Liquid Ring Vacuum Pump
Start Up
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Greetings!
This month’s article is going to focus on what is required to successfully install and start a liquid ring vacuum pump system. Many of the pitfalls associated with liquid ring pumps are due to mistakes made during the pump’s installation and initial start-up. These issues can have immediate repercussions or can cause issues that linger throughout its operational life.
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Initial Installation Concerns
A lot of potential issues and headaches can be avoided by paying close attention to a liquid ring pump’s initial installation. These issues often get overlooked and can result in failed pumps or a shorter operating life. Here are some of the most common issues, outlined below in detail.
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Pump and Motor Alignment – Liquid ring vacuum pumps need to be aligned to the motor to ensure good reliable operation. Packages provided by Graham are aligned at our factory, but should be checked before the piping is installed and then checked again once the pump’s suction and discharge piping has been connected. A laser alignment tool or dial indicators can be used to achieve good alignment. If misalignment is found, the motor should be shimmed to align with the pump, not the other way around. Misalignment will put unnecessary forces on the pump which will cause premature bearing and mechanical seal failures as well as shorting the operating life of the pump.
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Soft Foot – Soft foot is caused when the pump’s feet are not sitting flat against the baseplate. If the feet are tightened down, they will put twisting forces on the pump. This can cause premature bearing and mechanical seal failures as well as shorting the operating life of the pump. Our policy at Graham is to furnish a machined baseplate so the LRVP and Motor can be properly aligned and that soft foot is avoided. After installation, the absence of soft foot should be confirmed. It can be checked by aligning the unit, then loosening the hold down bolts to see if the alignment changes. If soft foot is identified, the baseplate should be modified, or shims can be used to correct this issue.
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Pipe Stress – Forces on the pump’s suction or discharge connections from connected piping should be avoided. The liquid ring pump should not be used as a piping support, the connecting piping should be supported independently. Suction and discharge bolts of the LRVP should be loosened prior to startup. If you can remove the bolts, this will prove the LRVP is not being subjected to piping stresses. If the piping moves, or the bolts cannot be removed, the piping should be modified. Excessive piping stress can cause mechanical seals and bearings to fail prematurely and will put additional forces on the pump’s rotating element which can lead to untimely failures.
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Suction Piping – When designing and installing the pump’s suction piping, loops or low points should be avoided to prevent liquid from accumulating. If low points allow liquid to gather, a slug of liquid may be drawn into the pump suction during startup. While liquid ring pumps are designed to handle wet process gases, and some liquid in their suction is acceptable, a solid slug may cause damage and premature failure.
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Start-Up Problems
Once a liquid ring pump has been properly installed, it will need to be started for the first time. During this initial start-up, there are some potential hazards that operations need to be aware of.
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Debris - During initial start-up or after an outage where work has been performed on the system, there is the threat of debris. Specifically, hard metal debris poses the most threat to the pump. Debris entering the pump can damage the pumps impeller and shaft. Piping should be flushed out before the LRVP is piped to the system, and a conical in-line strainer may be temporarily installed upstream of the pump in the suction line to prevent debris from entering the pump’s suction.
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Fig. 2 Impeller Damage by Debris in the Suction Line |
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Flooded Start-Up – While a liquid ring pump requires service liquid to operate, starting the pump fully flooded can prevent the pump from starting or cause mechanical failure. Liquid ring pumps are compressors and liquid is not compressible. When you start one of these pumps completely flooded with liquid, the pump is unable to compress the liquid and either the pump’s motor trips out on high load or the pump physically fails. To prevent flooding, package design should be such that the liquid level at shutdown is at the LRVP centerline. Any external sources of liquid should be isolated when the pump is shut down. Before starting the pump, it should be drained to the pump’s centerline or below to ensure that no flooding is present before the pump is started.
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Fig. 3 Damage Due to Starting Pump Flooded |
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Inadequate Service Liquid – Maintaining a good service liquid flow is vital to ensuring good operation. If no service liquid flow is present while the pump is being operated, it will likely compromise the pump’s mechanical seals. If the flow is too low or the supply temperature is too hot, the pump will cavitate. Cavitation damage won’t be noticed right away but will present itself with time. Ensuring that service liquid is introduced immediately after the pump is started is vital to it working correctly and avoiding damage.
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Isolation Valves - Many liquid ring vacuum pump installations have isolation valves to isolate the process at the suction and discharge of the pump skid. When starting up a liquid ring pump both suction and discharge isolation valves should be fully opened. Specific attention needs to be given to the discharge valve and downstream piping to ensure that the discharge is not blocked in. If a pump is started while isolated at the discharge, the discharge pressure will rise up until either the motor trips out at high load or the pump fails. Verifying that any isolate valves are open should be included in any start-up procedure.
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Start-up Procedures
- Turn the shaft manually to ensure it rotates freely. If the pump is binding or seized, refer to the troubleshooting chart found in the Graham Operation, Installation, and Maintenance Manual.
- Fill the pump with service fluid to the shaft centerline, but do not overfill. If the pump is designed for external mechanical seal flush, the flush liquid should be started at the same time as the LRVP motor. If a recirculated system is used, start the cooling water flow to the supplied heat exchanger.
- The normal service liquid level should be no higher than the shaft centerline. The pump may be started with a low service liquid level as long as a supply is available immediately after start-up.
- Open any valves in the suction and discharge lines.
- Confirm the pump rotation with the arrow on the casing by jogging the motor.
- Start the motor, ensure service liquid supply, and set regulating valve, when used, for optimum pump performance. Open and adjust the shaft seal cooling liquid valve, when used.
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Pump Control Issues
Once a pump has been installed and started without issue, the control room and DCS system is likely to be receiving feedback from multiple instruments. These readings can be used to execute a control scheme. There are several issues that can cause problems at this point in time.
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Gauge Calibration – Pump control requires feedback from transmitters in order to function and that control is only as good as the instrument calibration. Even if an instrument was calibrated by the supplier, after shipment and lapsed time between it being ordered and the initial start-up the instrument has the potential to lose its accuracy. It’s recommended with a new installation or during an outage to make sure that all gauges and transmitters are calibrated.
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Vibration Monitoring – Some installations will have vibration probes installed on the pump and/or motor to monitor vibration levels. While this is a good practice it can lead to some frustrating moments if the vibration trips are not set up correctly. When a liquid ring pump is first started, the liquid in the pump is not moving. The service liquid needs to be accelerated at start up so that the liquid ring inside the pump can be established. During this period, pump vibrations will be elevated compared to the normal operating levels. If the vibration trip points are set to go into effect immediately after the pump is started it can cause the pump to trip out in the initial few seconds of operation. Often this is overcome by including a delay on a vibration trip until after the pump has been started and the liquid ring established.
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Cavitation – Another common failure with LRVP's is cavitation damage due to too high capacity vs too low non-condensable flow rates. In this case the LRVP pulls to a deeper vacuum level and flashes the seal liquid. Cavitation can be prevented by installing a vacuum relief valve or recycle valve to keep the suction pressure at a reasonable level, typically 5 mm Hg above the vapor pressure of the seal liquid at the LRVP discharge temperature.
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Recycle Controls – Many pump installations have recycle controls, which are used to maintain a specific inlet pressure. In order for the recycle control to work correctly, any manual block valves should be opened and the recycle control valve should be fully opened at start up. Once the liquid ring pump has been started, the recycle control valve can then be slowly closed until the desired vacuum is achieved. Often the recycle control valve is one of the only actuated valves on the liquid ring pump skid. If the valve happens to be pneumatic, then the control valve will require the air supply be operational.
Hopefully these suggests help ensure that your liquid ring pump installation and start-up go smoothly. Graham is available both by phone and on-site and can assist with any installation or start-up questions or concerns. Please don’t hesitate to contact Graham for assistance.
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More content is available at Graham's YouTube Channel. If you are interested please don't hesitate to click on the YouTube link and subscribe. |
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Graham Gulf Coast Office
P.O. Box 3940
Covington, LA 70434
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Eric Johnson 585-815-9432
Alicia Johnson 585-815-9463
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