Hi Daniel,
When you view this item on homedepot.com, you will see a black box outlining a section labeled " Info & Guides." In this section you will see 4 links that you can click on. One of the links is "Installation Guide." If you click this link, it will take you to our HOME-FLEX Underground System Design and Installation Manual. One of the most important statements in the manual is that you must perform your installation in accordance with the plumbing code that is in effect for the location of the installation. For Example, if the required amount of cover below grade for your location is 18 inches, and your trench is 20 inches deep, then the answer to your question is no. You may not install the 90 degree elbow vertically, because then the fitting and pipe will not remain 18 inches below grade. The pipe and fitting will be too shallow in the ground. Our instruction sheet and our Installation manual also provides our contact information. You can give us a call and we will answer your questions. We are here from 8-5 Los Angeles time M-F (661) 257-3923 Ask for Jon or Dave