When a model is opened by a user, a lock is not placed on the files and another user is able to open and work on it. Is this expected? Is there a way to make it so a project to only be opened by a single user?
Currently it is not possible to prevent multiple users from opening a project that is saved to a network drive.
When a model is opened, the model files are copied from the location where they are stored, to the Windows Temporary folder (type %temp% in the address bar in File Explorer and look in the Bentley subfolder). When the user is working on the model, it is actually working off the copy from the Windows Temp folder, not the one in the folder where it is saved to. When the model is saved, it is copied from the Temp folder to the folder where the model is saved to, overwriting the old files.
So, if the model is stored on a shared network folder and user A opens it, user B will still be able to open the model from the network folder. At that point, they will each be working off of separate copies of the model in their respective computer's Temp folder. If user A makes changes and saves the model, it will be written back to the network folder. If user B is making changes at the same time and saves the model after user A, then user B's changes will essentially overwrite the changes made by user A, since their version of the model will be copied from their Temp folder and overwrite the current version of the model stored on the network drive.
If the project needs to be saved to a network drive, you may want to look into ProjectWise. ProjectWise provides a managed environment for multiple users to work on models. ProjectWise allows for models to be checked out by a user. When this is done, the model will not be available to other users, though it will still be listed in ProjectWise. Information on who has the file checked out is also available, so it will be easy to know who is working on what files at a given time. More information on ProjectWise can be found in the link above or through your Bentley account manager. Information on integration between the OpenFlows products and ProjectWise can be found here: ProjectWise Integration.
If ProjectWise is not an option and you cannot change the network drive configuration to stop files that are already accessed from being accessed again, it is sometimes better to have the model files saved to the local drive. If a user is working on a project, they can move the SewerCAD files out of the network drive so they are literally no longer there for others to use. They can then work on file and save it back to the network drive.
If the other users do not need to make changes to the model but rather simply view it (and element attributes), consider periodically exporting to i-model or Bentley Map Mobile.