Learning from Preferred’s Main Steam Line Plug Operating Experience

Preferred Engineering has extensive experience with the installation and removal of Main Steam Line Plugs. Over the years, we’ve developed a number of best practices that are key to maximizing efficiency and cutting critical path time. Here are some of the most important procedures we’ve established in recent MSLP projects:

Camera Positioning

Camera placement is one of the most critical considerations in MSLP operation. We have found that the most effective camera placement is two color cameras at the nozzle elevation, outboard of the nozzle pair being installed/removed. The near camera provides closeup shots of the plug position while the camera staged at the far nozzle provides an overview shot of the installation. The function of the camera is simply reversed when moving to the second nozzle. Stud mounted cameras can serve a similar overview function.

Hose Management

It’s important to establish how the vent hoses are being routed and who is responsible for this task during installation. We have observed that, during an outage, time can be lost while deliberating where and how the hoses should be routed. We recommend that you have a plan of how to route the hoses before approaching this step in the MSLP installation process.

The vent hoses on the MSLPs are cumbersome to handle, especially after the plugs have been installed, because they come out wet and contaminated. You should make a decision about handling the vent hoses before plug removal. They are reusable, but most plants elect to cut them off at the MSLP and discard them. Since they are a pressure boundary and are typically safety related, there is a lead time associated with reordering them. Please note that the ends of the vent hoses have brass ball valves that are used to isolate the hose to allow the plant to perform the LLRT against the plugs. These valves are non‐safety related and are typically in stock; however, we suggest that the ball valves be removed from the ends of the vent hose and saved in the plug storage boxes for next use.

Proper Platform Use

Some refuel machines have overhangs that make using the refuel machine a difficult platform to work from. The overhang adds challenges to the crews as they try to work the equipment into and out of the MSL nozzles by interfering with their ability to position the crane and equipment close to the bridge for easier manipulation.


Using the MSLPs to backfill the main steam lines is an ongoing practice. Typical times to backfill the lines range from 1 to 1‐3/4 hours. Lowering the open vent hose valve slightly under the water while backfilling is an easy way to determine when the lines are full. The bubbles flow steadily as the lines are filling and stop when filled.

Coupling Position

Coupling position is critical. We have seen multiple instances over the years of damage to the IR tool coupling and/or plug shaft due to faulty positioning of the IR tool couple. The couple must only be in the up (coupled) position when it is physically coupled to the plug shaft. If the coupling is in the coupled position but not coupled to the plug, then moving the main cylinder toward the uncoupled plug will result in significant damage to the plug and/or tool.

Critical Spare Parts

We cannot overemphasize the importance of keeping critical spare parts onsite and accessible, especially during outage season. We have provided each MSLP installation with a number of critical spare parts, and we can assist you in making sure that you have what you need on hand.

Preferred Engineering’s Main Steam Line Plugs have been in use for nearly three decades, isolating steam lines during refueling outages and facilitating MSIV maintenance and testing. Their passive design requires no input energy to maintain a firm seal once installed, and their dual seals offer redundant isolation. The MSLP’s polished stainless steel construction promotes effective decontamination, with built-in adjustability that allows the plugs to accommodate variations in nozzle geometry. To learn more about the advantages of our MSLP products, contact Preferred Engineering today.

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