Go here:

http://www.elsevier.com/wps/find/bookdescription.cws_home/677491/description#description

Buy the book and read Chapter 6 and 10 (along with the rest of the book).

This applies to piping but you should apply it to all disciplines.

Each industry ( and it's associated accounting/purchasing types) has it's own method but you may be able to do a "Request for Proposals" to develop the scope of work. This is where you actually pay the company with the winning proposal to design the project which compensates them for their design work ( fair and ethical) and gives you the right the freely distribute the approved scope of work to all bidders for like quotes so purchasing is comparing apples to apples.

I used to try to get 5 quotes and submit the top 3 to purchasing so they could call around and do their mandatory "cost savings" negotiations. The capital request was then forwarded to accounting and if it looked good they would send it to corporate for their required approvals. (this was all a painful, inefficient but mandatory process)