Starting Up and Shutting Down Vacuum Systems

August, 2019

Greetings!  Our topic for this month is the proper way to start up and shut down an ejector vacuum system. There are many different configurations for vacuum systems. In this article we will cover the basics that will apply to most systems.

Starting Up

Check the motive steam inlet line. Blow clear the lines to insure no particles plug the motive nozzle throats. This should be done as close as possible to the motive steam inlet connection to insure there is nothing plugging the line. For example for new installation, there could be weld slag or chips in the line and if the unit has been idle rust or scale particles may be in the line. If strainers, separators, or traps are present, they should be inspected and cleaned after the lines are blown clear. When condensers are present insure the vapor outlet of the aftercondenser and all the condensers condensate outlets are open and free of obstructions. Make sure condensate legs are sealed. Insure the cooling water is flowing to the condensers before starting the ejectors. Open the vent valves when first starting the cooling water flow and close them once the cooling water flow is set and all the air is vented. Open the ejectors suction and discharge isolation valves if present. Note: If the unit has dual ejector elements with condensers present, make sure the condensers have been designed for both ejector elements operating. If the condenser has been designed for one ejector element operating, open the suction and discharge valves to one only and isolate the other ejector element. Fully open the motive steam valve to the last stage ejector (Z stages). Note: For optimum performance during an evacuation cycle, the motive valves should always be open starting with the last stage ejector (Z Stage) and proceeding to the next stage Y, X, etc. When a pressure gauge is present near the motive inlet, which is recommended, check to insure the motive steam inlet pressure is at or slightly above design operating pressure. The motive steam inlet pressure should not be below design.

Turbine exhaust surface condensers typically have twin 100% ejector elements. In this case all four ejectors can be operated at start-up to reduce the time required to evacuate the system. This is especially the case when a hogger ejector is not present. After the system has been evacuated to the normal operating pressure, one of the ejector elements (one first (Y) stage and one second (Z) stage) can be taken out of service to conserve motive steam usage. This should be done by closing the valves in the following order: First stage (Y stage) suction. First stage (Y stage) motive inlet. First stage (Y stage) discharge. Second stage (Z stage) suction. Second stage (Z stage) motive inlet. Second stage (Z stage) discharge (if present).

Shutting Down

Shutting down is broken into two ways, depending if vacuum upstream of the first stage ejector is desired or not.

Option 1: To be used to maintain the vacuum upstream of the first stage ejector rather than allowing the pressure to rise to atmospheric pressure. Note an isolation valve has to be present at suction. In this case the isolation valves should be closed in the following order. Close first stage (Y stage) suction valve. Close first stage (Y stage) motive inlet valve. Close first stage (Y stage) discharge valve. Close second stage (Z stage) suction valve. Close second stage (Z stage) motive inlet valve. Note: If more than two stages are present STOP HERE and continue to repeat steps, 3, 4, and 5 before proceeding to step 6 Close second stage (Z stage) discharge valve (when present). Note: If the system contains an isolating valve at the first stage suction only, the procedure would be to close this valve and then either shut off the motive to all ejectors at once or shut them off by stages, starting at the first stage. When all motive valves have been shut off, the cooling water may be turned off also. If the unit is going to be shut down for a short period of time to service the ejectors or for some other reason, it is not necessary to shut off the cooling water. Energy savings should be considered when making this decision. If the unit is going to be down and freezing of the cooling water is possible, then measures must be taken to prevent freezing or the unit drained as much as possible to prevent damage.

Option 2: To be used when maintain vacuum upstream of the first stage ejector is not required. In this case the isolation valves should be closed in the following order. Close the main motive valve to all the ejectors or close the motive valves to each individual stage, starting at first stage and continue on to second and so on. The cooling water may be turned off as explained in preceding paragraphs.

Switching Ejector Elements

Should it become necessary or desirable to shift from one two-stage element to another while the unit is in operation, proceed as follows: Open discharge valve of the standby second stage ejector (if provided). Open second stage motive valve. Open second stage suction valve. When this has been accomplished, this standby second stage ejector begins to take suction from the intercondenser along with the other second stage element. Open first stage discharge valve on standby element. Open first stage motive valve. Open first stage suction valve. At this point, both two stage elements are in parallel operation. The operating element can now be secured by closing the valves as follows: Close first stage suction valve. Close first stage motive valve. Close first stage discharge valve. Close second stage suction valve. Close second stage motive valve. Close second stage discharge valve (if provided).

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Graham Gulf Coast Office P.O. Box 3940 Covington, LA 70434 Eric Johnson 585-815-9432 Alicia Johnson 585-815-9463

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