Vacuum Pump Shutdown Sequence: Stopping Production Safely

Table of Contents

• Why Shutdown Is a Procedure
• Process Preparation Before Shutdown
• Sequence for Cutting Vacuum and Venting the Line
• When to Shut Off Service Water and Cooling
• Electrical Isolation and Lockout
• Checks and Records After Shutdown

Stopping a vacuum pump may look as simple as starting it, but a shutdown carried out in the wrong order can cause backflow in the vacuum line, liquid entering the pump or an unwanted pressure change in the process. This article describes the sequence to follow to stop the pump and the process safely.

Why Shutdown Is a Procedure

A pressure difference exists between the pump and the process, and this difference changes quickly at the moment of shutdown. When the pump stops, backflow can occur in the line, and condensed vapour or dust in the process can return toward the pump. For this reason, shutdown should always follow a written sequence.

The procedure protects both the equipment and the operator. When it is known in advance who closes what and in which order, the chance of error is reduced, even in an emergency.

Stopping a vacuum pump is as important an operation as starting it, and it should not be done at random. A mistake made during stopping can damage the internal parts of the pump, the process line or the people working nearby. For this reason the stopping operation should be tied to a written procedure and carried out in the same way by everyone.

One purpose of the procedure is to reduce the risks that can arise during stopping. The order in which the valves are closed, the order in which the power is cut and the time at which the cooling water is stopped should all be clearly set. In this way everyone knows what they are doing at each stage of the operation.

There is a big difference between a planned stop and an emergency stop. In a planned stop the process is prepared, the line is emptied and the pump is shut down safely. In an emergency stop time is limited, and for this reason an emergency procedure must also be prepared in advance.

The stopping procedure depends on the type of pump. The stopping steps of a monoblock pump and of an oil lubricated vane pump are different. In the oil type, the oil temperature is expected to fall, whereas in the liquid ring type the time at which the service water is cut is important. A separate procedure should be prepared for each type.

Process Preparation Before Shutdown

When the decision to shut down is taken, first check the state of the process. Make sure the product or vapour under vacuum has reached a safe level. Communicate with the process team to tell them the pump will be stopped and what effect this will have on the process.

If there is any build-up of liquid or solids in the process, drain it. Confirm that the drain points are open so that condensed liquid in the line is not drawn back into the pump.

Before stopping, the process must be prepared. The basis of this preparation is stopping the process equipment connected to the pump, bringing the product to a safe point and emptying the residues inside the line. If the pump is stopped suddenly, the material in the process can flow back or become jammed in the line.

During preparation, the condition of the product should be checked. A pump stopped before production has finished can leave the line full of half finished product. This causes blockage and quality problems at the next start. The stopping should begin only after the condition of the product has been clarified.

Another part of the preparation is informing the operators. The time of the stop, its duration and the work to be done afterwards should be shared with everyone concerned. A stop made without this communication can cause another team to start the pump without waiting.

The order of cutting the vacuum is a critical part of the procedure. The general rule is that the vacuum line is closed first and then carefully opened to the atmosphere. Fully opening the line to the atmosphere while the pump is still running can place a reverse load on the pump and cause damage to the internal parts.

Sequence for Cutting Vacuum and Venting the Line

The general rule is to cut the vacuum from the process first and isolate the line in a controlled way. First close the suction valve of the pump, then stop the pump. In this way the pump is separated from the line under vacuum, and the pressure difference in the line can be balanced in a controlled manner without causing backflow.

If the line has to be opened to atmosphere, do this after the pump has stopped and through a controlled valve. Opening quickly can blow dust or liquid back out of the line. To know in advance how the system will behave, use the plant's own procedure as the basis.

Opening to the atmosphere should be done slowly. Opening the valves suddenly creates a fast flow of air in the line, and this flow can drag dust or liquid residues from the line towards the pump. A slow opening reduces this risk and lets the system gain pressure in a balanced way.

The position of the shut off valve in the vacuum line is part of this sequence. It should be checked that the valve is closed and that the handle of the valve opened to the atmosphere is in the right direction. A valve left in the wrong position can cause an unexpected loss of vacuum at the next start.

After opening to the atmosphere, it should be confirmed that the pressure in the line has balanced. A gauge should show a value close to zero. Moving to the next step without this check can create an unexpected pressure problem when the pump cover is opened or when maintenance is done.

When the service water and cooling are to be turned off is an important point of the stopping procedure. If the cooling is cut before the pump has completely stopped, the internal temperature rises quickly. For this reason the cooling and the service water should usually be shut off some time after the pump has stopped.

When to Shut Off Service Water and Cooling

On liquid ring pumps, service water should stay open until the pump has come to a complete stop. Shutting off the service water before the pump stops can lead to dry running inside and friction. After the pump has stopped, close the water line and drain it in a controlled way.

On pumps with a cooling system, cooling must continue until the temperature of the pump has dropped. Suddenly cooling or fully shutting off a hot pump can cause thermal stress in the internal parts.

How long after stopping should the service water be cut? This time should be set according to the size of the pump, the temperature values and the recommendation of the manufacturer. In general it is common practice to keep the water flowing until the internal temperature of the pump has fallen to a safe level. This period should be clearly written in the records and in the procedure.

In liquid ring pumps, cutting the service water completely can let the water left inside the pump dry out and leave deposits. These deposits can build up on the inner surfaces and cause problems at the next start. For this reason, cleaning and drying the inside of the pump after it has stopped should be planned.

The timing of the cooling water shut off becomes even more important in the winter months. In an environment with a risk of freezing, the water left inside the pump can freeze and seriously damage the internal parts. In this case it is essential to drain the pump completely and empty the water lines.

Cutting the electrical supply is the last electrical step of the stopping sequence. After the power to the motor has been cut, it should be confirmed that the pump has fully stopped and is not turning. Touching a turning pump or opening its cover can cause serious injury.

Electrical Isolation and Lockout

After the pump is stopped, the electrical supply should be isolated at the main switch and, where possible, locked out. Locking out prevents another person from restarting the pump by mistake. If maintenance is to be carried out, confirm that the energy has really been isolated using a voltage tester.

This step is indispensable, especially during maintenance or repair work. Lock and tag application should follow the plant's own procedure.

Electrical locking is done to prevent accidental starting during maintenance. Locking the switch, hanging a warning label and, if necessary, keeping the lock key with the person responsible for maintenance is a safe practice. This lock should not be removed until the maintenance is complete.

The energy isolation must be done not only at the motor switch but also in all the circuits that feed the pump. If the control circuit, sensor supply or heating elements are fed from a separate source, it should be checked that these sources are also switched off. An incomplete isolation can lead to unexpected running or electric shock.

The electrical isolation and locking should also be documented in a record. Who cut the power, at what time, how the lock was put on and who will remove it should be written down. This record creates a chain of responsibility for safety.

The check after stopping is made to confirm that the pump has been shut down safely. The position of the valves, the state of the power isolation, the time at which the water flow was cut and the pressure of the line are the main headings of this check. Every item on the check should be ticked on a list.

Checks and Records After Shutdown

Once shutdown is complete, inspect the outer surface of the pump, the connection points and the drain line. If there are signs of liquid leakage, a burning smell or damage, record them and inform the maintenance team.

Record the reason for the shutdown, the time and the steps taken. This information is a valuable reference at the next start-up and when investigating a possible fault. Plants that keep a consistent shutdown record diagnose problems far more quickly.

The record after stopping forms the shut down history of the pump. The date and time of the stop, its reason, the last operating values and the operations carried out should be written down. This record is used as a reference at the next start and makes it easier to understand the cause of any problem.

Observations made after shut down should also be recorded. Information such as the sound the pump makes as it stops, the internal pressure or the state of the oil gives valuable data for later maintenance. If an irregular sound or oil contamination was observed in particular, this must be noted.

In conclusion, stopping the pump correctly should be seen not only as a shut down operation but as a safety and maintenance process. When the procedure is followed, the pump is ready for the next start without problems, and both the people and the equipment are protected.