roots vacuum pump—and your production schedule—will thank you.