There is one structural question about purchase order approval and everything else is detail: does it happen before the order goes to the supplier. Before, it is a decision. After, the commitment already exists and the approver is reviewing something they cannot undo without a commercial cost. Many organisations approve after issue for understandable reasons, and the fix is speed rather than sequence.
Why the sequence is the control
Once a supplier holds an order they may act on it, and in most commercial relationships you are committed whether or not anyone internal has agreed. An approval given afterwards can catch errors but cannot prevent the spend, which was the point. A process that approves after issue is a review process, and it is worth calling it that honestly.
Making it fast enough to survive
Approval before issue only holds if it is quick. Named approvers rather than groups, automatic cover during absence, escalation to a named alternative after a set period, and thresholds that keep small orders away from senior people. An approval that adds days will be bypassed, and the bypass will be undocumented.
Amendments, the quiet gap
An order approved at one value and later amended upward has escaped its threshold. A rule that re-triggers approval above a defined increase closes it. Without one, orders grow past their approved value one small change at a time, which is a finding waiting to be made rather than a decision anybody took.
Questions people ask about purchase order approval
Can small orders be auto-approved?
Many organisations do it to keep the process usable, with a periodic review of what passed through. Make it an explicit risk decision.
Who should approve?
The budget holder for the cost centre, against thresholds taken from your existing delegated authority rather than a parallel scheme.
What about framework agreements?
Usually a lighter approval, since the commercial terms were approved once. Write that down as a rule rather than leaving it informal.